Patent GB-684914-A (GB)
Title: Improvements in variable-frequency power-supply system
IPC: H02P27/04
Filed: 1949-09-20
Abstract: 684,914. Control of A.C. motors. BRYANT CHUCKING GRINDER CO. Sept. 20, 1949 [Oct. 29, 1948], No. 24168/49. Class 38 (iii) [Also in Group XXXV] A variable frequency supply suitable for industrial electric motors driving machine tools, comprises a pair of gas-filled discharge tubes with control grids connected together in push-pull and coupled through an amplifier to a variable frequency A.C. or pulse source, the output of variable frequency being developed in a centre-tapped transformer winding connected across the valve anodes from a D.C. supply connected through a current limiting impedance between the centre-tap and the valve cathodes by the firing of the valves at their grid frequency, commutating condensers being connected between the anodes to quench the valves alternately. A resistance-capacity tuned oscillator 6 with an impedance coupled phase inverter and amplifier tube 7 supplies a signal of variable frequency through an amplifier stage 45 to a second stage voltage amplifier 49 comprising valves 50, 51 connected in push-pull between input transformer 48 and their output coupling transformer 52. The output from transformer 52 is fed to the grids of power valves 54, 55 of the thyratron type connected in push-pull, which act as inverters to convert the d.c. supply connected between their cathodes 63, 64 and the centre tap 77 of their output transformer 76 primary winding to a.c. of the oscillator frequency. This is supplied through transformer 76 to a split phase motor 93 having capacitors 106 in one supply lead. Commutating condensers 84 quench each of the valves 54, 55 in turn immediately the other valve fires. The d.c. supply is obtained through three phase rectifiers 119, fed through transformers 121 from a three-phase supply 120. Closure of the main switch 133 in the three phase supply energizes transformer 130 which supplies heating current to power valves 54, 55 and transformer 123 which supplies heating current to rectifiers 119 and also energizes transformer 160 of the plate supply unit 161. This comprises a rectifier 162 and supplies anode and heating current to the oscillator valves 6, 7, amplifier valve 45 and driver stage valves 50, 51, and fixed negative biasing potential through lead 61 to the output 60 of transformer 62 connected to the grids of valves 54, 55. Transformer 123 also supplies power to the relay circuits. Thus the system is energized (as indicated by lamp 184) in readiness for starting the motor 93 by closure of relay 135, which has holding contacts 136, in the threephase supply to rectifiers 119 upon operation of start switch 138. The oscillator frequency and consequently the motor speed is controlled by moving fine adjustment 31 and coarse control 38 which vary respectively the values of resistances 12, 16, and capacitors 35, 36 in the oscillator circuit. Control 38 at the same time alters the value of commutating condensers 84 and phase splitting condensers 106 to suit the changed frequency. Shunt load resistor 112 and an optionally variable p.f. correction condenser 114 are connected across the load. The motor is stopped by opening the stop switch 155 in series with holding contacts 136, or by operation of overload relays 126, 142, 142. The current rate of increase and peak value during firing are controlled by resistor 79, inductor 80 and overload relay 126 in power valve anode supply. To counteract the effects of heavy starting current, the commutating condensers 84 and phase splitting condensers 106 are increased by operation of relay 179.