Slip ring motors or wound rotor motors are a variation of the standard cage induction motors. The slip ring motor has a set of windings on the rotor which are not short circuited but are terminated to a set of slip rings for connection to external resistors and contactors. The slip ring motor enables the starting characteristics of the motor to be totally controlled and modified to suit the load. A particular high resistance can result in the pull out torque occurring at almost zero speed, providing a very high locked rotor torque at a low locked rotor current. As the motor accelerates, the value of the resistance can be reduced by altering the start torque curve in such a manner that the maximum torque is gradually moved towards synchronous speed. This results in a very high starting torque from zero speed to full speed at a relatively low starting current. This type of starting is ideal for very high inertia loads allowing the machine to get to full speed in the minimum time with minimum current drawn.
The drawback of the Slip Ring Motor is that the slip rings and brush assemblies need regular maintenance which is an additional cost not applicable to the standard cage motor. If the rotor windings are shortened and a start is attempted, i.e the motor is converted to a standard induction motor, it will exhibit an extremely high locked rotor current, typically as high as 1400% and a very low locked rotor torque, perhaps as low as 60%. In the case of most applications, this is not a suitable option.
Jayashree has the expert’s solution to boost the torque of slip ring motors by electronic as well as by HFSR type Soft Starter. Using Jayashree make Soft Starters, you can precisely achieve the required torque in desired time and current limit. Ask for the SMSC technical note for further details.