Winding arrangement with different number of turns in coils 2

[zlatkodo]

When redesigning the windings of large three phase low voltage motors, often happens that the number of turns / coil must be a number that is not whole, even in cases where the winding should be done with the maximum number of parallel circuits. We can get, for example 2.5 turns / coil.
In this case, the two-layer winding should be done with different number of turns in coils, for example, with 2 and 3 turns, so that in every slot we have a total of 5 turns. Such an arrangement depends on whether the number of coils in the pole-phase group is odd or even, whether the pitch is odd or even number, etc.
I'm interested in, whether there are some practical rules for determining such arrangement?
Zlatkodo

 

[edison123]

The few LV motors I have done with different no. of turns in the coils had only odd pitch like 1-10 so that no. of conductors per slot remain the same. But they all had more than 3 turns per coil.

Muthu

http://www.edison.co.in

 

[wolf39]

zlatkodo:

Do you mean re-design or re-wind? A re-wind does require an identical number of turns for a given slot number and voltage. For a re-design you can easily adjust the slot number and/or the number of parallel circuits to arrive at a whole number of turns per coil.

Regards

Wolf

http://www.hydropower-consult.com

 

[zlatkodo]

Here is a simple example of what I asked.
Motor of 250 kW, Y-660 V, 4 pole, 60 slots have the
following original data:
lap, double-layer, 4 par. circuits, pitch 1-14, 6 turns / coil (6 + 6 in slot).
The motor we need to rewind to 380 V , (Y connection).

The new number of turns / coil should be: 6 x 380/660 = 3.45 turns / coil.
This number should not be rounded to 3 or to 4, because motor will not be good in both cases. Also, number of parallel circuits can not be increased (except in special cases).

In practice occurring even more complicated cases.
Zlatkodo

 

[edison123]

You can just reconnect the old winding (if it is good) in 380 V delta without actually rewinding it. If the old winding has failed, you can still rewind it for 380 V delta with the same 6 turns per coil, 4 circuits.

Muthu

http://www.edison.co.in

 

[electricpete]

Muthu's answer sounds like a good one for this motor. I think delta winding is a little vulnerable to 3rd harmonic circulating currents especially if the iron becomes saturated. But lots of motors operate as wye-start / delta run (steady state in delta)... particularly smaller motors like this (150kw).

More generally, Nailen discusses "fractional turn" windings here:

http://findarticles.com/p/articles/mi_qa3726/is_200803/ai_n25138697/

I have the hardcopy of this article here at my desk including figures if there are any questions about the figures which are omitted from the on-line version. (Electrical Apparatus magazine is free for motor profressionals - highly recommended imo)

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[wolf39]

zlatkodo:

I wished you had specified in detail what you had in mind.
A star to Muthu and Pete for proper and competent response.

Regards

Wolf

http://www.hydropower-consult.com

 

[zlatkodo]

Yes, Muthu, that is true but only if we want the stator in the delta.
My example is just to explain what I asked. What if:
- voltage ratio is not 1.73 or
- connection is delta in the original and the new winding (most often the case in practice) or
- the pitch is odd or
- the pole-phase groups are unequal, with the same or even with different pitches etc.
I wonder whether someone has dealt with such cases in detail.
Yes, Electricpete, I saw that article, but unfortunately with no pictures.
I think that in some cases is almost impossible to make even such a simple redesign (like change winding for new voltage) with no significant deterioration in motor characteristics. Do you agree with me?
Zlatkodo

 

[edison123]

I don't know why you want to use only wye in a redesign. But if you insist, you could go for single layer mush winding wye with 7 turns for the case specified.

Given that the standard voltages are not that many and given that a lot of manipulative factors like wye/delta, parallel circuits, single/double layer, pitch factor etc. are available, I have not had any problem so far in ac motor redesigns. (DC motors are much restrictive in that matter).

Of course, I do agree there might be still cases that are not fit for a redesign. If you have any such case, I would be interested in taking a crack at it.

Muthu

http://www.edison.co.in

 

[zlatkodo]

Thank you all for the answers.
By the way, Muthu, I think you certainly know that the double layer winding can not be replaced with a single-layer just like that : the number of turns multiplied by 2.
This can be done only in rare cases when the double-layer winding was made with the full pitch. In other cases, the number of turns should be corrected again for 2-15% (sometimes even more), which mostly depends on the pitch of the double-layer winding. After these corrections you get another number of turns that may significantly differ from whole number, again.
Zlatkodo

 

[edison123]

Why would the pitch factor change for the same pitch in single layer ? The distribution factor would but not the pitch factor.

Muthu

http://www.edison.co.in

 

[electricpete]

Seems like winding a double layer fractional pitch winding to a single layer winding of the same pitch is not always possible... 60 slot, 4 pole, 15 coils sides per pole, full-pitch=1-16, actual pitch = 1-14... you can’t fit the other 14 required coil sides in between 1 and 14. There were some clever alternatives given in the other thread, but not always possible, and some of them (option C) resulted in dramatic decrease in distribution factor. So it seems like that’s a good option to consider going to single layer random wound, just need to work through the specifics of each case as you suggested. I’m sure I haven’t said anything here you don’t already know.

Attached are some of the figures from Nailen’s article.


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[zlatkodo]

Thanks Pete, your attachment is useful, as always.
For some reason, it is difficult to find specific, useful information and schematics for rewinding.
Perhaps, in the future, I could publish my own collection of schematics and winding details.
Zlatkodo