VFD on wounded motor 1

[lukin1977]

We have a machine that uses an induction motor with wounded rotor.
The control panel now has resistors for starting and stoping of the motor
We need to control the ramp up, ramp down and dwell speed of this motor. We are plannig to take out the resistors and install a VFD. The wounded rotor´s rings will be close

Is this possible to do? Any comments? It will work?

Thanks

 

[Skogsgurra]

It will work. No problem.

Only thing to care about is the rather aggressive PWM waveform and what it does to your motor's insulation.

I think that a new, modern motor with an insulation system that is "inverter resistant" should be considered. That will give your machine a new life.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[lukin1977]

Is there anything we can do to "improve" the insulation? I should mention that the motor was not used in the last 4 or 5 years

Brand: BBC
Power: 36 kW

 

[Skogsgurra]

Yes, use a filter between VFD and motor. Preferably a sine filter. But a decent du/dt filter will also work. It depends somewhat on the mains voltage. Lower voltages, up to 200 - 300 V is usually not a problem. Above 300, you may need to do something.

What age is the motor?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

No, you cannot change the motor insulation capabilities without having the motor rewound.

You can however buy as good of an output filter for the VFD as you can afford, that will allow your motor to last a lot longer. When it does finally succumb to the new stresses, have it rewound with Inverter Duty magnet wire.

A few other tips:
1) Keep the distance between the VFD and the motor as short as is practically possible. The more distance there is, the higher potential damage to the winding insulation.

2) Do not short the rotor windings on the stationary side of the slip rings. It is best to short them at the rings themselves.

3) When you do that, be very careful about what you use so as to not create any significant imbalance in the rotor weight.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[lukin1977]

I dont know exactly the year of the motor. I think it is from the 70s or 80s

Is it possible to use the original resistor as a brake? The motor is on a wire drawing machine and when the wire breaks we need to stop the machine as fast as possible. Can ew do this with the resistor? Or as Jraef said we must shorted the ring on the motor itself?

The length of the cable from VFD to the motor will be 2 meters

Jraef: Can you explain some more how to do the shorted of the ring?

 

[lukin1977]

Nameplate pic

 

[lukin1977]

rings pic

 

[Skogsgurra]

You can probably "recycle" the resistors and rearrange them into a suitable two-terminal braking resistor that you connect to the brake chopper on the VFD. You cannot use them on the motor as such any more.

Most VFDs have a DC injection braking possibility. That should be adequate on a wire drawing machine. Talk to the VFD supplier about that.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

That's a small motor, not much room to work with. I would remove it and take it to a proper motor shop to have them braze shorting bars accross the collector rings and them rebalance the rotor. You could try that yourself, but rebablncing is something you are not likely to be able to do unless you have a fairly sophisticated shop. All you have to lose is the motor, in which case you buy a VFD rated replacement and be done with it.

Or just do that now and sell this motor and controller in working condition befor you permanently alter it. There are people who legitimately like to use Wound Rotor Induction Motors and will appreciate having a working one.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

By the way, it's obvious that English is not your primary language, so I'll help you out here a bit. I know it can be confusing because we have a lot of spelling and pronunciation issues that don't seem to be consistent.

To WIND (pronounced "wyend") something like a motor coil is the present tense, WOUND (pronounced like "round" or "pound") is the past tense. This is also unfortunately the exact same spelling as "wound" as in to "wound someone by shooting them in the arm", but is pronounced "woond" with the "oo" sound like in food or book.

"Wounded" is ONLY pronounced like "woonded" . Your motor is "wound" not "wounded". At least not yet...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[electricpete]

Great job by jraef explaining wound vs wounded. I had to laugh when I read the subject line. Not at the author, but at our language and the funny challenges it can create.

I couldn't quite figure out your question on braking. But I wanted to mention: If you do rely on dynamic braking with dc applied to stator, it would not stop the rotor as quickly with rotor shorted as it would with resistor inserted in the rotor circuit. (for similar reasons that the DOL starting torque reduces when we short the rings of wound rotor motor).



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(2B)+(2B)' ?

 

[lukin1977]

ok, thanks for the english lesson

one question: what will happen if we shorted the rings using the original shorted method. The rings are now shorted with a contactor in the control panel

the option of selling the motor is very difficult here

 

[jraef]

Shorting them after the brushes as is done when no VFD is involved will lead to an early death. The VFD induced harmonics* in the rotor windings must pass through the brushes and there is an effect similar to EDM that will destroy the connection in relatively short order.

*to the purists yes, I know this is inaccurate, I'm using the term loosely for brevity (I'm on an iPad and I'm lazy).

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[lukin1977]

ok, thank you very much to all for your help

 

[electricpete]

Interesting about effect of stator vfd on rotor brushes. I didn't realize that.

Even if no vfd, brushes can be a high maintenance item that you'd be glad to get rid of

=====================================
(2B)+(2B)' ?

 

[Skogsgurra]

I'm not so sure. I think that Jeff was somewhat overly "pedagogic". Always good to remove the brushes. Less maintenance.

Anyhow, a new motor doesn't cost much. So, why bother with all the possible/potential problems at all?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

I'm not so sure. I think that Jeff was somewhat overly "pedagogic".

Pedagogical. And I am not!

Take up the subject (of removing brushes) with MarkE, I learned it from him. But subsequently I have come across two instances where WRIMs were retrofitted with VFDs and the rotor was shorted after the brushes. In both cases there was a lot of arcing in the brushes within months of the change and instability in the speed control. I suggested they short the collector rings, they did, and the problems went away.

Empirical truths are what I strive to pass on, not pedagogy. Although if you were to talk to my daughter, she would likely agree with you...

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[PersonalProfile]

Consider the following (not directly aimed at WRIM, though similar issues - particularly when the windings were not selected with VSD in mind):

http://www.climatec.com/nm/docs/ST-311-102.pdf

Can the existing infrastructure not deliver your requirements? A suitable WRIM/SRS could potentially achieve your requirements.

Have you considered a VSD c/w a new motor?

There is alternative technology, including:
1. Double feed / secondary resistance injection (though would assume similar issues with windings).
2. Solid state secondary resistance, for example ABB Ecoss:

http://www05.abb.com/global/scot/sc...041b1d1/$file/compact_starter_ecoss_final.pdf

Regards,
Lyle