GB950734A - Improvements in or relating to control means for electrical apparatus - Google Patents

Abstract

950,734. Control of D.C. motors. SMITH'S DELIVERY VEHICLES Ltd., H. W. HEYMAN, and J. HAMBLETON. May 29, 1961 [March 1, 1960; Aug. 20, 1960; March 11, 1961], Nos. 7245/60, 28893/60 and 8975/61. Heading H2J. [Also in Divisions G3 and H3] A control system for a battery-operated electric traction motor comprises a variable frequency pulse generator, the pulses from which are used to trigger a gated diode which in turn controls the motor current. Closure of switch S1 charges capacitor CE through silicon controlled rectifier SCR3, cutting off transistors Q3, Q4. Rectifier SCR3 remains conductive, however, until the charging current through capacitor CE decreases to a predetermined value. Capacitor C1 commences to charge, at a rate determined by the value of resistor VR coupled to an accelerator pedal, until it is discharged by transistors Q1, Q2, into the gates of rectifiers SCR1, SCR2. The battery voltage is then applied across the motor circuit, and the capacitor CE discharges inductively through the elements SCR1, SCR2, L1 and D3, so reversing the charge across capacitor CE. Capacitor C2 discharges at a rate determined by resistor R1 until transistor Q4 renders rectifier SCR3 conductive and the reverse potential held by capacitor CE turns off the rectifiers SCR1, SCR2. The capacitor CE then recharges in the original direction and the cycle is repeated. Motor current is maintained by diode RD2 during the interpulse period. Fluctuations in the battery voltage produce an inductive effect and tend to reduce the stored charge on capacitor CE, so preventing turn-off of the rectifiers SCR1, SCR2. The inclusion of diode RD1 ensures that the stored charge on capacitor CE is not reduced by the battery transients. For battery charging purposes, the points A, B, C and D are connected as shown in Fig. 2, the rectifiers SCR1, SCR2 serving for both speed control and battery charging. The battery is supplied through mains transformer T, an auxiliary winding of which supplies a bridge rectifier whose output is clipped by Zener diode Z. Potentiometer P controls the potential applied to transistor Q1 for charging capacitor C1 until transistors Q4, Q5 produce a sharp pulse. This pulse fires the rectifiers SCR1, SCR2, which are switched off automatically at the end of the half cycle. By varying the setting of potentiometer P, the time delay of the pulse from the start of the cycle can be changed. Current feedback from resistor R1 to transistor Q3 modifies the phase of the firing pulse. When charging is complete, a voltage sensitive relay actuates switch S to raise the potential of the base of transistor Q4, so delaying the pulse.