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Old 08-23-2012, 01:35 PM
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Default AC induction motor rewinding questions

Hi,

we are ready to do a test rewind for an industrial AC induction motor. We have a motor, an aluminum frame 7.5 kW nominal 4-pole 50 Hz motor that weighed 50 kg.

We already did strip old windings from the stator, and built a simple winding machine, and will be getting a 22 kg reel of new enamel wire and impregnation varnish hopefully tomorrow.

Getting more power out of industrial motors by "overclocking" has been debated, so we will try it out to see what happens. So I'm not giving any claims here, just what we are going to test. I hope to hear any opinions before we go, for example, would you go for even lower / not so low nominal voltage.

In any case, we'll probably limit ourselves at around 150 Hz, or 3x the original frequency.

To state the actual problem again (why do we need a rewind): As for the original design, 380V 7.5 kW 1500 rpm @ 50 Hz, going to 4500 rpm without field weakening would require 1140V @ 150 Hz and produce 22.5 kW of power. This would require a 1600 volt battery pack, which is practically impossible.

We will use a 320 V battery pack. Hence, we need to reduce the original voltage by the factor 5.

The motor has 36 stators slots, this is 3 slot pairs per pole pair per phase. There were 3 strands of 0.80 mm diameter wire in parallel, making 34 turns per slot pair (34 turns * 3 parallel wires = 102 wires in a slot).

Do you agree that the right way to go is to reduce the number of turns by the factor of 5, too? This would be 7 turns per slot. Our new wire is 1.18 mm in diameter. In practice, we would test how many wires fit the slot, and parallel as many as possible, probably around 8...10 parallel. There was quite a bit of air in the slots of this motor - clearly it wasn't optimized for maximum efficiency - but we can make a tighter fit to increase the amount of copper there.

Any comments on this design before we go?


Ah, and we have an another one too. It was originally a 2-pole 440V 13kW 3000 rpm @ 50 Hz and has the same frame size despite the higher power. We would make identical windings for this one, thus converting it to 4-pole. I can see that there is less iron in stator in this 2-pole design (slimmer "teeth") but we still want to try it out. But do you see any other problems? Are there inherent, large differences 2-pole vs. 4-pole in the rotor?

Any comments appreciated. Will share the results.

Last edited by Siwastaja; 08-23-2012 at 01:40 PM.
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Old 08-23-2012, 01:56 PM
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Default Re: AC induction motor rewinding questions

Quote:
Originally Posted by Siwastaja View Post
The motor has 36 stators slots, this is 3 slot pairs per pole pair per phase. There were 3 strands of 0.80 mm diameter wire in parallel, making 34 turns per slot pair (34 turns * 3 parallel wires = 102 wires in a slot).
.............
Any comments on this design before we go?
I don't get "slot pairs" You have 9 slots/pole and 3 phase. Each slot has 2 coil sides in it, doesn't it? Total of 36 coils, 2 coil sides per coil, or 72 coil sides, into 36 slots, giving 2 coil sides per slot. Is it lap wound? (all coils identical?) What is the coil span? Did you make a wiring diagram before you stripped it? Got your slot liners and top sticks?

Quote:
Are there inherent, large differences 2-pole vs. 4-pole in the rotor?
All else being equal, on the 2 pole rotor, the end rings support twice the current so could be larger. Oft times the motor company uses the same die for 2, 4 or even 6 pole and carries the extra for the higher pole counts. Going from 2 to 4 should not be a problem unless the rotor slots are different. The 2 pole design could use deeper slots which would mess with 4 pole performance. Hard tellin' unless you cut one in half

Good luck.
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Old 08-23-2012, 04:02 PM
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Default Re: AC induction motor rewinding questions

Quote:
Originally Posted by Siwastaja View Post
Do you agree that the right way to go is to reduce the number of turns by the factor of 5, too?
Yes, your plan sounds good to me. The only problems should be the mechanical details of rewinding, and keeping the same winding pattern, as Major has mentioned.
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Old 08-23-2012, 04:46 PM
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Default Re: AC induction motor rewinding questions

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Originally Posted by major View Post
Each slot has 2 coil sides in it, doesn't it? Total of 36 coils, 2 coil sides per coil, or 72 coil sides, into 36 slots, giving 2 coil sides per slot.
Hm, no no. Both of the motors we stripped have the simplest possible single layer windings. (If I understood you correctly?)

Like this, for the 4-pole motor (drawn with Emetor.com)



... and the identical set of second pole pairs (not drawn above) connected in series with the first ones. (Actually, the connections were accessible so that it could have been modified for half the voltage by changing them to parallel, but it wouldn't have been enough still.)

Quote:
Is it lap wound? (all coils identical?)
No, it was a concentric winding. We are going to replace it with lap winding as it seems like a bit less hassle to wind identical windings. Otherwise, it doesn't seem to make much difference? Concentric are easier to machine insert?

Quote:
What is the coil span? Did you make a wiring diagram before you stripped it?
Coil span should be 9 slots for lap winding, right? However, as it was concentric, it was 8, 9 and 10, yes?

Wiring diagram as shown above -- it was easy to get as it was single layer winding and not completely gummed up with the varnish.

Quote:
Got your slot liners and top sticks?
I thought about using 100 Ám PET (Mylar) sheet for slot and phase insulation, and reuse the top sticks from the old windings. Didn't throw them away. However, now I do realize that having a wire and varnish specified up to 200 deg C, that mylar sheet would be the limiting factor... Oh well, we saved most of the slot insulation, too, and most of them are in a good shape. Reusing would at least be ecological .

Quote:
Good luck.
Thanks! I think we will need it .

Last edited by Siwastaja; 08-23-2012 at 04:55 PM.
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Old 08-23-2012, 05:31 PM
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Default Re: AC induction motor rewinding questions

Hey, I also came up with a new idea.

As this is a four pole design, I will be probably connecting the identical pole pairs in series, as they were originally. However, since I'm going to have a very small number of turns -- just 7 per slot pair (ah, by slot pair, I'm simply meaning the two slots 90 deg apart in a 4-pole design that share the exact same winding with opposite current directions) or 3*7 = 21 per three adjacent slots (one pole of one phase) -- would it be better to have 14 turns instead of 7 and connect the two pole pairs in parallel instead of series?

Or, would this make any difference? At least connecting them in series guarantees the same current flowing through them.
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Old 08-23-2012, 06:16 PM
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Default Re: AC induction motor rewinding questions

Here is something i didnt see you mention, The IN HAND TURNS have to handle the incoming current, what does the controller put out 200? 500?
amps? A regular motor can only hanlde 20-30 amps.

You have to do a delta connect for the 3 phases, 4 poles seires,
dont forget the poles s-s f-f (start start) (finish-finsh) when laying the
4 poles, Notes on my rewind span 1-9 so the first pole starts, slots
123 789 second pole slots 10 11 12 16 17 18 ect

The second phase start slot 789 13 14 15 ect.

You can see more on my web site.
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Old 08-23-2012, 08:17 PM
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Thumbs up Re: AC induction motor rewinding questions

You might want to check the skew of the rotors of these motors. It may be different for the two pole vs the four pole. I've rewound some smaller motors which were actually single phase and I made them three phase with variously more poles, up to 12. I had some strange experiences, but I think I figued out what was wrong. You might want to read though my thread on SED about 7 years ago when I did it:
http://sci.tech-archive.net/Archive/.../msg01591.html

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Old 08-24-2012, 03:10 AM
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Default Re: AC induction motor rewinding questions

I hope your original motor was not concentric wound, which would make it a pain in the rear. I like lap wound 3-phase motors, but machines can make concentric wound coils easily vs. lap wound coils. So if your motor is below 50 HP then you most likely had an original concentric wound stator before you modified it. My little black book on rewinding motors does not have the information on how to convert a concentric wound stator to a lap wound stator. Good luck.
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Old 08-24-2012, 05:21 AM
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Default Re: AC induction motor rewinding questions

Quote:
Originally Posted by Dennis View Post
I hope your original motor was not concentric wound, which would make it a pain in the rear. I like lap wound 3-phase motors, but machines can make concentric wound coils easily vs. lap wound coils. So if your motor is below 50 HP then you most likely had an original concentric wound stator before you modified it. My little black book on rewinding motors does not have the information on how to convert a concentric wound stator to a lap wound stator.
Why would this be any more difficult than just making the coils identical (practically, like the middle one)? Refer to the picture; over 5 minutes in MS Paint:




All this multilayer stuff is making my head spin. Why it is done . Neither of these motors did have anything special, they were just like the first textbook example with single layer windings with equal number of turns in every slot.

Everything looks too simple to me now. Given all the information, it seems I am probably missing something crucial.

Some sources claim double layer windings can achieve better efficiency, some say it doesn't matter. For example:
http://pe.org.pl/articles/2011/3/16.pdf table 4; efficiency is the same, but single layer has better PF. EDIT: the fractional multilayer
has practically the same efficiency as integer single-layer but with a bit less copper; so I would guess that the core losses might go down a little bit but I2R losses a bit up due to layer insulation in slots leading to less space for copper.


Or:
http://engineering2.dartmouth.edu/in.../isie2006a.pdf

"a well designed multi-layer winding with a sufficient
number of layers can have lower losses than a single-layer
winding. This does not mean that any random multi-layer design
is superior it only holds if the winding is properly designed,
taking into account all harmonics and optimizing the number of
layers and/or the layer thickness."



The newer of the two motors is manufactured in 2004 in Germany by VEM. It is very difficult to believe this would have any kind of severe compromise w.r.t. efficiency.

Luckily, these motors had simple single-layer windings so I don't have to care about this at all .

Last edited by Siwastaja; 08-24-2012 at 07:19 AM.
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Old 08-24-2012, 07:39 AM
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Default Re: AC induction motor rewinding questions

This is what i am talking about the inhand turns, this is a ev motor
that is able to handle the amps 500+ delta connection. This pic
is the six leads tyed together delta, see the size of the leads?
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Last edited by rbgrn; 03-22-2013 at 11:42 AM. Reason: Unsafe image removed
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