Using VFD with slip-ring motors 1

[greta]

We have a 3ph 44kW slipring motor been used on a turnover cooling bed of a billet caster.
The motor is presently controlled thru a multi-tapped rotor resistors system. Our customer would like to get rid from such old stuff replacing it with a VFD, shorting the sliprings.

What are the draw-backs of this mod ?
Will the VFD provide the same torque as the multi-tapped rotor resistor system?
Need to oversize the VFD ?

 

[Marke]

Hello Greta
Yes, you can replace the slip ring starter with shorted rings and a vfd. It is quite common to short the rings on the rotor itself and remove the brush assembly.
The torque will not necessarily be as high as can be achieved by a slip ring starter, but if you really need to have greater than 120% torque at the lower speed ranges, then use a vfd rated for higher currents during acceleration. The running torque should not be a problem.

Best regards, Mark Empson

http://www.lmphotonics.com

 

[TheDOG]

Try thread237-7851

 

[niallnz]

One thing to remember when putting VFD's onto slip ring motors is that modern VFD's using IGBT's produce a very fast rate of change in voltage. Slip ring motors tend to be older and their insulation system may not be up to the voltage stress that the VFD will subject them too. You may need a VFD with a dV/dt output filter ie incressed cost.

Also not all VFD's are created equal, in an application where the torque load varies quickly (step load change) the transient change in speed, and time to recover to the speed set point can effect the quallity of your end product.

I would recommend using the drive manufacturer that I work for, but that would be advertising

 

[jbartos]

Suggestion: As pointed out in the previous posting, there are different principles of ac motor drives. Since your motor will not fall into the category of "inverter motor duty," it may be a good idea to select a type of drive and accessories proven to this kind of motors. The typical motor drive topologies are VSI, CSI, matrix converter based drive, etc.

 

[don01]

hi greta,
be careful here!!! does this machine take a long time to wind up?? that motor was chosen at a lot of extra cost to get that starting torque. I think I've said this before but I've had a whole plant of mills growling and not turning after the designer threw the cast iron bits out and moved the machines 800 kms away.(and used vfds)
There isn't a lot of science in what I'm saying here but I would suggest these machines have been in service a long time, during that time they have consumed heaps of contactors, labour, and downtime stuffing up budgets and production plans, but they're still there.
If you get the motor data and resistor values then talk to the local trades school or a gun motor engineer (and make sure he /she is) they can quickly run the calc to give you the starting torque and currents. (I aint that good)
So on that note of caution I'll leave you with it - it might work but check it out very carefully.

All the best
Don