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AC Motor Power Efficiency

AC Motor Power Efficiency

AC Motor Power Efficiency

Where is the best place to find information on "AC Motor Efficiencies" with respect to rewinding the stator?
Does anyone have any information to share on the topic of rewinding a motor verus replacemnt?
Replies continue below

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RE: AC Motor Power Efficiency

Suggestion: Please, notice that the high motor efficiency is achieved due to several factors, namely:
1. The physical design, perhaps with the application of appropriate electrical finite element analysis software. This factor can hardly be improved by rewinding only.
2. The magnetic material of the magnetic path.  Also, this factor can hardly be affected by rewinding of the stator.
3. Higher conductivity of winding conductor. This is the potential efficiency improver. However, cost is to be watched. E.g. silver is more expensive than copper.
4. A price of the high efficiency motor versus rewinding the existing motor. Also, this is a potentially decisive factor.
5. Conductor insulation. This is also factor that may be affected by rewiring of the stator.
6. Accurate design, better bearing, lubrication, etc. are also important; however, they may prove to be less significant.

RE: AC Motor Power Efficiency

Thanks jbartos for your reply.
To furthur the question; what is the rule of thumb on loss (or possible gain) in motor efficiency when a motor has sustained stator damage (short) and is rewound?  I know the answer is dependant on a lot of factors particular to material quality in the construction of the motor and the quality of work performed by the "rewinder".  So narrow it down to an already high efficency motor from a quality vendor.  I have been told that in such a case a loss in efficency of 2 to 4% is expected? What do you think?

RE: AC Motor Power Efficiency

In most cases a stator can be rewound with little or no loss in efficiency assuming that the core has not been damaged. In fact, most rewind efficiency loss is associated with core losses due to some form of damage. Core damage in these cases is caused either by the original winding fault or by the rewind process itself.

Core damage can result from a winding ground fault within the stator slot. Essentially, the power dissipated in the fault fuses the core laminations and in some cases actually vaporizes material such that there is a hole. If the fusing is not cleared so that the individual laminations are insulated from each other again then eddy currents and core losses will result. In some cases if not corrected the resulting localized hotspot can reach high enough temperatures to damage the new winding in a very short time.

The process of rewinding can also have a detrimental effect on the stator core in some cases, especially if done improperly. During rewinding, original windings are often burned out of the stators. Excessive temperatures or multiple rewinds can degrade the insulation between the laminations of the stator and increase the core losses. As well, I know that there are some motors out there today that have laminations constructed of lower grade material that cannot be burned out during a rewind because damage will result.

There are several ways to test a stator for core losses and most large motor shops will do so for motors being repaired. If a core is damaged, it may or may not make sense to repair it as described below.

With respect to the topic of rewinding versus replacement, the two primary factors that come into play are cost and availability (time). For example, it is usually not cost effective to perform a major repair on a 40hp or lower NEMA/IEC frame induction motor because you can buy a new motor off of the shelf anywhere for about the same money as a repair. Even if the new motor costs 30-50% more than a repair, it still may make sense because a replacement can be had immediately and the cost is relatively low for these motors. Give that 40hp a special frame and it is a different deal since a replacement may not be available immediately or may carry a premium price.

For intermediate motors it is a case by case basis. For example, I have seen some people buy a replacement motor that cost 3 times more than the repair of the original because they could not wait for the required repair time and the replacement was immediately available. I have also seen people spend more on a repair than the cost of a new motor because, in this case, the delivery of a new motor was greater than the time required for repair.

For large motors the repair is usually the only option since the cost of these motors can exceed $100,000 and delivery time is measured in months. For these motors almost any repair can be cost and time effective, including shaft replacement, core/rotor restacking, and rotor rebarring.  

RE: AC Motor Power Efficiency

DOE/EASA has a tech brief which according to the table of contents addresses both subjects (efficieny during rewind and rewind vs replace).  I didn't read the whole thing but you can get it at

Most repair shops aren't going to make any promises about efficiency.  They usually don't have the original design/test data so they won't make any predictions or guarantees.  But there are some specific items that you may be able to get some control over.

One minor point on how efficiency can be improved when rewinding an older motor.... we don't need to use a higher-conductivity metal like silver.... just more copper. This is often possible because the newer insulating materials take up less room and leave room for more copper.  You can ask them to specify copper cross section per turn in their quote... and then use that as a comparative basis for selecting your rewind shop.  But don't let them cheat on the groundwall or turn insulation to do it.

If you don't want to go to all that trouble, one item you can put in your spec is to not allow slot fillers.  You can allow packing material for tightness but slot fillers are a missed opportunity to put in more copper.  The distinction can be a little grey so you can ask that they get your approval on packing material.

If you have significantly upgraded the thermal capability of the insulation system then likely the motor will be over-cooled if the fans are unchanged.  It is possible to modify the fan to trim back a little bit of the excess cooling flow (it costs power to drive the fan).  But would require a lot of care since again the rewind shop is not likely intimately familiar with the cooling design.  Better luck if the OEM is rewinding the motor.

Rhatcher is right on the money that you can lose efficiency during burnout of stator windings.  Some things to consider putting in your spec:  1 - Max temperature of burnout should be 650F unless approved by the client.  Some core materials are more sensitive than others to high temperatures.... make the shop do their homework to convince you that the temperature they propose is safe for this core.  Another thing to put in your spec: If post-burnout core losses (watts per pound) exceeds incoming (before burnout) core losses by more than 5%, then the seller is responsible to repair the core such that core-losses are reduced to within 5% of incoming core losses.  That will give them some incentive.  Also you can spec the hotspots and change in hotspots if you want.  There is also a whole lot of discussion on how core loss testing should be done (what is rated flux etc).  

If efficiency is really important to you and you're willing to finance a substantial redesign anyway, you might ask 'em to consider changing the volts per turn. That is not always possible but there may be ways to rearrange the circuits to accomplish it.  Higher volts-per turn generally gives better efficiency at high load (lower volts-per-turn generally gives higher efficiency at low load).

I agree with the others that in general you expect a rewound motor should be as efficient as the original.  Possible core burning during rewind addressed above.  Most phase-to-ground failures are relatively tame and don't cause any core damage.  

I can't improve on rhatcher's response to rewind vs replace.

In large motors sometimes there is a third option for coil cutout at a fraction of the cost and time for full rewind.  It will require a little engineering evaluation but often a failed coil can be cut out of a large motor with no adverse effects other than slight derating of the motor.  Sometimes two other symmetrically-spaced coils need to be cutout.  EPRI has a very detailed report on the subject.  Our plant has had one good experience with cutout coils (worked for three years until we decided to rewind for other reasons) and one bad experience with it (cut out failed coil, failed the hi-pot on adjacent coil, cut out adjacent coil, failed surge test on another adjacent coil, lost a month, still shelled out a quarter million for a rewind).

RE: AC Motor Power Efficiency

Suggestion: The above preceding two postings could possibly be supplemented by checking with some established electrical motor rewinding firm regarding any documentation it may have to provide it with rewound motors which would include the efficiency test result. This test result is one of the major criteria how the rewinding was performed. E.g. rewinder may do the high quality rewinding work. This means the coil turns are professionally would side by side and layer by layer without any crisscrossing. Such rewound motor may have the efficiency as good as the new motor with a negligible margin of error or within efficiency meters errors. This is what would be the above concern about 2% to 4%.

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