Motor with rectangular slots 1

[zlatkodo]

What is the most appropriate method for securing the winding in the slots with a rectangular shape?
Zlatkodo

 

[electricpete]

If it's form wound it usually has wedge grooves.
If it's random wound, it's usually semi-enclosed slots that help retain a top stick.
I don't see either wedge grooves or semi-enclosed slots.

I guess if the coil were packed tightly side-to-side and vpi'd, that might be enough without a wedge for form wound? ... but I've never seen one like that.

Is it random wound for form wound?

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[electricpete]

Is it random wound for form wound?

I guess it must be form wound. If it were random wound, it would have slots shaped more like a trapezoid (to allow uniform tooth width).

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[petronila]

Hello Zlatktodo

I am agree with pete, the rectangular slots are used in form wound windings due to the coils will be fit pretty good in the slots.

In this case the picture show an old design and a the winding was developed in round wire(I don´t think this small motor was designed with formed coils, this action will be expensive.

Regards

carlos

 

[electricpete]

Yes, good point – looks very small in size range of low voltage / random wound.

From a magnetic standpoint, rectangular (vs trapezoidal) slot doesn’t make sense for random wound. To cut up the circular core, we need to make either the teeth or the slot trapezoidal (the other can be rectangular). In form wound, we need rectangular slot for coil insertion, so we are forced into trapezoidal tooth which is not efficient due to tooth width wider at base that tip (so designing to avoid flux density near the tooth width forces us to use extra unnecessary iron at the tooth base). In random wound motor, there is absolutely no reason to use rectangular slot since it forces trapezoidal teeth,which waste iron. Maybe it is just a case that small motors are not “optimized” the same way as large motors.... other factors like standardization may weigh heavier for small motors.


I'll be interested to hear what type it really is. Looks very weird to me.

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[electricpete]

Correction in bold:
(so designing to avoid flux density near the tooth width forces us to use extra unnecessary iron at the tooth base).
should've been
(so designing to avoid saturation near the tooth tip forces us to use extra unnecessary iron at the tooth base).

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[rhatcher]

The size of the motor looks like a low voltage, random wound type. I am guessing that the stator bore ID is 6 inches or so at the most.

Zlatkodo can hopefully provide the provide the nameplate voltage to confirm or deny this. Theoretically, he could actually provide us with the specific rewind data since I assume that he, a rewinder, is working on this motor in his shop or place of employment.

I have seen small random wound motors with square slots. For small motors, the electrical characteristics are not as demanding and the square slot core lamination is easier and less expensive to cut than one with a radiused slot.

Also, for small motors, an open slot as shown is easier to insert the wire than a closed slot. However, easier is a relative term since it is difficult to wind a motor this small no matter what the slot opening is.

The slot does appear to have a very small wedge groove. If so, use a flat piece of thick nomex cut to fit tightly into the groove similar to the way that a hard phenolic wedge would be applied to a larger, form wound motor.

Also, use slot liners, top and bottom separators, and slot fillers as required to get a tight slot fill from top to bottom.

Finally, it is always a good practice to use mylar phase paper between the phases and to tie the random winding firmly with at least one tie per coil.

 

[zlatkodo]

Unfortunately the motor is not next to me, but I guess that it is designed for a 380 V, 50 Hz.
Zlatkodo

 

[electricpete]

Random wound doesn't exert pressure against the sides of the slot. What's left to hold the conductors in place?... the wedge/top stick / nomex in that tiny slot... that doesn't inspire confidence for me. Therefore I would vote for vpi or similar resin treatment to help "glue" the winding in place.

If for some reason, you can't do that, at least make sure the slot liner wraps all the way around and perhaps tie it, rather than wrapping the slot liner around just far enough to meet the top stick... a litle bit of insurance against the possibility of that topstick coming out. But vpi would be better imo.

Just my opinion. Does anyone think it doesn't need vpi? (just curious).

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[electricpete]

If for some reason, you can't do that, at least make sure the slot liner wraps all the way around and perhaps tie it, rather than wrapping the slot liner around just far enough to meet the top stick... a litle bit of insurance against the possibility of that topstick coming out. But vpi would be better imo.

For example you could wrap paper completely around a coil side before placing it in the slot. (probably couldn't wrap both coils in the same sheet). That is not a choice for normal semi-closed slot where you have to feed conductors in through the narrow gap at the top, but would be an option for this rectangular slot. Again the idea is to try to make the slot section winding a little mechanically stronger. vpi of course even better.

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[electricpete]

Also this guy "Henk" has a pretty extensive discussion of wedges:

http://www.mandc.co.za/pdfs/Electric_Motor_Failure_Wedges.pdf

Somewhere in there he made a statement that random wound motors with open (vs semi-closed) slots typically use magnetic wedges for purposes of reducing harmonics. Of course that makes the method of securing the wedges all the more vital.



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[electricpete]

I think the harmonic considerations are similar for random wound as for form wound, but the small random wound motors tend to have smaller number of stator slots and rotor bars, so the open slot construction tends to be less tolerable for the small motors. (At least, that's the only reason I can come up with the random wound motors with open slots would typically require mag wedges, when most form wound motors with open slots do not.)

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[electricpete]

Another comment that perhaps goes without saying... the design of wedges (magnetic or non-magnetic) is determined by the OEM and should be recreated during rewind.

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[electricpete]

I think the harmonic considerations are similar for random wound as for form wound, but the small random wound motors tend to have smaller number of stator slots and rotor bars, so the open slot construction tends to be less tolerable for the small motors. (At least, that's the only reason I can come up with the random wound motors with open slots would typically require mag wedges, when most form wound motors with open slots do not.)

Actually, I can think of another reason why open slots without magnetic wedges may be less tolerable on small motors than large motors: if you compare length of the flux path (for a given pole) in iron between large motor and small motor, there will be a dramatic difference between large and small motor... but if you compare flux path in air there is not much difference. So airgap is relatively larger on small motors and has a biger impact on power factor. The mag wedges are a means to decrease the effective airgap (via the Carter factor) and improve the power factor slightly.

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[Motorwinder]

Cut your slot cells to just under the wedge groove. When inserting coils, place a nomex wedge (formed nomex) over the coil. Cut some wedge strips using .032" thick G10 epoxy board, that will slide into the wedge grooves. (Good to cut these before you start winding). Slide the wedges over the nomex wedge.

If you don't have any wedge grooves, cut your top sticks wide enough to wedge them in from the top. Tapping them down with a tamping tool.
Listo

 

[starkopete]

Hi.
Motorwinder is giving you good advice. I'd like to add - that if you have room in the slot, cut your slot liners above the laminations so you can fold them over the top of the coil and then your "wedge" will hold it down.

Thanks