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Author Topic: Windings : enamelled copper cross sectional area  (Read 5304 times)

Ian

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Windings : enamelled copper cross sectional area
« on: August 30, 2007, 04:24:52 PM »
I am a little confused and am seeking clarification, please, from your good selves….

My ST alternators are rated at 7.5 KVA. The rated output current (at around 230 volts) is about 32 amps (stator). The field winding (rotor) is nameplate rated at 2.7 amps at about 62 volts.

The cross sectional area (diameter) of the winding wire is usually proportional to the current the winding has to carry. So, I would expect the stator windings (required current carrying capacity of 32 amps) would be substantially heavier than those for the stator (required current carrying capacity of 2.7 amps). But it is the other way round!

The 4 rotor windings have substantially larger cross sectional areas than the stator coils. I would have expected commercial sense to limit the rotor windings to the minimum required and also to limit centrifugal forces by keeping the rotating mass down. In my view, the cooling effectiveness of the rotating bobbins on the 4 rotor coils is good, and there is plenty of room left on the winding former so there is unlikely to be a heat build up in the middle of the field windings.

When I refer to the stator windings, I am talking about the main output windings and NOT the harmonic or “tell-tale” bulb windings, which, as one would expect, are of low cross sectional area compared to the main stator windings.

1.   Why are the rotor windings larger than the stator windings ?
2.   What am I missing ?

Regards,
Ian

Doug

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Re: Windings : enamelled copper cross sectional area
« Reply #1 on: August 30, 2007, 05:29:28 PM »
Its easy to spin coils for the field and the wire size was probably chosen as much for mechanical strength as convinience.

Stator windings are different....
I am afraid I can't convert to metric or even gause for you but I know the lower limit for curent to copper in an open frame machine is around 300-400 CM per amp. This is around th minimum you can run with class F insulation. Smaller wire means smaller slots and this means more and better use of the Iron in the stator core.
The stator also acts as a heat sink for the windings.

The trade off is trying to find the ballance point where insulation hot spots, iron density/pole fulx and the carring current of the copper. This is less critical in the rotor, however you want to largest possible wire size because the rotor doesn't cool as well as the stator ( inspit of what you would normaly think ) so the larger the wire the less the effects of teperature are going to change the resisitance of the winding. The voltage aplied to the winding is also lower so any change in resistance is going to have a larger effect than if the voltage ws higher.

I don't know, they may simply have used the least costly wire too.

There are a lot of things to consider when designing an electrical machine....

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Jim Mc

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Re: Windings : enamelled copper cross sectional area
« Reply #2 on: August 31, 2007, 04:34:34 AM »
Might the stator be wound from several parallel-connected windings?

Doug

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Re: Windings : enamelled copper cross sectional area
« Reply #3 on: August 31, 2007, 05:26:16 AM »
That could be too, sometimes when wire size is in the  larger ranges ( 14 or so ) it can start to be a handful to get it to nest well in the slots. 16 - 20 are the nicest sizes...
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Ian

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Re: Windings : enamelled copper cross sectional area
« Reply #4 on: August 31, 2007, 10:16:43 AM »
Thanks once again guys, I had not even considered parallel windings !

Other than making it easier to wind, are there any other positives or neagtives with parallel windings ?

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Ian

Doug

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Re: Windings : enamelled copper cross sectional area
« Reply #5 on: August 31, 2007, 03:04:11 PM »
Two or three wires in hand sometimes you can increase the amount of copper in a coil for a given size slot.

These machines are small I wouildn't expect to see more than 2 wires in hand. I could see maybe 2 17 for something like this off the top of my head. Sometimes you migt use tow different sizes too like am 18 and 17.

Again it all comes back to CM per amp ratio. In a machine like this with F insulation I would expect the range to be 300 to 400 cm per amp.

Lower classes of insulation will require higher ratio, possibly lower iron density.

Highly involved stuff, but the math is easy.
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Ian

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Re: Windings : enamelled copper cross sectional area
« Reply #6 on: August 31, 2007, 03:14:43 PM »
Doug - I am showing my ignorance again. Could you please explain what you mean by (and its significance) the following...- not because I NEED to know but because I WANT to know:

* cm per amp. (eg you indicated 300 - 400 cm per amp)
* Insulation rating

Many thanks

Regards,
Ian

Doug

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Re: Windings : enamelled copper cross sectional area
« Reply #7 on: August 31, 2007, 03:37:17 PM »
C M circular mill

Others go by the older amps per square inch of copper to calculate the current density of the cooper

Not sure how this is done on your side of the pond.

Insualtion rating the the maximum temperature the insulation can run at before it starts to age and brake down at an accelerated rate.

I don't have any tables or books to explain what the classes are any more, I have been out of the electric motor rewind trade a long time.

I like things to run fairly hot, but obvioulsy in their rated range. Machines have to run in the ranges for wich they were designed for best efficiency and reliability 
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