- Sep 17, 2008
Yup. I knew that. Only halfway thru the first cup of coffee...The proper term here is inductive reactance.
As you go to higher RPM your drive frequency has to go up (for a fixed number of poles). Consequently the inductive XL reactance goes up. IIRC, the RPM of a brushless motor is related to the number of rotor and stator poles, by a relationship I don't recall at this time. So you, again IIRC, can optimize the number of poles and drive frequency for a specific RPM to reduce reactance.
Actually, there is no power lost in a pure reactance. It is the resistance of the windings that causes the loss. To avoid the sharp voltage spike when the square wave drive current shuts off (V=L*dI/dT can get quite huge) you can use a step approximation to a sine wave to drive the motor.