help with wound rotor w/slip rings starter design

[artecerv]

I have this old rubber mill de-commissioned about 8 years ago that we are planning to bring back to service. Its starter has been used as source for spare parts and there is not much left of it. The motor is an 800 HP 2.4kV wound rotor with slip rings and its starter used to step resistors (relay switched) to take the motor to full speed (at this point the whole resistor bank was fully short circuit).

Now I need to re-build the starter, but I don’t know the design criteria to choose the number of steps and the size of the resistor used in each of them.

Any tip/ hint will be much appreciated.

Thanks in advance,

 

[jraef]

Do you need to operate it at any step below fully shorted, or is it just to get it started?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

Hello artecerv

Download my electrical calculations software and you can use that to calculate the secondary resistors. You will need to separately determine the power rating of the resistors.

http://www.LMPhotonics.com/busbar32.zip

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

I have never done this or given much thought to it. I did find some info in Liwschitz Garak - Electric Machinery Volume II 1946 page 221 gives a recipe.

Find or estimate spo = "pullout slip" - I haven't heard the term pullout applied to wound rotor induction motors but from the conteext it looks like he means the slip corresponding to max torque with when rotor shorted (i.e. if you had torque speed curve for motor with shorted rotor, find the peak torque, and read the slip corresponding to that peak).

In his example he uses spo = 10%. spo would be somewhere in the range of twice the slip at full load so it seems to me the range would be 2% - 10%. If you have a torque speed curve you're all set, otherwise... pick a number for spo?

Now use R = r2 * (s - spo)/spo
where r2 is measured dc secondary rotor resistance and R is external resistance and s is slip.

Thus the initial resistance is given by susbsituting s=1. The ideal start (fastest start) would change R to keep it somewhere close to the calculated value as s decreases. You're going to have to guesstimate how your load accelerates and go from there.

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[electricpete]

I didn't see Mark's post until after I entered mine (we must have been typing at the same time). I haven't seen his but I would trust his info.

I'm on a sssslllooo

http://www dialup

connection, so I'm going to wait until work tomorrow to look at his 2MB file.

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Eng-tips forums: The best place on the web for engineering discussions.

 

[Marke]

Hi electricpete

The big secret that some forget is the final stage. If the slip for maximum torque is to large, you will get a very high current at the point that you short out the rotor. It is important to select your final stage for a torque maximum at a relatively low slip. Beyond that, you can evenly space the rest of the peak torques down the speed range.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[artecerv]

Hi All,

Thanks for the info and the attachments.

jraef asked what the purpose of the starter was:
its purpose is to control the torque (current) during start up. This type of motors is used to drive high inertia loads.

Electricpete,

Do you know whether your book (Electric Machinery Volume II) is still been published? I will visit the library to see if I can get me a copy … do you recommend it?

I think I will play with Mark’s software for a while.

Kind Regards,

 

[electricpete]

I doubt that the book is still published but you may be able to snag it on ebay or amazon.

The focus of that particular book is a mathematical analysis of electrical and magnetic aspects of electrical machine design.

There is nothing about bearings, lubrication etc. Nothing about maintenance. Very little about manufacturing and assembly and repair techniques. Nothing about insulation systems. Not a helluva a lot of application information (the wound rotor resistor thing is an exception).

But Lots and lots of math and theory addressing in gory detail wide-ranging subjects from harmonic fluxes, winding factors, motor circle diagram etc. If that's what you are after this is the best book in that area I have seen. (Better than Krause who relies heavily on computer simulation vs analytical solution, Better than Fitzgerald that only scratches the surface on the winding/pitch factor issues compared to Liwschitz Garak). I consider myself to have a pretty good background in math but there are many parts of this book that are a tremendous challenge for me to understand due to the endless strings of equations. I have had the book for 5 years and studied almost all the pages at one time or another but I think I only understand 10% of it. That's ok with me because I'm still learning from it.

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[electricpete]

I spelled the author's name wrong. It is:
Michael Liwschitz Garik

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Eng-tips forums: The best place on the web for engineering discussions.

 

[jraef]

artecerv,
Since all you need the Wound Rotor for is to accelerate the motor, consider replacing it with a Solid State Soft Starter. You can still use the WR motor, but just leave one resistor step in place for starting, then a shorting contactor for full speed full torque operation. This eliminates the long term problems of the WR controller and maintenance. This is the way most users are going with replacing older WR controllers now as long as it is not being used for speed control by running continuously at various resistance steps.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[PowerfulStuff]

jraef,
Don't soft starters have a lower starting torque than can be expected from the rotor resistance starter? I haven't got any references with me at the moment for the maths but I understood this to be a key advantage and why they are still made.
Regards,
Martin

 

[jraef]

Well yes, but of course the reason that you step through various stages of resistance in the old fashioned WR controller was so that you slowly increase the torque until you get to full speed. With the soft starter, you pick an optimum torque curve by selection of one resistance value, then let the electronic ramping prevent excessive torque from getting to the drive train components. As with any form of soft starting, high starting torque requirements are not good applications.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[Marke]

It really depends what you are looking for.
A would rotor machine and secondary resistance starter can be used to slowly increase the torque and provide a gentle acceleration. This a primary starter can co also. The primary starter would however, have a higher start current than the staged secondary resistance starter.
The other application for a secondary starter is to provide a high start torque, say 200% from zero speed through to full speed. This the soft starter can not do.
The advantage of this high start torque is the reduction in start time for very high enertia loads. The mechanics have to be able to withstand the high start torque though.
The third application of the secondary resistance starter is to change the speed torque curve of the motor so that it operates at a higher slip (lower speed) This is a very inefficient system though and VSDs will often provide a better option.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[artecerv]

Jraef,

Besides we are talking about an 800 HP medium voltage (2400V AC) rotor wound induction motor, how expensive will be a soft-starter for it?

I’m guessing but .. to use a soft-starter connected to the stator’s phases first I’ll have to short circuit the rotor winding (so the motor behaves like a squirrel cage induction motor). Stator currents are much higher than rotor current, hence I’ll be ending paying $$ more (bigger power modules) … I don’t think I have heard of anybody doing this.

There is another method from which I have only read about it. It is called slip energy recovery system for wound rotor induction motors. It is a solid-state device (sort of VFD) that like the step resistor starter controls the rotor’s currents. The advantage is that it is more energy efficient than the step resistor because feeds the energy back into the supply (I suppose in the same fashion that regenerative VFD’s do) … another great advantage is that since they control the rotor’s currents rather than the stator’s currents they are smaller and cheaper.

I certainly would like to know more about this system … any body??

King Regards,

 

[jraef]

artecerv,
No, you do not need to oversize the soft starter. You do NOT short the windings during ramp up. As I said previously, you leave one resistor in the circuit giving you the optimum torque curve, then short it only AFTER it gets to full speed. It is done all the time because the soft starter is much more reliable than the old WR controller. As to cost, this isn't an appropriate place for quotes, but suffice to say that it will be much less than a replacement WR controller, and usually about 1/10th the cost of a VFD.

Slip recovery systems are still made, but the returns were never as good as the claims and the available market for them is relatively low, so most of the people making them either went out of business or got into some other business to stay alive. The old Marathon product was one of the first designs and widely hyped here in the US a number of years ago, but they sold their design to FloMatcher in Oregon 10+ years ago. FloMatcher still sells them I think.

http://www.flomatch.com/

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla