Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
(OP)
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!!!
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!!!





RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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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RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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.
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
"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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
I encourage everyone to checkout MotorMaster 4.0+ at http://
There are several studies which show that a good rewind will result in an efficiency within about 1% of the original efficiency.
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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 *
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
I figured as much. Guys like us are worth our weight in gold...
or better...
old field guy
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
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.
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
In this instance, we are replacing a 1250hp boiler feedwater pump motor fed from our 2300V switchgear. One of the biggest reasons why I suspect O&M wants to replace the motor is because it was installed in the early 60's.
RE: Large AC Squirrel-Cage Induction Motor Repair Versus Repalce
It might be worth it to do some thorough testing on the motor. I'm not sure about the efficacy of on-line testing for finding motor faults or wear, but I'm sure someone else here could answer that.
Also, at 1250HP, you may be able to re-wind and re-rate the motor to higher power and better efficiency. We've rewound a couple of large compressor motors here with good results. It was done before my time, but I recall that we were able to give a pretty substantial boost to the motor HP.