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Capacitor Start Induction Motors and Soft Starters

There is a common misconception that a soft-starter used on a capacitor-start induction motor will damage the motors start-winding, start capacitor, or lead to premature failure of the motor. While this is possible if proper precautions are not taken, generally speaking this is simply not true. 

A capacitor start induction motor requires a large capacitor in order to provide maximum starting torque. Simply put, the start capacitors job is to provide a phase-shifted current through the start winding of the motor. The resulting out-of-phase magnetic field is what creates the starting torque required to get the motor shaft rotating. Because of the ratings of the start capacitor, it must not stay energized for more than 3 seconds (per the EIA) without risking damage to the capacitor itself, and therefore must be removed from the start winding circuit electronically or via a centrifugal switch that disconnects the capacitor/start winding when motor speed reaches 80-90% of full speed. 

The start winding itself is typically of a smaller diameter copper wire that is not able to withstand full current for an extended period of time, however, this amount of time is far greater than just a few seconds. Therefore it is primarily the starting capacitor, and NOT the start winding, that is of concern when adding a Soft Starter, so as long as the starting capacitor is not energized for more than 3 seconds, we shall not worry.

Because of the starting torque required with many capacitor start motor applications, an extended soft start is counter-productive because the motor typically needs the maximum surge current in order to start rotating. However, not one application benefits from the excessive (40-90 Amps) of inrush current (0 - 0.150s) that reduces the working life of switch/relay contacts. 


capacitor start induction motors

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