Can a VFD drive a slip ring motor?

Variable frequency drive allows for programming of the Volt per Hertz ratio and starting boost. Seek input from several sources especially the crane manufacturer. In many cases they can assist with these conversions. In all cases you will have to manufacture a shorting ring to interconnect the windings.


Short out the rotor windings being careful to use symmetrical bridges for dynamic balance and even current through bridges. Now the hard part. You need to be certain the motor has sufficient reserve KW in its existing operation to allow for the reduced voltage starting ramp of the VFD. Next the VFD must have a brake control system that measures motor torque to release brake and motor speed to set brake. Test with gradually increasing weights to determine performance max capability.

Since this seems to be a matter of using an existing slip ring motor, from start not designed for VFD, the most important concern in this is the insulation of the windings, not which drive principle that is preferred. A slip ring motor is normally not designed for the extreme high du/dt values stressing the winding insulation at VFD drive (fast switching).

There is a high risk that partial discharge will break down the winding isolation material causing flash-over if it is not designed for VFD. One point more is that one of the bearings must be isolated, if not there is risk that circulating currents destroys the bearing(s) by making burn marks. For those reasons Gozuk strongly recommend you to investigate in a square cage motor designed for the purpose.

But why would you want to keep the slip ring part of the motor when you have a VFD? Your VFD can achieve the purposes of a slip ring and resistance starting, so I would shot out the wound rotor at the slip rings and run the motor as a normal induction motor, that way I eliminate unnecessary points of potential breakdown. We all know the problems associated with resistance starting, a VFD will eliminate all that.