# number of poles, revisited, again



## dcb (Dec 5, 2009)

I was perusing aeva this morning and looking at the comparisons of abb motors gave me an idea.
Since number of poles comes up in the context of rewinds, and my understanding is that torque 
is to a large degree a function of rotor surface area (length and diameter), I thought moment of inertia might be a good 
reference point for comparison when considering changing number of poles for a given motor.

http://www.matrixps.com/products/abb/pdf/Combination 400V 50Hz_GB 022004.pdf
400v 50hz (mi=moment of inertia)

```
2 pole	M2AA 160 MA	11kw	36nm	2915rpm	0.039mi	73kg
*2 pole	M2AA 132 SB	7.5kw	25nm	2855rpm	0.016mi	42kg
4 pole	M2AA 132 M	7.5kw	49nm	1450rpm	0.038mi	48kg
6 pole	M3AA 132 MA	4kw	40nm	960rpm	0.038mi	46kg
8 pole	M3AA 132 S	2.2kw	29nm	720rpm	0.038mi	46kg
```
*for reference
2 and 4 pole have about the same power density by weight. the 42kg 2 pole makes 7.5kw, though 
the higher rpm can be impractical. The 11kw presumably has more back iron(25kg worth?). 

here are the 11kw versions, you can see the moment of inertia vs torque is roughly equal for 2,4,6 poles, 
but goes way down at 8 poles, as does the power density. And the 2kg difference between the 2 and 4 pole probably won't
compensate for the gearing.

```
2 pole	M2AA 160 MA	11kw	36nm	2915rpm	0.039mi	73kg
4 pole	M2AA 160 M	11kw	72nm	1460rpm	0.067	75kg
6 pole	M3AA 160 L	11kw	109nm	970rpm	0.107	102kg
8 pole	M3AA 180 L	11kw	146nm	720rpm	0.224	147kg
```
37kw comparisons, the moment of inertia isn't remotely linear with torque or size, 
and the 2 pole is starting to pull farther ahead in the kw/kg category (though fixed voltage/frequency in effect).

```
2 pole	M2AA 200 L	37kw	120nm	2945rpm	0.115mi	170kg
4 pole	M2AA 225 S	37kw	240nm	1475rpm	0.35mi	216kg
6 pole	M3AA 250 SMA	37kw	359nm	985rpm	1.16mi	280kg
8 pole	M3AA 280 SMA	37kw	481nm	735rpm	1.52mi	375kg
```
last one I wanted to look at was matching by torque (~73nm), perhaps this should have been first.

```
2 pole	M2AA 180 M	22kw	72nm	2925rpm	0.06mi	111kg
4 pole	M2AA 160 M	11kw	72nm	1460rpm	0.067mi	75kg
6 pole	M3AA 160 M	7.5kw	74nm	970rpm	0.089mi	88kg
8 pole	M3AA 160 M	5.5kw	74nm	710rpm	0.091mi	88kg
```
for a given voltage/hz the two pole makes the most power and is rather efficient, but we don't use a fixed hz. For a given weight the 4 pole pulls ahead in torque.
Though motor weight is just one part of the puzzle. Obviously bus voltage and battery availability and budget and etc are factors, and changing number of poles is rarely practical.

Pack voltage is a frequent consideration, so it kinda looks like the two pole is the winner in that regard, since it can also 
double the rpm while making the same torque, but would have to be larger than a 4 pole. But if you are rewinding and using an inverter (not fixed frequency) then you can keep the weight down with a 4 pole for the same effect, I think, and/or delta<->wye, etc., typical KV stuff, of course your pack and controller need to be able to handle the current. 

6/8 pole will lose efficiency as you try to ramp up the hz to get more power out of them, and they are playing catch-up in the power/weight department. 

2 pole looks like a possibility though, if you have a handle on the gear reduction. About %30 more weight for a given torque than a 4 pole, but if both are hz limited, you will still have ~2x as much power from the 2 pole on the top end. But you can't rewind a 4 pole into a 2 pole afaik. And I don't really have a handle on that kind of gear reduction.

edit, it seems the 2 pole is more severely hz restricted than the 4 pole, i.e. 125% vs 200% (obviously that gets pushed), so the 4 pole is going to catch up to the 2 pole as frequency increases, and probably the 6 and 8 pole motors to a larger degree. But that might just be a balance thing vs a magnetic path thing.


----------



## PStechPaul (May 1, 2012)

I have wondered why higher pole count motors seem to have lower power/weight, especially in small sizes. I think 4 pole is generally a good choice because you can fairly easily overclock it to 2x and 3600 RPM at which point it will have a higher power/weight ratio than the 2 pole. 

One consideration is that higher pole count motors have higher torque and the frame may need to be heavier to accommodate that. But also I think it relates to the amount of overlap of adjacent poles based on the number of slots in the stator. The 12 pole motor I made with a 36 slot stator had each belt span 3 slots, so the overlap was about 67%. A 2-pole machine has little overlap, so may have up to a 3x advantage in power/weight. It may be helpful to add a column in your chart for power/weight. It looks like a factor of about 2 to 2.5 from 2 pole to 8 pole.

I think you will find less variation in really large motors of 50-100 kW and up. They have many more stator slots and can be wound more efficiently.


----------

