April 2012 Electrical Apparatus

April 2012 Electrical Apparatus

This is a summary of the Electrical Apparatus April 2012 featured technical article,  by Richard L. Nailen, P.E.   

A basic a-c motor control circuit includes three elements connected in series: an electromagnetic contactor to connect the motor windings to a power source; starting and stop switches to energize and de-energize that contactor; and overload contacts responsive to motor current, that open the contactor circuit when the motor is in danger of overheating. Behavior of such a circuit is complicated by the common location of the contactor itself (with the overload contacts) at a distance from the switches. Capacitive coupling between long interconnecting wires can interfere with normal circuit operation.

Control circuit power, to operate the contactor, is often supplied by a separate control transformer, with one side of the secondary grounded for safety. Some of the various ways of interconnecting the circuit components can therefore cause undesired or dangerous contactor operations in the event of a second, accidental ground within the circuit.

One hazard is the unexpected accidental starting of a de-energized motor. A second is the failure of the contactor to open and stop the motor when the stop switch is operated. A third danger is the flow of ground fault current through the overload contacts, which can weld the overload contacts closed before the current is safely interrupted. Still another concern is the creation of a ground path that bypasses the overload contacts entirely, so that overload protection no longer exists.

To mitigate these hazards, various circuit options have been recommended. Of particular importance is the location of the overload contacts within the circuit. Also, use of dual start or stop switches can prevent several undesirable consequences of accidental circuit grounds.

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