Large AC Squirrel-Cage Induction Motor Repair Versus Repalce 1

[engineerithink]

Hi All,

I was wondering if anyone knew of any books/websites/whitepapers/etc. which can direct me on making an informed recommendation to repair or replace large voltage AC squirrel-cage induction motors. What I am particularly interested in are approximate cost estimates for replacing different components of the motor like the rotor, stator, or bearings. I understand that there are many factors which determine cost such as availability of materials and demand for repair work. However, if I have approximate cost I can determine whether or not I should even bother requesting a quote to repair the motor. Something as simple as repairing rotor bars usually equals x amount of dollars will do fine. This way I can say something like the downtime to repair the motor plus the approximate cost to replace ___ motor components justifies replacing the motor instead of repairing it.

Thank you!!!

 

[itsmoked]

Part of the problem is you don't open a motor up and say, "Gee, the rotor bars are used up, lets put in a new set." It's more like going thru the entire motor and improving those things that need it. So I am having a hard time seeing how you'll be able to assess the job without pulling the whole thing apart,(a major job in itself).

I think you need to check the places you can get it rebuilt. Ask for estimates. Add the estimated time, add some extra time, price => impact. Verses a new motor delivered, Y hrs swap.

It would help to know where you are, and what type, size, and application, motor you are talking about. Someone can probably ballpark the prices. There is a typical size for typical motors that you just don't bother with rebuilds.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricpete]

Go here:

http://www.easa.com/

then clik on "Industry Info" (bottom of left side) and it will pull up a page with several documents on the repair vs replace decision.

I think the discussion there is weighted more heavily towards efficiency aspects than most people would consider. Older motors are less efficient and also repair and rewind processes can degrade the efficiency (particularly core insulation degradation from winding burnout during rewind if not properly controlled or core lam insulation is poor to begin with).

At our plant, anything under 50hp would probably be an automatic replacement if the winding had faulted (of course if it's just a bearing problem, we would repair it ourselves down to about 20hp). Anything 500hp and over is an automatic refurbish or rewind. (The upfront capital costs for new motor really drive this one). In between is sort of a grey area.

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[petronila]

Hello,

For this propouse I think the better is to make a list of the most representative motors in your plant and separe it in three major groups of 2, 4 and 6 poles.

Choice 5 of each one from 100 to 1000 HP and send the list to your repair shop with complete name plate information asking for a quote of rotor repair.Also ask for the motor`s manufacturers for a rotor replace price.

Normally the rotor`s repair cost vary due to the copper prices vary at trade, But could be 30 or 40 Percent of a new one motor.

Two good IEEE papers about the matter: Analysis of rotor Failures in Squirrell Induction Motors by Bonnett & Soukup and Repair or Replace By Yung & Bonnett.

The thread 237-167837 could help you too.

Regards

Petronila

 

[engineerithink]

itsmoked/All,

I work for a power plant company. Luckily, For now it's not my job to determine the problem because we have O&M engineers who do vibration analysis, MCE testing etc. Depending on what they report the problem to be, I have to determine whether or not we should replace or repair the motor. I could just get a quote for repair and compare that to purchasing a new motor, but I have some fears about doing that. First off, I will only know how much it cost to fix a particular problem and there are so many different components of a motor which can break. Secondly, I am afraid that if I ask for too many quotes and end up rewarding vendors with too little jobs, they will start to ignore me or give me rediculous prices because they don't want to spend the time doing accurate quotes when they know I am just using them to justify replacing a motor. If I do get a quote for a vendor, I want it to be because there is a fine line between repairing or replacing the motor not because I am too inexperienced to know that replacing ____ components always warrents replacing the motor.

Thanks for the help

 

[electricpete]

I have always been on the customer side (like you) vs the repair/supply side. But I have a feeling that most of your suppliers would be happy to give you quotes if there is the potential for a lot of business in the long term. I would not worry about overtaxing them. But perhaps it will ease their burden if your communicatge that these are requests for prelimary "budgetary" type quotes instead of firm quotes. That's a double-edged sword as well since they have incentive to give you a low cost now and jack it up after you've made a decision.

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

 

[jraef]

I think electricpete's advice on size range considerations reflects what I see in the marketplace.

50HP and down, repair is usually uneconomical because replacement motors are inexpensive and probably more efficient. 500HP and up, replacement usually becomes the lesser of the two choices and can often be done faster anyway, so down time becomes a cost issue as well. 60-250HP, energy efficiency issues play a bigger role because that is the range in which there are often new higher efficiency motors available for replacement and even if the cost to repair is lower, the cost of ownership may shift the balance towards replacement. 3000HP to 450HP is the more gray area. So what I see a lot is people who have standing policies of replacement at 50HP and under (a few are 75HP), then probable replacement up to 250HP, then automatic refurbishment at 500HP, leaving only the remainder as the ones they go out for comparative bid evaluations on.

 

[oftenlost]

I am on the repair side of the business and generally, if asked for a budgetary price from a customer, I usually try to give him a worse case idea of what he could be facing so that he can get his contingency plans in order. As far as I am concerned, open motors of standard design probably should be replaced up to 250 HP. The differential between my repair cost and acquisition cost of a new low end, open motor is not that great.

 

[oldfieldguy]

I concur with electricpete in general.

A previous employer set the criteria at 75 HP for common frame induction motors. Below that rating, bearing changes were all we did.

Exceptions were made for specialty motors like oddball frames and such.

old field guy

 

[waross]

I agree with oldfieldguy. Several generations ago, our breakpoint was 10 HP but the principal was the same. This was a plant that was very abusive of motors. We found that our rewind shop gave us a more robust motor than a new motor.
At that time and place, a 10 HP rewind was slightly more expensive than a new motor but the small extra cost was worth it.
I suspect that any standard motor large enough to have replaceable rotor bars would be cheaper to repair than replace.
It may be an interesting exercise to compare the price of replacement rotors and stators to the price of complete motors in the smaller sizes.
respectfully

 

[oldfieldguy]

waross addresses the basic premise: cost of rewind vs. cost of new motor. Our engineering staff went a step or two further though, addressing the fact that we buy new high efficiency motors and rewind shops do not achieve the same efficiencies in rewinding. Looking at the power costs over expected motor life brings more numbers in on the side of replace versus rewind.

"Large motors" depends on context. In most of my career in the petrochemical and powerplant industries, "large motors" were generally 3000 horsepower and above. These are definitely candidates for rewind, short of some catastrophic damage to the motor steel or rotor.

I presently work for a natural gas pipeline bunch, and when I came on board, a "large motor" to these people was anything above ten horsepower. Since I arrived, we are now operating three in the 7000-9000 horsepower range. Nobody knows anything about them, and it elevates me to the status of high priest of all things concerning electrical power.

I wear the mantle graciously...

old field guy

 

[itsmoked]

I will be sure to not hire you oldfieldguy if hiring you portends, million dollar a month, power bills!!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gepman]

I am surprised by waross's statement that his rewound motors were longer lasting than his original motors. I believe he either had a very good motor shop or poor quality motors (or both).

I encourage everyone to checkout MotorMaster 4.0+ at

http://www1.eere.energy.gov/industry/bestpractices/software.html#mm

which is free software that can be used by an industrial facility to manage their motor inventory including deciding when to repair or replace, when to replace for efficiency, maintenance history, etc. It has a database of over 20,000 existing motors. There is also an international version (national in this case being the US).

There are several studies which show that a good rewind will result in an efficiency within about 1% of the original efficiency.

 

[oldfieldguy]

itsmoked-

worry not. I'm off the market.

On the electric vs. natural gas recip/gas turbine prime mover thing: Pollution permits become somebody else's problem. Fuel cost vs. electricity cost is a figure that can be evaluated based on real numbers, and right now, electricity is cheaper.

If you've tried finding parts and qualified contractors to work on fifty-year-old piston engines in the 2500-5000 horsepower range or gas turbines, you'll recognize some more hurdles, not to mention getting spare parts for them.

Compared to the care and feeding of those animals, my dumb ol' electric motor, with its one moving part and much simpler controls, well, the operators love them.

Me being the electrical guy, I'm fond of them too...

Sorry about the off-topic thing...

old field guy

 

[edison123]

My biased 2 cents as a rewind shop owner for 30 years

I don't agree that the motor efficiency goes down when rewound. If done with proper knowledge, the efficiency can actually be improved.

In my shop, we always redesign the winding for more copper in the winding which reduces the copper loss. This had been possible due to 1. Excessive lower class of insulation which we replace with optimum higher and better insulation leaving scope for increased slot fill 2. OEM had cut corners using as minimum as copper possible leaving more space.

All our rewound motors and generators have had a longer life than the OEM's.

Recently, AEMT (UK) did a study about burn-off ovens (not that I use them)and found that contrary to common belief, the efficiency of the motor did not go down after a burn-off.

My cut-off points for replacements - up to 25 hp; failed cast aluminium rotor bars for any size; excessive core damage requiring core replacement.




* I would go green if only I were not yellow *

 

[itsmoked]

oldfieldguy; Hope you realize I was joking..

edison123; Thanks for coming forth with that. It is exactly what I would expect. Same with automotive engines. Get a new one tear it apart, blueprint it while putting it back together, and they'll always run better and put out more.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rmw]

Specific to the OP's point made in his second post, one determining factor has to be the factor of down time required for the rewind. For common run-of-the-mill motors, with spare units readily available, it should be a no brainer. However, with special application motors typical to his industry; motors that drive high HP users like fans, pumps, etc and that have a one off design have to be analyzed based on the time required to rewind vs the available outage duration.

I am not trying to insult anyone's intellegence because I think most who have participated in this thread already know that, but for younger engineers who are exploring this territory for the first time it should be pointed out.

rmw

 

[oldfieldguy]

itsmoked--

I figured as much. Guys like us are worth our weight in gold...

or better...

old field guy

 

[HCBFlash]

engineerithink;

At least at this point of the discussion, most of us could reply with a bit more applicable insights if you'd at least give us a hint of the size motors you have in mind, or at least the size motors where you feel the decision is a bit more difficult for you.

 

[itsmoked]

I asked him that on the first response...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com