We have been told that to achieve a 10:1 speed range for an AC Permanent Magnet motor is very significant. Yet, I seem to routinely see motors rated with a wider speed range than this.

Is there a confusion of different types of motors, or are people using different methods of rating them? I would expect the speed range to go pretty much all the way from 0 to rated speed, if cooling can be maintained.

Thanks,
Ron

 

Lets clarify a point:
10:1 speed range does not mean that you have to omit the first 10%; it is the guaranteed range where achievable torque is constant and nominal.

Therefore, if you do not mind about torque for speeds lower than 10%, you can use a 10:1 motor down to 0,001%(provided that you make forced ventilation). If you mind about the torque at lower speeds, than you have to look at the speed-torque diagrams for the motor.