Slip Ring Resistors

Slip ring motors, or wound rotor motors, need to have resistors in their rotor circuit to enable them to develop high slip torque. If the rotor is shorted out, the motor will have a very highLocked Rotor Current and a low Locked Rotor Torque.
If you then apply a reduced voltage starter to the stator, the shaft torque will be much lower than for a standard cage type motor.
The resistors in the rotor circuit modify the start torque curve of the motor. As the resistance is increased, the slip at which the maximum torque occurs is increased. At zero ohms, the maximum torque is at very close to full speed. By selecting a number of stages with the torque occuring across the slip range from zero to 100%, you can design a start system where the motor can produce maximum torque for minimum current fromzero speed to rated speed. In order to prevent a very high current surge when shorting the last resistor stage, it is important to position the final stage close to full speed. If you position the final stage maximum torque at half speed, there will be a big jump in torque when the last contactor is closed.

When selecting resistors for a slip ring motor, you must select a resistor that has the correct resistance and also is capable of absorbing enough energy.
When starting a machine, the full speed kinetic energy of that machine is dissipated in the secondary resistors. This is usually a significant amount of energy.

To determine the values of the rotor resistances, you need to know the rotor voltage and rotor current of the motor, and the number of steps required.






The resistors can be connected in a star configuration, or in a delta configuration. If you are going to apply a soft starter to the stator of the slip ring motor, you select 1 stage only and use that value. This will result in a lower torque and higher start current than would be achieved using a proper multistage slip ring starter.