what is carrier frequency in VFD?

[danjenkins]

I would like to know in more depth what carrier frequency is and its role in VFD.

Any guidance would greatly be appreciated.

[jbb]

To vastly simplify, the carrier frequency is the frequency uses to turn the power switches on and off (ie it’s like PWM frequency).

It depends a LOT on power
- small drives (<100kW) will be in the 4kHz - 16 kHz range and likely use Space Vector PWM (SVPWM, you can search for it online).
- Drives 1000 kW and over drives might be 400Hz - 1kHz. At these frequencies SVPWM doesn’t work too well, and other schemes such as Optimal Pulse Pattern control are used. One particularly twisty extension of this is Model Predictive Pulse Pattern Control (caution: vector diagrams and considerably implied mathematics).
- Drives over 30000 kW might be thyristor based and go at line frequency (50 - 60 Hz). I’m not sure what the current favourite is for controlling these.

[MagicSmoker]

@jbb's answer is mostly correct, I would just emphasize that a VFD with a fixed carrier frequency implies a lower frequency sinusoidal reference is modulating the duty cycle of a higher frequency square wave (the carrier). Typically (if not exclusively), this means the VFD employs a scalar (V/Hz) control scheme, rather than a vector (field-oriented) control scheme.

Note, also, that space-vector pulse width modulation is a switch control scheme that maximizes the use of the available bus voltage and not a motor control scheme, per se. So you could theoretically have a drive that uses a scalar motor control scheme with SVPWM, but it would be most unusual to say the least; carrier-based PWM is far more likely in such a case. In contrast, many older drives use a field-oriented motor control scheme with carrier-based PWM, so that combination is quite common.

@jbb - the venerable old thyristor-based cycloconverter is still commonly used at the very high power level, though it is being supplanted by the matrix converter.

[jbb]

Thanks MagicSmoker, that does clarify things a bit.  I skipped over some stuff.

At Medium voltages (say 2kV and up), Modular Multilevel Converters (MMC) are starting to make inroads.  But they are quite complicated... I think the thyristors will be safe for a while yet 

[lukebarber]

20 kHz is common for a variety of reasons.

1) The frequency is above the audible band.

2) Power devices have been optimized for this frequency.

3) The switching frequency is high enough to allow reasonably high current loop bandwidths-1-2 kHz.

4) Noise suppression components are smaller.

Here's a good post on selecting VFDs.
https://precisionautomation.net/2016/01/27/how-to-choose-the-right-control-method-for-vfds/