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Wound rotor motor vs. induction motor 3

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bdn2004

Electrical
Jan 27, 2007
799
We have a 40 HP hoist motor that is currently of a wound rotor design and a load brake. The plant people want to change it out to a standard induction/vfd design.

The Vendor is telling us we have to bump up the size an additional 20 HP. How do I confirm this?

Also, he is saying the gearbox has to be changed out because of the additional horsepower. And it's not cheap either.

Anyone got any experience with such an application? And volunteer info to think about?

 
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WRIMs can produce higher torque a standard Design B Squirrel Cage Induction Motor (SCIM). So hopefully, that statement was made after careful observation and evaluation of the specifics of your application. Hoists are not your average load, they are more torque dependent than something like a pump or fan. So while I would not make a blanket statement that all SCIMs being used to replace WRIMs must be 50% larger, it could very well be true under the right circumstances, with hoists being a good bet to fall into that category.

Think of it this way: A VFD can, at best, allow an SCIM to produce Break Down Torque from the motor for a short period and FLT indefinitely. But a Design B SCIM will produce about 220% FLT as BDT. In a WRIM, each resistance value has it's own torque/speed curve that develops BDT at different speeds, but that BDT is typically around 300% of FLT; more like a Design D high torque motor. If in your hoist application you would be needing that BDT capability of the WRIM, then to replace it with a SCIM would mean up-sizing the motor.

Now... a 50% increase in HP to net about 36% more BDT? I'm not so sure about that part...


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Thanks Jraef...

Is the wound rotor motor still used in new applications?
Why do they refer to it as brake down torque?
 
People still buy new WRIMs sure. Like most other technologies they have their place and their fan base. But with as cheap as VFDs and SCIMs are now, WRIM users are getting to be fewer and fewer.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
What is the aim of the replacement? What problems is it supposed to address?

As for the new gearbox, why not set a torque limit with the VFD to stay within the capability of the current box?

Or is the whole aim of this replacement to increase the capacity of the hoist?
 
ccjersey,

The aim of the replacement is to rid them of a 40 year old controller and motor that they have to troubleshoot more often than they would like. And because this wound rotor/brake setup is different than anything else they deal with on a regular basis, its unfamiliarity amplifies the frustration.

That is a good question about the gearbox. The manufacturer of the hoist is telling us that. I wondered something similar like why can't they gear it down in some way? The gearbox is on the order of 2X the cost of a new motor and drive. What you describe sounds simple enough...I'll check.

I don't believe there is a problem with capacity, for if there were, we could easily justify the project on that basis.


 
And because this wound rotor/brake setup is different than anything else they deal with on a regular basis, its unfamiliarity amplifies the frustration.
Really common problem.

The gearbox issue doesn't make sense. The reason for needing more HP is because the WRIM would have had more torque. It's the torque that could damage the gears. So if the gearbox was designed for the application and the torque output of the WRIM, then why would you need to oversize the gearbox if you up-size the SCIM to match the torque of the WRIM? Smells like someone trying to snake an unnecessary order. I'd be skeptical too.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
You could just put a VFD on the current motor. There are 2 options for the rotor. The best is likely to just install a small fixed resistance and leave the brushes. The other option is to short out the slip rings and run without any resistance. A good VFD should work fine without any reistance, but the motor could be more difficult for the VFD to properly control.

I would install the VFD and try it with the brushes shorted. If it works OK then remove the brush holders and short the slip rings together. If it doesn't work very well then try adding a little rotor resistance.

Otherwise, I agree with Jeff's comments that the VFD and larger motor should be fine on the present gear box. It should be a simple matter of setting torque limits to limit the new motor to the existing gear box's rated torque.
 
Lionel,

That is an interesting idea. Would you happen to have a link to an article or something that can give this idea a little more umphfff?
 
I really don't have any papers like that. Basically, a WRIM works like a high torque SCIM except the rated slip is modified via the external resistors.

There are a couple of other thing that should be looked at.

If the WRIM is old and has not been re-wound recently then a good output filter should be used between the VFD and the motor. An old winding may not last very long on a VFD. The motor voltage has a large effect on this. If it's 230V then you'll likely be OK but if it's 600V then there is a much higher chance an older winding will fail. It's not a very large motor so if the windings did fail it could be re-wound with much better insulation.

The WRIM will only produce it's peak torque when the stator is supplied enough current. This may require a substantial over-sizing of the VFD. It might be a good idea to put a meter onto the motor and log the accelerating currents to get an idea of what current is required.
 
Some discussion of it here.



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376
 
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