AVR-Chat

--- In AVR-Chat@yahoogroups.com, Robert Adsett <subscriptions@a...> 
wrote:

>...
> 
> I do understand this.  However, the switching frequency and the 
drive 
> frequency do not need to be the same.  Remember that the motor is 
> essentially acting as a low pass filter.  I'd rather see a MUCH 
higher 
> switching frequency being used to generate the 40 Hz drive wave.
> 
> The switching frequency may be part of your problem but more 
information is 
> needed.

...maybe it helps if the original Poster (KC?) has a look at 
http://www.joliettech.com/ac_motor_control_characteristics.htm where 
they show the waveform of a PWM inverter.

The (in this case) 40Hz Motor waveform is not generated directly, 
but by changing the on-time of a PWM at higher frequency. The 
inductance of the motor acts like a "current capacitor" and smoothes 
out the peaks. Allthough the voltage is a high frequency (typical in 
the low kHz area) rectangle signal, the current has a sine waveform 
(as long as the PWM is controlled that way). Maybe it's clearer now?