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Willmall overload relays protect running motors from sustained overload, co-operating with the contactor that starts and stops the machine.
It protects a running motor against sustained overload, using a time-current characteristic so that harmless starting currents do not cause a trip while prolonged overcurrent does.
The adjustable range is set to the motor full-load current and adjusted during commissioning so the relay alarms only on genuine overload conditions.
The overload relay sits in the control circuit of the contactor; when it trips, it drops out the contactor coil and the motor is powered down.
Manual reset requires an operator to re-arm the relay after a trip, while automatic reset re-arms after the thermal element cools, chosen according to the duty and safety requirements.
Depending on the design, differential thermal elements detect the extra heating caused by single-phasing and trip the motor, a useful function on three-phase supplies.
