PF is caused by reactance in motor windings, and is going to be very similar to running directly off the line, the non sinusoidal wave form may alter it some but will still be similar.
100% understood on all this. You're comparing VAin to POWERin, and comparing VAout to POWERout.
if power factor is 1.0 then VA and watts are the same. Therefore on the input side VA and power are nearly the same because input power factor is normally very close to 1.0. Output power factor is dependent on the load being driven.
Powerin = powerout, discounting conversion losses.
Yes, I understand the relationship of power, VA, and PF on the input (or output). I am talking about the comparing of in and out of the VFD. It's a very strange (but true) phenomenon of input CURRENT being less than output current, and I'm trying to wrap my mind around what's really happening in a way that I could explain it to somebody in a simple way.
I think you're now getting to the root of my problem in understanding. The part I bolded (VAin < VAout) is eyebrow raising. I don't think that's possible. I don't say that with absolute certainty though, which is why I'm asking. If what you said is correct, this would mean that the VFD is creating VAs, which I don't think it can do?Powerin = powerout, discounting conversion losses.
VAin = powerin, or very close anyway.
VAout value will exceed powerout value because PF < one. VA and power are related but are not the same thing.
VAin will usually be less than VAout.
Unless the VFD is off than they should be similar.
I agree with you.
www.controleng.com
Thanks, but we're not really questioning whether the output current is higher than the input current. We know this to be true.From this webpage, Even at full load conditions, the input current will typically be lower than the motor current because the input still does not have any magnetizing current component in it.
The truth about five common VFD myths | Control Engineering
No matter how commonplace variable frequency drives (VFDs) have become to you, somewhere someone is using one for the first time.
Reading the article, I gather what happens is the motor's magnetizing current comes from and goes back to the capacitor bank each cycle like a tuned tank circuit. This increases the VARs on the output, but not the input. Here is the entire paragraph from the webpage:
www.controleng.com
Only a sparkies but between the capacitor bank that is the first thing feed from VFD input then to IGBT'S and the better line reactors that have capacitors would think the drive input power factor hopefully would be over 90% PF and not have too much distortion.
Yesss..... that's it! That clicks now. Thinking of it having like a tank circuit component to it all, and the charging of the drive's capacitors (dc links) essentially as the prerequisite to making it all happen. Once they are ready to go, the caps supply any needed VARs in the output to match the motor inductance, and so the only thing that the drive's input needs is the real power requirement for the torque load. Hence, the input current can be less than the output current, since the output current has the real power AND the VARs bouncing back and forth between the drive's caps and the motor.
The drive isn't creating the apparent power. The inductive load (motor) is.
Got a p/n for this motor that creates apparent power?
"The(re) is not any direct connection between input and output sections of a VFD."
Not really true. It is like saying the same water goes into a lake as goes over the dam that makes the lake.
Can you explain how Power Factor Correction Capacitors work? If so, it’s the same issue. In the VFD, the DC bus capacitors are storing and releasing the reactive power for the motor, the same as when you have PFC capacitors.
Yes, I was really not considering that it's the dc-link caps in the drive that make this happen with creating VARs, which increases the VA. I had in my mind that VAout couldn't be more than VAin, but that's just not true. I had no problem explaining how this works with PFC caps.
