Control systems for electric generators

Abstract

The excitation winding of an alternator is energized through a silicon controlled rectifier (SCR) from the output of the alternator. The SCR is bypassed by a constant current network to allow the alternator to start up; even with no excitation current, when the alternator output voltage is due to residual magnetism alone, this network can pass a current into the excitation winding, and build the current up gradually to a value above the latching current of the SCR. When this has happened, the SCR can latch on after being fired, and takes over control of the excitation current. Because the network is a constant-current network rather than a purely resistive network, it can be designed to pass sufficient current at low alternator output voltages, without having to dissipate excessive power at higher output voltages. Also, when the output voltage is at its normal operating value, the constant-current network is rendered non-conductive, to avoid the excessive power dissipation which might occur at full voltage. A protective circuit is also included, which is tripped if the voltage on the excitation winding should become excessive as the result of an overload. The tripping takes the form of the turning-on of a programmable unijunction transistor (PUT), which remains on, drawing current from a smoothed supply taken from the alternator output, until the alternator speed is reduced to a very low value. At this low speed, the constant-current network cannot pass sufficient current to allow the alternator output voltage to build up, so that the output voltage does not suddenly increase when the PUT turns off.