Patent GB-1358494-A (GB)
Title: Washing machine control circuit
IPC: H02P7/295
Filed: 1972-06-30
Abstract: 1358494 Control of D.C. motors PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 30 June 1972 [3 July 1971] 30777/72 Heading H2J [Also in Division G3] A washing machine control circuit generally similar to that described in Specification 1,195,179 is provided with means to make the power supplied to the heating resistor 2 independent of the operating condition of the D.C. motor 1, this means comprising a triac 7 in parallel with the motor 1 and a triac 3 (or thyristor if only one direction of motor rotation is required) and a control circuit 8 for the triacs. In operation the triac 3 is fired at the zero point of each supply half-cycle of the appropriate polarity, so that current flows through the members 1, 2 until the triac 7 is fired at a subsequent point in the half-cycle, the motor current thereafter declining and the resistor current increasing. According to the amount of heating power required, the triac 7 may be fired prior to the start or during the following half-cycle or may remain non-conductive during this half-cycle. Typical waveforms of the current through the resistor, motor and triac 7 are described, Fig. 4 (not shown), the triac 7 being fired at its instant of current zero. With the motor stationary the heating current is controlled by the firing of triac 7 alone. In a modification, Fig. 3 (not shown), the triac 3 is replaced by a pair of anti-parallel connected diodes (3, 3<SP>1</SP>) selectible by a change-over switch (9) according to the direction of motor rotation required. Fig. 5 shows the circuit diagram of the control circuit 8 applied to the embodiment of Fig. 2 (block M). A trigger circuit TR derives a square-wave voltage from the A.C. supply 6, 6<SP>1</SP> and applies it to the base of a transistor Tr 4  in a circuit D having transistors Tr 6 , Tr 7  controlling the gates of triacs 3, 7 in response to the voltage at Tr 4 . Speed stabilization of the motor is effected by a signal derived from a circuit S including tachogenerator G and members L, D 1 , C 1 , R 1 , R 2 , R 3 , this signal being applied to the base of a transistor Tr 1  to determine its collector current and hence the charging time of a capacitor C 2  which controls the transistor Tr 7  and hence the firing instant of the triac 7. Alternatively the motor back EMF may be used as the speed indicative signal. The cycle of running and stopping of the motor and its direction of rotation is determined by an astable multivibrator A controlling trigger circuit TR via a bi-stable multivibrator B, an inverter Tr 14 , Tr 15  providing interchange of the cycle periods of multivibrator A. Capacitors C 2 , C 3  limit the durations of the trigger pulses to prevent short circuiting of the motor by simultaneous conduction of the triacs 3, 7 with consequent reversal of motor current causing it to stop. Transistors Tr 8  and Tr 9  determine respectively the condition of the motor (running or stopped) and the method of supplying the resistor 2 (through the motor or the triac 7). Maximum or minimum power to the resistor 2 during the "motor stopped" condition is determined by the presence or absence of a voltage at terminal Q controlling a transistor Tr 10 , which condition corresponds to the connection of the input of the multivibrator A to transistor Tr 8  and to a transistor Tr 11 .