Power factor for VFD drives 4

[power2020]

Dear Folks,
Two issues are confusing me in regard of VFDs.
1. What is the Power factor of PWM type VFD type with Line choke (source side)? As DC link capacitor is there, and motor always take reactive power from capacitor, I assume unity power will maintain at line side. Anyone pl. confirm.
2. What is the fault current contribution of the PWM VFD drives? As snubber circuit is connected across SCRs in rectifier circuit, is it possible to contribute fault currents by motors?

 

[Skogsgurra]

There are two power factors involved. The displacement power factor aka cos(phi1) and the distorsion power factor, lambda.

The "phi1" stands for phase angle between voltage and current of the fundamental frequency, usually 50 or 60 Hz.

The distorsion power factor is lambda = P/S and results from line current being distorted so that S, defined as Urms times Irms, is sometimes several times larger than P.

The lambda thus gets down to .3 or even .2 in severe cases (lightly loaded motor) while cos(phi1) always, for reasons that you mentioned in your OP, stays close to 1.

Note: lambda is actually the total power factor. The simplification made here is valid when displacement power factor is unity.

Gunnar Englund

http://www.gke.org

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

 

[power2020]

Thank you Skogsgurra for reply. It is useful info for me. If we provide APFC (Automatic power factor control) devices with CT and PT connections, which power factor it follows?

And how can I get the information about VFD motor fault current contribution?

Once again thanks...

 

[Marke]

Hello power2020

The true power factor for loaded motor and drives with DC bus chokes is typically in the order of 0.75 - 0.8 but it depends on the design of the the DC bus choke and/or the line reactors.
Some drives have no reactors at all and they have a very low true power factor due to the very short current pulses of very high magnitude on the input side.

The power factor can often be improved by the addition of AC line reactors, but there is a definite limit to the improvement. Additional improvement requires the use of a filter, or an active front end. The filter can be an active filter or a passive filter.
Traditional power factor correction capacitors provide correction for displacement power factor, but not for distortion power factor.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Skogsgurra]

"And how can I get the information about VFD motor fault current contribution?"

A standard one quadrant VFD does not contribute to fault current. It is only when you have regeneration that a VFD can supply any current to the grid. And that current is limited by many factors. Primarily the built-in current limit in the front end of the drive.

So, if you have VFD:s without regeneration, no contribution. If you have regeneration, current contribution no more than current limit of VFD front end.

Gunnar Englund

http://www.gke.org

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

 

[ozmosis]

Total Power Factor(lambda) when looking at PWM VFD's at full load:-
1) No AC line reactor or DC link chokes= approx 0.65~0.7
2) 2% AC line reactor = approx 0.75~0.8
3) 5% AC line reactor or DC link chokes= approx 0.85~0.9
The lower the harmonic distortion, the closer to unity the total power factor will be.

 

[LionelHutz]

Is the power factor control you are contemplating an active controller or a bunch of capacitor steps.

If it is the first case (active filter), then the filter can be sized to reduce the current distortion and harmonics as seen by the incoming power as well as correcting the power factor. A VFD load can be a good application for such a filter.

If it is the second case (capacitor steps) then they will have to be filtered so that the harmonics do not cause damage to the capacitors. After you do that, the capacitors will only correct the 50 or 60hz power factor, as Marke has already stated. However, the fundamental power factor really does not need to be corrected so installing this type of capacitor bank for a VFD application would be a waste of time.