Single Phase current draw for a 3 phase output VFD: technical discussion
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Across-the-line motor PFC capacitor compared to VFD DC bus filter cap. Latter has a rectifier stage ahead of it, former does not.
Two quite different scenarios. \You can't conflate them.
Unlike PFC, unidirectional current flow. DC if you wish.
And, unlike PFC, it doesn't return its energy to the supply.
They serve two totally different functions.
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I'm not conflating them. Just comparing characteristics, function, performance. Though you may not concede to the idea, they do have some crossover in these areas.
The main purposes are different, yes. But a capacitor is going to perform as a capacitor no matter what its general purpose is, and that is the part which you do not want to concede. You are even saying the proper words to prove it: "...it doesn't return its energy to the supply."
No
a portion does
still a reactive load on the grid during conduction period
and therefore improves power factor
if not there supply current will increase
The DC cap bank doesn't - at least to seem to understand that bit. It justn't do what PFC does with a direct across the line motor. That's what you and Ing can't seem to, don't want to grasp.
Let me ask you both a question. You have a dozen VFDs on a process line, powers from 100kW to 400kW. They are all fed from a common DC buss. How does altering the VFD capacitor bank affect the power factor in such a case? Can you at least begin so see the disparity?
Ing posted a link (#post#249 I think) from a VFD manufacturer. It has errors about power factor correction - clearly not written by someone with design level understanding. The trouble is that such disinformation is accepted as the gospel truth by some. And that saddens me.
"The moving finger writes........"
Helps if you are........
It does the same thing
there are a few misconceptions
a rectifier has a pf of 1, it does not
that it does not flow reactive power, it does
that line side current flow is uni-directional, it is not
those multiple articles are written by experts with far more understanding than you or I
it is easy to criticize the work of others when you present none of your own to be reviewed
other than nuggets of non-relevant information couched as knowledge
condescending
as far as your multiple motor example
Net pf imrovement
know any?
if you are implying you are, apparently not lol
But I'm not saying it does it in the same way. If the power factor of a VFD input is better than the same motor connected across the line at the same speed and loading there has to be a reason. And that reason is the DC bus filter cap(s). There are no active components in existence that I'm aware of that can improve the power factor of a motor. A passive component or second power source has to be involved. However you want to explain that is up to you.
We are talking power factor comparison motor vs. VFD input. Not just the input.
If you completely remove the capacitor bank, will you measure a better or worse power factor at the AC source?
How so on a DC supply?
Batteries
caps
motor-gen set
rectifier
????
assumed an ac sourced rectifier with filter caps
You wouldn't measure anything. The PWM drive wouldn't work.
Yes.
A bigger bucket will make the PF worse, not better as you claimed due to the worse input current waveform distortioin.
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