How Induction Motors Are Operated At Higher Frequency Using VFD drives.

The concept is that AC motor speed is a function of the number of electrical poles in the motor and the applied frequency. So if you increase the frequency on a motor, it goes faster.

There is a cost however, and this is often ignored at peril to the application. A motor is actually designed around a SPECIFIC frequency and voltage and that can be expressed as a ratio of Volts and Hz, or V/Hz. So a motor that is designed for 380V 50Hz has a V/Hz ratio of 7.6:1 (380/50). As long as you maintain that ratio, the motor will put out rated torque and the windings will not saturate. But when you increase the Hz woithout increasing the V, you begin to lose torque and the motor is running in "constant power" mode. So let's say you have a 10kW motor rated 380V 50Hz and you run it at 100Hz. Since your voltage is limited to 380V, the V/Hz ratio is decreasing above 50Hz so it is going to be losing torque at a similar ratio. At 100Hz then it is still a 10kW motor but because you have twice the speed, but you have 1/2 of the torque. Depending on the application, this can (and often does) lead to stalling the motor when people are unaware of this. there are ways around it, but this is an important part of the basic "concept" you were asking about.

This phenomenon by the way continues to the point where at some speed the motor no longer has enough torque to move even its own rotor inertia and the motor stalls even with nothing connected to it. So for example just because a VFD can put out 1000Hz (as some do), that does not mean you can take a motor designed for 1400 RPM at 50Hz and run it at 28,000 RPM, it will just sit there and hum at you until it burns up.

Well that's one less homework question to worry about

Eh... maybe. But I find that a LOT of people who were not yet exposed to the concept are unsure of how it works. I taught basic VFD theory for industrial electricians and one of the common problems I had was that nobody ever taught them basic motor theory to start with.