Slip-ring Motors: can they be started with a soft starter? 

Yes, provided that the torque available from the motor under the new configuration is sufficient to accelerate the load. This may be difficult to determine and a trial may be required.

Soft starting is not suitable for applications where:
- the slip-ring motor was installed to deliver speed control.
- the load requires extreme start torque.

To develop starting torque, some resistance must remain in the rotor circuit during motor starting. This resistance must be bridged out using a contactor (AC2 rated for rotor current) once the motor is running close to full speed.

Rotor resistance (R) can be sized using the following formula:
R (per phase) = 0.2 x

Where

VR = open circuit rotor voltage
IR = full load rotor current

Power (per phase) = 20% x motor kW
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