carrier frequency and output frequency

[abfer]

What must be the relationship with the carrier signal and modulator(output) signal. I mean at least what must be the ratio between them? Can i get a 1KHz spwm signal with 7KHz carrier signal?

 

[Skogsgurra]

I assume that it is about inverter drives.

It is almost like the Nyquist frequency - but reverse. If your motor inductance filters the carrier frequency very well, then you can go close to motor frequency = 1/2 carrier frequency. At least in theory. And with the motor acting as a brick-wall filter.

But since the motor is a first order low pass filter (6 db/octave) it will not work. You will probably need something like carrier = 10 times motor frequency or better.

1000 Hz motor frequency at 7 kHz carrier is doable, but be prepared to have some interference between carrier and motor frequencies. An output filter could perhaps improve the situation.

Gunnar Englund

http://www.gke.org

 

[electricuwe]

For a ratio as small as seven you will have to change motor and carrier frequency sychronously to avoid interference. An output filter, as suggested by skogsgurra, will not help in that case since the frequencies disturbing operation are intermodulation products of carrier an motor frequency below the motor frequency.

Asynchronous modulation usually require ratios of at least 20.

 

[abfer]

Ok, i understand, thanks. I decided to make it so.

 

[Skogsgurra]

I figure that at these high speeds (1000 Hz results in 60 kRPM in a two-pole motor) the low-frequency torque variations can be tolerable - or not. It all depends on the masses involved. But, of course, there are frequencies where the interference will be very annoying.

Gunnar Englund

http://www.gke.org