Power Factor - Arc Furnace

[ZL1Killa]

The basic question I have is : Can the power factor be different from the primary side to the secondary side of a circuit? Read it as simply that and answer.

The circuit that we are trying to understand is our Electric Arc furnace. We have the installed PRIMARY side power factor monitoring (which has shown us a ~0.77 ~ 0.8 power factor). We then have a SECONDARY power factor monitor that we have installed, and we are 100% certain is configured correctly, our secondary power factor is 0.98 to unity.

The circuit from our 1MVA transformer consists of the following in order: (I have a single line diagram showing the circuit I can email anyone)
Duke Energy 1MVA Xfrmr to our MCC
Then to our power factor gauge, which leads then to the 125kVAR reactor (inductance) and then to a 250kVA transformer
Then to our Secondary power factor monitor and out to the electrode arms (2). This is a single phase Arc furnace FYI.
I need some help with power factor, and explaining this concept to non-electrical people. I completely understand real & imaginary numbers, along with how power factor is calculated, however, others do not believe me.
If you could advise, or possibly provide someone who could advise, I would greatly appreciate any help. Or if my calculations (examples) are correct, a simple reply would be enough.


I have a document (PDF) that I have shown all my calculations & information on the circuit, however I have not made believers out of me.

 

[gar]

110525-2012 EDT

Consider the following circuit:

A sine wave AC voltage source, and a series load consisting of a resistance and inductor.

If you measure the voltage across the resistor, and compare the phase relationship with the the current thru the resistor, then what is your result? And what is the power factor using these measurements?

Next do the same on the input to the resistor and inductor in series. What is the result?

I might have thought the arc would have been worse than your high reading, which it might have been if it was mostly a pure arc. It appears it is very close to being just a resistance.

.

 

[philly]

The basic question I have is : Can the power factor be different from the primary side to the secondary side of a circuit? Read it as simply that and answer.

The circuit that we are trying to understand is our Electric Arc furnace. We have the installed PRIMARY side power factor monitoring (which has shown us a ~0.77 ~ 0.8 power factor). We then have a SECONDARY power factor monitor that we have installed, and we are 100% certain is configured correctly, our secondary power factor is 0.98 to unity.

The circuit from our 1MVA transformer consists of the following in order: (I have a single line diagram showing the circuit I can email anyone)
Duke Energy 1MVA Xfrmr to our MCC
Then to our power factor gauge, which leads then to the 125kVAR reactor (inductance) and then to a 250kVA transformer
Then to our Secondary power factor monitor and out to the electrode arms (2). This is a single phase Arc furnace FYI.
I need some help with power factor, and explaining this concept to non-electrical people. I completely understand real & imaginary numbers, along with how power factor is calculated, however, others do not believe me.
If you could advise, or possibly provide someone who could advise, I would greatly appreciate any help. Or if my calculations (examples) are correct, a simple reply would be enough.


I have a document (PDF) that I have shown all my calculations & information on the circuit, however I have not made believers out of me.

I would think that you can se a different p.f. on the primary and seconary. The transformer will have a magnetizing current that will be 90deg out of phase with the source. This magnitizing current is in parallel to the secondary of the transformer so if you have a purely resistive load on the secondary and then it it possible to measure current in phase there however on the primary of the transformer you will be measuring some of the out of phase magnetizing current and thus have a lower p.f.

 

[philly]

110525-2012 EDT

Consider the following circuit:

A sine wave AC voltage source, and a series load consisting of a resistance and inductor.

If you measure the voltage across the resistor, and compare the phase relationship with the the current thru the resistor, then what is your result? And what is the power factor using these measurements?


Next do the same on the input to the resistor and inductor in series. What is the result?

I might have thought the arc would have been worse than your high reading, which it might have been if it was mostly a pure arc. It appears it is very close to being just a resistance.

.

In your example if you assume the resistor is 1ohm@0deg and the inductor is 1ohm@90deg then the series current flowing through the circuit is .71A @-45deg. So if you measure the voltage and current across the series combination assuming the voltage source is 1V @0deg you will see the current lagging the voltage by 45 degrees and thus measure a power factor of .7.

However if you measure the voltage across only the resistor you will have a voltage of (.71A@-45deg)(1ohm@0deg)=.71V@-45deg. So you can see that when looking only across the resistor the measured voltage and current are both at and angle of -45deg and therefore appear in phase with a unity power factor.

So using this to look at the OP's question it appears that he is only measuring voltage and current across the resistive arc on the secondary and thus sees close to a unity power factor.

You can probably forget my bit about about the parallel magnitizing current for it will probably only confuse the issue here.

 

[ZL1Killa]

the secondary measuring point is located on the output to the electrode arms. Yes, mostly resistive to ground, so therefore unity power factor.

My point or answer I'm looking for (that I hopefully in my experience already know) is that the primary and secondary sides of a transformer are only joined by the induced voltage, no physical wire connection. So in essence the two sides of the transformer are two separate circuits.

the primary metering of power factor is connected prior to the inductor. And as Duke also has their metering, they also see ~0.7 power factor on the primary.

I agree with the statements you have made. the confusing part that I'm not understanding is that the transformer simply based on turns ratio primary to secondary induces a voltage to the secondary, there is no angle difference at that point, the secondary load will determine that based on the circuit.

 

[gar]

110526-1512 EDT

ZL1Killa:

There are ideal transformers (none exist), and there are various real world transformers.

Ideal transformers behave based on the turns ratio, and zero R, L, and C.

A real world transformer can be made close to an ideal one for certain operating conditions, or it can be designed to have characteristics quite different than an ideal unit.

For power frequencies and practical materials a typical transformer will have a small amount of leakage flux that does not couple the primary to the secondary. This in turn creates what is called leakage inductance and this is an inductance in an equivalent circuit that is in series with the load current.

The reason I suggested that you analyze a series R, L load and what the power factor was at different points is because you can create an equivalent for your arc system that looks something like that.

There are books that discuss transformers and equivalent circuits for them.

.