Thermally Responsive Electrical Control Circuit

摘要

1,168,972. Protective systems. ROBERTSHAW CONTROLS CO. 21 Oct., 1966 [23 Oct., 1965 (2)], No. 47347/66. Heading H2K. A fail-safe motor protective arrangement has a controlled rectifier 16 with its gate-cathode circuit connected between the centre tapping of the secondary winding of a transformer 24 and a point 58 in a resistance network which includes a temperature sensitive resistor 52 positioned in the windings of the motor 10. A standing bias for the gate of the rectifier is provided by resistors 30, 32 so that the rectifier 16 normally conducts on alternate half cycles and energizes a relay 26 to maintain contacts 40, 42 closed. In the event of overheating the resistance of the resistor 52 increases causing the point 58 to go negative so that the rectifier 16 ceases to conduct. The relay 26 opens its contacts 40 to cause a main contactor 12 to open and opens its contacts 42 to remove a short circuit from a resistor 64 in the resistance network. This last action increases the operating differential so that a considerable drop in temperature must occur before power is restored to the motor. To ensure adequate protection against single-phasing, a further temperature responsive resistor 68 may be connected as shown.