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)
Code:
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.
Code:
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).
Code:
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.
Code:
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.
