I've got an old machine (it's a turnover cooling bed of a continuous casting machine) whose gearmotor is driven by an AC slip-ring motor with a set of stepping rotor resistors to increase the staring torque and for plugging.
My customer is asking to get rid from this old stuff; he intends to short the slip rings together and drive the motor through a frequency drive.

Is this feasible ? Can the VFD provide the same torque as the old stepping resistors starter ? Shall I oversize the VFD ?

 

Wound rotor motors typically are high torque-high slip motors. They have starting torque ratings about double standard motors and VFDs typically limit the starting current and torque. Older motors may not stand up to the high voltage spikes generated by the switching devices in the VFD. The secondary resistors with contactors are extreemly reliable. All this said, you will have to get information and an evaluation from a VFD company for your application but it seems you will reduce reliability, reduce starting torque, possible reduce the motor life and add another level of complexity to the system for the people supporting the equipment. The slip rings and brushes would continue to require maintenance after being jumpered out.

 

Hi,it is OK. Your motor is inductive motor with varible rotor resistor to improve the start performance, which means first connect resistor for start, then when speed up, short circuit the resistor. Anyway, max Torque can't be changed with resistor, the max torque is consistant.
VFD characteristic is keeping constant torque when power of motor is under rated power and keeping constant power when power of motor is over rated power. So using VFD can keep a high start torque without needing for resistors.

If you have any questions, you can email me:
[email protected]

 

Dear Claudio,
We had a similar situation with one of our clients .There were 10 motors each of 22 kW . We shorted the rotors at the slip ring so that it would behave like a squirrel cage one then used Vacon frequency drives. It worked well. It is now three years since and so far no problems have been recorded.
Regards,
Gabriel

 

I would suggest going to this website ( http://www.payback.com ) and looking at their product. I have seen them used with 100 horsepower fan motors, and are an interesting alternative to VFD's. You get the high torque start with the modulation of a VFD.

Just so you know I am not affiliated with the product in anyway. I have just seen them used and thought it might work for you.

 

Responding to Claudio's Nov 13, query:

A VFD can certainly be considered. One possible pitfall is the assumption that shorting of the slip-rings is all that is required. This approach is only true if the resistance of the rotor-winding alone can provide adequate starting-torque.

I suggest you: 1) determine the reason the slip-ring motor was originally specified; and 2) consider the age of the motor's windings because their insulation will be subjected to steep wave-fronts produced by the VFD.

As an aside, in response to Lei's, Sun, Nov 17, 6:16 pm, comments:

I disagree with your statement that the "VFD can keep a high starting-torque w/o resistors." It is true that the "maximum" runup-torque is independent of rotor-winding resistance (search archive for math proof on this point). Contrarily, however, "starting-torque" is very dependent on it.

Regards, Phil Corso, PE
(Boca Raton, FL)
tal-2(AT)webtv.net

 

I have performed this retrofit many times on overhead bridge cranes with great success. This does not mean that it is good for every circumstance, however it is a very common practice in the bridge crane industry.

 

I used to design wound rotor starters, and a VFD can work, but as some people mentioned, not always. A Wound rotor motor has double or more starting torque avaible to it, and this torque is (as mentioedn in one reply) a factor of the motor so the VFD will not hurt the rated starting torque....having said that, most VFDs only hold 200% (if that) current for a very limited time before they kick out, so the starting currents required for the motor to generate its huge starting torque WILL be limited unless you put in a huge drive. There is ussually a reason for the origianl use of WRIM motors, normally high stall/starting torque, so be careful if you put in a VFD, because, you will limit the motors torque drastically. Also, as someone mentioned, some older WRIM motors wont handle the VFD as the insulation in the motor wont take the spikes.