Operation Of Generator

SRCR GENERATOR WIRING DIAGRAM

A1 Regulator Assembly

C1 Surge Capacitor

CR1 Control Rectifier

CR2 Control Rectifier

CR3 Power Rectifier

CR4 Power Rectifier

CR5 Field Rectifier

CR6 Build-Up Diode

CR7 Surge Suppression Diode

CR8 Surge Suppresion Diode

CR9 Blocking Diode

E1 Main Heat Sink

E2 Build-Up Heat Sink

E3 Auxiliary Heat Sink

F1 Fuse

F2 Fuse

FL1 Noise Suppressor Assembly

K1 Build-Up Relay

L1 Filter Choke

L2 Sensing Reactor

L3 Revolving Field

L4 Stator

R1 Regulator Gain Resistor

R2 Regulator Gain Potentiometer

R3 Voltage Droop Potentiometer

R4 Voltage Level Potentiometer

R5 Resistance Wire

R6 Surge Resistor

T1 Isolation Transformer

T2 Voltage Droop Transformer

T3 Regulator Transformer

TB1 Terminal Block

Introduction

The Statically Regulated Controlled Rectifier Generator gives improved performance and longer service life by applying a method of excitation which is controlled by an automatic voltage regulation system that contains no moving parts. The generator voltage build-up system uses only one moving part: a relay, which operates only when the generator is started or shut down.

Generation Of Voltage

The generator is constructed with the armature coils wound on stator (L4) and the field coils wound on the rotor, designated in the wiring diagram as revolving field (L3). The field coils are wound on magnetic steel that will retain a small amount of residual magnetism. The revolving field is connected directly to the engine flywheel through the generator shaft and coupling.

As the engine turns revolving field (L3), a small amount of alternating current voltage is generated in stator (L4) by the influence of residual magnetism in the revolving field. A portion of this alternating current (AC) is rectified to direct current (DC) and this portion is directed back to the revolving field to increase its magnetism. The procedure of excitation can be traced on the SRCR Generator Wiring Diagram by following the path of AC power from stator (L4). This AC power from phase 1 and phase 2 passes through noise suppressor (FL1) through fuses (F1 and F2), wires (37 and 38) to the full wave bridge rectifier (CR1, CR2, CR3 and CR4) mounted on heat sinks (E1 and E2) which rectifies the AC power to provide full wave DC power. Center tap voltage surge suppression diode (CR7) across power rectifiers (CR3 and CR4) limits abnormal transient peak voltages on the bridge rectifiers. Fuses (F1 and F2) are the "fast blow" type and provide protection against secondary damage of the excitation circuit if any one component should fail or malfunction.

The full wave DC power from the rectifiers continues from heat sink (E3) through regulator gain resistor (R1), wire (30), and noise suppressor (FL1) to the positive "+" end of revolving field (L3). To sustain current in revolving field (L3) when neither (CR1) or (CR2) is conducting (because of no circuit from stator phase 3) there is a circuit from the negative "-" end of the revolving field to the positive "+" end of the field. The circuit is from terminal (7) through noise suppressor (FL1) to terminal (8), through wire (29), field rectifier (CR5), regulator gain resistor (R1), through wire (30) to terminal (6) and through the noise suppressor to the positive end of the revolving field. This circuit maintains a flow of current due to self induced voltage of the magnetic field. A mechanical analogy of this circuit is; an engine flywheel as it maintains crankshaft rotation between the power strokes of the individual pistons.

Surge suppression diode (CR8), surge resistor (R6) and surge capacitor (C1) between terminals (6 and 8) on noise suppressor (FL1) minimize voltage surges (spikes) in the excitation circuit to protect the field rectifier (CR5).

CONTROL RECTIFIER SYMBOL
1. Anode. 2. Cathode. 3. Third terminal (gate).

Control rectifiers (CR1 and CR2) are in effect "on-off" valves that can either allow current to flow or can stop the flow of current through the excitation circuit. A control rectifier has the usual rectifier terminals, anode (1) and cathode (2), and a third terminal (3) that, for explanation purposes, will be referred to as the "gate". When gate (3) receives an electric impulse, it takes approximately three micro-seconds (.000003 second) for a control rectifier to "turn on" and allow current to flow. The control rectifier stays "on" until no current is flowing; then it turns "off". Because of no circuit from phase 3, current does not flow once during each complete cycle. Therefore, control rectifiers (CR1 and CR2) are "off" once each cycle and each gate must receive a signal to "turn on" the controlled rectifiers some time during the next cycle.

The timing of the signal to the gate of each control rectifier (CR1 and CR2) is a function of regulator assembly (A1). As generator load increases, regulator assembly (A1) signal the "gates" of the control rectifiers earlier in the cycle, permitting a longer excitation time the revolving field thereby providing the required additional excitation to maintain rated voltage with increased load. When generator load decreases, regulator assembly (A1) signals the "gates" later in the cycle and excitation time is less. Even when control rectifiers (CR1 and CR2) are "off" and current from phase 1 and phase 2 is blocked, revolving field excitation current is subtained for a complete cycle by the circuit that includes field rectifier (CR5). (Remember the flywheel analogy).

Build-up relay (K1) has the only moving part (except for the rotating field) in the entire exciting and regulating system. The relay has contact points that operate only when the generator is being started or stopped. The normally closed contact points in build-up relay (K1) are connected, in effect, from phase 2 to the "gate" of control rectifier (CR2) and through resistance wire (R5) to the cathode of control rectifier (CR2) at auxiliary heat sink (E3). The relay coil is connected in the excitation circuit by wire (18), between terminal (8) on the noise suppressor and pin terminal (A) on the build-up relay (K1). The other end of the build-up relay coil connects to stator (L4) neutral, by wire (17), between relay pin terminal (B) and noise suppressor terminal (10). When generator output voltage is low, the "gate" of control rectifier (CR2) receives a continuous signal during the positive half cycle of phase 2. The signal from phase 2 passes through wire (38), heat sink (E2), build-up diode (CR6), wires (39 and 21), through the closed contacts of build-up relay pin terminal (9) through blocking diode (CR9), through wire (23) to the "gate" of control rectifier (CR2) and through resistance wire (R5) and wire (22) to the cathode of the control rectifier (CR2). A small voltage drop across resistance wire (R5) causes the "gate" of control rectifier (RC2) to have a positive voltage in respect to the cathode of the control rectifier. This voltage is enough to "turn on" the control rectifier. Blocking diode (CR9) protects voltage regulator (A1) from a damaging current surge should field rectifier (CR5) become shorted. As soon as the generator voltage in the relay coil circuit reaches pick-up voltage, the coil causes the contact points to open and regulator assembly (A1) now supplies the signals to both (CR1 and CR2) "gates". When the engine is stopped, pick-up voltage to the coil stops and the contact points close. Pick-up voltage is generated at a speed somewhat less than engine low idle speed.

Voltage level control (5) is a manual control to adjust voltage level potentiometer (R4) when it is necessary to adjust generator voltage to obtain correct line voltage.

Regulator gain control (6) is a manual control to adjust regulator gain potentiometer (R2). Regulator gain control (6) and voltage level control (5) are adjusted in sequence to obtain precise generator voltage regulation when the engine is equipped with either a 3% mechanical speed droop or an isochronous (0% speed droop) engine governor. See OPERATION AND MAINTENANCE INSTRUCTIONS.

When two or more generators are to be operated in parallel, it will be necessary for the voltage of each generator to decrease a specified amount as the generators are loaded. This decrease in voltage, as a generator is loaded, is called voltage droop. Voltage droop control (4) is a manual control to adjust voltage droop potentiometer (R3). Correct generator voltage droop can be obtained by adjusting voltage droop control (4) from counterclockwise for no voltage droop toward clockwise for increased percentage of voltage droop. Voltage droop control (4) must be adjusted in sequence with voltage level control (5) and regulator gain control (6). See OPERATION AND MAINTENANCE INSTRUCTION.

GENERATOR VOLTAGE ADJUSTMENT CONTROLS
4. Voltage droop control. 5. Voltage level control. 6. Regulator gain control.

It was previously stated that control rectifiers "turn on" in approximately three micro-seconds. This extremely fast "turn on" causes shock loading on stator (L4). AC voltage shocks will generate harmonics at radio frequencies. For many applications, these harmonics would be very undesirable.

Choke coils, in series in the excitation power supply, in noise suppressor assembly (FL1) help inpede these high frequencies. Capacitors, in the noise suppressor, by-pass these high frequencies to the generator frame. The noise suppressor is sealed and is serviced as a unit. To make good use of the noise suppressor, always ground the generator frame.

Regulator Assembly

After the generator voltage builds up enough to open the contact points of build-up relay (K1), the relay has accomplished its function. When the build-up relay contact points are open, regulator assembly (A1) supplies both control rectifier "gates" with electric impulses.

The regulator assembly contains resistors, rectifiers, capacitors and transistors in circuits connected to terminals (1 through 12) on the side of the regulator assembly. Because of many components and the complexity of the circuits, the complete assembly is sealed in nonconductive synthetic resin and is serviced as a unit.

From stator (L4) of the generator, a circuit (connected from both phase 1 and the stator neutral) including sensing reactor (L2) and voltage level potentiometer (R4) leads to terminals (12 and 11) on regulator assembly (A1). This circuit from the generator stator will establish an alternating current voltage reference. Here the AC voltage is divided in direct proportion to the reactance of the sensing reactor and the combined resistance in the regulator assembly and the potentiometer. Because frequency varies the reactance of the sensing reactor, the voltage applied to the arm of the potentiometer is independent of engine speed or frequency change. This AC voltage reference also connects to isolation transformer (T1) primary winding. The transformer isolates the regulating circuit and also prevents the voltage divider sensing circuit and the regulator circuit from becoming parallel circuits.

The AC voltage from the secondary winding of the isolation transformer enters the regulator assembly through terminals (1 and 2). Terminals (1 and 2) lead to four diodes that make up a full wave rectifier which changes AC voltage to DC. This DC voltage is filtered by filter choke (L1) connected to terminals (3 and 5). The filtered DC voltage supplies a network of transistors, resistors, capacitors and diodes. The transistors in this network amplify any voltage variations in the input from the isolation transformer. This amplified voltage controls a timing circuit in the regulator assembly. Signals from the timing circuit, through terminals (7 and 8), supply the "gates" of control rectifiers (CR1 and CR2) with electric impulses which cause the control rectifiers to "turn on" the excitation circuit to revolving field (L3) [as required] to maintain constant generator output voltage.

REGULATOR ASSEMBLY