Operation Of Generator

SRCR GENERATOR WIRING DIAGRAM EXCITER AND REGULATOR COMPONENTS
A. Stator. B. Rotating field. D. Power rectifier. E. Main heat sink. F. Field rectifier. H. Auxiliary heat sink. J. Controlled rectifier. L. Build-up relay. M. Sensing reactor. N. Voltage level potentiometer. P. Isolation transformer. Q. Filter choke. R. Regulator gain resistor. S. Regulator gain potentiometer. T. Voltage droop transformer. U. Voltage droop potentiometer. V. Noise suppression assembly. W. Noise suppression transformer. X. Regulator assembly.

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 (A) and the field coils wound on the rotor, designated in the wiring diagram as rotating field (B). The field coils are wound on magnetic steel that will retain a small amount of residual magnetism. The rotating field is connected directly to the engine flywheel through the generator shaft and coupling.

As the engine turns rotating field (B), a small amount of alternating current voltage is generated in stator (A) by the influence of residual magnetism in the rotating field. A portion of this alternating current (AC) is rectified to direct current (DC) and this portion is directed back to the rotating 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 form stator (A). This AC power from phase 1 and phase 2 passes through noise suppression assembly (V) to power rectifiers (D) [installed on main heat sink (E)] which rectify these two phases of AC power to provide two phases of half wave DC power. These two phases of DC power then continue through controlled rectifier (J) to rotating field (B). To sustain current in rotating field (B), when no current is flowing (because of no circuit from phase 3), another circuit from the negative (-) end of rotating field (B) [through field rectifier (F) mounted on auxiliary heat sink (H) to the positive (+) end of rotating field (B)], maintains a flow of current due to self induced voltage of the magnetic field. A mechanical analogy of this circuit is the action of an engine flywheel as it maintains crankshaft rotation between the power strokes of the individual pistons.

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

Controlled rectifier (J) is, in effect, an "on-off" valve that can either allow current to flow or can stop the flow of current through the excitation circuit. Controlled rectifier (J) 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 controlled rectifier (J) to "turn on" and allow current to flow. The controlled 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, controlled rectifier (J) is "off" once each cycle and gate (3) must receive a signal to "turn on" the controlled rectifier some time during the next cycle.

The timing of the signal to gate (3) is a function of regulator assembly (X). As generator load increases, regulator assembly (X) signals the "gate" of controlled rectifier (J) earlier in the cycle, permitting a longer excitation time to the rotating field and thereby providing the required additional excitation to maintain rated voltage with increased load. When generator load decreases, regulator assembly (X) signals the "gate" later in the cycle and excitation time is less. Even when controlled rectifier (J) is "off" and current from phase 1 and phase 2 is blocked, rotating field excitation current is sustained for a complete cycle by the circuit that includes field rectifier (F). (Remember the flywheel analogy.)

Build up relay (L) 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 (L) are connected, in effect, from phases 1 and 2 to the "gate" of controlled rectifier (J). The relay coil is connected from line to neutral. When the generator output voltage is low, the "gate" of the controlled rectifier receives a continuous signal through build-up relay (L) closed contact points, and the controlled rectifier remains "on". 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 (X) now supplies the signals to the "gate". 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 (N) 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 (S). 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.

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

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 (U). 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 INSTRUCTIONS.

NOISE SUPPRESSION ASSEMBLY
7. Choke coils. 8. Capacitors.

It was previously stated that controlled rectifier (J) is "turned on" in approximately three microseconds. This extremely fast "turn on" causes shock loading of stator (A). AC voltage shocks will generate harmonics at radio frequencies. For many applications, these harmonics would be very undesirable. To reduce these high frequency harmonics, noise suppression assembly (V), mounted on the generator frame, is utilized in conjunction with noise suppression capacitor (W). Choke coils (7) in the noise suppression assembly, in series with the excitation power supply, help impede these high frequencies and capacitors (8) bypass these high frequencies to ground at the generator frame. To make good use of noise suppression assembly, always ground the generator frame.

Regulator Assembly

After the generator voltage builds up enough to open the contact points of the build-up relay, the relay has accomplished its function. When the build-up relay contact points are open, the regulator assembly supplies the controlled rectifier "gate" with electric impulses.

The regulator assembly contains resistors, rectifiers, capacitors and transistors in circuits connected to terminals (1 through 10) on the side of the regulator assembly. Because of many components and the complexity of the circuits, the complete assembly is sealed in non-conductive synthetic resin and is serviced as a unit.

REGULATOR ASSEMBLY WIRING DIAGRAM (SCHEMATIC)

From stator (A) of the generator, a circuit (connected from both phase 2 and the stator neutral) including sensing reactor (M) and voltage level potentiometer (N) leads to terminals 10 and 9 on regulator assembly (X). 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 (P) primary winding. The transformer isolates the regulating circuit and also prevents the voltage divider sensing circuit and the regulator circuit from becoming parallel circuits.

SRCR GENERATOR WIRING DIAGRAM

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 (Q) 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. A signal from the timing circuit, through terminal 8, supplies the "gate" of controlled rectifier (J) with an electric impulse which causes the controlled rectifier to "turn on" the excitation circuit to rotating field (B) [as required] to maintain constant generator output voltage.