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Slip Ring Motor VFD Application


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Hi Marke,


We intend to use one existing 600 k W 400 V slip ring wound rotor motor with a variable frequency drive. We have got some pretty good applications where wound rotor were used in the past in lieu of standard squirrel cage motors for VFD applications.

The load is a heavy crusher requiring a relatively high torque to start from zero speed.My concern now is that the resistance of the rotor might be too low to drive the load and we will need additional resistance in the rotor circuit. However, I am told by the VFD guys that the variable speed drive will handle the torque problem but I would like to know how.

Grateful to have your valuable inputs.


Kind regards.



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Hi Bob


You can certainly use a slip ring motor with a VFD, but you need to short out the rotor. It is best to short out the rotor on the rotor so that the brushes can not arc on the output of the drive. This will certainly save energy over using the rotor resistance to alter the speed of the machine.

When you alter the machine speed by increasing the rotor resistance, you are increasing the slip of the motor and this results in an increase in the slip losses. At major speed reductions, the slip losses are considerable.

When you operate a motor on the output of a VFD, you never operate the motor at high slip so you do not need rotor resistance. The lack of external rotor resistance will not affect the magnitude of the maximum torque, just the slip at which the maximum torque occurs.

I would recommend that you ensure that the VFD is selected for a high torque. Drives from Elettronica Santerno and Emotron can be selected for torques higher than the usual 150%.


Best regards,

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