Slip Ring Motors and Soft Starters

Slip Ring motors are used when high starting torque is required. When started with rotor resistors they produce a starting torque of 2 - 2.5 nominal torque (Tn). Slip ring motors are used, for example, to start rock - crushers to allow motor starting while rocks are between the jaws.

Soft starters are designed to reduce starting torque. Typically, a motor started by a soft starter will produce 0.25 Tn. therefore it can not be used when a high starting torque is required. eg. Tn = K*U², K = Constant ~ 1, IV = 50% thus Tn = 1*(50%)² = 25% or 0.25 Tn.

Generally, we do not recommend using soft starters to start slip ring motors, however, it may be that a slip ring motor was used even though there was no requirement for high starting torque. eg. a squirrel cage motor could be used however a slip ring motor at the right KW rating was available in stock.

Using a Soft Starter

If an application requires a low torque (for example, starting an unloaded machine), then a soft starter may be used. However, it must be understood that the machine will not be able to produce high starting torque.

The soft starter should be connected to the stator, while the rotor resistors can be left or the rotor can be modified to operate as a squirrel cage by shortening the resistors.

When it is required to short the rotor resistors, it should be done at the resistor cabinet and not at the leads coming out of the rotor. It is necessary to leave a certain resistance in the rotor circuit otherwise it may be impossible to start flawlessly.

If the resistors are left active, the "End Of Acceleration" contact can be used to short them after starting process had been completed.

It would be a good practice to de-rate one size, for example use a RVS - DN 460 for a 390A motor. For altitudes exceeding 1000 meters (3281ft) de-rate 1% for every 100 meters.