Wound rotor/squirrel cage with VFD 1

[freetown]

Hi all,

I have 2 wound rotor 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 VFD for each.
It worked well. It is now one year since and so far no problems have been recorded.
My question is if i can replace the 22 kW wound rotor motor (shorted) with a 22 kW squirrel cage (with VFD of course)?
Because i've heard that if i want to do, it will require more powered squirrel cage motor (45kW) which will be the equivalent of 22kW wound rotor motor?

 

[jraef]

Hi all,

I have 2 wound rotor 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 VFD for each.
It worked well. It is now one year since and so far no problems have been recorded.
My question is if i can replace the 22 kW wound rotor motor (shorted) with a 22 kW squirrel cage (with VFD of course)?
Because i've heard that if i want to do, it will require more powered squirrel cage motor (45kW) which will be the equivalent of 22kW wound rotor motor?

The kW rating of a motor is a shorthand expression of a torque value at the rated motor speed. So just saying "22kW" is not enough information to make that determination. What is the base motor speed of the 22kW WRIM, and what is the base motor speed of the replacement SCIM that you want to use?

If the base motor speeds are the same, then 22kW = 22kW. But for example if the 22kW WRIM is at 1000RPM, it provides 210 N-m of torque. If the new motor is 4 pole, so 1500RPM, to get the same 210 N-m of TORQUE from the new motor, it must be 33kW. If we apply the same principals backward from what you were told, the 45kW motor requirement might mean that your 22kW WRIM is likely 750RPM.

 

[LionelHutz]

A 22kW induction motor rated for the same speed could replace it. You basically turned the motor into a 22kW induction motor by shorting the rotor slip rings.

The motor power rating is at the rated speed of the motor. HP or kW is directly proportional to rpm when using a VFD, so if your new motor was rated for 2X the speed of the old motor then it'd need to be rated for 2X the kW and you'd run it at 1/2 the frequency to get the same power and speed output as the old motor.

 

[waross]

Compare the torque curves of the two motors. Torque may be more of a limit than HP.

 

[LionelHutz]

At most, the only part of the torque curve that is of interest is between the breakdown torque and full speed. The VFD can hit it's current limit before the motor can reach breakdown torque.

 

[PersonalProfile]

I speculate that the justification for what you heard was due generally using WRIM with secondary resistance to increase torque during starting etc. This is apparently not occurring for the WRIM currently installed, as the secondary resistance is stated to be shorted. A VSD in principle may be selected to complete similar with an induction motor - both: SCIM, and WRIM.
The following states some other risks associated with using a "standard" motor with a VSD:

Effects of AC Drives on Motor Insulation

The output voltage waveform of today’s PWM adjustable frequency drive is not a sine wave, but a series of square wave pulses that produces a reasonable ap-
proximation of sine wave current. Although there is an extensive history of suc-cessful use of standard motors on this type of waveform, the possible effects on
the motor should be carefully considered.

https://library.e.abb.com/public/fec1a7b62d273351c12571b60056a0fd/voltstress.pdf

 

[waross]

Torque, not HP.
WRIMs are often used in applications demanding a lot of torque.
I suggest evaluating a contemplated replacement motor based first on the torque demanded by the load and second by the HP.
You may need a higher HP to develop the torque needed. Maybe not.
For example; If the peak torque demanded by the load and supplied by the WRIM is 300% of rated torque, then an induction motor that peaks at 200% of rated torque will not be suitable.