GB881922A - Static excitation system for an alternator - Google Patents

Abstract

881,922. Alternators; automatic voltagecontrol systems. SIEGLER CORPORATION. June 8, 1960 [June 12, 1959], No. 20113/60. Classes 35 and 38(4). An exciter system for an alternator is regulated to prevent excessive excitation in the event of a fault, sudden removal of the load, and on starting. Alternator 10 has a field winding: 11 excited through current transformers 17 in output lines 14, and rectifier 20. A voltageregulating shunt circuit 25 is also provided between the field winding 11 and output lines 14, controllable to allow current flow in either direction. Voltage regulator 25 comprises a " positive " group of controlled silicon rectifiers 26 and a " negative" group of controlled silicon rectifiers 30, in parallel with the " positive " group but conductive in the opposite direction. The emitter-base circuits of rectifiers 26, 30 are connected through diodes 67 to individual windings 50, 53 on individual saturable pulse transformers 44, 47 each of which has four windings. Input windings 57, 60 are connected between pairs of alternator output lines 14 so that rectifiers 26, 30 conduct 120 degrees after the begining of each positive half-cycle of the voltages at their anodes. The conduction is controlled by providing bias windings 69 on each transformer, connected to a constant voltage D.C. source through a potentiometer 72 and resistor 76. The voltage output of the generator is sensed by connections to the output lines 14 through diodes 82 and this voltage is applied across resistor 76 in opposition to the voltage from the constantvoltage source. If the generator output voltage decreases then positive bias is applied to windings 69 advancing the firing point of rectifiers 26 ahead of 120 degrees and retarding that of rectifiers 30. This increases the flow of current from output lines 14 to field winding 11. Similarly current flows in the other direction, reducing the excitation, if the output voltage increases. If the load is suddenly removed the generator field can decay rapidly by the flow of current in the shunt path. Additional negative bias windings 86 are provided, connected through a zener diode 91 across resistor 88 in the field winding circuit so that the excitation is also reduced if the excitation current exceeds a predetermined value. Excessive torque in the hydraulic drive of the alternator due to excessive excitation may also be prevented by providing a pressure-actuated relay in the system, energizing windings 86 from a D.C. source. Specification 868,400 is referred to.