Technological Innovation

Can I control motor speed with a soft starter?

No. In operation, the AC induction motor constantly tries to reach, but never reaches, its synchronous speed which is proportional to the applied AC voltage frequency. To control the speed of an AC induction motor both the voltage and the frequency must be varied as VSDs (variable speed drives) do. A soft starter only reduces the applied voltage to the load while accelerating or decelerating the load and the motor is always driven at the constant line frequency. In effect the motor is starved for current while accelerating towards its synchronous speed. Depending on the load, it will settle within its normal slip range which is between its nameplate speed (full load speed) and the synchronous (practically at no load) speed.


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