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How to choose an overload relay for single phase motor ?


The selection of Direct on line starting components for a single phase motor,we consider the current of the motor mentioned on the motor nameplate,where we choose the breaker,contactor and overload,each separatly according to the current value.Please note that in this case no coordination type is guaranteed.

Regarding the overload,Standard 3 phase thermal overloads,  are designed to detect a current imbalance in the 3 phase supply. As a result of this, if a single phase supply is wired such that line and neutral pass through just 2 of the 3 elements the overload can trip.

To avoid this problem when using a standard overload with a single phase motor load, the wiring through the overload must be arranged so that all three elements are in the circuit. In practice the contactor is also part of this wiring recommendation as the overload will usually be directly coupled to the contactor ;

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