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

[Smart $]

'Ideal' here refers to active power input and output of black box proper and not that of its associated power lines. In that case input power and output power equal but power factors need not be as no power loss in the lossless black box as a result.

:thumbsup:

 

[Jraef]

So... after all of this happy conflagration and side tracking, have we reached a consensus that we CAN indeed estimate the Single Phase input current of a VFD that is powering a Three Phase motor? I think so.

I think, after reading all of this and not being swayed by the "it's not something you can generalize" arguments, that the original premise, the 1.732 ratio, is in fact incorrect, not just for sizing the VFD as I had postulated, but also for estimating the input current. It is in fact going to be close enough to 2.0 for me to say that this is the number that should be used by electricians when sizing OCPDs and conductors if the ACTUAL data from the VFD mfr is not available. I can back this up empirically as well, by virtue of having looked up several well known brands of VFDs with readily available technical data sheets that do in fact confirm this relationship (for the most part, as I said, close enough) in their published material. The "1.732" number is indeed thrown around very loosely in websites, but I see that as a "layman's term" that I believe, from the evidence, it is simply because of the relationship factor of single phase to 3 phase, so many people leave it at that, not understanding that it is wrong on all accounts. But as has been pointed out here it is not that simple because of the complexities of the way the rectifier functions. I now accept that we cannot make assumptions about what the INPUT power factor will be when feeding a VFD with single phase power (something I had not properly contemplated prior to this exercise), and that has a direct effect on the input current.

So circling back to the myth, if you look at a 2HP single phase motor and a 2HP 3 phase identical motor, the current for those motors would differ by a factor of 1.732, but that relationship cannot be applied to the difference in input current to a VFD. In my humble opinion, I therefore believe it MORE correct to assume the ratio is 2:1, and let us all lay the 1.732 thing to rest as "accurate in a way that doesn't apply to this PARTICULAR set of circumstances".

 

[mike_kilroy]

I now accept that we cannot make assumptions about what the INPUT power factor will be when feeding a VFD with single phase power (something I had not properly contemplated prior to this exercise), and that has a direct effect on the input current.

Close, but not limited to 1ph... identically true for 3ph inputs.

See Schneider ele input current examples of identical load on 3 different input impedance sources.

Post #210


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[Sahib]

I think the VFD may improve power factor as the capacitor in it may compensate the lagging power factor of motor load. Line current may be lower than load current.........

 

[mivey]

So... {etc.}

Sounds reasonable to me for the stated purpose.

 

[Besoeker]

So circling back to the myth, if you look at a 2HP single phase motor and a 2HP 3 phase identical motor

Then they are already not identical...........

 

[Besoeker]

I think the VFD may improve power factor as the capacitor in it may compensate the lagging power factor of motor load. Line current may be lower than load current.........

It usually is but that isn't because of the bucket capacitor bank.

 

[Sahib]

It usually is but that isn't because of the bucket capacitor bank.

Bucket capacitor bank?

 

[Besoeker]

Bucket capacitor bank?

Most common or garden VFDs have the same format.
Input plain rectifier, DC filter, and output IGBT PWM inverter.
The DC filter comprises a (bucket) bank of electrolytic capacitors and sometimes a series choke.

 

[Sahib]

I can't see any thing that prevents the bucket capacitor of VFD from also compensating the lagging power factor of motor load on output side.

 

[Besoeker]

I can't see any thing that prevents the bucket capacitor of VFD from also compensating the lagging power factor of motor load on output side.

It smooths the DC and provides a low impedance source for the IGBT PWM inverter. That's it's purpose in life.

 

[Sahib]

It smooths the DC. That's it's purpose in life.

I think it does more. That is easy to verify by checking load current and line current while VFD in operation. Line current would be lower I believe.

 

[Besoeker]

I think it does more. That is easy to verify by checking load current and line current while VFD in operation. Line current would be lower I believe.

OK. Check it.

 

[Smart $]

It usually is but that isn't because of the bucket capacitor bank.

So... what is it because of? :huh:

 

[Besoeker]

So... what is it because of? :huh:

An uncontrolled input rectifier.
It really is that simple.

 

[gar]

170614-0845 EDT

Sahib:

A half wave, full wave, or N wave rectifier feeding a capacitor input filter where the output loading on the capacitor (energy consuming, or resistive component) is such that the time constant of the RC circuit is long compared to the pulse input rate (in other words very little voltage ripple across the capacitor) means the rectifier only draws current pulses for a very short time at the source voltage peaks. These current spikes being at the voltage peaks and short in duration (in phase with voltage) really don't provide any useful compensation for linear inductive load current, but create their own, and possibly worse, havoc on the supply system.

To the extent that these pulses have an effect on the power factor of the fundamental frequency linear components it is because these pulses look like added load resistance at the fundamental frequency.

.

 

[Smart $]

An uncontrolled input rectifier.
It really is that simple.

170614-0845 EDT

Sahib:

A half wave, full wave, or N wave rectifier feeding a capacitor input filter where the output loading on the capacitor (energy consuming, or resistive component) is such that the time constant of the RC circuit is long compared to the pulse input rate (in other words very little voltage ripple across the capacitor) means the rectifier only draws current pulses for a very short time at the source voltage peaks. These current spikes being at the voltage peaks and short in duration (in phase with voltage) really don't provide any useful compensation for linear inductive load current, but create their own, and possibly worse, havoc on the supply system.

To the extent that these pulses have an effect on the power factor of the fundamental frequency linear components it is because these pulses look like added load resistance at the fundamental frequency.

.

Simply amazing. Bes' chooses to use as few words as possible and disseminates no useful knowledge, while you obscure the gist of the matter with technical jargon. What you just stated amounts to power factor correction, inadvertent as it may seem.

 

[gar]

170614-0858 EDT

Smart $:

Displacement power factor that is created by a linear inductive or capacitive load is not normally considered to be corrected by adding a lot of power wasting resistive load. However, added resistive loading does somewhat improve power factor. But that is only because of the increased real power load in the numerator of the PF eqauation.

To correct reactive power factor one has to use the opposite type of reactive component.

.

 

[Besoeker]

Simply amazing. Bes' chooses to use as few words as possible and disseminates no useful knowledge

I answered your question without unnecessary verbosity.
What else did you want?

 

[Smart $]

I answered your question without unnecessary verbosity.
What else did you want?

Implied within the sentence quoted.