Hi Andy,
If you average a steady state PWM signal (apply a low pass filter) you get a DC voltage. At 0% PWM you get 0V, at 50% PWM you get 1/2 voltage and at 100% PWM you get full voltage and so on.
Now if you slowly change the pulse width your DC voltage will slowly change to follow it.
A VFD changes the pulse width in a sinusoidal pattern and the motor inductance acts as the filter. As long as the PWM frequency is much higher than the sine wave frequency (say 10kHz or highter) all the motor will see is the sine wave. By varying the pattern the VFD can control the sine wave frequency and voltage.
Les
Andy Wander wrote:
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Les:
Thanks for the explanation-what I still don't understand about it is how the
"Sine Wave", which is generated by the "filtering" of the motor, will vary
in frequency, if the only thing that is done to it is to change the PWM duty
cycle. This would seem to me to get us to a "Sine wave" which has different
values for it's Positive and Negative half-cycles, but I can't see how the
frequency would change unless the frequency of the PWM changed.
Andy Wander
Verrex Corporation
