Re: Too much time to think!
its my understanding that in an AC motor, voltage and frequency are linearly related, mostly to prevent magnetic saturation.
so, if the motor is 1800rpm @240v 50hz then then 900rpm is @120v 25hz
you should be able to run the motor past its rating, but you cant do it using hz or voltage alone, it must be both, in the correct measure. so, for the motor above, you could run it at 3600rpm, providing you could supply it with 480v and 100hz.
Ive never actually run an AC motor past its rating, but i always thought that because of their simplicity they should be very tolerant to overload. i would imagine its limits would be the class of insulation in the field windings