It is true*. The V/f ratio is constant.I was always under the assumption that a VFD voltage output stayed constant, but today I was talking to coworker saying that the output voltage varies with the output frequency, is this true?
And therein lies the probably source of confusion on this issue, I have seen it a lot. An AC drive's output is a PWM signal, Pulse Width Modulation. It is not true AC, it is a series of DC pulses in strings, first positive, then negative. Each pulse is ALWAYS the same voltage potential, that of the DC bus. So in a 480V drive for example, the DC bus voltage is the same as the peak voltage of the incoming sine wave, or 480 / .707 = 678V. But an inductive load, such as a motor, cannot really respond to a DC pulse very fast because of hat is called the Inductive Time Constant (look it up), so by the time the pulse has an effect on the motor magnetic field, it has gone away and a new pulse has replaced it. That string of pulses then gets "averaged" out by the motor's reaction to it into a pseudo sine wave, called an RMS voltage. This is very close to looking like AC, in fact so close as to make the motor work fine. So then the rate at which the strings of pulses change direction (+-) determines the output frequency, the width of the pulses determines how high that RMS pseudo AC voltage gets in the motor. Combine them and you can change speed and maintain torque.In general, no. With some VFDs, you might get a few % over 480V on the output, but no more.
(With the caveat that the DC switching voltage described above is being ignored.)