Hi,
the power factor of VFDs is very poor, for those manufacturers who publish it, it ranges from about 0.5 to 0.6.

The issue is that VFDs draw current very unevenly rather than a sinusoidal current waveform, also called harmonic distortion. For the small VFDs
we use having such a low power factor is not a big issue, sure we're paying for 20A but only 'getting' 11A, not to mention that we need bigger cables and breakers etc.
In a big industrial VFD/motor combinations it becomes expensive and so correction techniques have to be applied.

The attached pic is the formula for the Distortion Power Factor (DPF) for a given Total Harmonic Distortion (THD). Schenider are one of the few companies that
publish all the details about THD and power factor for their devices.

The simplest of the correction techniques is a line reactor, basically three coils (three phase assumed) of about 5mH to 10mH. That would take a power factor
of 0.55 to about 0.8 to 0.85, quite a gain.

The next step up is active power factor correction. You might be surprised to know that there are many hundreds of millions of the circuits in existance, one in every PC to start
with. Quite some years ago the US realised that the PC power supply, being a line connected rectifier and capacitor, would introduce unacceptable harmonic distortion on
the AC network, and so regulated that power factor correction be employed, and that has been in place for decades now.

The idea is quite simple, that the power supply be preceeded by a boost converter modulated in such a way that the current draw is quasi-sinusoidal. The IC to do so can be had for
under $1.00 each.

Large power factor correction circuits use the same idea but have to handle many tens or hundreds of amps and cost a bomb.

Here in New Zealand we have seen a dramatic increase in the use of VFD controlled 'down bore pumps' for irrigation. These pumps range up to 500kW.
A VFD for such a pump might cost $50,000, but then you have to add another $25,000 for the power factor correction unit. Not surprisingly the farmers object,
but unless they mitigate the current distortion the power supply companies will refuse them service. The existing installed base of VFDs prior to this restriction becoming
universal is such that in rural areas in New Zealand PCs, TVs etc tend not to last long before they get fried by all the harmonic currents.

As I posted earlier I don't have a screened VFD/spindle cable, but that has not been a problem. What I found though was that when I put a line reactor preceeding the VFD
I reduced a lot of noise effects in my control circuits. I concluded that the harmonic currents of the VFD were leaking through the power supplies of my PC and other electronics
and that a line reactor all but stopped those harmonic currents, and naturally improving the power factor.

Craig