Sorry Anonymous, but your friend is right... VA remains the same. Your calculation is flawed, that is, you have erroneously multiplied the 12-V supply by the 220-V current. The 12-V current should have been 50/12=4.25 A! Thus, 12x4.25 = 50VA.

Regards, Phil Corso, PE ([email protected])

 

50 watts is 50 watts. That is the amount of power that is being consumed. A transformer changes voltage and current, not power (other than you lose a small amount of power due to losses in the transformer). So:

50VA at 12V draws 50VA/12V = 4.17 amps (at 12 volts)

50VA at 220V draws 50VA/220V = 0.23 amps (at 220 volts)

Note though that a lot of household compact fluorescent bulbs (I am not speaking of halogen bulbs here) will show two "power" ratings. One is the rating for how much power it actually consumes, and the other is the power consumption of an equivalent incandescent bulb. A compact fluorescent bulb that is approximately equivalent in visible light output to a 60W incandescent bulb will actually consume 13 or 14 watts. The larger number (60W in this example) has nothing to do with actual power consumption though. It is simply present as a guide to help you to select the correct bulb size when replacing an existing incandescent bulb.

 

Why is it that people say transformers are rated in VA because they deal with voltage and ampere? All electrical devices deal with voltage and ampere (I thought everyone knew that).

Also why do some people say that VA is independent of the power factor? VA, the apparent power (modulus of the complex power), has a direct relationship with the (real) power and power factor (p.f).

Apparent power = S = ã3*V*I
Power factor = p.f = cosĮ

Real power = P = ã3*V*I*cosƒÆ

Replacing ã3*V*I in the Real power equation with S and cosƒÆ with p.f.

Real power = P = S*p.f
There's the relationship between the two. I'm not sure how you can talk about apparent power without power factor.

P.S. There's nothing with using VAR instead of VAr when referring to reactive power.

 

Because it is taken from this role p=VIcosQ

 

Thank you, that may be more informatiom than I need.

I have a Transformer 250KVA & wanted to know the Max current, or the real watts. I believe it's 1:2 ratio. With 120Vac applied to H1.3 & H2.4 windings in parallel. Secondaries X1, X2, X3, X4 in series.
110 Vac output + or -5 vac. I have not loaded it up as yet.

If you have time to maybe send me the math formula or example.

In the project, I am feeding this transformer with a Auto Trans. of 4.2 Amp.

Thank You from Larry
AgonyAt48 @ aol. com

 

>Do you find that it's common practice (in your industries) to express reactive power in VA instead of VAR? <
---- snip ----

I am totally agreed with u Nathan.
Reactive power is expressed by VAR and active power is expressed by watt.

 

the details are given here

http://electrical-engineering-tutor...7/rating-of-transformers-why-kva-and-not.html

 

It is very Interesting Question & Answer has been replied in many ways many times.

First of all your question completes as follows:
" Why all Loads are usually mentioned in Hp or KW and Why all Power Sources are Labelled as KVA"

Suppose a case of Power Source of 500 KVA:

A 500 KVA Power Source shall provide Maximum 500 KVA or Safe Operation at 80% 400 KVA fixed. But Depending upon Load and their Power Factors; shall provide KW which may vary from 400 KW to 240 KW (Unity Pf to 0.6 Pf)or further less if Pf falls further lower.

Similarly Load shall draw their fixed KW whatever the source of KVA is present and what ever the Power Factor falls.

 

It's A simple Answer to it As my point of View.
THe losses In the Transformer are OF two types. Core losses And winding losses.....Here comes the point. THe core losses is due to voltage (V)..and winding losses due to current (I) denoted by AMperes (A) so THe transformer ratings is iN ....VA

 

Power transformers transfer electrical power (energy per time) from one voltage level to another. Apparent power (volt-amperes) is transferred, however, it is the load on the transformer that determines the KW/KVAR mix and the same rated transformer(KVA) could have a different KW/KVAR mix depending on the load characteristics. The transformer could care less as it supplies volt-amps, hence the transformer is rated in KVA. The ohmic losses (I2R) or other losses (and there are a number of these) do not have anything to do with why a transformer is rated in KVA, but does have much to say about how the transformer is constructed and its limits of application.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions
All information is supplied on a Cravat Emptor basis.

 

Thanks so much everyone. I found several good answers (and FYI corrections) about rating transformers. Honestly helpful.

Bur

 

pls i'm trying 2 solve a project,& i want to know how much power a 33kv transformer can supply

 

> hy are transformer ratings always given in VA and not in Watt?

b/c transformer losses in voltage & current

 

Why are transformer ratings always given in VA and not in Watt?

What is the minimum transformer rating suitable for a bungalow apartment in Nigeria?

 

1. Because transformer heating is mostly proportional to current and you can have current with little real resistive power being delivered. Apparent power better represents the transformers capability.]

2. I have absolutely no idea.

Regards
cww

 

CWW's 1... Actually total xfmr-loss (omitting auxiliaries) is equal to core-loss (essentially fixed) plus load-loss (proportional to the square of load-current)!

Regards, Phil

 

That's why I said mostly :^) Core loss also has some small dependence on current in reasonable designs and sometimes more with non-linear loads. And heating is important in small air cooled transformers because it is extremely difficult to remove from the windings. There is still a bit of magic in transformer design. A good way to think of it is that the voltage is a given, current is what's important, regardless of phase angle.

Regards
cww

 

CWW... Your explanation using "mostly" is "precisely" why I used "actually!"

Phil

 

With the state of the art in transformers being what it is. One of the very few things that you can say exactly is that all the losses add up to total loss, so I'll concede your point, neglecting radiative losses at these low frequencies :^)

Regards
cww

 

transformer rating is not in kW BECAUSE OF THE FOLLOWING REASON.
Though zero power is delivered to load at zero power factor, rated output and losses takes place if, rated voltage and currents were applied. Generally heat produced by transformer losses, like copper and core losses, limits the transformer output. Hence transformer rating is in KVA because core and copper losses depends upon voltage and current respectively.