Question for experienced rewinders 5

[zlatkodo]

I would like to know how often exist in practice (on the U.S. market and in other areas outside Europe), three-phase low voltage induction motors with 96 slots and for which number of poles , power and purpose ?
Thanks in advance.
Zlatkodo

 

[edison123]

For 96 slots, I would say 6 poles min and 18 poles max but most likely it is 8 to 12 poles.

As for HP, need the machine size.

Purpose - Any purpose, I guess.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Hi, Edison,
Thank you for your quick response. But just one thing:
I think that it is impossible to make a symmetrical (balanced), three-phase winding for 6 and 12 poles and 96 slots. Am I right?
Regards.
Zlatkodo

 

[edison123]

Zlatkodo - No. it's possible to balanced winding for 6 & 12 poles with 96 slots. The only requirement for 3 phase balanced winding is that the no. of slots be divisible by 3. It's not necessary that all poles in a phase have the same no. of coils. Of course, the coil groups in such a case have to be arranged in a manner to have a space balance also to prevent electromagnetic vibrations.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Hi, Edison,
Do you have an example diagram for a symmetrical, balanced coil with 6 or 12 poles, 96 slots?
I think that windings for 6 or 12 poles and 96 slots are asymmetric windings. Therefore, they must be unbalanced. The only question is: what we can do to reduce imbalances? There are not many cases where the imbalance can be effectively reduced. But even in these cases, these windings are still unbalanced and should be avoided.
Zlatkodo.

 

[edison123]

zlatkodo - No, it is not an asymmetric winding. You have 32 slots per phase. It's just a question of apportioning judiciously them between 6/12 poles. And it is done all the time without any imbalances whatsoever.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Edison wrote:
1/ it's possible to balanced winding for 6 & 12 poles with 96 slots. The only requirement for 3 phase balanced winding is that the no. of slots be divisible by 3.
2/- No, it is not an asymmetric winding. You have 32 slots per phase.

I disagree with that. It is NOT only requirement for 3 phase balanced winding is that the no. of slots be divisible by 3.
There are other very important conditions that must be met.
Here we are talking about the three-phase windings with fractional number of slots per phase and per pole , such as: q = 1? or q = 2? etc .
This fractional number we can write in this form: q = A b/c
If q is an integer then the winding is always symmetrical. But if q is fractional number then symmetrical three-phase winding MUST meet two additional requirements:
1/ - Number of poles must be divisible with “c” and
2/ - Number "c" should not be divisible by 3 .
In our case (for 6 and 12 poles and 96 slots) is q = 5 1/3 and q = 2 2/3.
Obviously this is asymmetrical winding.
In all such cases it is necessary to make additional calculations of asymmetry-size . Then we must decide whether such a winding corresponds to a certain motor regarding type ( squirrel-cage or wound-rotor), power and purpose. Asymmetry has a bad impact on the motor: increase of vibration, noise, current, winding-heating ,serious loss of torque …
I think it was shown that balanced windings are not possible with certain numbers of slots, notably for 6 and 12-pole windings.
If we decide to use such a winding arrangement , previously we must done additional calculations.
Regards
Zlatkodo

 

[wolf39]

Edison (Muthu) is right.

For a 6 pole motor the number of slots per pole and phase is 5 1/3. For phase A you have 5 slots for the first pole, 5 slots for the second pole and 6 slots for the third pole. This you repeat once with this phase for poles 4, 5 and 6 and you've filled 32 slots total for phase A. Phase starts with 5 slots, next 6 slots, last 5 slots. With repetition you again filled 32 slots. Phase C starts with 6 slots, next 5 slots, last 5 slots. As you can see, each phase has 32 slots in series and all phases are symmetrical winding-wise.

Regards

Wolf

http://www.hydropower-consult.com

 

[zlatkodo]

I do not doubt that such a winding we can cram in stator, but I say that it is asymmetrical, unbalanced winding. The fact that it has the same number of coils in each phase, means nothing.
Winding is not symmetrical only because it visually looks symmetrical. There are also symmetrical, balanced windings, which in general do not look visually balanced.
It is asymmetric, unbalanced winding with all the bad qualities that such windings have.
If we decide to use such a winding arrangement , previously we must done additional calculations.
Zlatkodo

 

[edison123]

zlatkodo - Can you please define asymmetrical unabalanced winding ?

96 slot, 6/12 pole is known as fractional slot winding and thousands of motors are running with such a winding.

I will simplify futher

96/6 - 4x5 and 2x6 coil groups per phase

96/12 - 8x3 and 4x2 coil groups per phase

Now it is just a question of arranging these coil groups around the periphery to get a "symmetrical, balanced" winding.

Muthu

http://www.edison.co.in

 

[wolf39]

zlatkodo:

Phase A: 2 x (5 + 5 + 6) = 32 slots

Phase B: 2 x (5 + 6 + 5) = 32 slots

Phase C: 2 x (6 + 5 + 5) = 32 slots

Total 96 slots

Each of the 6 poles has a slot number of 16.

It doesn't matter whether the phase winding is the result of adding 5 + 5 + 6 voltage vectors or 5 + 6 + 5 or 6 + 5 + 5 voltage vectors. The resulting phase voltage is always the same.

Regards

Wolf

http://www.hydropower-consult.com

 

[zlatkodo]

Your question:
“zlatkodo - Can you please define asymmetrical unabalanced winding ?”
If you carefully read my previous posts, you can find there a precise definition of three-phase unbalanced windings.

But I'd like to read your definition, because you say:
“The only requirement for 3 phase balanced winding is that the no. of slots is divisible by 3.”
Does this mean in practice that unbalanced thee-phase windings do not exist at all?
Regards.
zlatkodo

 

[edison123]

No. There are no unbalanced three phase windings in rotating machines.

Yes. The only requirement of 3 phase balanced winding is that the no. of slots be divisible by 3.

I am out.


Muthu

http://www.edison.co.in

 

[wolf39]

Muthu:

I may add that the number of slots per phase and the number of turns per phase is equal.

Regards

Wolf

http://www.hydropower-consult.com

 

[edison123]

Hi wolf - Agreed. More definitive.

Guess you don't like things unsaid.

Muthu

http://www.edison.co.in

 

[electricpete]

As zlatkodo said, there are more prerequisites to being able to create a symmetrical winding than having number of slots per phase (coils per phase) being equal.

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[electricpete]

Use left/right arrows to navigate to page 100 here:

http://books.google.com/books?id=_y...=Symmetrical Fractional Slot Windings&f=false

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[electricpete]

Using the terminology of the link, for the case
Q = 96
p=12
m=3
q = 96/36
we have z/n is q expressed as lowest fraction 8/3
z = 8
n = 3

Look at table 2.6 conditions of symmetry, last item (three phase m=2).
It requires that n/m is not a whole number
we have n/m = 3/3 =1 is a whole number

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[electricpete]

Here is a more direct link to table 2.6 (scroll to bottom of the page):

http://books.google.com/books?id=_y...symmetry for fractional slot windings&f=false

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[electricpete]

Regarding the original question - Look on the right side of this table and you will see 96 slots used in 75 - 300hp range, 8 pole:

http://www.reliance.com/mtr/pcrss.htm

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