GB1445711A - Electric motor thermal overload protection circuit - Google Patents

Abstract

1445711 Protective systems KENWOOD MFG CO Ltd 22 March 1974 [22 March 1973] 14001/73 Heading H2K An electrical thermal overload protection circuit for an electric motor comprises a voltage divider formed by series connected resistors R1, R2 and positive temperature coefficient (PTC) resistor 30, the junction between resistors R1 and R2 being connected to a thyristor 25 to control the current to the motor armature 24. In operation under normal conditions the resistance of the PTC resistor is very small compared with R 1 and it has negligible effect on the circuit, the thyristor 25 being biased by the voltage divider to conduct and the motor being on; however under overload conditions the resistance of the PTC resistor rises rapidly as it is heated by a heater resistor 31 in the supply lead 21 to the motor and protection circuit, and eventually disturbs the bias on the thyristor so as to cause it to cease conducting and switch off the supply to the motor, after which the PTC resistor continues heating itself and keeps the motor cut off until the supply has been manually switched off for long enough for it to cool down. Triacs or SCRs may be used in place of the thyristor 25, Fig. 2, not shown, with an associated trigger circuit. In a further embodiment for use with a motor having a tachogenerator speed control circuit, Fig. 3, not shown, the PTC resistor and a further resistor form a potential divider their junction being connected to the inhibit pin of an integrated circuit controller so that when the PTC resistor is heated during overload the voltage applied to this inhibit pin causes the control circuit action of the controller to be overridden and firing pulses to a triac in the motor circuit to be inhibited so that the supply to the motor is interrupted.