GB1411742A - Electrical overload detection apparatus - Google Patents

Abstract

1411742 Protective arrangements ELECTRIC ACTUATOR CO Ltd 18 Sept 1972 [16 June 1971] 28325/71 Heading H2K A protective arrangement, e.g. for an electric motor, comprises means TR3, TR5 for comparing a voltage signal proportional to the current to the protected A.C. load with a reference voltage, a transistor TR4 receiving pulses when the voltage signal instantaneously exceeds the reference voltage and causing a supply of current to a capacitor C3 in response to these pulses, and a further transistor TR6 which provides an output signal when the capacitor C3 has been charged by a pulse of sufficiently long duration due to overload conditions. A feed-back circuit D3, R16 from the transistor TR6 to the transistor TR4 causes these transistors to remain conducting after the overload condition has ceased. When the transistor TR6 conducts, the output signal causes a transistor TR9 to conduct so cutting off a transistor TR10 and de-energizing a relay coil RE1. Restarting is effected by closing a switch S1 to cause a transistor TK1 to conduct so causing a transistor TR7 to cease conducting due to diversion of its base current to a capacitor C2 for a short period, the circuit through the transistor TR6 being interrupted for this period.