Patent GB-1200858-A (GB)
Title: Current supply control
IPC: H02P7/292
Filed: 1969-03-13
Abstract: 1,200,858. Control of motors; rectifying. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 13 March, 1969 [16 March, 1968], No. 13214/69. Headings H2F and H2J. A D.C. servomotor 1 is supplied from an A.C. source through a controlled rectifier 5, and is operated to have a constant torque characteristic. The control circuit for the rectifier 5 comprises a capacitor 7 which is charged periodically at a steady voltage through a variable resistor 8. Each charging period commences at a reference instant determined by the counter E.M.F. of the motor becoming equal to the supply voltage as it approaches the end of a cycle, and the instant is advanced in time as the counter E.M.F. increases. Towards the end of the next cycle of the supply voltage, the capacitor 7 becomes charged sufficiently to break down the diode 10 and trigger the rectifier 5 which conducts until the supply voltage and counter E.M.F. become equal. Within a given range, the current-time integral of the pulses applied to the load is independent of the reverse voltage, the motor producing a constant driving torque without the need for a separate current-limiter. The voltage across the rectifier 5 is applied to a double-diode limiter 15 associated with a differentiator 16, 17, so that negative peaks are applied to the base of transistor 19 which is cut off when the decreasing supply voltage becomes smaller than the counter E.M.F. Thus, the transistor 24 becomes conductive to discharge the capacitor 7 at the reference instant. In another arrangement, the motor 1 is supplied reversibly through rectifiers 33, 33<SP>1</SP>, or 34, 34<SP>1</SP>, from transformer 27 associated with capacitor 35 and choke 36. A voltage derived from tachometer generator 37 is subtracted from the voltage from transformer winding 31. Voltage difference limiter 41, 41<SP>1</SP>, is associated with a differentiating circuit comprising capacitors 42, 42<SP>1</SP>, emitter follower stage 44, 44<SP>1</SP>, and a double gate comprising diodes 46, 47, and 46<SP>1</SP>, 47<SP>1</SP>. Pulses from the resistors 48, 49, are applied to transistors 67, 68, shunting capacitors 69, 70, in a timing network which also comprises transistors 77, 78. When the voltage across capacitors 69, 70 exceeds a given value, transistors 79, 80, conduct to fire the controlled rectifiers 33, 33<SP>1</SP>, or 34, 34<SP>1</SP>, through transformers 81, 82. A differential amplifier comprises transistors 85, 86, the base of the transistor 86 being associated with an integrating capacitor 98. The smoothed tachometer voltage and a control voltage are applied through resistors 100, 101, respectively to resistor 97 for varying the degree of amplification. The integration factor is varied by resistor 99, whilst resistor 106 adjusts the mean ignition angle for all the controlled rectifiers corresponding to the stationary state. If the motor is stationary and a forward speed is demanded, transistors 86, 85, become less and more conductive respectively, whereby transistor 77 operates to advance the triggering of rectifiers 33, 33<SP>1</SP>, whilst transistor 78 retards the firing of rectifiers 34, 34<SP>1</SP>. With a delay determined by the time constant of the circuit 97-100, the tachometer voltage reduces the effect of the control voltage. If the motor current becomes excessive, choke 36 is heated and resistor 105, having a positive temperature coefficient, responds through transistor 89 to cut off the transistors 85, 86. The system may be modified to control an asynchronous motor by varying its frequency. In other arrangements, the load is an accumulator or is shunted by a capacitor.