Power Factor verses Efficiency

[GoldDigger]

Power is the capacity to do work. In this regard, not only KW can do work but also KVA and even KVAR can do work under appropriate conditions. By KVA, KW and KVAR are all powers, I just mean that.

I am not arguing with that at all (except maybe for KVAR being able to do work).
I also was trying to make the distinction between the slight difference between KW and KVA which have the same dimensions and are intimately related, as compared to energy (foot-pounds) and torque (pound-feet) which have exactly the same units but are fundamentally different.

 

[Sahib]

I am not arguing with that at all (except maybe for KVAR being able to do work).
I also was trying to make the distinction between the slight difference between KW and KVA which have the same dimensions and are intimately related, as compared to energy (foot-pounds) and torque (pound-feet) which have exactly the same units but are fundamentally different.

I did not argue that, because two entities have the same units of measurement, they should be same. Apples and oranges have same units of measurement for weight, but they are not same. On the other hand, KVA, KW and KVAR are same in that they can be converted into one another: this follows from the principle of conservation of energy.

 

[GoldDigger]

On the other hand, KVA, KW and KVAR are same in that they can be converted into one another: this follows from the principle of conservation of energy.

Ah, there you have gone too far. I will agree that they can all be referred to loosely as power. And that those units integrated over time have the units of energy. But you will not be able to persuade me that they are forms of energy which are subject to Conservation of Energy. It will not happen.

 

[Besoeker]

I'll disagree with that unless you provide better information. Allow me to explain where I'm coming from.

The unit, symbol, and dimension for the three power units are:

volt ampere, VA, ML²T^-3
volt ampere reactive, var, ML²T^-3
watt, W, ML²T^-3

thus they all break down to the fundamental units of kg*m²*s^-3 given that you like metric and all.

Well, of course, if you multiply Volts and Amps you for either VA or Watts you get Volts times Amps in both cases.
But what this fundamentally misses is the time varying nature of the circuits where W, VA, and power factor are applicable.
You get one Watt when you have one Amp and one Volt flowing in the circuit at the same instant.
In an AC circuit, the values of I and V, thus W, vary over the period of the cycle. For the case of one Amp and one Volt the power is given by -1≤W≤1. The VA =1, regardless of the Watts. The VA is thus not a measure of power.

Timing.

On the use of SI units, well there are, as far as I'm aware, no imperial units for Amps or Volts.

 

[Besoeker]

First please come to terms with KVA, KW and KVAR as all powers.

Power is measured in Watts or multiples thereof.
Joules/second. Or Nm/s.
Neither kVA nor kVAr can be expressed thus. Neither is a measure of power. Please try to come to terms the fundamentals.

Here's a simple question. Neglecting losses, how many Watts does a 20kVAr consume? How much energy in kWh does it use in say, one hour?

 

[Sahib]

you will not be able to persuade me that they are forms of energy which are subject to Conservation of Energy. It will not happen.

You mean KVA, KW and KVAR can not be converted to one another?

 

[Besoeker]

You mean KVA, KW and KVAR can not be converted to one another?

OK. Convert 20kVAr into kW.

 

[Sahib]

OK. Convert 20kVAr into kW.

Hmm..
Only a fraction of 20kVar is stored as energy in the magnetic/electric field of inductor/ capacitor and reappears as kW when required. What happens to the rest of 20kVAr?

 

[Besoeker]

Hmm..
Only a fraction of 20kVar is stored as energy in the magnetic/electric field of inductor/ capacitor and reappears as kW when required. What happens to the rest of 20kVAr?

No conversion, eh?

TBH, I'm out. Post #2 provided a succinct and correct answer for the OP.

 

[Sahib]

No conversion, eh?

Partial conversion. I do not know the why of it.

I'm out.

No problem. Other members may figure it out.

 

[kwired]

Partial conversion. I do not know the why of it.



No problem. Other members may figure it out.

Exactly why it is called "apparent power" There is voltage and current but it is doing no work. The only energy consumed is because of line losses - but those losses are real power.

 

[Sahib]

Exactly why it is called "apparent power" There is voltage and current but it is doing no work. The only energy consumed is because of line losses - but those losses are real power.

But KVA is the power limit without exceeding specified losses. isn't it?

 

[rbalex]

I’m fundamentally in Besorker’s camp, but I appreciate mivey bringing up dimensional analysis. In the mass-length-time relationship he introduced, the unit vectors were left out; that is, the lengths introduce “direction”. In other words, we are also dealing with the dot product of the two “lengths”.

• Where the unit vectors are coincident V-A yields Watts.
• Where they are perpendicular they yield VArs.
• Essentially VA is a handy construct derived from multiplying only the numerical values of Volts and Amperes.
• It is only coincidental that Volts [Joules/Coulomb] x Amperes [Coulomb/sec] yields a numerical value consistent with power. [Joules/sec]

The original issue compared efficiency with power-factor.

• Efficiency is an energy, not power, ratio. [Energy-out/energy-in] Power is energy per unit time. Only since unit time is usually the same for both the numerator and denominator in power-out/power-in does it yield numerical efficiency.
• Power factor is the ratio of the numerical values of Watts/V-A.

We manipulate the values so casually we often forget what they really are.

 

[weressl]

No. It persists at least for the maximum demand period and so has the corresponding time element and so the utility has to 'rate' the efficiency of its usage by the consumer with the power factor.

One word, my dear chap: hogwash.

 

[ggunn]

One word, my dear chap: hogwash.

Perhaps his hog was dirty?

 

[GoldDigger]

[*]It is only coincidental that Volts [Joules/Coulomb] x Amperes [Coulomb/sec] yields a numerical value consistent with power. [Joules/sec]

Quite to the contrary, it is absolutely fundamental physics.
The fact that Amperes are defined as Coulombs per second tells us the number of charged particles per second that pass a given point or traverse a given path.
The Volt, as a measure of Joules per Coulomb tells us exactly how much work is done on a set of charged particles (independent of their mass or speed) that moves from one voltage (potential) to another. The product is the amount of work done per second. That is power. No two ways about it.

To say otherwise would be like arguing that multiplying a weight (force) times a distance traveled only coincidentally comes out to be a measure of work done (energy transferred.)

 

[rbalex]

Quite to the contrary, it is absolutely fundamental physics.
The fact that Amperes are defined as Coulombs per second tells us the number of charged particles per second that pass a given point or traverse a given path.
The Volt, as a measure of Joules per Coulomb tells us exactly how much work is done on a set of charged particles (independent of their mass or speed) that moves from one voltage (potential) to another. The product is the amount of work done per second. That is power. No two ways about it.

To say otherwise would be like arguing that multiplying a weight (force) times a distance traveled only coincidentally comes out to be a measure of work done (energy transferred.)

Since only the Ampere, meter, second and Kilogram are base SI units, Watts, Volts, work, energy, etc. are ultimately derived units. There is no “fundamental” unit of work or energy either. As I said, we manipulate the terms conveniently, often pragmatically, and because it “works” we forget the essential nature.

 

[mivey]

Well, of course, if you multiply Volts and Amps you for either VA or Watts you get Volts times Amps in both cases.
But what this fundamentally misses is the time varying nature of the circuits where W, VA, and power factor are applicable.
You get one Watt when you have one Amp and one Volt flowing in the circuit at the same instant.
In an AC circuit, the values of I and V, thus W, vary over the period of the cycle. For the case of one Amp and one Volt the power is given by -1≤W≤1. The VA =1, regardless of the Watts. The VA is thus not a measure of power.

Timing.

Yes, timing is important. The average of the instantaneous values of VA taken over one cycle gives you the watts so you need to distinguish between instantaneous values and average values. Easy enough to mix the two if not paying attention to the time functions, complex functions, etc.

 

[mivey]

the unit vectors were left out; that is, the lengths introduce “direction”. In other words, we are also dealing with the dot product of the two “lengths”.
...
We manipulate the values so casually we often forget what they really are.

That is a good point. One reason behind the apparent value is to explain why the same watt consumption was heating up equipment more with AC than DC in some cases. The real and reactive power relationship was then described.

The voltage was causing iron losses and the current was causing conductor losses so the apparent power is used to determine equipment capacity. In fact, early meters used both a voltage heating element and a current heating element to get an approximate VA measurement. Mechanical methods were also used like an aluminum ball being rotated by current and voltage disks and the ball, in turn, rotated a VA disk. Modern electronic meters use methods like I described in post #35.

 

[mivey]

OK. Convert 20kVAr into kW.

I can convert that quantity in 1/2 cycle.