Vfd & Slip Ring motor

Hi,

I have used Vfds for Squirrel cage induction motors. I am now required to use a Vfd with a Slip ring motor. My question is, what do I do to the slip rings, do I short them or do I leave a minimum resistance in them. The application is for a Hoist.

Thanks

Ron

simply - Yes you bridge out (short) the slip rings.
You may need to up the Torque setting in the VFD to fully magnetize the windings

The correct answer requires a little study. Just shorting out the slip rings (I assume this is a three slip ring motor or a wound rotor motor, not a two slip ring synchronous) may leave the motor "too tight" or, in other words, with not enough slip to control properly.

Sometimes leaving just a little resistance in the rotor (mounted in place of the slip rings) will provide enough slip to develop good torque and not be too hard for the VFD to control. I would aim for 1-2% slip from no-load to full-load.

Hi,

Thankyou both Ian & DickDV for your replies.

Yes Dick its a 3 slip ring motor & I agree with leaving a minimum resistance to assist in the slip.

Ron

I have had experience with this Dick - it is OK - But you can check with the VFD supplier as it may need to be up-sized

iant, as with any drive/motor/load system, you need to understand the torque/speed requirements of the load before sizing anything.

Since this is a hoist application, it should be fairly easy to quantify that data. If the wound rotor motor was routinedly being taken to 220% torque at near zero speed, then, yes, the VFD will have to be sized to accommodate that. But, if the peak torque is less than that, then the VFD sizing can come down into more normal ranges (150%, 110%, 100%, etc.)

I think, as a general rule, I would provide a minimum of 150% overcurrent capacity on a hoist whether it needed it or not. Two years down the road, that hoist may need to be used for something completely different than today and the extra capacity would be much appreciated.

As usual I d1dnt read the OP's post properly.
the slip rings are for the secondary resistance load control.
You are 100% correct Dick (sorry)
It is important to understand this aspect.
It is very important to check he specifications with the VFD supplier.

I had slip rings an old alternator sitting here, so I thought I've have a go at reconditioning it.
My local auto electrical place wanted €80. I bought the parts for €20.

This alternator is found in lots of LR vehicles, Ralph will tell you exactly which ones.

It's pretty straightforward. You'll need a high wattage soldering iron and a selection of sockets to use as drifts for the front and rear bearings. To remove the front bearing, it needs to be supported by a piece of pipe, bigger than my largest 32mm socket.