A motor starter can be manual or electromechanical. In applications where a manual starter is used, an ON/OFF lever or button must be manually changed from the ON or the OFF position. This will turn off or on the power source by making or breaking the contacts. .
A disadvantage of manual starters is that during a power failure, they would resume operation as soon as the power comes back online, potentially creating a safety hazard. Manual starters do not allow for automatic control, however, so industry best practices limit the use of manual starters in favor of automatic starters.
In an electro-mechanical starter, the contactor acts much like a relay. The control circuit provides voltage to a coil, which energizes and magnetically pulls in the contacts completing the circuit to the motor and allowing it to draw power. This isolates the control and power circuits completely, allowing for low voltage (typically 120V) control to a higher voltage (typically 208V or 460V) power circuit.
The overload relay protects the motor from overloading. Since current is what generates heat, the overloads monitor the current going to the motor to see how much heat is being generated.
An overload can trip for many different reasons, it could be running above full load amps, the inrush current could be drawn for too long, or one of the phases could be pulling more than the others. If an overload condition exists, the overload relay is tripped, automatically de-energizing the coils in the contactor and shutting power off to the motor. This is done to prevent damage to the motor.
Unlike a manual starter, the contactor contacts in an electro- mechanical starter move back to their normal position following a power failure or overload, which keeps the motor powered off when power flows again.
In applications where an automatic restart could lead to a safety hazard, a start pushbutton can be added to prevent automatic restart of the motor when the power comes back online. This would also prevent multiple loads from coming online at the same time after a power failure, leading to voltage dips and potential equipment damage, as discussed previousl
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