Copper instead aluminum

[zlatkodo]

An increasing number of small motors wound with aluminum wire comes in repair shops. Aluminum wire is replaced with a copper wire of reduced cross sectional area so that a portion of the slot (usually upper) is free of wires ( see here).
What do you think, whether it is good practice and how it affects the motor performance?
Zlatkodo

 

[electricpete]

It would reduce slot leakage reactance, since more of the stator coil will be linked by more of the cross-slot flux.

This would tend to reduce starting current but unfortunately also starting torque.

If resulting reduction in stator current brings portions of the bridge section (side portions of teeth extending over the slot opening) out of saturation during start, then the effect is larger than we would calculate with linear magnetic model.

Whether it would end up being significant or not, I don't know.

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[electricpete]

One correction
"It would reduce slot leakage reactance, since more of the stator coil will be linked by more of the cross-slot flux. "
should have (obviously) been:
"It would increase slot leakage reactance, since more of the stator coil will be linked by more of the cross-slot flux. "

The remainder of my post remains the same (the concern is still reduction in starting current and resulting reduction in starting torque.)

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[electricpete]

Also, the same effect (increase in stator leakage reactance) would result in lower running power factor. The non-linear enhancement of increase in leakage reactance due to change in saturation of bridge section (which applies during start) would not be as relevant for running conditions where saturation effects are small.

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[electricpete]

Also you have a smaller contact area between the conductors and the side walls of the slot, which can mean less effective heat transfer from conductors to the iron core and higher resulting conductor temperature (assuming same I^2*R).



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[electricpete]

a portion of the slot (usually upper) is free of wires ( see here).

My discussion of increase in slot leakage reactance was for pictured scenario where top of slot is empty, conductors on the bottom.

If you put empty area at bottom of slot and conductors at top, then the slot leakage reactance would decrease. Result would be increase in locked rotor current.

If you put conductors at some position in between top and bottom by adding filler stick at top and bottom, then in theory you could keep leakage reactance exactly the same. But the reduction in heat transfer between conductor/core would be even more pronounced since you lose the additional contact area at the bottom of the slot due to thermally insulating non-metallic filler stick. (can metallic filler stick be used?... I’m not sure).

How significant is the change in leakage reactance - I'm not sure. One thing to note is only the slot leakage reactance is affected (endwinding leakage reactance is not).

Just some thoughts for consideration. No firm answers...

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[electricpete]

(can metallic filler stick be used?... I'm not sure).

Probably not a good idea due to potential for eddy losses in the filler stick, and possible hysteresis losses if made of steel, and possibility to short core lams if not insulated from core.

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[petronila]

Hello

I would like to add another comment, the aluminium wire is used due to cost reduction, the aluminium wire is cheaper than the copper, some practizal way to calculate the desired new mm2 copper wire is :

R= ρ L / A where ρ is resistivity L is lenght and A is area (mm2).

So for both materials:

R Cu = ρ Cu x L / S Cu and R Al = ρ Al x L / S Al

R / L is:

R Cu/ L = ρ Cu / S Cu y R Al / L = ρ Al/ S Al


ρ Cu / S Cu = ρ Al / S Al

Due to ρ Cu = 1.71 x 10-8 Ω-m and ρ Al = 2.82 x 10-8 Ω-m

The mm2 are :

S Cu = 1.71 x 10-8 x S Al / 2.82 x 10-8 or

S Cu = 0.606 x S Al

So for practical cases you can reduce two sizes when replace the aluminium, then a 17 AWG is replaced by 19 AWG.

But in my own practice I was replace the Al by Cu wire with the same AWG gauge so if we have 17 then I use 17, and this is supported in: 1. The same wire gauge don´t produce that space in the slot and further mechanical problems 2.The motor will run more cool and could support overloads(The effect is like you can increase two wire sizes in original copper winding) and 3. The 17 AWG wire cost is less than 19 AWG , small sizes are expensive than big ones.

Best Regards

Carlos

 

[Motorwinder]

I try to keep the cross section the same. I figure it will increase efficiency.

Is this valid thinking?

 

[Motorwinder]

I just saw Carlos' post. Two sizes is what I use to reduce it. Is there any detrimental effects of keeping the same size? Most the motors I've wound that had aluminum wire were small.

 

[electricpete]

Is there any detrimental effects of keeping the same size?

I’d say no, other than higher rewind cost... which may pay for itself in energy savings later. Makes it simpler to ensure the slot remains tightly filled without adding a huge amount of fillers, which raises some questions about heat transfer, leakage reactance etc. Those questions may be easily answerable by people with more experience than me, but going with same size eliminates concerns which are indeterminate for me and sounds safer to me.

By the way, EPRI 480 volt motor rewind specification includes the "Conductors shall be electrical grade copper and of the same size as provided by the original manufacturer. Conductor size may be modified only with the approval of Purchaser’s Technical Contact"

Other customer specs may have similar requirements. I read that to mean that this spec would require substitution of copper for aluminum and further require the copper have the same size as original.

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