To elaborate a little on what itsmoked said:
There are two frequencies of concern in a VFD, and the PWM frequency is not the "Variable Frequency". The PWM frequency is a fixed frequency substantially higher than the maximum variable fundamental frequency into the phases. This higher frequency can be thought of as a carrier frequency, as in radio AM and FM station frequencies, that are higher than the fundamental audio frequencies they "carry".
The most important term in the PWM abbreviation is "modulation". The purpose of modulation is to provide some fraction of the (relatively) fixed supply voltage to a phase at a given moment. One strategy is linear modulation, where for example, to provide 50% of the supply voltage, you turn the power transistor half-on, dropping half of the supply voltage in the transistor, providing the other half to the phase. As itsmoked alludes, the problem with this method is the very high power dissipation in the drive that is produced. (These are the "Class A" audio amplifiers that you see in audiophile stereo stores, with the huge heatsinks that are almost as big as the electronics box.) Still, there are linear modulated drives for motors, usually when the switching noise of a PWM power stage cannot be tolerated.
As itsmoked points out, PWM is popular because it keeps the power transistors in low-loss states for the vast majority of the time. In PWM, if you want to provide 50% of the supply voltage to the phase, you turn the transistor full on for 50% of the cycle, and full off for the other 50%. You count on the low-pass filtering of the motor's electrical inductance and the system's physical inertia to time-average the effect of this command pattern.
Now let's look at the "fundamental" frequency this modulation is carrying. In a typical VFD, this will vary from a few Hz up to maybe a few hundred Hz. The magnitude of these AC waveforms (and therefore the maximum PWM duty cycle) will be approximately proportional to the frequency (this is called the volts/Hz ratio). Remember that the PWM duty cycle for each phase will vary sinusoidally over the phase cycle.
Because a motor is always a generator, it will produce a back-voltage waveform on its phases of magnitude proportional to the speed. To produce any torque, the supply voltage from the drive must be greater than the back EMF voltage to be able to force current into the phases. Even at no load, the drive must be producing phase voltage waveforms of magnitude (and therefore of peak PWM duty cycle) proportional to the speed.
Curt Wilson
Delta Tau Data Systems