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Large motor efficiency loss on re-wind.

Large motor efficiency loss on re-wind.

(OP)
Here's a question that was emailed to me. Not really in my wheel-house so I'm hoping someone can help with this. Note, there is a bit of a language issue involved. (My writing is in blue.)

There is a problem wich I found when I transform an induction motor from 380v to 400v.

I work in a company of induction motors, so, when we change the winding of the small motors (2,4,6 pôles ) and 4 and 6 pôles of big motors ( from 380v to 400v) , the efficiency is good, but not the case for the 2 pôles of the big motors , can you guess what’s the matter with a big motor (upper then 22 KW) in 3000rpm(2 pôles)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Large motor efficiency loss on re-wind.

What the heck would they be changing to go from 380V to 400V??? And WHY????
Those voltage levels are virtually interchangeable, I don't see the value in fixing something that isn't broken. But that's not what you asked...

That said, it is virtually impassible to attain the exact same efficiency of any motor that is rewound. The processes will never be exactly the same. I read some articles about this years ago when Energy Efficient motors first came out, because it was an accepted aspect of that design "improvement"; the high efficiency rating was lost the first time they were rewound. You might be able to find those articles again, your Google-fu is strong.

Side note chuckle... 22kW = "large" motors... spin2


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Large motor efficiency loss on re-wind.

22kW of losses don't even begin to warm up a "large" motor. winky smile

RE: Large motor efficiency loss on re-wind.

Let us know the more details on particular redesign of 2 pole motor.
Obviously, something is changed in flux densities, either in winding diagram ( for example the pitch) or in turns/coil ( sometimes is impossible to change the t/c for 5 % exactly and "rounding" this number could be the cause).
BTW, what is the reason for redesign from 380 to 400 V?
Click on the link below to find more about the general limitations in case of winding redesign:
AC Motor Redesign

RE: Large motor efficiency loss on re-wind.

The "pôle" spelling is a giveaway. French? Then, the voltage "harmonization" is the reason to change voltage. And the IE3/IE4 craze probably*.

After the UK decided to isolate the continent from the British Isles, we could as well go back to old continental 380 V 50 Hz. The brits never changed much - they were supposed to go down from 415 V so we could meet half-way - but I haven't seen much of that.

So, there we are, worse PF in old 380 V motors because they are now overexcitated and many guys think that they need to do something about that. Actually, it is about less than a one percent efficiency improvement. So it is difficult to see any real reason for the rewinding. Except, perhaps, the feel-good factor. And fear that some inspector some day will ask you why you still are using 380 V motors when you are supposed to use IE4 motors?

Europeans (and notable us Swedes) are keen on staying on the right side of the thin line between lawful and criminal behavior. Ridiculous, yes. But sometimes, someone gets nervous. And then you get problems like this. It is a sad result of political correctness ambitions.

22 kW a large motor? OK, weighs enough to be large to me. But not large as in large.

_____________________________
*I am looking at the consequences that IE4 motors bring with them. EDM in bearings seem to increase. I see three reasons why. But motor manufacturers are somewhat reluctant to discuss the matter. Any input from the forum?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Large motor efficiency loss on re-wind.

Davidbeach's comment made me re-read the original post, now I realize they may have meant 22kW in losses, not that it was a 22kW motor... oops.

But of course without context, we still don't know what that means because as he said, 22kW in losses on a large motor is possibly inconsequential.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Large motor efficiency loss on re-wind.

I still read it as 22 kw motor fwiw.
Way too little info to understand (let alone answer) the question as others mentioned.

=====================================
(2B)+(2B)' ?

RE: Large motor efficiency loss on re-wind.

Sorry Jeff, it was just a snide remark about 22kW being a large motor. There are also people out there who think 120V is high voltage.

RE: Large motor efficiency loss on re-wind.

Some, no many years ago, I was running a repair ship in a motor and generator factory. I remember we worked out that over the life of a motor (motors fail more often than generators due to factors like more stops and starts.

Anyway, outside repair shops offered their fast turn round repair time. They stripped windings by a burn out method which baked the core and increased the core losses. So we did a few test and quickly showed that a stator recore and rewind was more economic in the long term than a straight stator rewind, due to the better core loss.

RE: Large motor efficiency loss on re-wind.

I subscribe to that, Hoxton.
Plate/plate insulation gets worse when you burn out the windings. You need to be very careful with temperature and most shops use excessive heat to get the job done as quickly as possible. That increases eddy current losses and lowers efficiency quite a lot. You may notice that when doing a ring test, but many shops don't mind and deliver sub-standard motors.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Large motor efficiency loss on re-wind.

(OP)
This is sounding more plausible in the OQ's (original questioner) situation. Since he's seeing a worse result in the 2 pole arena is there anything about this 'burn-it-out' scenario that would produce increased loss in efficiency specifically in the 2-pole winds?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Large motor efficiency loss on re-wind.

LMGTFY...


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Large motor efficiency loss on re-wind.

jraef,

I tried to follow your link. How do links work?




(Yes, I am messing with you - that page is hilarious. I will use that.)

RE: Large motor efficiency loss on re-wind.

I am wondering if some of the motors had a lower efficiency before the rewind. We don't know the efficiency of the motors before the rewind. It may be that they are finding out that two pole motors have a lower efficiency than slower motors regardless of a rewind.
As has been pointed out, not enough information ???

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Large motor efficiency loss on re-wind.

Quote:

This is sounding more plausible in the OQ's (original questioner) situation. Since he's seeing a worse result in the 2 pole arena is there anything about this 'burn-it-out' scenario that would produce increased loss in efficiency specifically in the 2-pole winds?
Degradation of stator core lamination insulation during winding burnout is a function of burnout oven temperature, burnout oven duration, and original laminar insulation design (C3 or C5). None of those factors are affected by number of poles. Some might argue there is some difference in what kind of flux pattern those cores subsequently see when you put them in operation (2-pole will tend to have a higher flux density in the "back-iron" than higher pole), but imo it's kind of a stretch to tie that in with so little info.

The standard way to assess stator winding core laminar insulation degradation from burnout is comparison of core tests before and after burnout.Typically we look at core watts per pound limit, look for less than 5% increase in core "watts loss" and also examine the core for hotspots. Any kind of total motor efficiency measurement would be a much more indirect way to examine core loss since there are a variety of losses present even at no-load.

I'm not clear what is the context where efficiency would be measured following rewind anyway. We do not do that even for our large motors. And if they weren't measuring efficiency, what specifically did they observe that led them to question efficiency. Also op has not weighed in to clarify the size of these motors.


=====================================
(2B)+(2B)' ?

RE: Large motor efficiency loss on re-wind.

Regarding the size of the motor, large or small?
For a frog in a pond, size of pond is like universe where as a frog which lives in the ocean, ocean is its universe. So the perception of the size is a relative term.

Regarding the lower efficiency:
One should not expect the same efficiency from the rewound motor. The guaranteed original efficiency is for the new motor. Now the question is what is the reduction in the efficiency from the original?

Are you talking about the original efficiency from the motor data sheet? For the old motor, the efficiency must have already deteriorated due to the ageing effect.

I would suggest you to apply your technical common sense and not the technical knowledge. Set an acceptable criteria for the reduction in the efficiency. If it is not substantial, (i.e. below the acceptance criteria) you should not waste time and simply accept.

RE: Large motor efficiency loss on re-wind.

Re-reading the original post, it is not clear if the efficiency is compared to the original efficiency or if it is just checked after the rewind with no previous benchmark to compare with.
Possibly the reason for the lower efficiency of the 2 pole motors was not that good before the rewind.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Large motor efficiency loss on re-wind.

The "large motor" means low number of t/c, especially if it is a 2 pole.
Note that 2-pole motor has about two times less t/c then 4-pole (for the given HP)
and often it is not feasible to change the t/c exactly for 5%.
Rounding this number will cause the change in flux-densities and accordingly in efficiency.
This is my guess regarding the 2 pole motors.

Winding Design and Motor Repair

RE: Large motor efficiency loss on re-wind.

Efficiency could change when the rewind changed the wire size to make it an easier wind. Converting one wire into two etc. to make the circular mills smaller to fit through the slot in the stator. If the motor isn't used on a VFD or a critical speed control, it may not make a difference.

RE: Large motor efficiency loss on re-wind.

It is me again!

When one sells / buys a large < 1000kW motor, you often have an "expert" involved. This expert can be anything from a one man consultancy to Bechtel (other Architect-Engineers are available!). Difficult questions may be asked about efficiency, support programs, warrantybetc.

When your maintenance / operations manager has a production line powered by "a large < 1000kW motor" wyou have a different situation. Your engineer thoughts of efficiency / reliability go out of the window. It is 4pm on a saturday, Monday 8am your director will ask what has happened, while he is on the golf course..... Try explaining that you have shut down production for a week to get the perfect repair when he has production targets, and a senior Vice President asking difficult questions....

This is why I had difficulty selling factory repairs against the repair companies.

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