July 2009 Electrical Apparatus

July 2009 Electrical Apparatus

This is a summary of the Electrical Apparatus July 2009 featured technical article,  by Richard L. Nailen, P.E.     

Deceptively simple in principle, circuitry for a motor and its control can be quite complex in practice. To ensure safety of persons and property, the National Electrical Code prescribes the basic components of a motor circuit. They include a controller to start and stop the motor; an overcurrent device to deenergize an overloaded motor; another protective device for shutdown when a fault occurs; and a disconnect to open the entire circuit for servicing.

The simplest controller is an electromechanical contactor. A separate control circuit includes pushbuttons to operate the contactor coil, together with contacts of the motor’s overload protection device. One side of that circuit is often grounded for safety. The NEC requires the contactor coil and overload contacts to so located in the circuit that an accidental ground elsewhere cannot start the motor unexpectedly. Some connections would also allow such a ground fault to hold the contactor closed when the stop button is pressed. Other connections can allow ground fault current to weld the overload contacts closed or to bypass them. Consequently, proper wiring of motor control circuits has been the subject of much debate.

A more controversial Code Provision involves the relative locations of the controller, the circuit disconnect, the motor itself, and the driven machine. In sometimes confusing language, lengthy provisions in NEC Article 430 (with exceptions and alternatives) require these locations to be clearly visible from one another. Each revision cycle of the NEC brings new proposals to change the wording. But clarity remains elusive. Means of locking switching devices in a safely open position are increasingly popular.

Another concern arises when the stop/start pushbuttons are remote from the controller. In any a-c circuit, capacitance between conductors allows the flow of leakage current. When control conductors are long and in close proximity, that current can be sufficient to hold the controller closed when the stop button is pressed. Control manufacturers can provide design data for avoiding the problem impedance. Control voltage may be fed back directly or indirectly from the counter EMF of the accelerating motor.

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