794 AC Off-Highway Truck Systems Caterpillar

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The Power Train Electric Drive System will contain hazardous voltage levels during machine operation and for a short period of time after engine shutdown. Do not remove any covers that will expose energized high voltage electrical components while the engine is operating. Any type of maintenance on the following components can only be performed after the Power Train Electrical System Service Shutdown procedure has been followed: High voltage compartments in the inverter cabinet The rear axle housing that contains the electric drive traction motors The generator The retarding resistor grid, the grid blower motor and the grid system cabling The excitation field regulator The high voltage cables and connection enclosures Failure to follow these instructions could result in personal injury or death.

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In order to avoid the buildup of hazardous live voltages on the exposed surfaces, all grounding wires and grounding straps must be properly connected at all times during engine operation. Any disconnected grounding wires, including the grounding wires for all high voltage components and the grounding strap for the inverter cabinet must be properly reconnected before the engine is started. Failure to follow these instructions could result in personal injury or death.

The electric drive train system operates at high DC voltage levels and at high AC voltage levels. The system is designed to keep these voltages isolated from the truck frame and isolated in protected areas where any incidental contact cannot occur.

After reading this section of the manual, the user should be familiar with the operation of the drive train system. The user should be familiar with the components that are used in the system. This section will enable the user to understand the procedures and practices that must be followed before performing any maintenance procedures or troubleshooting procedures.

These procedures are located in the Troubleshooting, "Electrical Shutdown and Voltage Discharge" section of this manual. Read and familiarize the procedures. The procedures must be followed completely before performing maintenance procedures on the high-voltage electrical system on the truck.

System Operation Overview

The 794 AC Off Highway Truck is equipped with an electric drive train system. The electric drive train system eliminates the need for a mechanical transmission and rear differential.

In the FORWARD speed range, the electric drive train system enables travel speeds of 1.6 km/h (1.0 mph) to 60 km/h (37 mph) without shifting.

In the REVERSE speed range, the electric drive train system enables travel speeds of 1.6 km/h (1.0 mph) to 12.9 km/h (8.0 mph).

Illustration 1	g02082557
A simplified diagram of how an AC variable frequency drive system works

The electric drive train system is an AC variable frequency variable voltage drive system. The system is designed to provide variable levels of three-phase AC voltages to each of the electric traction motors. Electronic control of the traction motor supply voltage enables the electric drive train system to meet the travel demands of the machine operator.

The Inverter Cabinet houses most of the controls and components that are used to create and control the AC voltage supply for the traction motors.

Electronic control of the electric drive train system is provided by three Electronic Control Modules (ECM). The three electronic control modules are:

• the "Drivetrain ECM"

• the "Motor 1 ECM"

• the "Motor 2 ECM"

The "Drivetrain ECM" is located in the cab front ECM compartment. The two motor control modules are located in the "Inverter Cabinet".

High-speed operational communication between the three control modules is conducted on the CAN B Data Link circuits and the CAN A Data Link Circuits.

The "Drivetrain ECM" is the system priority control module. The "Drivetrain ECM" monitors the operational status of the complete system. The "Drivetrain ECM" receives the input command signals from the operator controls and monitors the status of various system components. The ECM coordinates the operation of the electric drive system with the operation of the other machine systems.

Based on the status of the system and the demands on the system, the "Drivetrain ECM" issues control commands to each motor ECM. Each "Motor Control ECM" provides the control capabilities to carry out the commands.

The main function of the Motor 1 ECM is to monitor and control the operation of the left-hand Traction Motor 1.

The main function of the Motor 2 ECM is to monitor and control the operation of the right-hand Traction Motor 2.

Each motor control ECM also provides other system functions that are discussed later in this section.

System Startup

The "Drivetrain ECM" will monitor and control the operation of the generator. The generator will be controlled to maintain the level of the DC Power Bus at a voltage that will meet the system loads. To enable the generator to provide the most efficient operation, the "Drivetrain ECM" will send engine speed requests to the Engine ECM.

At engine startup, the "Drivetrain ECM" will perform a "Drivetrain System Test". This test will verify that the operation of the generator is correct. The test will also determine if the operation of the DC Power Bus is correct.

Once the engine speed has stabilized (700 rpm) after startup, the "Drivetrain ECM" commands the EFR to send 2 amps of current to the generator exciter winding. The Drivetrain ECM will then monitor the system for expected voltage and current. If the voltage or current is not what is expected, the ECM will activate one of the following four Events:

• E1311, Level 3 - Low DC Power Bus Voltage Detected During Drivetrain System Test

• E1312, Level 3 - High Generator Phase Current Detected During Drivetrain System Test

• E1313, Level 3 - High DC Power Bus Current Detected During Drivetrain System Test

• E2140, Level 2 - Drivetrain Limited Due to System Fault

Refer to the Troubleshooting, "Event Code List" for more information on the system test Events.

The Drivetrain System Test will take approximately 5 seconds to complete. If the shift control lever is moved to a travel position during the test, the test will be aborted. Normal machine operation will then be enabled if no system faults are activated.

Once the system test is completed, and no abnormal conditions are detected, the "Drivetrain ECM" will command the "Generator Excitation Field Regulator" (EFR) to boost the generator AC voltage outputs to 1000 VDC on the DC bus.

The generator output voltage is rectified to create the DC Power Bus voltage. Depending on the system requirements, the "Drivetrain ECM" will try to maintain the bus voltage in a range of between 1000 VDC and 2700 VDC. When the truck is in the travel mode, the ECM will attempt to maintain the voltage of the DC Power Bus at approximately 2700 VDC. The maximum amperage that can be supplied by the DC Power Bus is 1050 amps.

The "Drivetrain ECM" controls the voltage on the DC Power bus using several different system components.

To raise the DC Power Bus voltage level, the ECM will command the EFR to send more current to the generator excitation winding. The result will be an increase of the Generator output voltage.

During machine operation, the stored energy in the DC Power Bus Capacitor and other component capacitors helps to maintain the bus level. When immediate heavy system power demands are required, the filter capacitor will help to provide extra power until the "Drivetrain ECM" can increase generator output.

Some system functions require the voltage of the DC bus to be briefly decreased. In addition to using the EFR to decrease generator output, the "Drivetrain ECM" will use the Chopper Module to decrease the DC Power Bus voltage.

The "Drivetrain ECM" will command the "Motor 1 ECM" to activate the negative side power transistor in the Chopper Module. The activation frequency will determine how much the bus voltage is decreased. The chopper operation enables quicker control than is possible by changing the output of the generator. For this function, the duty cycle or duration of activation will be limited since the grid cooling fan will not be turned ON.

At startup, the "Drivetrain ECM" will energize the current drive circuit for the "Electric Drive Cooling Fan Solenoid". This action will start the operation of the system hydraulic cooling fan.

At engine low idle, the fan speed is approximately 1250 rpm. The drivetrain ecm decreases the current to increase the fan speed based on various conditions. The maximum fan speed is 3400 rpm but this speed is only commanded if the components are nearing the level 2 event thresholds. Disconnecting the solenoid will create a diagnostic and will result in full fan speed.

The fan draws outside air into vents in the front of the "Inverter Cabinet". The air flows through heat exchangers on top of each phase module. The air flow dissipates the heat generated by the operation of the power transistors in the phase module. The cooling air is then channeled through ducts into the generator and back to the axle housing to cool the traction motors.

At startup, the "Drivetrain ECM" will use a sinking driver circuit to ground an enable circuit to each motor control ECM.

Each motor control ECM will perform system checks to determine the status of the components that are being controlled. If no fault conditions are present, each ECM will use a sinking driver circuit to ground an enable acknowledge circuit. The circuit is connected to the "Drivetrain ECM". the grounded circuits indicate that each motor control ECM has received the grounded enable signal from the "Drivetrain ECM" and is ready for operation. The enable and the feedback circuits always remain grounded during system operation. If one of the grounded signals is interrupted, the "Drivetrain ECM" will disable the operation of the drivetrain.

If a fault condition is detected, an "Event" code or a "Diagnostic" code will be activated. If the fault condition is severe, drive train operation may be disabled.

All these initial checks, startups, and enabling of the controls takes place in a short time. As the initialization is occurring, the traction motors are in a zero power state.

Travel Mode Control

Illustration 2	g06390832

Once the operator moves the shift control lever out of the PARK position, the "Drivetrain ECM" commands the "Motor 1 ECM" and "Motor 2 ECM" to begin control of the traction motors. A small amount of current is sent to the traction motors to "flux" or energize the stator. The "flux" ensures that the motors are ready for a quick response when required.

The "Drivetrain ECM" receives a PWM input from the throttle pedal position sensor. When the operator depresses the pedal, the "Drivetrain ECM" will command the "Engine ECM" to increase the Engine speed to approximately 1300 rpm. The increased Engine speed enables an increase in the power output from the generator.

As the accelerator pedal is further depressed, the "Drivetrain ECM" commands an increased output of the generator to boost the DC Power Bus up to a level of 2600 VDC depending on machine speed..

As the accelerator pedal is depressed, the "Drivetrain ECM" will send torque commands over the "CAN B" Data Link circuits to the two motor control ECMs. The "Drivetrain ECM" will base these torque commands on several factors. The power on the DC Power Bus, the position of the shift control lever, the position of the accelerator pedal and the position of the service brake pedal will all affect the "Drivetrain ECM" response. The service brake pedal controls the retard electrical braking and mechanical braking. During the first 80% of travel the retard electrical braking is controlled. During the last 20% of engagement of the service brake, mechanical braking is engaged.

When each Motor ECM receives these torque commands from the "Drivetrain ECM", the two ECMs will use six power transistor "pairs" in three-phase modules to provide the AC power to the traction motors.

Each transistor pair consists of two power transistors that are connected in parallel. The parallel connected transistors provide more current handling capability to handle the high current loads that are required for motor operation.

Each transistor pair is turned ON by one ECM gate driver circuit and provides one status feedback input circuit back to the controlling ECM.

At startup or during operation, if the controlling ECM does not detect the expected transistor status feedback signal, the ECM will activate a "Level 3 Event". The "Drivetrain ECM" will disable drivetrain operation.

Three transistor pairs are used to switch the positive side DC Power Bus voltage. Three transistor pairs are used to switch the negative side DC voltage. Each of the positive side pairs is also labeled as "Power Transistor 1". Each of the negative side pairs is also labeled as "Power Transistor 2".

One positive side switching pair of transistors and one negative side switching pair are used to create one of the 3 AC phases for the motor.

The gate drivers for each of the transistor pairs require a voltage pulse to turn ON (close) the transistors. Each motor control ECM will use driver output circuits to send the voltage pulses to the internal electronic gate drivers for each transistor pair.

Each motor control ECM is paired with an interface module. To provide isolation between the high-voltage phase module circuits and the low-voltage driver circuits, the ECM transistor driver circuits are sent through an interface module. The interface modules convert the voltage sources to a light pulse that is sent to the phase modules through fiber optic circuits.

In the phase module, the light pulse is converted back to a voltage pulse. The gate driver voltage pulses will switch the power transistors ON when the voltage pulse is high. The power transistors will be turned OFF when the pulse is absent or low.

The ECM will activate the gate driver outputs for the three pairs of positive switching transistors and the three pairs of negative switching transistors at a high-frequency sequence that will create the three phases of PWM output voltage. The traction motors detect an RMS voltage from the PWM pulses that resembles an AC sine wave (Refer to illustration 1).

For each motor phase, when the "Motor Control ECM" modulates the output PWM pulses to increase the time that the pulse width is high, the AC voltage of each phase will increase.

For each motor phase, when the "Motor Control ECM" increases the transistor switching frequency, the current for each phase will increase. Higher frequency and current results in greater motor speed and greater torque output.

When a request to change the direction of machine travel is received from the "Drivetrain ECM", each motor control ECM will electronically switch the two of the three phases for each traction motor. Switching the two phases will result in the motors reversing direction of rotation.

The high degree of variable control ensures that the requested motor speeds and torque requirements can meet the travel requirements of the truck.

Traction Control System

The Traction Control System (TCS) is controlled by the "Drivetrain ECM". The TCS function is active when the shift control lever is in a travel position. The ECM will suspend TCS operation when the service brakes are used.

The TCS is a software function. No operator controls are used to control the function. The function is activated by the "Drivetrain ECM" when required. An indicator on the cab display is illuminated when the function is active.

The TCS function enables the “Drivetrain ECM” to limit the slippage of the rear wheels during travel. Under slippery conditions, the “Drivetrain ECM” will limit slippage on downhill slopes.

The ECM monitors the inputs from the following components to control the TCS function:

• The position of the shift control lever as reported by the "Chassis ECM".

• The signals from the (4) wheel speed sensors.

• The signal from the service brake pressure sensor as reported by the "Brake ECM".

When the "Drivetrain ECM" detects slippage of the rear wheels, the ECM will adjust the torque commands to the motor controls. More torque will be applied to the rear wheel that is slipping the least as the torque to the slipping wheel is decreased.

When the truck is driven into a turn, the "Drivetrain ECM" will use the PWM input signals from the two front speed sensors to determine the angle of the front wheels. The ECM will calculate the change in the rotation speed for the traction motors required as the rear wheels travel through the radius of the turn. The "Drivetrain ECM" will command each "Motor Control ECM" to adjust the speed of the motors accordingly. This rear wheel speed adjustment eliminates wheel skid as the rear wheels travel around the radius of the turn. The speed adjustment allows greater turning performance and will greatly reduce the wear on the rear tires. If the signal from a front speed sensor is lost, the truck can still be operated. However, the TCS will not function during a turn.

Retarding Function

The dynamic retarding function can be activated at any machine speed. When the retarding function is activated, each "Motor Control ECM" will greatly reduce the current sent to the traction motors. The "Drivetrain ECM" will calculate the speed of the truck and the amount of retarding power needed to meet the operator request.

When dynamic retarding is requested, the "Drivetrain ECM" reduces the output of the generator. The ECM will use the "Chopper Module" to reduce the voltage of the DC Power bus to less than 1500 VDC before the retarding contactors are CLOSED. Reducing the voltage before the contactors CLOSE reduces wear on the contactor tips. The ECM may decrease the voltage of the bus as low as 850 VDC depending on the retarding request. However, the bus voltage will not be allowed below this threshold.

Once the contactors CLOSE, the "Drivetrain ECM" raises the voltage level of the DC bus proportionally up to approximately 2800 VDC depending on the amount of retarding power required.

The retarding contactors are always CLOSED during retarding. Whentheretarding contactors CLOSE, electrical current will be directed through "Contactor Grid 2" resistor elements. The current that is directed through the grid will supply operating DC voltage to the "Retarding Grid Blower Inverter" (RGBI) through circuits that are tapped off the grid elements. The RGBI will create three phases of AC voltage for the grid blower motor to dissipate the large amount of heat that is created by the resistor elements.

The "Contactor Grid 2" resistor elements are located closer to the fan since the contactors provide the first 50 percent of the rated grid retarding power.

The electrical energy created by the traction motors is dissipated as heat by the grid resistor elements. The result is a resistive load being placed on the traction motors which slows the motor speed and the wheel speeds.

With little current supplied to the traction motors, the rotation of the traction motors causes a voltage to be generated at levels that will reverse the current flow back to the DC Power bus. The current will pass through diodes that are connected across each of the phase module power transistors. More rotational speed of the rear wheels produces more current which requires more retarding power that must be dissipated in the grid resistor elements.

For retarding demands that are greater than 50 percent, the Chopper Module is used to channel more of the current produced by the traction motors through the "Chopper Grid 1" resistor elements. The Chopper Module provides the upper 50 percent of retarding power that can be utilized.

When the Chopper Module negative side power transistors are gated ON (CLOSED), the negative (-) DC bus is shorted to the positive (+) DC bus through the "Chopper Grid 1" resistor elements. The positive side power transistors are not used in the "Chopper Module".

The "Motor 1 ECM" will adjust the duty cycle of the chopper to control the amount of current and voltage directed through the chopper grid elements.

A "Chopper Module" 100 percent duty cycle (negative side transistors held ON) results in the full rated retarding power (4750 kw) being directed through the grid.

When the dynamic retarding function is no longer requested, the Drivetrain ECM must briefly reduce the DC bus voltage to less than 1500 VDC before the retarding contactors can be opened.

The retarding contactors and the "Chopper Module" are used in the same manner during the "Engine Performance Test". For this test, the contactors and the "Chopper Module" are used to build up a resistive load on the generator to load the engine. The test will determine if the engine will operate at the full load rated output.

Crowbar

In the electric drive train system, the "Crowbar" is used as an over voltage control device. When the "Crowbar" is activated, the positive (+) DC Power Bus is shorted to the negative (-) DC Power Bus through the "Chopper Grid 1" resistor elements. This series of events results in an immediate discharge of the bus.

If the voltage of the DC Power Bus reaches 3000 VDC, a "Level 2 Event" for high bus voltage is activated by each motor control ECM. If the voltage level of the DC Power Bus reaches 3100 VDC, a "Level 3 Event" for high bus voltage is activated by each motor control ECM. The "Level 3 Event" results in the "Interface Module 2" turning ON the Crowbar to discharge the bus immediately. When the "Crowbar" is activated, drive train operation will be disabled. The engine must be shutdown to turn off the crowbar do to the nature of a thyristor.

System Components and Operation

The electric drive train system includes the following main components that are visible on the truck:

Illustration 3	g03671808
The main visible electric drive train system components. (1) Inverter Cabinet - houses all the traction motor control components, system status input devices, and the Retarding Grid Blower Inverter. (2) Retarding Grid Assembly - contains the load resistor elements that are used for loading during dynamic braking and for discharging the DC Power Bus voltage. A Grid Blower Motor is used to cool the resistor elements. (3) Electric Drive Cooling Fan (not visible). Located under the chassis. The hydraulic fan is used to provide cooling airflow for the components in the "Inverter Cabinet", the generator, and the traction motors (4) Generator - when the Engine is operating, the generator supplies three-phase AC voltages to the "Inverter Cabinet". (5) Traction Motor 1 - contained inside the rear axle housing and controlled by the "Motor 1 ECM". (6) Traction Motor 2 - contained inside the rear axle housing and controlled by the "Motor 2 ECM". (7) Final drive

Inverter Cabinet Components

Note: The two top compartments on the front of the "Inverter Cabinet" contain low voltage control components. All the other compartments in the cabinet contain high-voltage components. The "Electrical Shutdown and Voltage Discharge" procedure must be performed before any of the high-voltage compartments are entered.

Illustration 4	g06390923
Systems components that are present in the front compartments of the "Inverter Cabinet" (8) Bulk capacitors. The bulk capacitors are supplied system power by the interface modules. The capacitors are in place to provide a 3 second emergency power supply for the ECMs. This power supply will allow a stable phase module shutdown if the system power is interrupted. This compartment is in the low voltage compartment. (9) Terminal blocks - Used for connections to the voltage sensors, current sensors, the system control 24 VAC circuits, and the 24 VDC and ground wires. The cabinet ground boss and the RGBI pilot relay are on the floor of this compartment in front of the terminal blocks. (10) Interface Module 2 - used between the "Motor 2 ECM" and the phase modules for the "Traction Motor 2". The interface modules are used for conversion and isolation of the copper transistor driver and feedback circuits to fiber optic circuits. Also used for isolation of the phase module power AC power supply circuits. This compartment is a low voltage compartment. (11) Motor 2 ECM - monitors and controls the operation of the "Traction Motor 2". (12) Interface Module 1 - used between the "Motor 1 ECM" and the phase modules for the "Traction Motor 1". The interface modules are used for conversion and isolation of the copper transistor driver and feedback circuits to fiber optic circuits. Also used for isolation of the phase module power AC power supply circuits. (13) Motor 1 ECM - monitors and controls the operation of the "Traction Motor 1". (14) PMA2 - Motor 2 ECM will use the power transistors in this phase module to create the "Traction Motor 2"-phase A power circuit. (15) PMB2 - Motor 2 ECM will use the power transistors in this phase module to create the "Traction Motor 2"-phase B power circuit. (16) PMA1 - Motor 1 ECM will use the power transistors in this phase module to create the "Traction Motor 1"-phase A power circuit. (17) PMB1 - Motor 1 ECM will use the power transistors in this phase module to create the "Traction Motor 1"-phase B power circuit. (18) Retarding Contactors 1 and 2 - used to switch the DC Power Bus through the "Retarding Grid 2" resistors. (19) PMC2 - Motor 2 ECM will use the power transistors in this phase module to create the "Traction Motor 2"-phase C power circuit. (20) "Retarder Chopper Module". "Motor 1 ECM" controls the operation of "Chopper Module". The ECM uses the chopper to modulate DC bus voltage through the "Chopper Grid 1" resistor elements. Used to help control the voltage level of the DC bus, add dynamic load during retarding and discharge the bus when required. (21) PMC1 - "Motor 1 ECM" will use the power transistors in this phase module to create the "Traction Motor 1"-phase C power circuit. (22) Connection compartment for the retarding grid system circuits. Also contains the chopper current sensor (CHPRCT). This sensor is used to monitor the current that is present in the chopper grid resistor circuits from the "Chopper Module" and the "Crowbar". (23) Connection compartment for the Generator three phase cables, "Traction Motor 1", "Traction Motor 2", and three phase cables to bus bar connections. The Generator and the motor phase current sensors are located in this compartment. (24) Compartment that contains the "Pressurization Filter". This filter is used to filter the air that is used to create the positive air pressure in the cabinet.

Illustration 5	g06390910
Systems components that are present in the rear sections of the Inverter Cabinet (25) Inverter active resistors - used for the inverter active lamp voltage drop circuits (26) Discharge resistor assembly - used in the voltage divider circuits in the ground fault voltage circuit and for a DC voltage discharge path from the positive DC bus to the negative DC bus (27) "Retarding Grid Blower Inverter" (RGBI) - self-contained inverter that creates the three phases of AC voltage that is used to power the "Grid Blower Motor" (28) "Ground Fault Capacitor" (29) "Ground Fault Shunt Resistor" (30) "Ground Fault Voltage Sensor" (31) "DC Bus Voltage 1 Sensor" (DCV1) (32) "DC Bus Voltage 2 Sensor" (DCV2) (33) "Traction Rectifier 2" - used to create the negative side voltage of the DC Power Bus from the three-phase AC Generator output (34) "Traction Rectifier 1" - used to create the positive side voltage of the DC Power Bus from the three-phase AC Generator output (35) "Retarding Grid Blower Inverter Capacitor" - used as a suppression device to suppress high-voltage spikes in the RGBI DC power supply circuits (36) Contactor current sensor (CONTCT) that is used to monitor the electrical current that is passing through the closed retarding contactor circuit (37) DC bus current sensor (DCCT) that is used to monitor the electrical current that is present in the DC Power Bus (38) Crowbar Assembly (thyristor)

Electronic Control Modules

Illustration 6	g06390931
Locations of the electric control modules (39) "Drivetrain ECM" (40) "Chassis ECM" (41) "Brake ECM" (42) "Motor 2 ECM" (43) "Motor 1 ECM"

Electronic control of the electric drive train system is provided by three electronic control modules (ECM). The three electronic control modules are the "Drivetrain ECM", the"Motor 1 ECM", and the "Motor 2 ECM".

The "Drivetrain ECM" is located in the cab front ECM compartment. The two motor control modules are located in the "Inverter Cabinet".

The "Drivetrain ECM" is the primary control module for the operation of the "Electric Drive System". The "Drivetrain ECM" controls the following machine operations:

• Sends and receives operational communication with the "Motor 1 ECM" and the "Motor 2 ECM" on the CAN B Data Link circuits and the CAN A Data Link circuits.

• Sends and receives communication information with the other machine control modules on the CAN A Data Link circuits.

• Enables and controls the operation of the generator and the DC Power Bus voltage level based on system demands and conditions.

• Enables and exercises overall control of the speed and the direction of each of the rear wheel sets. Torque commands are sent to each motor control ECM based on input signals from the operator controls, the position of the steering cylinder and based on drive train system feedback.

• Controls the speed of the engine using "Cat Data Link" communication with the "Engine ECM" and by controlling the generator load. The "Drivetrain ECM" receives input information from the shift control lever position, the accelerator pedal position, the speed of the traction motors and other system conditions.

• Controls the operation of the hydraulic electric drive system cooling fan based on the speed of the Engine.

The "Motor 1 ECM" controls the following system operations:

• Sends and receives operational data to and from the "Drivetrain ECM" on the CAN B Data Link circuits and the CAN A Data Link circuits.

• Creates and regulates the three phases of AC power that are sent to the left-hand "Traction Motor 1" based on the torque commands that are received from the "Drivetrain ECM". The "Motor 1 ECM" also monitors the motor speed status, the current of each of the three phases and the voltage of the DC Power Bus.

• Monitors the operating temperature of the "Traction Motor 1" windings and bearings.

• Enables the "Interface Module 1" operation and enables the "Interface Module 2" operation.

• Regulates and monitors the operation of the retarding contactors that are used during retarding and the discharging of the DC Power Bus when required.

• Regulates and monitors the operation of the "Chopper Module" to control the voltage level of DC Power Bus, control the retarding function, and discharge the DC bus when required.

• Controls the operation of the grid blower inverter pilot relay to reset the "Retarding Grid Blower Inverter" when required.

• Monitors the input status from the "DC Bus Voltage 1 Sensor" (DCV1) and from the "DC Bus Voltage 2 Sensor" (DCV2) DC Power Bus voltage sensors.

• Monitors the input status from the "Ground Fault Voltage Sensor".

• Will send a request to activate the "Crowbar" to the "Motor 2 ECM" to discharge the DC Power Bus immediately when required.

The "Motor 2 ECM" controls the following system operations:

• Sends and receives operational data to and from the "Drivetrain ECM" on the CAN B Data Link circuits and the CAN A Data Link circuits.

• Creates and regulates the three phases of AC power that are sent to the right-hand "Traction Motor 2" based on the torque commands that are received from the "Drivetrain ECM". The "Motor 2 ECM" also monitors the motor speed status, the current of each of the three phases and the voltage of the DC Power Bus.

• Monitors the operating temperature of the "Traction Motor 2" windings and bearings.

• Enables the "Interface Module 2" operation.

• Controls and monitors the operation of the "Crowbar" to discharge the DC Power Bus immediately if required.

• Monitors the input status from the "DC Bus Voltage 1 Sensor" (DCV1) and from the "DC Bus Voltage 2 Sensor" (DCV2) DC Power Bus voltage sensors.

• Monitors the input status from the "Ground Fault Voltage Sensor".

Interface Modules

Illustration 7	g06390944
View of the Interface Modules (10) "Interface Module 2" (12) "Interface Module 1" (44) The J1 connector - connects the copper transistor driver circuits and feedback circuits from the "Motor Control ECM". The system power and ground supply circuits from the terminal blocks and the system power and ground supply circuits to the phase modules are also connected through this connector. (45) The fiber optic circuit connections to the phase module transistor driver circuits and the transistor feedback circuits.

Illustration 8	g03695209
"Interface Module 1" and "Interface Module 2" fiber optic connector assignments

Each "Motor Control" ECM uses an interface module to aid in the control of the transistors in the phase modules. "Motor 1 ECM" uses the "Interface Module 1". "Motor 2 ECM" uses the "Interface Module 2".

The interface modules are identical. Each interface module uses the same version of software. Therefore, the interface modules can be switched, without flashing software, to determine if a suspected problem is present in a specific module.

The J1 connector grounded location code circuits indicate to the software which position and ECM that each interface module is paired with. Based on this information, the appropriate control software functions for the specific module position will be activated.

The functions of the interface modules are:

• Primary function is to isolate the low-voltage ECM control circuits and the phase module power supply circuits from the high-voltage circuits that are present inside the phase modules. This isolation ensures that there is no possibility that a short circuit between a high-voltage circuit and a low-voltage circuit can occur. The fiber optic circuits also ensure that electrical fields cannot accidentally cause transistor activation.

• Supply system control voltage to the bulk capacitors. In the event of a system power loss, the bulk capacitors will provide approximately 3 seconds of emergency power to allow the interface module to activate an orderly shutdown of the power transistors. The emergency power avoids possible short circuits of the DC bus that can be caused by erratic switching of the transistors at a control voltage that is too low.

• "Interface Module 1" controls the operation of the "Chopper Module" based on commands from either of the motor control ECMs or the "Drivetrain ECM".

• "Interface Module 2" controls the operation of the "Crowbar" based on commands from "Interface Module 1", either of the motor control ECMs or the"Drivetrain ECM".

Illustration 9	g03695324
Motor Control ECM's and Interface modules enable feedback circuits

The operation of the interface modules is enabled by the motor control ECMs. When the key start switch is moved to the ON position or when the Engine is started, the ECM will determine if any drive train related diagnostic codes or "Level 3 Events" are active. If no faults are active and the system conditions are correct, each "Motor Control" ECM will use the following logic to enable or disable the operation of the interface modules:

• "Motor 1 ECM" will GROUND the normally high enable circuit at contact J1-63 (IMEN1) to enable "Interface Module 1" and supply one of two required enable signals for "Interface Module 2".

• "Motor 2 ECM" will GROUND the normally high enable circuit at contact J1-63 (IMEN2) to supply the second enable signal to enable "Interface Module 2"

• If one or more of the ECM enable signals for either interface module is interrupted or lost, "Interface Module 2" will activate the "Crowbar" to disable drive train operation.

• Each interface module acknowledges to each "Motor Control" ECM that the enable signals have been received by outputting greater than 4 VDC from the Diagnostic Feedback pin J1-10 (circuits EVNT1, EVNT2).

• If either "Motor Control" ECM detects that one or both of the feedback circuits from the interface modules are interrupted or lost, the retarding contactors and the chopper module will be activated to disable drive train operation.

• Once enabled, each interface module will monitor the status of the 24 VDC system voltage input and the isolated 24 VAC power supply output that is sent to each phase module. If either of these power supplies is detected to be too high or too low, the interface module will send a diagnostic feedback voltage signal (circuits EVNT1, EVNT2) back to the "Controlling Motor Control" ECM. A feedback circuit voltage of greater than 4.0 VDC indicates that the power supply voltages are in tolerance (22.4 VAC to 23.2 VAC). A feedback circuit voltage of less than 2.0 VDC indicates that a low voltage has been detected (21.5 VAC or less), the enable is not present from the "Motor Control" ECM, or there is something wrong internally to the interface module.

• A diagnostic feedback voltage signal of less than 2.0 VDC or a loss of the diagnostic feedback signal from either interface module will result in activation of the Retarding Contactors and the Chopper Module to disable drive train operation.

Illustration 10	g02085773
Interface module isolation of the phase module control circuits

Each "Motor Control" ECM uses two separate power transistor pairs in each phase module to create one of the three phases of AC voltage for each of the traction motors.

Each transistor pair has an internal electronic transistor gate driver circuit. The gate driver requires a voltage input from the ECM to turn the transistors ON. Each transistor pair sends status feedback to the ECM using the feedback circuit. Therefore, each phase module requires two driver circuits from the ECM and supplies two feedback circuits back to the ECM.

System voltage 24 VDC is supplied from the machine fuse panel. The power circuits are connected to the interface modules through the terminal blocks in the cabinet. The interface modules supply isolated 24 VAC power and ground supply circuits to the phase modules. The AC power is required to power the internal electronics for the gate driver control circuits and the feedback circuits. The power is supplied to each transistor pair in the phase module.

Fiber optic gate driver circuits and feedback circuits provide isolation between the low-voltage control circuits and the high-voltage circuits in the phase modules.

The interface modules convert the voltage pulses of the ECM driver circuits to light pulses. The light pulses are sent to the phase module through plastic fiber optic circuits. The internal electronic gate driver for each transistor pair converts the optical pulse back to a voltage pulse to activate the transistors.

The status feedback signal is sent back to the interface module as light pulses through a separate fiber optic circuit. The interface module converts the light pulse back to a voltage pulse and sends the signal back to the ECM on a copper circuit.

The interface modules receive the system power supply and use internal transformers to provide isolation of the power supply circuits. One isolated 24 VAC power supply is supplied to each phase module. The AC power provides the power for the internal electronics of the transistor pairs . A transformer inside the phase modules is also used to isolate this power supply.

At engine start, the controlling ECM will perform an automatic test to check the status of each transistor driver and feedback circuits. If the expected results of the tests are not detected, the drivetrain system will not be enabled.

A Cat^® ET test is available that enables a technician to perform a key ON test for each transistor pair. The test will allow the user to turn ON single ECM transistor driver circuits. The user can check the operation of the driver circuit, the power transistor pair, and the feedback circuit. When a circuit is turned ON, the fiber optic circuits can be disconnected and the light pulses can be visually verified to be present.

"Motor 1 ECM" and the "Interface Module 1" also control the operation of the "Chopper Module". The "Chopper Module" is a phase module that is identical to all the other phase modules, however, only one of the transistors pairs is used. A fiber optic driver circuit and feedback circuit is used to control the "Chopper Module". The "Chopper Module" is discussed further in the "Phase Modules" section.

"Interface Module 2" controls the activation of the "Crowbar" according to signals that are received from either of the motor control modules. The "Crowbar" is used to discharge the DC Power Bus immediately if certain severe fault conditions occur in the system. A fiber optic driver circuit is used to activate the "Crowbar". When the "Crowbar" is activated, the DC Power Bus positive bus and the negative bus are connected together through the "Chopper Grid 1" resistor elements. Activation of the "Crowbar" effectively shorts the bus and immediately discharges the bus voltage in a short time period. The operation of the "Crowbar" is discussed in the "Crowbar" section that follows.

Note: The fiber optic connectors on the interface modules and on the phase modules require special handling to avoid damage when connecting or disconnecting the connectors.

Phase Modules

Illustration 11	g03672211
The phase modules and the chopper module that are assigned to each "Motor ECM"

The phase modules and the "Chopper Module" in the "Inverter Cabinet" contain the power transistors used to create each individual power phase for the three-phase traction motors.

In the "Inverter Cabinet", the phase modules are labeled "PMA1", "PMB1", "PMC2", "PMA2", and so on. The "PM" stands for phase module. The "A", "B", or "C" stands for the motor phase. The "1" or "2" stands for "Traction Motor 1" or "Traction Motor 2".

To identify the correct phase module in the cabinet, in this manual the phase modules will be referred to in the following format:

• "Motor 1"

• "Phase A Phase Module" (PMA1)

• "Motor 2 Phase A Phase Module" (PMA2)

Illustration 12	g02087133
ECM control circuits for the operation of one traction motor control phase module

The phase modules and the "Chopper Module" installed in the "Inverter Cabinet" are physically identical. Each of the phase modules contains two transistor pairs. Each pair consists of two power transistors that are connected in parallel between DC Power Bus and the AC output terminal. The parallel connected transistors are used to handle the high current loads required for traction motor operation.

An AC bus is connected between the two sets of transistors. This AC bus is connected to one of the 3 AC phase cables that will go out to the traction motor.

The set of transistors connected on the DC positive side of the AC connection will be used by the "Motor Control ECM" to switch the positive DC voltage in a sequence that will create the positive side of the PWM output voltage.

The set of transistors connected on the DC negative side of the AC connection will be used by the "Motor Control ECM" to switch the negative DC voltage in a sequence that will create the negative side of the PWM output voltage.

The ECM will activate the gate driver outputs for the three pairs of positive switching transistors and the three pairs of negative switching transistors. The ECM will use a high frequency sequence that will create the three phases of PWM output voltage. The traction motors detect an RMS voltage from the PWM pulses that resembles an AC sine wave (Refer to Illustration 1).

Each transistor in the phase module has a diode connected between the emitter and the collector. The diode allows electrical current to flow in the reverse direction when the traction motors are producing a voltage output during dynamic braking.

Illustration 13	g06390961
Phase Module (46) Cooling air heat exchanger (47) Low voltage connector for the fiber optic driver circuits, the fiber optic feedback circuits, the copper power supply circuits, and the copper temperature sensor circuits. (48) Ground connection (49) Negative (N) DC Power Bus connection (50) AC output connection (AC) connection - phase A, B, or C for motor control (512) Positive (P) DC Power Bus connection

Each phase module, including the "Chopper Module", has an internal 1667 microfarad capacitor connected between the positive and the negative DC Power Bus connections. The capacitor is in place to help to absorb the voltage spikes that can be caused by the switching of the transistors. All the phase module capacitors are connected in parallel to each other and to the filter capacitor. The combined capacitance sum provides a large amount of total DC filter capacitor. The filter capacitor helps to stabilize the DC Power Bus voltage and provide more power during periods of heavy demand.

Illustration 14	g02088526
Use a 331-6561 Voltmeter to verify a low Module DC capacitive voltage

In addition to performing the "Electrical Shutdown and Voltage Discharge" after Engine shutdown, always verify that the voltage at an individual phase module is discharged.

Note: Before any contact is made with the phase module bus connections or the phase module connector, use the high-voltage meter to measure for a DC voltage. Ensure that a voltage of 50.0 VDC or less is present before any work is performed.

Illustration 15	g06390968
Phase module low voltage harness connector (52) Fiber optic driver circuits (534) Fiber optic feedback circuits (54) Temperature sensor circuits (55) Power supply circuits

The fiber optic circuits and the system voltage power supply circuits connected between the interface modules and the phase modules are connected at the phase module in a low-voltage connector. These connectors enable connection of the two copper power supply circuits, the two sets of fiber optic driver circuits, and the feedback circuits for the two transistor pairs in each phase module. In addition, two temperature sensor circuits are connected back to the "Motor Control ECM". Enter the phase modules through this connector.

Illustration 16	g06390973
Phase module low voltage connector (52) Fiber optic feedback circuits (53) Fiber optic driver circuits (54) Copper AC power supply circuits (55) Copper temperature sensor circuits (56) Alignment posts that are used to aid in the alignment of the connectors. The posts are not used for electrical connection.

Each transistor pair requires a transistor gate driver input from the ECM to turn the transistors ON and OFF. Each transistor pair sends status feedback to the ECM. Consequently, each phase module requires two driver circuits from the ECM and supplies two feedback circuits back to the ECM.

The ECM driver and feedback circuits are fiber optic circuits. For more information on the fiber optic circuits, refer to the "Interface Modules" section that precedes this section.

Each transistor pair contains internal electronic gate driver control circuits that will control the operation of the transistors. The gate drivers require a 24 VAC power supply to operate. These isolated power circuits are supplied to each phase module from the interface module that is paired with the "Controlling Motor Control" ECM.

At key ON or at engine startup before actual transistor operation begins, the "Controlling Motor Control" ECM will test the operation of each transistor pair that is under control. The ECM will send a driver circuit signal pulse to each of the power transistors pairs. If an internal problem is not detected, the electronics for each pair will answer the signal pulse with a feedback status pulse on the feedback circuit.

If this status pulse is not received by the Motor Control ECM in a predetermined time, the ECM will not enable the drivetrain operation.

During machine operation when the ECM is sending driver signals to the transistor pairs, the electronics for the transistors are constantly sending a corresponding feedback signal back to the ECM. This feedback signal indicates that the operation of the transistor pair is correct.

If the ECM does not detect the feedback signal or if the signal is not what is expected by the ECM, the involved motor control ECM will immediately command the "Interface Module 2" to activate the "Crowbar". The DC Power Bus voltage will be immediately shorted through the "Retarding Grid 1" resistor and the operation of the drive train is disabled.

During the initial test or during machine operation, a failed feedback signal will also result in the activation of a "Level 3 Event" by the "Controlling Motor Control" ECM. For motor control transistors, E900 through E905 for "Drive Motor Phase (A, B, or C) Power Transistor (1 or 2) Signal Mismatch".

A Cat^® ET test is available that enables a technician to perform a KEY ON test that will turn ON a specific ECM transistor driver circuit to test the operation of the driver circuit, the power transistor pair, and the feedback circuit. When a circuit is turned ON, the fiber optic circuits can be disconnected and the light pulses can be visually verified to be present.

During machine operation, the power transistors in the phase modules are cooled by air passing through the heat exchanger that is at the top of each phase module. An antifreeze mixture is sealed in the phase module and the heat exchanger.

The heat exchanger lines up with the air intake vent on the front of each phase module compartment. As the hydraulic cooling fan draws the air in through the front of each phase module compartment, the internal liquid mixture is cooled. The liquid is naturally circulated through the phase module to cool the internal components.

A "Sand Filter" on the front of the intake air vents eliminates large particles from entering the cooling system. However, this air is largely unfiltered air. The sand filters should be checked regularly for obstructions that could cause a reduction of air flow.

Each of the six traction motor phase modules and the "Chopper Module" has an internal temperature sensor. Each of the temperature sensors is a 1000 ohm (at 0.0° C (32.0° F)) Resistive Temperature Device (RTD).

The RTD is isolated on the heat sink, a section of the module where there is no possibility of short circuit to a higher voltage. The temperature sensor positive and negative circuits are connected through the phase module low-voltage connector directly to the ECM. The circuits do not pass through the interface modules.

The "Controlling Motor Control" ECM monitors the temperature sensor circuits for abnormal conditions. The ECM will activate a diagnostic code for a specific temperature sensor circuit if an unexpected condition is detected.

The components inside the phase modules including the temperature sensor are not serviceable. If a phase module internal component has failed, the phase module will have to be replaced.

Illustration 17	g02088496
Phase module disconnected and ready for removal with the front connections connected with a wire (arrow)

If a phase module or the "Chopper Module" is disconnected for removal, the DC Power Bus connections (P, N) and the AC output connection (AC) on the front of the phase module must be connected together with a tie wire. This tie wire connection is in place to ensure that the internal capacitor remains discharged by placing all connections at the same potential.

The phase modules weigh approximately 135 kg (298 lb). A removal tool is available to aid in the removal and replacement of the phase modules. Refer to Disassembly and Assembly, KENR8715 for instruction on the removal and replacement of the phase modules.

Chopper Module

Illustration 18	g03695385
ECM control circuits for the operation of the "Chopper Module"

The "Chopper Module" is a phase module that is the same as the other phase modules. The same hardware used for the phase modules is in the "Chopper Module".

The difference between the "Chopper Module" and the other phase modules is application of the module. The "Chopper Module" is not used for traction motor control.

The "Motor 1 ECM" activates and controls the "Chopper Module" for the following functions:

• Help the "Drivetrain ECM" to control and maintain the voltage of the DC Power Bus.

• Control the amount of dynamic retarding energy used to load the generator and the engine.

• Used to load the generator and the engine during the "Engine Performance Test".

To accomplish these functions, the "Negative Side Transistor Pair 2" is switched ON and OFF to connect (switch) the "Positive Side DC Bus" through the "Chopper Grid 1 Load" resistor elements to the "Negative DC Bus".

This shorts the "Positive DC Bus" to the "Negative DC Bus", through the grid resistors, which will produce a chopped PWM signal. When the "Motor 1 ECM" switches the transistors ON, the voltage will be low. When the transistors are switched OFF, the voltage will be high.

The switching frequency is modulated to control the duration of the low to high-voltage cycle. The longer that the transistors are turned "ON" (signal low) determines how much the voltage level of the DC bus will be pulled down.

Depending on the status of the DC Power Bus voltage or depending on the dynamic retarding requirements, the "Motor 1 ECM" will switch and modulate the transistors at a frequency up to 150 Hz. The control of the frequency and modulation will increase or decrease the current that is dissipated through the grid resistors.

The switching frequency duty cycle will determine the amount of electrical current that is dissipated through the resistors and the resulting amount of heat that is created.

When the "Chopper Module" is used for control of the DC bus voltage, the retarding contactors are not used. When the retarding contactors are not used, the Grid Blower Motor will not turn ON to cool the resistor elements. (The fan operation is covered in the following "Retarding Grid Blower Inverter" section).

When the "Chopper Module" is activated without fan operation, the "Motor 1 ECM" will control and limit the duty cycle of the "Chopper Module" to avoid overheating the "Grid 1 Resistor" elements.

The "Drivetrain ECM" will try to maintain the voltage of the DC Power Bus in the range of 2700 VDC to 2800 VDC. If the voltage level increases beyond this range, the "Drivetrain ECM" will adjust the generator output and command the "Motor 1 ECM" to switch the "Chopper Module" transistors ON and OFF at a rate that will aid in decreasing the voltage. If the voltage continues to increase, the ECM will accelerate the switching rate in an attempt to meet the commanded bus voltage.

When the "Automatic Retarding Control" (ARC) is activated, the retarding contactors are used to dissipate 50 percent of the retarding load through the grid that is coming from the motors. The "Chopper Module" is used to control and increase the amount of retarding load between 50 percent to 100 percent.

When the "Engine Performance Test / Grid Dry Test" is active, a similar process is used to load the engine. As the "Drivetrain ECM" commands a gradual increase in generator output, the retarding contactors will CLOSE and supply the first 50 percent of the retarding load back to the generator. The chopper module transistors will then be switched at a rate that will gradually increase the amount of current being sent to the retarding resistor element. This switching sequence gradually increases the generator load and the load on the engine until the rated engine output is reached.

The fault condition logic used for the operation of the "Chopper Module" is:

• During the startup "Motor 1 ECM" transistor test or during "Chopper Module" operation, a faulty feedback signal or the loss of the feedback signal will result in the activation of a "Level 3 Event" code (E1285 - Chopper Module Mismatch) by the ECM.

• The "Motor 1 ECM" continuously monitors the circuit current signal from the chopper grid current sensor (CHPRCT). Current in this circuit is enabled by the operation either the "Chopper Module" or the "Crowbar". Both devices will not be activated at the same time.

• If the "Chopper Module" is in operation during retarding and "Motor 1 ECM" is detecting a chopper grid 1 circuit current that is less than what is expected, the ECM will activate a "Level 2 Event" (E907 - Low Chopper Module Output Current).

• If the "Chopper Module" or the "Crowbar" have not been activated and "Motor 1 ECM" is detecting a current that is greater than 100 amps in the circuit, the ECM will activate a "Level 3 Event" (E911 - Unexpected Chopper Module Output Current).

• If the chopper grid resistor elements should overheat during Chopper operation, the "Motor 1 ECM" will stop the "Chopper Module" operation to allow the grid elements to cool. The ECM will also activate an E1083 Event (High Contactor Retarding Grid Accumulated Thermal Energy).

Retarding Contactor Assembly

Illustration 19	g06390990
Typical view of the retarding contactors (57) Retarding Contactor 1 (58) Pilot Relay (59) Retarding Contactor 2 (60) Auxiliary contacts (61) Harness connector

The Retarding Contactor Assembly consists of two identical high-voltage magnetic contactors with auxiliary contacts and a pilot relay.

The "Motor 1 ECM" will use a sinking driver output to energize or de-energize the pilot relay. When the relay is energized, 24 VDC system voltage will energize the coils of the main high-voltage contactors. A mechanical linkage that is connected to the auxiliary contacts for each contactor will cause the auxiliary contacts to CLOSE.

To prolong the life of the contactor tips that can occur if the DC bus voltage is too high, the "Drivetrain ECM" will reduce the output of the generator and use the "Chopper Module" reduce the voltage of the DC Power Bus to less than 1500 VDC before the retarding contactors are CLOSED or OPEN.

When energized, the contacts CLOSE which will send electrical current from the positive DC Power Bus through the "Retarding Grid 2" load resistor elements and back to the negative DC Power Bus.

During retarding, the closing of the contactors will provide the first 50 percent of the possible load on the traction motors. This load will cause a resistance to rotation of the motor.

At system shutdown, closing the contactors will discharge the voltage of the DC Power Bus.

When the "Pilot Relay" is energized, the auxiliary contacts for each main contactor will also be mechanically CLOSED. This situation results in two separate feedback circuits that are connected to the "Motor 1 ECM" being grounded. The two grounded circuits indicate to the ECM that the main contactors are closed.

Each of the retarding contactors is equipped with an electrical arc suppression device or arc chute on the top of the contactor. When the contacts are opened while conducting current, a high-voltage electrical arc can occur. An uncontrolled arc can cause damage or premature wear to the contacts. When an arc occurs, magnets that are outside the chute pull the arc upward into the chute. As the arc is directed into the chute, the arc is elongated and cooled which extinguishes the arc.

The arc chute can be removed from the contactor housing. An interlock will prevent contactor operation when the arc chute is removed.

The main contacts in the retarding contactors can wear during use. The contacts are serviceable. A 500-hour service interval is recommended for checking the condition of the contacts.

Refer to the Disassembly and Assembly, KENR8715, "Retarding Contactor - Disassemble" manual for a procedure to service the contacts.

The "Motor 1 ECM" monitors the signal from the "Contactor Grid Current Sensor" (CONTCT). If the contactors should be CLOSED and the ECM is not detecting the expected current from the current sensor, the ECM will activate a "Level 2 Event" (E908 - Low Retarding Grid Contactor Output Current).

If the ECM does not expect the contactors to be CLOSED and current is detected in the circuits, the ECM will activate a "Level 2 Event" (E912 - Unexpected Retarding Grid Contactor Output Current Detected).

The functions of the retarding contactors are:

• The primary function is to provide 50 percent of the available load to the traction motors when the "Automatic Retarding Control" (ARC), manual retarding, or the blended braking function are activated.

• When the "Engine Performance Test" is active, the retarding contactors will CLOSE resulting in a 50 percent loading of the generator.

• If a controlling ECM detects a system condition that is severe enough to cause the drive train to be disabled, the retarding contactors will CLOSE aiding in the discharge the DC Power Bus voltage.

• When the machine is SHUT DOWN, the retarding contactors will CLOSE aiding in the discharge of the DC Power Bus voltage.

Crowbar

Illustration 20	g06390997
View of the rear of the inverter cabinet (62) Low voltage connector (fiber optic and copper circuits) (63) Crowbar thyristor (64) CHN bus bar connection (65) DCN bus bar connection (DC Power Bus negative)

The "Crowbar" is a solid-state thyristor which is also referred to as a "Silicon Controlled Rectifier" (SCR). When the thyristor is OFF (OPEN), no current will flow through the device. When turned ON (CLOSED), the thyristor will conduct current in only one direction.

In the electric drive train system, the "Crowbar" is used as an over voltage control device. When the "Crowbar" is activated, the positive (+) DC Power Bus is shorted to the negative (-) DC Power Bus through the "Retarding Grid 1" resistor elements.

Either the "Motor 1 ECM" or the "Motor 2 ECM" can command "Interface Module 2" to activate the "Crowbar". To activate the "Crowbar", an ECM will send a voltage pulse to "Interface Module 2". "Interface Module 2" will convert the voltage pulse to a single light pulse in a fiber optic driver circuit. The fiber optic circuit is connected to the "Crowbar" assembly. The optical pulse is converted back to a voltage pulse in the electronic "Crowbar Gate Driver" circuit.

The fiber optic driver circuit is connected between the interface module and the "Crowbar" assembly provides isolation between the low-voltage control circuits and the high-voltage circuits that are connected to the "Crowbar". The fiber optic circuits also provide isolation from stray electrical fields that could cause accidental activation of the "Crowbar".

When the "Crowbar" is turned ON, the thyristor will remain in the latched ON (CLOSED) state. The battery disconnect switch must be turned OFF to unlatch (OPEN) the thyristor.

The crowbar will turn OFF when the DC link voltage is almost zero and the firing pulse from the OIM has been removed. The firing pulse is removed during a normal shutdown once the diagnostic has completed, or after loss of control power during a fault shutdown. In the latter event, the control power is lost when the disconnect switch is turned OFF or 30 seconds after key switch OFF (unless the DC link voltage sensors have failed).

Note: The true fail-safe way to turn the crowbar OFF is to wait until the DC link voltage is zero and then cycle the battery disconnect switch. However, most times, a shutdown of at least 30 seconds will do the trick (even for drivetrain and inverter fault shutdowns).

Either "Motor Control" ECM can use a crowbar driver circuit to signal the interface module to activate the "Crowbar Module".

The thyristor has an anode (A) connection connected to one side of the "Retarding Grid 1 Resistor" elements. The other side of the resistor elements is connected to the positive (+) DC Power Bus. A cathode connection is connected to the negative (-) DC Power Bus.

When the gate of the thyristor is biased by the voltage pulse, the electrical current from the positive (+) DC bus is directed through the "Retarding Grid 1 Resistor" elements and on to the negative (-) DC bus through the conducting thyristor. This series of events result in an immediate discharge of the DC Power Bus voltage.

The control logic and the fault condition logic that is used for the operation of the "Crowbar" is:

• During machine operation, if one or more of the enable signals from the motor control ECM to either interface module is interrupted or lost, "Interface Module 2" will activate the "Crowbar".

• If the voltage level of the DC Power Bus reaches 3000 VDC, a "Level 2 Event" (E655 - High DC Power Bus Voltage) is activated by each motor control ECM. If the voltage level of the DC Power Bus reaches 3100 VDC, a Level 3 - E655 Event is activated by each "Motor Control" ECM. The "Crowbar" will be turned ON to discharge the DC Power Bus voltage immediately.

• If the "Motor 2 ECM" detects a current imbalance of greater than 20 percent between the generator output phases, a Level 3 Event (E0978 - Generator Phase Current Imbalance) is activated. The "Crowbar" will activate discharging of the DC Power Bus immediately.

• The "Motor 1 ECM" continuously monitors the circuit current signal from the "Chopper Grid Current Sensor" (CHPRCT). Current in this circuit is enabled by the operation either the "Crowbar" or the "Chopper Module". Do not activate both devices at the same time.

• When the engine is turned OFF, the "Motor 2 ECM" will activate the "Crowbar". This activation is done to help discharge the DC Power Bus and to check the operation of the "Crowbar". If the "Crowbar" does not activate as expected, the ECM will activate the appropriate event.

• If the "Crowbar" has been activated and the expected current is not detected in the circuit, the "Motor 2 ECM" activates a "Level 3 Event" (E757 - Crowbar Not Responding to Command).

• If the "Crowbar" or the "Chopper Module" are not activated and the "Motor 2 ECM" is detecting a current greater than 100 amps, the ECM activates a Level 3 Event (E911 - Unexpected Chopper Module Output Current Detected).

A Cat^® ET test is available that enables a technician to perform a KEY ON test that will turn ON the "Crowbar" driver circuits. When a circuit is turned ON, the interface module fiber optic driver circuit can be disconnected and the light pulses can be visually verified to be present.

Traction Rectifiers

Illustration 21	g03788641
(33) "Traction Rectifier 2" - used to create the negative side voltage of the DC Power Bus from the three-phase AC Generator output (34) "Traction Rectifier 1" - used to create the positive side voltage of the DC Power Bus from the three-phase AC Generator output (66) DC Power bus positive (+) common bus bar connected to diode cathode connections (L-R) K1, K2, K3 (67) Three phases of the generator output voltage connected to the diode anode connections (L-R) A1, A2, A3 (68) DC Power Bus negative common bus bar connected to diode anode connections (L-R) A1, A2, A3 (69) Three phases of the generator output voltage connected to the diode cathode connections (L-R) K1, K2, K3

Illustration 22	g02062974
Circuit connections for the Traction Rectifier 1 and the "Traction Rectifier 2"

Two identical rectifier assemblies are used to rectify the generator three-phase AC output voltages (A1, A2, A3) to the positive and negative DC Power Bus voltage in the "Inverter Cabinet".

The anode ends of the "Traction Rectifier 1" diodes are connected to the AC output voltage (A1, A2, A3) from the Generator. The diode cathode ends are all connected to the positive side of the DC Power bus.

The "Traction Rectifier 2" diodes are connected in the opposite configuration. The cathode ends (K1, K2, K3) of the diodes are connected to the AC output voltage (A1, A2, A3) from the generator. The diode anode ends are all connected to the negative side of the DC Power bus.

Illustration 23	g03788656
Typical view Rectifier assembly orientation arrows properly matched when mounted

Orientation arrows are present on the cabinet wall and on each of the traction rectifier assemblies. These arrows are provided to ensure that the orientation of the identical rectifier assemblies is correct for each mounting position. The arrows ensure proper connections of the bus bars to the assemblies. Ensure that the arrows are matched during any removal and replacement procedure.

When the assemblies are mounted in the cabinet, cooling fins on the rear of the assembly extend into the cabinets internal cooling duct. The fins dissipate the heat that is created by the diodes during machine operation.

A test procedure is available that allows the user to check the operation of the individual diodes in the assembly. The test can be performed when a low DC Power Bus voltage is indicated. The test can be performed anytime that a problem is suspected in the traction rectifier assemblies.

Inverter Active Resistor Assembly, Discharge Resistor Assembly

Illustration 24	g03672254
Typical view of the resistor assemblies as located in the rear of the cabinet (A) Inverter Active Resistor Assembly (B) Discharge Resistor Assembly

Two resistor assemblies are located in the rear left-hand side of the "Inverter Cabinet". Each resistor assembly consists of four separate resistors.

The assembly on the left-hand side is the "Inverter Active LED Resistor" assembly. The assembly on the right-hand side is the "Discharge Resistor Assembly".

The two assemblies look identical, however, the two assemblies are not interchangeable. The resistance values of the resistors in each assembly are not the same.

Inverter Active LED Resistors

Illustration 25	g01789153
Inverter Active Lamps illuminated indicating that the Inverter DC Power Bus voltage is greater than 150.0 VDC

The "Inverter Cabinet" is equipped with two "Inverter Active Lamps" located above the "Retarding Contactor" compartment. The two LED lamps are illuminated any time that the voltage on the DC Power Bus is greater than 150.0 VDC. The lamps will operate regardless of the state of the key start switch.

The "Inverter Active Lamps" are used as a visual aid to indicate the status of the DC Power Bus. The "Inverter Active Lamps" are not designed or intended to be used as an absolute indication that the DC Power Bus has been discharged. Regardless of the state of the "Inverter Active Lamps", the DC Power Bus voltage must be measured manually using a high-voltage meter. The voltage must be verified to be below 50.0 VDC before any service procedure or maintenance procedure can be performed on any of the electric drive system high-voltage components.

Illustration 26	g02098134
Four "Inverter Active LED Resistors" located in the rear cabinet compartment.

To enable the "Inverter Active Lamps" to turn ON when the voltage of the DC Power Bus is greater than 150.0 VDC and turn OFF when the voltage of the DC Power Bus is less than 150.0 VDC, two resistors are connected in series with each lamp.

Each resistor is a 35K ohm resistor rated for 500 watts.

An LED protection resistor is connected in parallel with each lamp to protect the lamp from voltage spikes. These resistors are a part of the LED lamp assembly.

Discharge Resistors

Illustration 27	g02098173
Four discharge resistors located in the upper rear left-hand cabinet compartment

Illustration 28	g02098174

The Discharge Resistor Assembly consists of four 15K ohm resistors that are rated for 500 watts.

This resistor assembly is used for several functions. The primary function of the resistors is to provide a connection point for the Ground Fault Voltage Sensor. This voltage sensor is used to detect ground faults in the electric drive system.

One set of parallel resistors are connected in series with another set of parallel resistors. The DC positive (+) bus is connected to one end of the resistor configuration and the DC negative (-) bus is connected at the other end.

The voltage sensor is connected between each set of the series connected resistors. The voltage at the point of sensor connection is always exactly half of the total voltage of the DC Power Bus voltage. This center point creates a balanced voltage reference point of connection for the voltage sensor.

If a fault to ground occurs somewhere in the system, the balance of the DC bus voltage will shift from the original center reference point to either the (+) side or the (-) side of the bus. The Ground Fault Voltage Sensor detects this shift. The amount of shift from the original reference point indicates the amount of ground fault "leakage" that is occurring. The frequency and the direction of the shift will indicate whether the ground fault is in the AC section of the system or in the DC section of the system.

For more information on the operation of the Ground Fault Voltage Sensor, refer to the "Voltage Sensors" section that follows.

Another function of the Discharge Resistor Assembly is to provide a path to discharge the voltage of the DC Power Bus if other discharge components such as the Retarding Contactors or the Chopper Module should fail to activate.

The primary path for discharge is from DC link positive to DC link negative. There is some discharge to ground through the GFV sensor and RS1 resistor.

A complete discharge of the DC voltage only through the resistor assembly will take up to 15 minutes.

Voltage Sensors

Three voltage sensors are used in the electric drive system. The DC Bus Voltage 1 Sensor (DCV1) and the DC Bus Voltage 2 Sensor (DCV2) voltage sensors are connected between the DC positive (+) bus and the DC negative (-) bus. As discussed in the previous section for discharge resistors, the Ground Fault Voltage Sensor is connected to a voltage divider circuit. The sensor is used to detect ground faults in the system.

DC Bus Voltage 1 Sensor (DCV1), DC Bus Voltage 2 Sensor (DCV2)

Illustration 29	g03672299
View of the rear of the inverter cabinet (A) Ground Fault Voltage Sensor (B) DC Bus Voltage 1 Sensor (DCV1) (C) DC Bus Voltage 2 Sensor (DCV2)

The DC Bus Voltage 1 Sensor (DCV1) and the DC Bus Voltage 2 Sensor (DCV2) are used to monitor the voltage level of the DC Power Bus.

Each of the voltage sensors is monitored by the "Motor 1" ECM and the "Motor 2" ECM. The voltage status is provided to the "Drivetrain" ECM over the CAN B Data Link circuits.

The voltage sensors receive +15 VDC and -15 VDC power supplies from a "Motor Control" ECM. The sensor ground is connected through two "Wago" connectors, then to a ground boss located below the terminal boards.

The voltage sensors provide a current output that is proportional to the voltage of the bus. At 3000 VDC, the current output of the sensor is 50 milliampere.

The current output circuit is connected at one of the terminal blocks. To allow the ECM to interpret the sensor current output, a 150 ohm burden resistor is connected between the sensor signal circuit and a ground connection at the terminal block.

Each ECM will receive an input from the signal side of the resistor and an input from the ground side of the resistor. These two inputs allow the ECM to measure the voltage drop across the resistor. The voltage drop will be used to calculate the bus voltage.

Each +/- 1.0 VDC drop across the resistor will be interpreted as a voltage of 412.0 VDC by the ECM.

If the voltage status information that the "Drivetrain" ECM is receiving from each "Motor Control" ECM is slightly different, the "Drivetrain" ECM will consider the higher voltage to be relevant.

If the voltage status information that the "Drivetrain" ECM is receiving from each "Motor Control" ECM is different by greater than 200.0 VDC, the "Drivetrain" ECM will activate a "Level 2 Event" (E1126 - DC Power Bus Voltage Mismatch).

"Motor 1" ECM will monitor the signal circuit of the "DC Bus Voltage 1 Sensor" (DCV1) for diagnostics. If an abnormal condition is detected, the ECM will activate a CID 2934 (DC Power Bus Voltage 1) diagnostic code.

"Motor 2" ECM will monitor the signal circuit of the "DC Bus Voltage 2 Sensor" (DCV2) for diagnostics. If an abnormal condition is detected, the ECM will activate a CID 2935 (DC Power Bus Voltage 2) diagnostic code.

Ground Fault Voltage Sensor

The "Ground Fault Voltage Sensor" operates in the same manner as the two DC bus voltage sensors, however, the "Ground Fault Voltage Sensor" is used to detect ground faults in the electric drive system.

The "Ground Fault Voltage Sensor" receives +15 VDC and -15 VDC power supplies from the "Motor 2" ECM. A sensor ground is connected to the cabinet ground boss, located below the terminal blocks in the cabinet.

The positive high-voltage connection for the sensor is connected between each of the series connected resistors in the "Discharge Resistor Assembly". The negative high-voltage connection for the sensor is connected to a system ground. The voltage at the point of positive sensor connection is always approximately half of the total voltage of the DC Power Bus voltage regarding ground. This center point creates a balanced voltage reference point of connection for the voltage sensor.

If a fault to ground occurs somewhere in the system, the balance of the DC bus voltage will shift from the original center reference point to either the (+) side or the (-) side of the bus. The "Ground Fault Voltage Sensor" detects this shift. The amount of shift from the original reference point indicates the amount of ground fault "leakage" occurring. The frequency and the direction of the shift are used to indicate whether the ground fault is in the AC section of the system or in the DC section of the system.

The "Ground Fault Voltage" sensor provides a current output that is proportional to the voltage shift from the balanced voltage point of the bus.

The current output circuit is connected at one of the terminal blocks. For the ECM to interpret the sensor current output, a 453 ohm (5 watt, 1 percent tolerance) burden resistor is connected between the sensor signal circuit and a ground connection at the terminal block. This resistor provides a smaller and more accurate voltage measurement.

Each ECM will receive an input from the signal side of the resistor and an input from the ground side of the resistor. Two inputs allow the ECM to measure the voltage drop across the resistor which will indicate the amount of voltage shift from the balance point.

Each +/- 1.0 VDC drop across the resistor will be interpreted as a voltage of 145.0 VDC by each "Motor Control" ECM.

A small amount of electrical leakage to ground will not cause a problem in the system. Larger amounts can cause damage of the system components and may require immediate action.

The amount of voltage shift from the balanced point can be observed using the Cat^® ET service tool. The amount of shift will be displayed as a percentage.

For a ground fault that is detected in the DC section of the system, the percentage of shift and the related actions are:

• 0 percent to 30 percent - acceptable amount of leakage to ground, no indication will be activated.

• 30 percent to 60 percent - A "Level 1 Event" (E988 - DC Ground Fault) Event will be activated. Ground fault leakage at this level will not damage components in the system. The condition may require attention if the Level 1 event remains active for a long time period (hours).

• 60 percent to 100 percent - A "Level 3 Event" (E988 - DC Ground Fault) will be activated. Ground fault leakage at this level could damage components in the system. An immediate safe shutdown of machine operation is required.

For a ground fault that is detected in the AC section of the system, the percentage of shift and the related actions are:

• 0 percent to 20 percent - acceptable amount of leakage to ground, no indication will be activated.

• 20 percent to 40 percent - A "Level 1 Event" (E1184 - AC Ground Fault) will be activated. Ground fault leakage at this level will not damage components in the system. The condition may require attention if the "Level 1 Event" remains active for a long time period (hours).

• 40 percent to 100 percent - A "Level 3 Event" (E1184 - AC Ground Fault) will be activated. Ground fault leakage at this level could damage components in the system. Immediate safe shutdown of machine operation is required.

Note: A 100 percent AC or DC ground fault will indicate a direct short circuit to frame ground.

For more information on the action that should be taken when a ground fault event is activated, refer to the Troubleshooting, "E988 or E1184".

Current Sensors

Illustration 30	g06391023
Typical view of nine of the 12 system current sensors for the measurement of the traction motor phases and the Generator phases (70) Motor 2-phase current sensors - left to right: M2BCT, M2ACT, M2CCT (71) Generator phase current sensors - left to right: GENCCT, GENBCT, GENACT (72) Motor 1-phase current sensors - left to right: M1CCT, M1ACT, M1BCT

Illustration 31	g06391030
Current sensor locations (73) Chopper current sensor (CHPRCT) in the grid connection enclosure on the lower left front of the Inverter Cabinet.

Illustration 32	g06391032
Current sensor locations (74) Contactor current sensor (CONTCT) located in the rear right-hand side of the Inverter Cabinet. (75) DC bus current sensor (DCCT).

Note: For detailed schematic diagrams of the Inverter Cabinet connections for each of the system current sensors, refer to the Testing and Adjusting, "System Schematic" section in the back of this manual. Also refer to UENR3700 in SIS for the full machine schematic.

Twelve current sensors, also called current transducers (CT), are used to monitor the current of various circuits in the electric drive system. Ten of the CT's are rated for 3000 amps. The other two CT's are rated for 1500 amps. A label on the side of each CT lists the amperage rating as either "3 KA" or "1.5 KA".

An arrow is molded on the top of each CT. The direction of the arrow is matched to the direction of current flow in the monitored circuit when the CT is installed.

For troubleshooting, a CT can be switched with another CT that is rated for the same amperage. Do not switch a 1500 amp rated CT with a 3000 amp rated CT.

The 12 CT's, the rated amperage, the function of each CT and the controlling ECM are listed below.

CT's that provide input to the "Motor 1" ECM:

• M1ACT (3000 A) - monitors the AC current of the Traction Motor 1 phase A Inverter output.

• M1BCT (3000 A) - monitors the AC current of the Traction Motor 1-phase B Inverter output.

• M1CCT (3000 A) - monitors the AC current of the Traction Motor 1-phase C Inverter output.

• Contactor current sensor (CONTCT) (1500 A) - monitors the DC current through Contactor Grid. Current should only flow in this circuit when the retarding contactors are closed.

• Chopper current sensor (CHPRCT) (1500 A) - monitors the DC current through the Chopper Grid. Current should only flow in this circuit when the Chopper Module is active or when the "Crowbar" has been activated.

CT's that provide input to the "Motor 2" ECM:

• M2ACT (3000 A) - monitors the AC current of the Traction Motor 2 phase A Inverter output.

• M2BCT (3000 A) - monitors the AC current of the Traction Motor 2-phase B Inverter output.

• M2CCT (3000 A) - monitors the AC current of the Traction Motor 2-phase C Inverter output.

• DC bus current sensor (DCCT) (3000 A) - monitors the DC current of the DC Power Bus that is rectified from the Generator output.

• GENA CT (3000 A) - monitors the AC current of the Generator phase A output.

• GENB CT (3000 A) - monitors the AC current of the Generator phase B output.

• GENC CT (3000 A) - monitors the AC current of the Generator phase C output.

Each of the CT's are supplied with +/-15.0 power supplies from the controlling ECM. Each CT will supply a positive (+) signal circuit and a negative (-) signal circuit to the ECM.

When current is flowing through a monitored circuit, a hall effect element in the sensor detects the created magnetic field. The CT converts the field to a voltage output that can range from - 10.0 VDC to + 10.0 VDC. The ECM will equate the amplitude of the voltage signals to a specific current for the circuit.

Each "Motor Control" ECM monitors the signal circuits of each CT for diagnostics. If an abnormal circuit condition is detected, the ECM will activate a diagnostic code.

Retarding Grid Blower Inverter

Illustration 33	g03787594
Typical view of the Retarding Grid Blower Inverter located in the rear of the Inverter Cabinet

The Retarding Grid Blower Inverter (RGBI) is used to create the three-phase AC power supply for the grid blower motor. The grid blower motor is a three-phase AC induction motor. The motor turns the fan that is used to cool the grid resistor elements.

The grid resistor elements are used as a resistive load primarily when the retarding mode is used.

When electrical current is directed through the resistor elements, the electrical energy is converted to heat energy. The process creates a large amount of heat in a short amount of time. The heat must be dissipated quickly to prevent the resistor elements from overheating.

To control the grid blower motor, the RGBI receives a DC power supply from circuits that are connected to tapped connections on one of the contactor grid resistor elements.

When the retarding contactors close, DC bus voltage is sent through the "Grid 2 Resistor Elements". This supplies the DC voltage to the RGBI. The RGBI inverts the DC voltage to three phases of modulated PWM voltage in the same manner as the drive train system Inverter supplies for the traction motors.

The RGBI uses internal power transistors to invert the DC voltage to three phases of modulated PWM voltage that the blower motor will see as AC voltage. The frequency of the PWM voltage will determine the rotational speed of the blower motor.

The frequency of the PWM voltage and the resulting speed of the fan depends on the DC voltage received by the RGBI from the tapped contactor grid circuits.

Internal electronics in the RGBI measure the DC supply voltage and an internal software map determines the frequency of the output voltage at a ratio of 5.55 VDC to 1 Hz.

During retarding, the RGBI uses the maximum frequency for the inverted output voltage to operate the fan at the maximum speed.

The low voltage control circuits for the RGBI include a system control voltage power circuit that is fed through a pilot relay, a ground connection, three grounded harness code connections, and two CAN B Data Link circuits.

If a grid-related fault condition causes the "Drivetrain ECM" to disable the operation of the retarding grid, control power must be cycled OFF and back ON to reset the RGBI. The RGBI pilot relay is used for this purpose. When the fault condition is cleared, the "Motor 1 ECM" cycles the sinking driver relay circuit OFF and ON to reset RGBI operation.

The harness code connections indicate the truck model that the RGBI is installed on. The harness code allows the RGBI to activate the correct software.

RGBI communication to each motor control ECM is conducted on the CAN B Data Link circuits. The RGBI can only send information. The RGBI cannot receive information from the other control modules on the data link.

The RGBI sends information to the controls indicating the status of the harness code, the output frequency, the current for each motor phase output, the internal temperature, and the voltage of the DC power supply from the grid 2 taps.

The RGBI also will use the data link to send status information for any detected fault conditions. When a fault condition is detected, the "Motor 1 ECM" activates the appropriate event. The events that can be activated for RGBI fault conditions are:

• E1076 - Electric Retarding Grid Fan Motor Phase Current Incorrect

• E1077 - High Electric Retarding Grid Fan Motor Voltage Incorrect

System Capacitors

DC Power Bus Capacitor (If Equipped)

Illustration 34	g03788721
DC Power Bus Capacitor

The "DC Power Bus Capacitor" consists of two capacitors in one enclosure. The two capacitors are connected in parallel between the DC positive (+) bus and the DC negative (-) bus. The capacitors together provide 1667 microfarads of capacitance.

All the capacitors in the system that are connected between the positive and the negative DC bus are connected in parallel.

The capacitance of the "DC Power Bus Capacitor" is combined with the capacitance of the phase module internal capacitors and other filter capacitors. The "DC Power Bus Capacitor" provides about one eighth of the total filter capacitor. The filter capacitor acts as a reserve that will help to maintain the power that is available on the "DC Power Bus" during times of high demand.

In addition, the filter capacitor helps to smooth the "ripple" on the "DC Power Bus" that is created by the rectification of the generator three phase outputs.

When the engine and the generator are shut down, the large filter capacitor will still be charged with high voltage that is close to the operating level of the "DC Power Bus". This capacitive voltage must be discharged to avoid accidental or unintended discharge.

The following multiple processes are designed into the system to discharge this stored voltage at the time of machine shutdown:

• The retarding contactors CLOSE which shorts the DC positive bus and the DC negative bus through the "Contactor Grid 2 Resistors".

• The "Chopper Module" is activated which shorts the DC positive bus and the DC negative bus through the "Chopper Grid 1 Resistors".

• The voltage of the "DC Power Bus" will naturally discharge through the "Discharge Resistor Assembly" after about 15 minutes.

• The voltage of the "DC Power Bus" will naturally discharge through the "Inverter Active LED Resistor" circuits.

There is a slight possibility that multiple failures of the discharge systems could allow capacitive DC voltage to remain in the system after machine shutdown.

For verification that the voltage is discharged to less than 50.0 VDC, a manual measurement using a high-voltage meter must be done before performing maintenance procedures on the machine high-voltage electrical system.

Note: Refer to the Troubleshooting, Electrical Shutdown and Voltage Discharge section of this manual to verify that the capacitive voltage has been properly discharge.

Bulk Capacitors

Illustration 35	g03672312
Bulk capacitors located in the Inverter Cabinet low voltage compartment

System 24 V power is supplied to each of the bulk capacitors by the respective interface modules. The main function of the bulk capacitors is to provide emergency power to the interface modules if the "Motor Control" ECM was to lose system power.

During a power loss, the bulk capacitors will supply approximately 3 seconds of system power to the interface modules. The backup power will allow each interface module to shut down transistor operation under full control power.

A shutdown of transistor operation during a low control voltage condition can cause transistor misfires which could result in direct shorts across the DC bus.

Retarding Grid Blower Inverter Capacitor, Ground Detection Capacitor

Illustration 36	g03672317
View of the rear of the inverter cabinet (A) Retarding Grid Blower Inverter Capacitor - used as a suppression device to suppress high-voltage spikes in the RGBI DC power supply circuits (B) Ground Fault Capacitor

The "Ground Detection Capacitor" and the "Retarding Grid Blower Inverter Capacitor" are used as protection devices in the respective circuits. Both of the capacitors are identical.

Each module consists of three 1 microfarad capacitors in a single enclosure.

Two of the three capacitors in the "Ground Detection Capacitor" are connected in parallel to the "Discharge Resistor Assembly". The third capacitor is connected between the voltage divider circuit and a ground connection.

The connection configuration provides a low-resistance path to ground for high frequency voltage spikes on the DC bus. The voltage spikes are caused by the high frequency switching of the power transistors. The resistor assembly enables more stable transistor switching.

The "Retarding Grid Blower Inverter Capacitor" is connected between the tapped DC positive (+) and the DC negative (-) circuits that are connected to the "Grid 2" resistor elements.

When theretarding contactors CLOSE, the DC current from the power bus is directed through the contactor "Grid 2" resistors. The current energizes the tapped DC supply circuits for the "Retarding Grid Blower Inverter" (RGBI).

When the contactorsOPEN and CLOSE, there is a slight difference between the time that each contactor actually makes or breaks contact. This time differential can cause voltage spikes that are absorbed by the "Retarding Grid Blower Inverter Capacitor" which protects the RGBI circuits from possible damage.

Chassis Components

Retarding Grid

Illustration 37	g02106415
Typical views Upper photo - the Retarding Grid Assembly mounted on the truck Lower photo - one of the four grid resistor elements

Illustration 38	g03790489
Retarding Grid Assembly connections

The Retarding Grid Assembly consists of the grid blower motor, the fan, and two separate sets of resistor elements. One set of elements is "Contactor Grid" and the other set of elements is "Chopper Grid".

The maximum rated power load for the grid is 4.7 megawatt (6303 HP).

The resistor elements provide the resistive load that is used during dynamic retarding and engine load tests. The resistor elements are also used for the discharging of the DC Power Bus voltage during shutdowns or severe fault conditions.

Each grid assembly consists of four curved segments. Each of the four segments contains a grid 1 resistor and a grid 2 resistor. The resistors are electrically isolated from each other.

When the four segments are mounted in the grid housing, the segments form two separate electrically isolated circles.

The contactor grid resistors are mounted closer to the fan. These resistors will be used to dissipate the most energy when the retarding contactors CLOSE during dynamic retarding.

The chopper grid resistors are placed further from the fan. The "Chopper Grid 1" resistors are used to dissipate energy when the "Chopper Module" is in operation or when the "Crowbar" is activated. The "Motor 1 ECM" will limit "Chopper Module" operation when the fan is not in operation to limit the heat that is created.

When mounted, the segments must be placed in the correct position as each segment has different electrical resistance values. The resistances of each resistor element and the complete sets are listed.

Individual untapped grid elements 1, 3, 4 resistance at 25° C (77° F):

• Between contacts 1 and 2 - 0.66 ohms to 0.69 ohms.

• Between contacts 3 and 4 - 0.53 ohms to 0.59 ohms.

Individual tapped grid element 2 resistance at 25° C (77° F):

• Between contacts 1 and 2 - 0.71 ohms to 0.75 ohms.

• Between contacts 3 and 4 - 0.53 ohms to 0.59 ohms.

Chopper Grid resistance with cables disconnected from supply cables:

• Between contacts CHN and CHP - 2.2 ohms to 2.3 ohms.

• Between contacts 3 and 4 - 0.53 ohms to 0.59 ohms.

Contactor Grid resistance with cables disconnected from supply cables:

• Between contacts CNN and CNP - 2.7 ohms to 2.8 ohms.

• Between tapped contacts IN and IP - 0.71 ohms to 0.75 ohms.

Electric Drive Cooling Fan System

Illustration 39	g03672390
Electric Drive Cooling Fan System (1) Main duct that directs the intake air flow from the Inverter Cabinet to the fan housing (2) Inlet duct that supplies air flow through the cabinet pressurization filter to the cabinet interior (3) Air filter (4) Blower fan (5) Duct splitter - One direction circulates cooling air flow into the generator, the other circulates cooling air flow into the rear axle housing (6) Ducts that are used to circulate the cooling air flow into the rear axle housing to cool the traction motors (7) Hydraulic Motor for the Electric Drive Cooling Fan (8) Electric Drive Cooling Fan Speed Sensor (9) Electric Drive Cooling Fan Solenoid (10) Two oil coolers for the final drives and brake cooling oil

Illustration 40	g03733351
(A) From steering pump (B) To steering valve (C) To final drive lubrication motor (D) To front brake cooling (1) "Priority Valve and Manifold" (blower fan, final drive lubrication, steering) (2) Relief valve (3) Priority valve (4) Solenoid valve (final drive lubrication) (not used) (5) Makeup valve (6) Fan motor (7) Fan motor solenoid (8) Actuator (9) Fan speed sensor (10) Drive Train ECM

The "Drivetrain ECM" controls and monitors the operation of the "Electric Drive Cooling Fan". A hydraulic motor is used to power the fan.

The "Drivetrain ECM" sends current to the "Electric Drive Cooling Fan Solenoid" to control the rotational speed of the hydraulic fan motor.

The ECM controls the speed of the fan in relation to the speed of the engine. Below 1300 engine rpm, the "Drivetrain ECM" operates the fan speed at approximately 1250 rpm. Above 1300 engine rpm the "Drivetrain ECM" operates the fan speed at approximately 2800 rpm. Maximum fan speed is approximately 3200 rpm.

The "Drivetrain ECM" will use the "Electric Drive Cooling Fan Speed Sensor" to monitor the speed of the hydraulic motor and the fan.

Illustration 41	g02108159
Intake vent sand filter removal

The cooling system intake vents are on the front of the "Inverter Cabinet". Every compartment that houses a phase module and the "Chopper Module" has an intake vent. Each vent is covered by a removable sand filter that will stop large particles from entering the intake vent.

The sand filters should be checked regularly for obstructions that could cause a reduction of air flow.

Heat exchangers on top of each phase module and the "Chopper Module" are lined up with the air intake vents. Seals on the heat exchangers prevent the lightly filtered intake air from entering the interior of the cabinet. The cooling fan draws the air in through the phase module heat exchangers to provide cooling for the power transistors in each phase module.

After passing through the phase module heat exchangers, the cooling air is directed out through the back of the "Inverter Cabinet" into the main duct. The air flow is directed through the hydraulic fan housing. The fan forces most of the air flow on to the generator and the traction motors for cooling.

A portion of the airflow flows into the generator housing. The air circulates through the generator to cool the windings. The air is then exhausted out of the rear of the generator.

The air is then directed into the rear axle housing.

The air flow that is directed into the axle housing is circulated through the open ended traction motor stators to cool the motor windings. The air flows through the motors. The air is then exhausted out of the axle housing through the vented access cover on the rear of the housing.

Louvers on the cover direct the exhaust air flow to avoid creating more dust at the rear of the truck.

Illustration 42	g03672404
Inverter Cabinet Pressurization Filter

A circular flexible duct branches off at the main duct behind the fan housing. The duct connects to an inlet fitting on the rear left-hand side of the "Inverter Cabinet". This duct directs air flow through the "Pressurization Filter" in the front cabinet compartment. This filter enables clean air to be forced into the cabinet interior. This air flow will cause a positive air pressure to be maintained in the cabinet interior. The positive air pressure in the cabinet prevents outside contamination from entering the cabinet. The pressurized air in the cabinet is naturally exhausted out of the cabinet at a slow rate.

The Pressurization Filter must be checked and cleaned at regular intervals to ensure that adequate air flow is entering the cabinet. Consult the Operation and Maintenance Manual for the recommended cleaning interval.

Seals on every cabinet compartment cover provide the means to maintain the positive air pressure in the "Inverter Cabinet". Always ensure that the cover seals are in good condition before the cover is installed. Ensure that the compartment covers are completely and securely fastened in place before operating the truck.

The "Drivetrain ECM" monitors the circuits for the "Electric Drive Cooling Fan Speed Sensor" and the "Electric Drive Cooling Fan Solenoid" for diagnostics. The ECM will activate a diagnostic code for either circuit if an abnormal circuit condition is detected.

Each "Motor Control" ECM monitors the circuits of the temperature sensors in the respective phase modules and "Chopper Module". Each ECM will activate a diagnostic code for the involved circuit if an abnormal condition is detected.

If an over temperature condition is detected in a phase module or the "Chopper Module", the motor control ECM will activate either a "Level 2 Event" or a "Level 3 Event". The level of the activated event will indicate the severity of the problem.

Traction Motors

Illustration 43	g02111155
Traction motor - drive-end

Two identical three-phase AC induction traction motors are mounted in the rear axle housing.

The electric drive train system is designed around the need for precise control of the operation of the two electric traction motors.

"Traction Motor 1" provides the power to turn the left hand set of rear wheels. "Traction Motor 1" is controlled by the "Motor 1 ECM".

"Traction Motor 2" provides the power to turn the right hand set of rear wheels. "Traction Motor 2" is controlled by the "Motor 2 ECM".

The specifications for each traction motor are:

• The three phase windings are connected in a Wye configuration.

• Maximum rotational speed - 3180 rpm.

• Full load travel mode voltage - 1960 VAC.

• Full load retarding mode voltage - 2060 VAC.

• Maximum stall current - 1300 amps.

• Maximum torque output - 35,523 N-m (26,200 lbf-ft).

• Nominal power in travel mode - 1206 kW

• Nominal power in retarding mode - 2430 kW

• Weight (each) - 4100 kg (9,039 lbs)

Each traction motor has a "drive-end" which is the rotor shaft end that connects to the final drive. Each traction motor has a "non-drive end" which can be seen from the access opening at the rear of the axle housing.

During operation, the motors are air cooled. The two traction motors have an open frame design. To cool the motor windings, the air flow that is directed into the axle housing by the Electric Drive Cooling Fan. Then, the air flows through the motor and is exhausted out through the vented cover on the axle housing access opening.

Three 777 MCM gauge high-voltage power cables from the "Inverter Cabinet" are connected to lugs on the non-drive end of each motor.

To control and monitor the electrical current in each of the three phases for each traction motor, the "Controlling Motor Control" ECM will use the input signals from a current sensor or CT that is located in the "Inverter Cabinet".

For more information on the operation of the motor current sensors, refer to "Current Sensors" in the "Inverter Cabinet Components" section.

The PWM voltage pulses from the phase modules are seen by the traction motors as AC sine wave signals. The three phases of AC voltage create a rotating magnetic field in the stator of the motor. The magnetic field will force the rotation of the rotor.

For each motor phase, when the "Motor Control" ECM modulates the output PWM pulses to increase the time that the pulse width is high, the AC voltage of each phase will increase.

For each motor phase, when the "Motor Control" ECM increases the transistor switching frequency, the current for each phase will increase. Higher frequency and current results in greater motor speed and greater torque output.

When a request to change the direction of machine travel is received from the "Drivetrain ECM", each "Motor Control" ECM will electronically switch two of the three-phase outputs for each traction motor. This phase switch results in the motors reversing direction of rotation.

The rotor shaft of each motor is connected to the mechanical final drive system in each axle. To multiply the torque delivered by the motors, the final drives are designed to provide a 35:1 gear reduction ratio.

Illustration 44	g02108853
Typical view of the non-drive end of each traction motor as viewed from the rear axle housing opening (87) Three-phase, high-voltage cable connections from the Inverter Cabinet (88) One of the two-speed sensors for each motor (89) Enclosure that contains the terminal connections for the motor winding temperature sensors and the bearing temperature sensors

Note: The two traction motors are identical. When mounted, the location of the components on the non-drive end is reversed when looking at one motor as compared to the other motor.

Each "Motor Control" ECM monitors the motor speed and the operating temperature of the motor that is under control.

In addition, the "Drivetrain ECM" receives an input from one of two-speed sensors that are mounted on the non-drive end of each motor.

Illustration 45	g02109113
Typical view of "Traction Motor 1" speed sensors (view: inside the rear axle housing looking at the non-drive end of "Drive Motor 1") (1) Drive Motor Speed Sensor 1 (2) Drive Motor Speed Sensor 2

The speed sensors are the "Drive Motor Speed Sensor 1" (Motor 1 Speed Sensor 1) and the "Drive Motor Speed Sensor 2" (Motor 1 Speed Sensor 2).

For both of the traction motors, the "Speed Sensor 1" is monitored by the "Controlling Motor Control" ECM for control of the traction motors.

The "Drivetrain ECM" monitors "Speed Sensor 2" for both motors. The speed sensor enables the "Drivetrain ECM" to determine the speed of each traction motor and the wheel speed for control purposes instantly.

Each speed sensor provides two signal inputs to the ECM. The two inputs allow the ECM to determine the direction of motor rotation in addition to the motor speed.

Each ECM monitors the circuits of the monitored speed sensors for diagnostics. If an abnormal condition is detected in a speed sensor circuit, the ECM will activate a diagnostic code for the involved circuit.

A test procedure is available to help determine if a motor speed sensor has failed. Refer to the Testing and Adjusting, Motor Speed Sensor - Test section of this manual for the test procedure.

Illustration 46	g01899214
Use a small flat blade screwdriver to release or install the winding temperature sensor or bearing temperature sensor wires in the terminals

Each of the two electric drive traction motors has internal temperature sensors for the drive-end bearing and the non-drive end bearing. Each temperature sensor is a 100 ohm (at 0.0° C (32.0° F) Resistive Temperature Device (RTD)).

"Bearing 1 Temperature" sensor is monitoring the drive-end bearing and "Bearing 2 Temperature" sensor is monitoring the non-drive end bearing.

The range of resistance of the sensor that the ECM considers acceptable is 80 ohms to 180 ohms.

Each temperature sensor provides a passive analog (resistive) signal to the "Motor Control" ECM that is controlling the operation of the drive motor. The ECM will use an internal capacitance circuit to determine the resistance of the circuit. The ECM will use this information to determine the operating temperature of the traction motor bearings.

Two redundant return circuits are used for each sensor positive signal circuit.

The bearing temperature sensors are not serviceable. A backup temperature sensor is available for each of the original bearing temperature sensors. If a temperature sensor has failed, the harness circuit wires will be reconnected to the backup temperature sensor circuits at the terminal block in the enclosure.

To determine the operating temperature of the traction motors, each controlling "Motor Control" ECM will monitor two temperature sensors that are embedded in two of the motor stator windings.

The winding temperature sensors are of the same type of RTD sensor that is used to monitor the bearing temperatures.

The "Winding 1" and "Winding 2" temperature sensor designations refer to the ECM circuits, not a specific motor winding.

Like the bearing sensors, the winding temperature sensors are not serviceable. A backup temperature sensor is available for each of the original winding temperature sensors. If a winding temperature sensor has failed, the harness circuit wires will be reconnected to the backup temperature sensor circuits at the enclosure terminal block.

If a high temperature condition for the bearings or the windings is detected by the "Controlling Motor Control" ECM, one or more of the following high temperature Events will be activated by either ECM. The ECM that activates the event determines the traction motor that is involved.

• E0720 - High Drive Motor Bearing 1 Temperature Sensor (Level 2 or Level 3).

• E0721 - High Drive Motor Bearing 2 Temperature Sensor (Level 2 or Level 3).

• E1067 - High Drive Motor Winding 1 Temperature Sensor (Level 2 or Level 3).

• E1068 - High Drive Motor Winding 2 Temperature Sensor (Level 2 or Level 3).

If the motor control detects an abnormal condition in one of the bearing temperature sensor circuits or a winding temperature sensor circuit, the ECM will activate one of the following diagnostic codes for the involved circuits:

• CID 3004 - Drive Motor Winding 1 Temperature Sensor.

• CID 3005 - Drive Motor Winding 1 Temperature Sensor.

• CID 3006 - Drive Motor Bearing 1 Temperature Sensor.

• CID 3007 - Drive Motor Bearing 2 Temperature Sensor.

Limp Home Mode

The "Limp Home Mode" enables the operator or technician to use the "Advisor" or Cat^® ET to disable the operation of one of the traction motors. Limited travel of the truck with only one traction motor operating can be enabled under certain conditions.

The truck must be unloaded before attempting to activate the "Limp Home Mode".

The "Limp Home Mode" can be enabled when any of the following conditions are active on the truck:

• A "Level 3 Event" or diagnostic code is activated by ONE motor control ECM for components that are under the ECM control.

• A mechanical problem is present in ONE of the traction motors or in ONE of the final drive assemblies.

Certain mechanical conditions may require the disassembly of traction motor or final drive components before the "Limp Home Mode" can be activated.

The "Limp Home Mode" cannot be enabled when any of the following conditions are active on the truck:

• If a "Level 3 Event" or diagnostic code has been activated by BOTH of the motor control ECMs.

• If a "Level 3 Event" or diagnostic code for the Generator is active.

• If a diagnostic code for the EFR is active.

• If a "Level 3 Event" or diagnostic code for a "DC Power Bus" component or condition is active.

• If the parking brake is not set.

1. When using Cat^® ET to activate the "Limp Home Mode":

   1. Under the "Drivetrain ECM" configurations, select "Configuration Group 1"

   2. Highlight the "Drive Motor Disable Configuration" line.

   3. Select "Change".

   4. A "Change Parameter Value" menu will appear that will allow the user to highlight either "#1" for motor 1 (left), "#2" for motor 2 (right) or "None". Select the motor that is to be disabled or select "None".

   5. Select “OK”.

When activating the Limp Home Mode using the Advisor, similar menus are provided under the “Settings” menu.

1. Depress the "Home" button on Advisor.

2. Select "Service Mode". Depress "OK" until service mode is "Enabled".

   Note: A password may be required.

3. Press the "Back" button and navigate to "Settings".

4. Select "Drivetrain", then "Pwr Conv Disable Config".

5. Select 1 or 2.

   To disable output to inverter 1, select "1".

   To disable output to inverter 2, select "2".

   Advisor will return to the "Settings" screen.

The "Drivetrain ECM" will enforce the following logic when the "Limp Home Mode" is active:

• The truck must be in PARK to attempt to activate the "Limp Home Mode".

• If "Traction Motor 1" is disabled. The "Drivetrain ECM" will activate an informational E1182 Event for "Drive Motor 1 Manually Disabled".

• If "Traction Motor 2" is disabled. The "Drivetrain ECM" will activate an informational E1183 Event for "Drive Motor 2 Manually Disabled".

• The "Drivetrain ECM" will disable "Limp Home Mode" operation if any "Level 3 Event" or diagnostic codes are activated for the traction motor or controls that are active.

• The retarding function will be disabled when the mode is active. Service brakes must be used to slow or stop travel.

• When the mode is active, travel speed will be limited to 11.2 km/h (7.0 mph)

• Traction control (slip control) will be active when in the "Limp Home Mode".

• The "Limp Home Mode" will be disabled when the key start switch is cycled.