Single Phase current draw for a 3 phase output VFD: technical discussion

[Smart $]

170620-0932 EDT

Smart $:


This is true. But you also have to define values, the load circuit, how the pulse width varies with position, and how pulse amplitude changes with position.

Well, when I first presented the example, it was not my intent to discuss the power factor. It was only to show that a pulse could have the same RMS value as a sinusoidal. And one thing lead to another... :blink:

We assume you are still using a voltage sine wave of 1.414 peaks, Vrms = 1 V. Since the current is constant in amplitude its Irms = 1 A. Thus, Vrms*Irms = 1.

However, average power is 1 (current) * 1.414 (peak voltage) * 0.636 (average value of a 1/2 cycle of a sine wave) = 0.899 . Thus, the power factor is about 0.9, not 1.

With a sine wave voltage source the load current must be a sine wave if the power factor is to be 1. In other words the load must be a continuously connecter resistor.

You are correct (as was GD earlier). I had not considered the power waveform for a long pulse to be a half-sine ripple.

I don't think that is in dispute. But the power factor of a load on a DC supply with a large output capacitor does not reflect back thru the DC supply to be the power factor at the input to the DC supply. The power factor at the input to a DC supply is a function of the design of the power supply, and the load power (not factor) on the DC supply.

Hmmm... seems that we now have a dispute over what was being disputed. :rant:

jk :lol:

 

[Ingenieur]

bigger caps on the rectifier section WILL improve input power factor

 

[Smart $]

bigger caps on the rectifier section WILL improve input power factor

And I believe there was a minor dispute over whether or not the motor power factor [effect] would be passed to the input if there were no DC bus filter caps. It is simply the extreme converse of your statement.

 

[Besoeker]

bigger caps on the rectifier section WILL improve input power factor

Care to explain how that works?
A quantitative analysis would be preferred.
Assume, say, a 75kW 400V 3-ph drive with 10,000uF or 100,000uF.
By how much would the input power factor change?

 

[Ingenieur]

Care to explain how that works?
A quantitative analysis would be preferred.
Assume, say, a 75kW 400V 3-ph drive with 10,000uF or 100,000uF.
By how much would the input power factor change?

No
it is obvious
not enough info anyways
fla and existing pf

you mean like when I asked you for the math to back up your data?
you offered a spreadsheet
I prefer an mathematical explanation

 

[Ingenieur]

http://www04.abb.com/GLOBAL/seitp/seitp202.nsf/viewunid/57D1C4FC9FB087F885256D420065AF11/$file/Power+Factor+Improved+Via+AC+Drives+-+Release.pdf

 

[Besoeker]

No
it is obvious

Then you should have no difficulty in explaining it.

 

[Ingenieur]

Then you should have no difficulty in explaining it.

Like you did? Lol
it is not kosher cutting snippets from a post
especially leaving out the questions you can't answer
the link above explains it

it has explained numerous times in this (and other threads)
and in many links

EE 101

 

[Ingenieur]

A variable frequency drive (VFD) will improve the system power factor to which it is connected, primarily because the motor reactive current is supplied by the DC bus, rather than the supply system. This will only improve the displacement power factor. Modern VFDs with AFE draw almost sinusoidal current from the supply, so power factor on the source side can be controlled up to unity and the generated harmonics are minimal so the system distortion power factor is not greatly affected either. The sole purpose of the DC bus capacitors (in a VSI drive) is to supply the motor load reactive power, not to improve the distribution system power factor, although this is a beneficial consequence.

http://www.gohz.com/how-vfd-improve-the-system-power-factor

 

[Besoeker]

Like you did? Lol

I did. All the numbers and calculations are there. It is not raw data as you claimed.
And my offer, that you chose not to accept, still stands.
Now, care to substiantiate your power factor improvement claim?

 

[Ingenieur]

I did. All the numbers and calculations are there. It is not raw data as you claimed.
And my offer, that you chose not to accept, still stands.
Now, care to substiantiate your power factor improvement claim?

no they are not
and what is there does not support your assertion
in fact, just the opposite

read the 2 papers
and about 20 posts by others
it is obvious
you are over-thinking this
EE 101

 

[Besoeker]

no they are not
and what is there does not support your assertion
in fact, just the opposite

read the 2 papers
and about 20 posts by others
it is obvious
you are over-thinking this
EE 101

Do you want to accept my offer or not?

 

[Ingenieur]

Do you want to accept my offer or not?

it has been answered
caps improve the system pf
bigger will improve it more, but no need since they are sized for the load

 

[Ingenieur]

Do you want to accept my offer or not?

A variable frequency drive (VFD) will improve the system power factor to which it is connected, primarily because the motor reactive current is supplied by the DC bus, rather than the supply system. This will only improve the displacement power factor. Modern VFDs with AFE draw almost sinusoidal current from the supply, so power factor on the source side can be controlled up to unity and the generated harmonics are minimal so the system distortion power factor is not greatly affected either. The sole purpose of the DC bus capacitors (in a VSI drive) is to supply the motor load reactive power, not to improve the distribution system power factor, although this is a beneficial consequence.

http://www.gohz.com/how-vfd-improve-the-system-power-factor

lol

 

[Ingenieur]

Do you want to accept my offer or not?

read and learn

http://www04.abb.com/GLOBAL/seitp/seitp202.nsf/viewunid/57D1C4FC9FB087F885256D420065AF11/$file/Power+Factor+Improved+Via+AC+Drives+-+Release.pdf

 

[mivey]

bigger caps on the rectifier section WILL improve input power factor

bigger will improve it more, but no need since they are sized for the load

Where does it say bigger is better in these articles? Please keep in mind I'm no VFD design expert so be precise please.

http://www04.abb.com/GLOBAL/seitp/seitp202.nsf/viewunid/57D1C4FC9FB087F885256D420065AF11/$file/Power+Factor+Improved+Via+AC+Drives+-+Release.pdf

http://www.gohz.com/how-vfd-improve-the-system-power-factor

 

[Besoeker]

it has been answered
caps improve the system pf
bigger will improve it more, but no need since they are sized for the load

I admire your tenacity but not your accuracy.

 

[Smart $]

Where does it say bigger is better in these articles? Please keep in mind I'm no VFD design expert so be precise please.

I'm certainly no design expert either, but I believe the theory is the bigger the DC bus filter cap, the better suited it is to handle the reactive current introduced by the motor... through the PWM stage. Consider that even an ABE (active back end) must get the current to counter the reactive nature of the motor from somewhere. Otherwise that current must supplied by the input as there is no other local power supply or storage device... or at least none of sufficient capacity.


So what would you use for motor PFC? No different here except the rectifier stage prevents overcorrection.

 

[Besoeker]

So what would you use for motor PFC?

Nothing - if you are referring to a motordriven by a PWM VFD.

 

[Smart $]

Nothing - if you are referring to a motordriven by a PWM VFD.

I was referring to an across-the-line motor. No VFD