No implication required.
Single Phase current draw for a 3 phase output VFD: technical discussion
- Thread starter Jraef
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Given the definition of power factor, it would be 0. So any change from there is an improvement.
Actually indeterminate.
Point. But the fact remains that any power factor is an improvement... and a point for me.
Backing up a little, why won't it work?
mivey
Senior Member
OK, thought maybe there was something I was missing since the articles did not validate nor echo your premise. Thanks for clarifying that.
the original premise (one of them) was the caps in a vfd improve pf
proven
a supplemental was more C will yield a larger improvement
fact easily understood with basic ee knowledge
fact
Wrong
a bigger cap will reduce thd
pf = (1/(1 + (thd%/100)^2)^0.5
pf will increase
not as I claimed
but EVERY engineer with any knowledge of vfd's, filtering and Fourier
based on your understanding of vfd pf improvement I would say more than an implication is required
mike_kilroy
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Except it isn't. The link you posted is wrong in that respect. A fact easily understood with basic ee knowledge.
The motor current is not unity power factor. The period wher the current is in antiphase wth the voltage means it needs somewhere to go. It can't go back through the unidirectional input rectifier.Point. But the fact remains that any power factor is an improvement... and a point for me.
Backing up a little, why won't it work?
So, the DC bucket cap is that place. If you could get by without it manufacturers, ourselves included, wouldn't fit it and save oodles of $$
mivey
Senior Member
Did you run your VFD model? Will you post the results?
And this is the same reason why it improves power factor. The out-of-phase energy component must be handled locally.
It isn't and it doesn't. Think about how PFC works. The fact that it can exchange energy with the supply. Something a DC bucket can't do.
You can persist with your claim. That doesn't confer validity.
The vfd caps do exchange energy with the ac source
obvious
a rectifier has a pf <1
so reactive power Q flows thru it
Yes
it improved pf
I'll try
for some reason the images are small and unreadable
Bose in his text uses matlab/simulink to model drives
http://www.wiley.com/WileyCDA/WileyTitle/productCd-0780310845.html
a whole section on it
leads me to believe model results are valid
Nope. Just wrong. A diode bridge conducts in one direction only. They can not and do not return energy to the supply
Nope. The lower than unity power factor is because of the non -sinusoidal distortion.
You appear to be confusing that with conventional displacement power factor.
you misunderstand the physics, you are not alone
they do during the conduction period
wrong again
you apparently do not have a firm grasp of the basics
An fvnr across the line starter with local pf caps at motor
line current is reduced vs no cap, ie, reactive current from the supply is reduced
a vfd local at the motor
line current is reduced vs no vfd, ie, reactive current from the supply is reduced
replace starter and vfd with a 'black box'
the line side changes are indistinguishable
line current is reduced vs no cap, ie, reactive current from the supply is reduced
a vfd local at the motor
line current is reduced vs no vfd, ie, reactive current from the supply is reduced
replace starter and vfd with a 'black box'
the line side changes are indistinguishable
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