Patent GB-636495-A (GB)
Title: Improved electrical system for synchronising the speeds of prime movers
IPC: H02P5/50
Filed: 1947-07-31
Abstract: 636,495. Automatic speed control systems. GENERAL MOTORS CORPORATION. July 31, 1947, No. 20788. Convention date, Nov. 17, 1944. [Class 38 (iv)] In a system for controlling the speed of a prime mover, e.g. an aircraft engine, in accordance with a constant speed or the speed of a " master " engine, comparison is made between the frequencies of a single-phase current and a plural-phase current, one of which is proportional to the master speed and one to the slave speed, the beat frequency being used to fire a grid-controlled gas-discharge tube which enables one of two or more other gas-discharge tubes to be fired selectively to control speed increasing or speed decreasing means. Fig. 3 shows a system for synchronizing two aircraft engines 1, 2 by varying the pitch of the propeller of engine 2, engine 1 constituting the master, the speed of which is predetermined by a manually operated switch 34 controlling the reversible servomotor 32 of the pitchadjusting mechanism. Engine 1 drives an A.C. generator 45 which supplies the primary windings 83, 93, 103 of transformers 80, 90, 100 with single-phase currents of the reference frequency. Engine 2 drives a polyphase generator, the individual phases of which are connected respectively to the second set of primary windings 84, 94, 104. The corresponding sets of secondary windings on the transformers are connected in series and opposition in the grid-circuits of discharger devices V1, V2, V3, the former of which supplies a condenser C1 and the devices V2, V3 the grid circuits of a double amplifier V4, V5 through transformers 107, 108 and time-delay circuits R6, R7. The amplifier V4, V5 controls the relays 67, 68, which operate the servomotor 36 in one direction or the other to adjust the pitch of the propeller 4 and bring the engine 2 into synchronization. The arrangement is such that when the two engines are in exact synchronism, both as regards speed and phase, a condition of zero beat will occur in the grid circuit of V1 and this valve remains inoperative. When, however, the speed of engine 2 rises, beat grid voltage is produced in V1 and the valve is operated to pulse condenser C1. Moreover, valves V1, V2, V3 are pulsed in that order and condenser C1 discharges through V2, V3 remaining inoperative since the beat pulse on that valve is out of phase with the condenser discharge. In this manner rectified pulses are produced in the secondary 109 and relay 67 is pulsed to operate the servomotor. Similar action takes place when the speed drops below the correct value but in this case tube V3 is operated and the servomotor driven in the opposite direction. The delay circuits R6, R7 ensure positive operation of the relays 67, 68 and also bring about continuous operation of the relays and servomotor during periods of wide divergence from synchronous speed. A switch 39 is provided to facilitate change over to manual control of the servomotor by switch 40. In a modification, both engines may be synchronized with an audio frequency oscillator which provides the reference frequency. Variable pitch of the propellers is effected through a governor type switch (Fig. 2, not shown), the speed of which is controlled by a spring, the compression of which is adjusted through rack-and-pinion gearing by the servomotors 32, 36.