# Please explain volt-ampere and power factor



## Sliver (Mar 5, 2009)

Volt-amps is vector sum of watts and VARs, to add them use Pythagorean Therom: A^2+B^2=C^2 a=watts b=vars c=VA. Watts VARs and VA are all units of power, Watts is resistive power, VARs are reactive power, VA is apparent power. Coils and capacitors a reactive loads, and current will either lag voltage (coils) or lead voltage (caps). 

Power Factor = Watts/VA. Power Companies will fine businesses if the power factor is less than .9. Its used mainly by the POCO to see if they are supplying power that they don't need to be.

To correct power factor on a motor circuit you would hook up capacitors to supply the VARs that the motor needs for its coils, this works because as the capacitor is charging the coil is discharging.

http://en.wikipedia.org/wiki/Power_Factor will probably explain every thing better.


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## telecom_guy (Mar 26, 2009)

So in the simple case of a generator of say 34 kw and 42 kva:

if you had purely inductive loads on it, you would be most concerned about the kva rating to make sure you get all the potential kw out of it, right?

As far a power factor goes, it looks like the power company would have to do an assessment of the facility to gather the required info it would need to come up with the power factor, right? Has anyone out there had to deal with a situation of power factor correction? Please explain how it went.


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## Zog (Apr 15, 2009)

telecom_guy said:


> So in the simple case of a generator of say 34 kw and 42 kva:
> 
> if you had purely inductive loads on it, you would be most concerned about the kva rating to make sure you get all the potential kw out of it, right?
> 
> As far a power factor goes, it looks like the power company would have to do an assessment of the facility to gather the required info it would need to come up with the power factor, right? Has anyone out there had to deal with a situation of power factor correction? Please explain how it went.


Not sure what you mean by purley inductive loads, there is no such thing. The kW rating of the generator is based on the size of the prime mover and the kVA rating rating is simply a function of the voltage and current ratings of the equipment. 

A power company always knows the pf at a facility. They hvae meters for Voltage, current, and watts. pf=(watts/voltamps). I have done many pf correction projects it is pretty simple stuff, did you have a specific question about pf correction.


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## telecom_guy (Mar 26, 2009)

No, I dont have a specific question, I am just trying to understand it and I thought it would help to see some examples of problems and corrections. 

As to the "purely inductive load", I meant (and am probly misusing the term) a motor load. I am wondering if you have loads such as motors if the va is more than say a heating element. Am I understanding correctly that certain loads operate with more "lag" than others (such as motors) and others are equally balanced (like a heating element?), while still others operate with a "lead" (such as capacitors)?

If this is so, the said generator would have more than enough potential kva to operate an equally balanced load while it may not be the case with a load with a lot of "lag".

As you can see, I still don't quite get it, sorry!


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## Zog (Apr 15, 2009)

There are three ways of expressing power in an AC circuit:
· Real power (P) expressed in watts (W) is the power that is required to perform work. 
· Apparent power (S) expressed in volt-amperes (VA) is the total power flowing is the circuit. Transformers are rated in their apparent power expression using units in kVA (kilovolt-amperes), as this value is proportional to the total heating in the windings
· Reactive power (Q) expressed in volt-amperes reactive (VAR) is the power which does no useful work but is required to assist in performing work, such as setting up magnetic fields in motors and transformers. It is often called “imaginary power.”

These three types of power -- true, reactive, and apparent -- relate to one another in trigonometric form. We call this the _power triangle_:


The angle of this "power triangle" graphically indicates the ratio between the amount of dissipated (or _consumed_) power and the amount of absorbed/returned power. It also happens to be the same angle as that of the circuit's impedance in polar form. 

Using the laws of trigonometry, we can solve for the length of any side (amount of any type of power), given the lengths of the other two sides, or the length of one side and an angle.

When expressed as a fraction, the ratio between true power and apparent power is called the power factor. Because true power and apparent power form the adjacent and hypotenuse sides of a right triangle, respectively, the power factor ratio is also equal to the cosine of that phase angle. 

It should be noted that power factor, like all ratio measurements, is a unitless quantity.For the purely resistive circuit, the power factor is 1 (perfect), because the reactive power equals zero. Here, the power triangle would look like a horizontal line, because the opposite (reactive power) side would have zero length. For the purely inductive circuit, the power factor is zero, because true power equals zero. Here, the power triangle would look like a vertical line, because the adjacent (true power) side would have zero length. The same could be said for a purely capacitive circuit. If there are no dissipative (resistive) components in the circuit, then the true power must be equal to zero, making any power in the circuit purely reactive. The power triangle for a purely capacitive circuit would again be a vertical line (pointing down instead of up as it was for the purely inductive circuit). 

Power factor can be an important aspect to consider in an AC circuit, because any power factor less than 1 means that the circuit's wiring has to carry more current than what would be necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load. The poor power factor makes for an inefficient power delivery system. 

Poor power factor can be corrected, paradoxically, by adding another load to the circuit drawing an equal and opposite amount of reactive power, to cancel out the effects of the load's inductive reactance. This can be done at individual loads or with a master fixed or adjustable bank for the entire system.


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## GIXXERDC (Mar 1, 2009)

Or true power divided by apparent power is your power factor in basic terms (what you use to what it says your use)


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## nick (Feb 14, 2008)

telecom_guy said:


> I have yet to grasp the understanding of volt-amperes and power factor. Can anyone explain these in simple terms and provide a couple basic examples? What is it, how does it work, where do you find it, where do you use it, ect? Also, how do you correct power factor?
> 
> Thanks!


Well simple PF is difference between wattage input to circuit to wattage ouput or used . i know you have heard the words true power and apparent power the expression ?

Take a motor the nameplate tells you rated FLA amps on plate run that motor and it will be a different reading meaning theres a loss in input to output by the nature of the motor inductance and magnetic effects internal to motor . Theres a loss to get that motor to spin kinda heat , inductance , resistance ,magnetic inter reaction. 

Wattmeter measures real watts but does not measure ac reactance the loss part of circuit but used part of circuit meaning the part that makes heat by induction or its effects internal to that motor .

someone stated there is no pure inductance thats is true a motor is a inductance but has resistance ,inductance ,capacitance , which makes power factor a nitemare . so a large number of motors becomes a power factor nitemare . 

Which gets corrected with capacitance added back into circuit but it must be tuned to that circuit by formulas and testing .Also power factor correction is not set or fixed one time like these energy saver devices !! it must be able to adjust to motor load as load changes on and off . 

A tuned circuit meaning just enough capacitance added to bring the current flowing in circuit kinda close in phase with the voltage in circuit the sine wave part . Thats when they say power factor equals unit of one ,thats never going to happen meaning get to zero it will always be off from zero on a power factor meter . look at pf as leading or lagging on meter if its neg its inductive if its positive its capacitive . Take care be safe


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## telecom_guy (Mar 26, 2009)

Sliver said:


> Volt-amps is vector sum of watts and VARs, to add them use Pythagorean Therom: A^2+B^2=C^2 a=watts b=vars c=VA. Watts VARs and VA are all units of power, Watts is resistive power, VARs are reactive power, VA is apparent power.quote]
> 
> 
> What is reactive power measured in and how do you get the measurement? Thanks for everyones input, keep it coming!


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## Zog (Apr 15, 2009)

nick said:


> Well simple PF is difference between wattage input to circuit to wattage ouput or used .


No, that is efficiency, has nothing to do with Pf.


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## Zog (Apr 15, 2009)

telecom_guy said:


> What is reactive power measured in and how do you get the measurement? Thanks for everyones input, keep it coming!


It is measured in VAR's, it cannot really be measured only calulated, like resistance.


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## nick (Feb 14, 2008)

Well power factor = ratio of true power kw /divide by apparent power kva is that better Zog. thats kinda what we meant yes we agree watt in/watts out = effc. glad you pointed that out . how was everything else ? take care


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## Zog (Apr 15, 2009)

nick said:


> Well power factor = ratio of true power kw /divide by apparent power kva is that better Zog. thats kinda what we meant yes we agree watt in/watts out = effc. glad you pointed that out . how was everything else ? take care


 Besides PF correction not being able to be fixed, it was prett good. Fixed PF correction is more common and easier than variable systems. You can eiher use a large bank in the main sub (Works best when loads dont vary much like you said) or fixed smaller caps at the individual loads or groups of loads, they ar energized when the motor is energized and vice-versa.


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## nick (Feb 14, 2008)

Zog said:


> Besides PF correction not being able to be fixed, it was prett good. Fixed PF correction is more common and easier than variable systems. You can eiher use a large bank in the main sub (Works best when loads dont vary much like you said) or fixed smaller caps at the individual loads or groups of loads, they ar energized when the motor is energized and vice-versa.



Well yes Zog we kinda seen a few capacitor banks in our time and also voltage regulators with the banks but we kinda install not design this stuff just mostly new projects . take care


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## jfwfmt (Jul 5, 2008)

*by phase shifting*



telecom_guy said:


> Sliver said:
> 
> 
> > Volt-amps is vector sum of watts and VARs, to add them use Pythagorean Therom: A^2+B^2=C^2 a=watts b=vars c=VA. Watts VARs and VA are all units of power, Watts is resistive power, VARs are reactive power, VA is apparent power.quote]
> ...


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## telecom_guy (Mar 26, 2009)

what is the significance of rating a transformer or a generator in kva and kw instead of only kw?

When sizing a service, why figure lighting loads and receptacle loads, ect, in va rather than watts?


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## Zog (Apr 15, 2009)

telecom_guy said:


> what is the significance of rating a transformer or a generator in kva and kw instead of only kw?
> 
> When sizing a service, why figure lighting loads and receptacle loads, ect, in va rather than watts?


Transformers are rated in VA, Generators are rated in watts and sometimes they add VA. The reactive load does no real work so it isnt seen as a load on the generqators prime mover, but there is current flow there for the VAR's so it needs to be accounted for your ampacity ratings. 

Think of VAR's as a superball bouncing up and down. there is energy required to put it into motion (Seen as an inrush) but once it is motion it keeps bouncing with no additional energy input needed. this is the current flow going back and forth between feilds, as the filed collapses when the direction of AC reverses the collapsing feild provides the energy to cause the current to flow the other way, expand the other feild, and this keeps happening over and over.


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## telecom_guy (Mar 26, 2009)

Sliver said:


> Volt-amps is vector sum of watts and VARs, to add them use Pythagorean Therom: A^2+B^2=C^2 a=watts b=vars c=VA.


so if a watt is a measure of the work being done and VAR is a measure of the "imaginary power" also required to do the work even though not actually doing any "work", and the vector sum of both is VA, then VA is a measurement of the absolute all inclusive power on the circuit? Which would also mean that the measurement of watts does no account for all the power on the circuit, depending on the load? And lastly, a VARs measurement must not be very much is it. Can anyone provide an example of a VARs measurement, and maybe even a full example of a real life watts, VARs, VA formula in action?


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