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Old Mar 09, 2006, 09:02 PM
Jesper Frickmann
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Raleigh, North Carolina, USA
Joined Aug 2005
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The case for a single phase brushless

Hi y'all,

This may be a little theoretical, but I hope that some of you fellow nerds can either tell me that I am right or why I am wrong.

A Wye-wired brushless motor only uses 2/3 of its stators at any time. This means that inner resistance is 2/3 of what it would be using all stators, and Kv is 3/2 times as high.

Moreover, since the two active phases are 120 degrees apart, the induced current is only Sqrt(3)/2 times what it would be if all active stators were on the same phase. Hence, Kv goes up further by a factor of 2/Sqrt(3). So Kv of a 3-phase motor is 3/2 * 2/Sqrt(3) = Sqrt(3) times, and inner resistance is 2/3 times what it would be in a single-phase motor with the same number of turns.

If we want the same Kv for the two motors, we need Sqrt(3) times more turns on the 3-phase motor, and if we are only allowed to use the same amount of copper, then the cross section area needs to be decreased by the same factor. Therefore, the inner resistance is 2/3 * 3 = 2 times as high in the 3-phase brushless, compared to a single phase motor with the same Kv.

The single phase motor is therefore able to eat Sqrt(2) = 1.4 times as much current as the three phase motor with the same Kv. Or a single phase motor with the same Imax can have a Kv of only 0.7 times the Kv of a three phase motor.

The catch? The single phase motor can spin both ways. I think it would need a sensor system and a smart ESC, to make sure it starts off in the right direction.

So can anyone tell me if I am right, or why I am wrong?

Jesper
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