TUBE 3236797 - Caterpillar

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