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Vfd Power Factor Improvment


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#1 SBsparky

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Posted 30 August 2009 - 08:25 AM

Hi Guys
I was reading subrao post reagarding vfd power facor and it thought I best not hijack his topic. But I work for a panel building company and have also being told that when MCC's /industrial sites have vfd's running the over all power factor for the site improves. But how can the vsd's effect the power factor on the line side of their supply? (eg the power factor created by other non vsd load) To be honest I don't really belive my boss but can someone please confirm or denie this for me.

Thank you

SBSparky




#2 marke

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Posted 30 August 2009 - 08:55 AM

Hello SBSparky

Welcome to the forum.

Power factor is the ratio of KW to KVA, but to many people, power factor is the cosine of the angle between voltage and current.

There are two types of power factor, displacement power factor and distortion power factor. Combined, these equate to the ratio of KW to KVA.

Displacement power factor is where the current is sinusoidal and is at a phase angle to the sinusoidal voltage waveform.
Distortion power factor is where the current waveform is distorted. It may be in phase with the voltage waveform, or it ma also be phase displaced.

The input current to a standard VFD is a highly distorted current in phase with the supply voltage, so the input of a VFD has a displacement power factor typically better than 0.95, but a distortion power factor less than 0.85
You can not add power factor correction to the output of a VFD, and because the displacement power factor is better than 0.95, many consider that the VFD is correcting the power factor of the motor.

If you fit VFDs to motors at an installation, the displacement power factor will improve because you are reducing the inductive current drawn from the supply. This will show on some instrumentation as an overall power factor improvment, however if the instrumentation measures true power factor rather than displacement power factor, the power factor wil not improve to the same degree and may even reduce.

Best regards,
Mark.

#3 SBsparky

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Posted 30 August 2009 - 10:43 AM

Thanks "Mark fof you quick respose We typically install power factor correction banks to our mcc's which have automatic controllers for bring in the capactors in and out as required, these controllers are typically wired to see the current on white phase and measure the voltage on all 3 phases so I believe they only look at the phase angle between the voltage and current not the distortion correct? Also if I understand you correctlly adding a new motor to a plant and running it from a vfd will not improve the exsisting power factor of the plant?

#4 kana

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Posted 30 August 2009 - 01:44 PM

Hi SBsparky,

The power factor regulator switches in capacitors according to the displacement power factor (phase angle between current and voltage).

The distortion factor represents the harmonic content of the supply.

The VSD improves the power factor of the motor which it controls and not the power factor of the plant with several other loads.

Regards,
kana

#5 SBsparky

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Posted 31 August 2009 - 07:44 AM

Thank you all for your prompt respose all. As Mark E said (Power factor is the ratio of KW to KVA) So if I understand correctly installing a new motor on a vfd of say 250kw running at a pf of say .95, to a site that is currently running at 1Mw .75pf would change that ratio giving the appearance of improving the power factor angle correct?



#6 kana

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Posted 31 August 2009 - 11:18 AM

Hi,

Adding a new motor will increase the power drawn from 1MW to 1.25MW. Since the power factor of the new motor with VSD control is 0.95, then the overall power factor of the plant will improve. However, if the power factor of the new load is lower than 0.75, then the overall power factor of the plant would reduce.

Regards,
kana




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