Speed Control, Slip Ring Motors, and VFDs

Yes! you can very well do so! The rotor windings of the Slip Ring Motors have to be short circuited or connected to a nominal resistance. Then it is treated as a Squirrel Cage Motor only and a VFD can very well be used. It would also suit blower application. Many such systems are in operation too.

I agree and can confirm I have applications where I have shorted the sliprings with no resistors and operated OK. I have heard reports of some VFD may prefer a small resistance left but I have never needed with FUJI.

Rotor windings resistors (connected via slip rings) are not supposed to remain connected during normal operation. They are normally designed and sized for the starting period. Their connection/short-circuiting is controlled either by a timer or by a speed sensor. Resistors that are meant to be used for achieving a variable speed with fixed frequency power supplies are different and are sized accordingly. They provide a shifting of the operating/working point (intersection of the motor characteristic with the load characteristic - [torque-speed characteristic]) to a different speed.

The VFD is connected to the stator windings, and basically provides a variable frequency (coordinated with a corresponding voltage) to allow a variable speed of the motor. I do not see any reason on why a small resistance in the rotor circuit should be retained.

i do agree tomad

while running under normal conditions, the slip-ring are automatically short-circuited by means of metal collar, which is pushed along the shaft and connects all the rings together. next, the brushes are automatically lifted from the slip-ring to reduce the frictional losses and the wear and tear, hence, it is seen that under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor..

my question is any how we know VFD can be used for slip-ring motors.... and we know by short circuiting rotor winding there will be appreciation of current will be there... i want to know how much of increase of current will be there?

Dear guest,

You should NOT use a slip-ring motor for VFD application, after short circuiting the rotor winding. If the speed is very low the cooling is affected. Further there are certain conditions are to be satisfied for the MOTORS OF VFD APPLICATION. ANY DOUBT PLEASE PUT YOUR DOUBTS TO THIS FORUM.

DHAYANANDHAN.S,CR4 MEMBER,

INDIA