What is the difference between Va and watts.

[goodpickles]

everybody says that it is different, but nobody can tell me why, or how to convert between Volt Amps and watts.

 

[evandude]

I'm a little rusty so someone correct me if i'm wrong...

Watts represents "real" power, ie - power dissipated in resistors, etc. There is also VARs (Volt Amps Reactive) which represents the reactive power, ie the power that is not dissipated, merely thrown back and forth by reactive elements (inductors/capacitors)

VA's represents the combination of the two. if you look at it in terms of complex numbers and vectors, Real power is a vector on the real axis, Reactive power is a vector on the imaginary axis, and the sum of the two is what is represented by VA's.

An example I heard is that power companies try to use capacitors to alter the impedance of buildings they supply power to. this is to get the building to "look" as much like a resistive load as possible, thus reducing the reactive component of the power, and making the total power as close to the real power as possible; this is because in effect the cost of supplying the customer with electricity is proportional to the TOTAL power (VA's) while the customer really only pays for the REAL component of the power they get (Watts) so they want to make the total power as close to the real power as possible to save themselves money.

 

[checkmate]

The major users of electricity, ie the industries, are charged according to the VA usage. Only domestic and commercial electricity is billed accordin to real power. Hence it is the industries that try to alter the power supply. The process is called power factor correction. However, implementing power factor correction may affect the entire grid, and the process is usually regulated by the power suppliers.

 

[samcheetah]

VA is the apparent power that is being consumed. it is represented by the letter "S". Watts is the real power that is "actually" being consumed. it is represented by "P". VAR (Volt Ampere Reactive) is the imaginary power thats is being used by the reactive components of the system. this power isnt consumed at all. it is represented by "Q".

and S = P + iQ in the complex number system

VA's represents the combination of the two. if you look at it in terms of complex numbers and vectors, Real power is a vector on the real axis, Reactive power is a vector on the imaginary axis, and the sum of the two is what is represented by VA's.

thats called the power triangle. its a graphical tool in the analysis of complex power calculations

An example I heard is that power companies try to use capacitors to alter the impedance of buildings they supply power to. this is to get the building to "look" as much like a resistive load as possible, thus reducing the reactive component of the power, and making the total power as close to the real power as possible; this is because in effect the cost of supplying the customer with electricity is proportional to the TOTAL power (VA's) while the customer really only pays for the REAL component of the power they get (Watts) so they want to make the total power as close to the real power as possible to save themselves money

this is called power factor correction.

one thing more u will never see a transformer rated in Watts. Why? because it doesnt dissipate power. it just transforms it. and u wont see a resistor rated in VAs or VARs because it is a resistive element. so it could be said that anything that "really" (this is the magical word) dissipates energy is rated in Watts. and anything that doesnt dissipate energy but has something to do with it, is rated in VAs. like transformers and UPS for instance.

 

[goodpickles]

One other related question. A friend told me that one of his neighbors back when he lived in...had a battery charger that actually made the power meter run in reverse! The power company found out and gave the man a new charger. The reason that it did this was that the power factor was so far off.

I understand that something with a lower power factor that 1 show as if it was taking less energy as it really does, but would it actually make the meter run in reverse?

 

[Nigel Goodwin]

One other related question. A friend told me that one of his neighbors back when he lived in...had a battery charger that actually made the power meter run in reverse! The power company found out and gave the man a new charger. The reason that it did this was that the power factor was so far off.

I understand that something with a lower power factor that 1 show as if it was taking less energy as it really does, but would it actually make the meter run in reverse?

It sounds very unlikely, there's not much in a battery charger to affect anything.

It sounds like one of the fictional 'urban legends' that go around, no one has ever heard of it first hand, it's always a 'friend of a friend'.

 

[goodpickles]

that is sort of what I thought...but since the name of the person that it happened to was with the story I thought that there must be something to it...but on the other hand the person that said it has a reputation of stretching his stories a little.

Well I will have to talk to the person first hand sometime.

 

[John Sorensen]

Here's something I learned recently (I just started a job doing substation design so I'm learning a lot of things): Power distribution lines are subject to a phenomenon called "voltage collapse." Following a fault, when the voltage on the line is trying to come back, there can be a case where if the line is not "strong" enough (higher impedence), then all motors coming back on line together present such a reactive load that it can make it impossible for the voltage to come back up, leaving the line at 50% or less of nominal for some time (or indefinately, until something trips) doing all kinds of damage. I guess this is especially a problem where there are a lot of air-conditioners and such, whose motor controllers don't have a low-voltage release. So, one of the stop-gap measures that utilities do is put capacitor banks out on the lines and at substations, to provide VAR compensation. Turns out that switching on and off the capacitor banks has troubles of its own, though.

j.

 

[samcheetah]

Following a fault, when the voltage on the line is trying to come back, there can be a case where if the line is not "strong" enough (higher impedence), then all motors coming back on line together present such a reactive load that it can make it impossible for the voltage to come back up, leaving the line at 50% or less of nominal for some time (or indefinately, until something trips) doing all kinds of damage. I guess this is especially a problem where there are a lot of air-conditioners and such, whose motor controllers don't have a low-voltage release

this happens all the time in my city during summer. the power company in my city usually gives a "load shedding" schedule every summer. actually they turn off power to certain parts of the city on a timely basis. thats because in about each house there is an air-conditioner and some homes even have 2 or 3 (it gets really hot here at times). so by shedding some of the load they increase the life time of their very old equpment that is deteriorating each day. and then they call some engineers to optimize the power grid who usually screw up and the load shedding goes on for quite a while. now every home is just waiting for the power to be available again. and add all the heat to the equation. now the moment the power becomes available everyone turns on their air conditioners, water motors, washing machines..... u name it. kaboom!!!!!!!!! the power goes out again. and then we have to wait 2-3 hours for someone to fix it again.

to eliminate this situation the engineers at the power station do use VAR compensation by adding capacitors but it doesnt help. thats because u cant predict the type of load. u dont know how many people are going to turn on their air conditioners. for industrial installations one can find out what compensation is required for power factor correction. but homes are very unpredictable.

 

[zevon8]

Informative thread, nice explanations. I have always had a problem with manufacturers who give VA ratings for a transformer without giving the maximum load current. Makes it hard to determine exactly how much loss there is due to eddy currents, etc, and how much power is going towards nothing more than heat. Some makers use better materials and more efficient designs, with others it's suprising just how bad the transformer really is under full load.

Here's a link you may find interesting, ( especially John Sorensen ) speaking of power factor correction on the utility grid.

**broken link removed**

It shows has some pictures and video clip of large Jacob ladders created by just the line reactor dumping on a power transmission breaker at a substation. There is some other neat high voltage stuff too.

 

[efren sabio]

Yes, I used to wonder about this VA rating on power transformer.
Then somebody gave me a big transformer which says, '400VA, 40V,
5A'. The secondary has 3 terminals, the center one being common which I tested and it turned out to be 40V to each side.
Now I have a power amp stereo each 200W.