I agree that a fully loaded motor at 50 Hz will probably be overloaded when it drives the load at 60 Hz. I can't think of a load that isn't greater when it's driven faster.
I get hung up on the same voltage at different frequencies. A vfd will vary the voltage with the speed - lower Hz needs a lower voltage. Then, would a higher Hz need a higher voltage to overcome the counter emf?
My guess, from the example given by Hutch, is that dragging down the speed of the motor with the load might not return the motor to it's original HP. That is, a 50 HZ motor has more power at 50 Hz that it does at 60 Hz because it is designed for 50 Hz.
I have seen motors rated for both frequencies but they have different voltage ratings or HP ratings, or both. I haven't seen one with the same voltage and HP.
