Electrical generator control apparatus

865,846. Automatic voltage-control systems. WESTINGHOUSE ELECTRIC CORPORATION. March 16, .1959 [March 20, 1958], No. 8965/59. Class 38 (4). In an automatic voltage regulator for an alternator in which the field of a D.C. exciter is intermittently energized through a transistor, the on/off ratio of the transistor is controlled by superimposing a sawtooth waveform on a voltage proportional to output voltage error. The output voltage of a 3-phase alternator 10 is rectified at 42 and applied to a Zener diode bridge 50 to provide a D.C. signal which has a median value when the output voltage of the alternator is correct, and rises or falls as the output voltage deviates one way or the other. This signal is applied between the base emitter of a transistor 220 to which is also applied a sawtooth voltage generated by a double-base diode relaxation oscillator 210 so as to produce pulses which are width modulated in accordance with the output voltage error. The pulses are amplified by transistors 230, 240, 250 to control a transistor 260 which is in series with the field of the exciter 20. The field, and the control circuits are normally energized from the output of the alternator through a rectifier 70, the supply for the relaxation oscillator and the early stages of the amplifier being stabilized by a Zener diode 74. It is stated that an independent D.C. source may replace the rectifier 70. An unspecified device 420 responsive to a current transformer 422 produces an output signal when the load current becomes excessive and this signal is applied over diode 412 to so bias transistor 230 as to cut off the transistor 260. For starting, an auxiliary D.C. supply 321 is connected by a switch 324 to supply the field and also to bias transistor 240 so as to render transistor 260 fully conducting. This bias is removed when the alternator builds up by a transistor 310 responsive to alternator output voltage over rectifier 316.