VFD input power factor

[MikiBg]

Hi all!
I'm about to start a re-design of some pumping and compressor stations supplied via VFDs. It will be good to know input power factor for large VFD. I suppose that there is a PFC circuit, but guess that it compensates only fundamental. So, with voltage distortion neglected, there will be: PF=cos phi1/sqrt(1+THDi^2). For example, 180kW input active power with cos phi1=1 and THDi=0,3 will give PF=0,958 and input apparent power of 180/0,958=188kVA.
Is my logic correct? All comments/links/experiences are welcome.

 

[CJCPE]

The displacement power factor of a PWM inverter is 0.95 or higher. That is the inherent chracteristic of the input rectifer circuit. The DC bus capacitors supply the magnetizing current for the motor. The total power factor including the effect of harmonic distortion is as you have stated it.

To calculate the total power factor of the facility, you need to determine the total RMS current from the total fundamental current and the total harmonic current using I = sqrt(I1^2 + Ih^2).

You can not correct the harmonic component of total pf in the conventional way. You need to use tuned filters. If you have a problem with harmonic distortion from large drives, you should consider drives with 12 pulse or higher rectifiers or active front ends. Even using pf caps to correct displacement pf can be a problem if there are harmonics present.

There is some information on my web site. I will try to post more.

CJC

http://www.vfdriveinfo.com

 

[ozmosis]

MikiBg
as CJCPE points out, it depends on the type of VFD.
Here is a very approximate guide:
A standard VFD 6 pulse rectifier with no AC line reactor or DC link chokes: total PF will be approx 0.6
A 6 pulse VFD with DC link chokes or (4~5%iZ) AC line reactor: total PF will be approx 0.85~0.9
A 12 pulse VFD inclusive of phase shift transformer: total PF will be approx 0.92~0.95
An 18 pulse VFD inc.phase shift transformer: total PF will be 0.95~0.97
A VFD with Active front end or Active filter embedded: PF is typically controllable; i.e leading/lagging depending on the capacity of the front end.

This is only a very approx guide. As more and more technologies develop to mitigate harmonic distortion, you will see the PF improve as a direct result. However, the better it is the more you pay up front but potentially save in the long run.

 

[MikiBg]

I know about filters, passive or active, as a option, but what is Active front end? Does it assume IGBT rectifier instead of SCR, with input current displacement control, a technology that modern UPS use as well?

 

[Skogsgurra]

Yes, that is 100 % correct.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

It could also cover the boost converter topology as applied to a standard rectifier which I think is slightly different to what MikiBG is describing. fairchild's app note illustrates it better than I can here:

http://www.fairchildsemi.com/an/AN/AN-42047.pdf

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