VFDs - variable frequency (motor) drives - output a pulse-width-modulated carrier approximating a sine wave. The carrier frequency is typically variable. I've seen numbers as low as 1 kHz, and as high as 16 kHz. Higher switching frequencies inevitably lead to higher losses in the drive, lower efficiency, and possibly derating of the drive. Higher carrier frequencies also run into problems with long cable runs. I understand that audible noise is a concern; are there other reasons to ever run a higher-than-minimum carrier frequency?

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    \$\begingroup\$ Vacuum Florescent Displays are not usually run with PWM in the first place. \$\endgroup\$ – Olin Lathrop Feb 5 '16 at 19:23

Yes, the power loss in the inverter is the first. The second is the current overload due to parasitic capacitance of the cable. The H-Bridge outputs a square wave, right so what is the current sink into a capacitor parallely connected? Ic=C*dU/dt, see dU/dt for square wave is unlimited therefore you get very high current spikes when transistor switches on/off. With higher switching frequency those spikes become more and they load the bridge. You can overcome this problem by installing a series choke at the output, still the hard switching losses of transitors persists.

Edit: Sorry I didn't read the question very well, if you have a large motor with large inductance then there is no benefit running at higher frequencies, if you have a servo drive and you need fast response then you should step into higher frequency, typical current servo loop is 125us which means 8kHz update rate, it would have no sense to compute PI loop if the PWM frequency is smaller.

  • \$\begingroup\$ Ah, so servo motors have lower inductance than others? Is that by design, to match with higher frequency operation for better control? Because servo motors tend to be permanent magnet? A combination of those? \$\endgroup\$ – Stephen Collings Feb 5 '16 at 19:50

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