Question for Experts

The Transformer Carries both the Active and Reactive Components of Power. Hence Rating of Tranaformer is shown KVA. KW Represents only active component of Power.

Hi Romana

Well,

Induction Motor also Carries both active and reactive power know???? then why Motor Rating shown as KW

OK, now you've got ME curious, as well!

I still don't understand why KVA instead of KW. For example, if I have a transformer with a rating of 5000 KVA, I have no idea what part of that number represents Volts and what part represents Amps. Just as if it was marked 5000 KW, I still don't know what part is Volts and what part is Amps.

So, I have this 500 Watt power supply. By looking at the specs printed on the box, I know that the power supply provides 100 Amps @ 5 Volts. So I know what the Watts mean, and what I can do with this supply. How would VA or KVA tell me anything more?

I still don't understand why KVA instead of KW. For example, if I have a transformer with a rating of 5000 KVA, I have no idea what part of that number represents Volts and what part represents Amps

Well no, you have to have at least one other piece of information other than the kW rating (you cannot work out two variables), which is why voltage is always given.

Hello Vermin, my take on this is - transformer output is limited by the current (determined by winding size) which fixes the design kVA (= design amps x volts / 1000). Nominal power factor is usually 0.8.

At design duty (eg 1000 kVA, nominal 800 kW), it doesn't care about PF as long as kVA is not exceeded. If PF = 1, it can give 1000 kW, if PF = 0.5, it can only give 500 kW, but kVA = 1000 in both cases.

I think above is right, but it's possible suppliers might stipulate maximum load 1000 kVA or 800 kW, whichever is lower, so if PF = 1, max kVA = 800. If somebody knows, pls comment

Cheers..........Codey

Actually, KVA is the most useful piece of information, and so is the transformer impedance, which will be inductive reactive (unless someone has figured out a way to make them capacitive reactive). The transformer with no load has a power factor because it's inductive. The load on the transformer plays a large part in what the power company has to produce to supply the transformer. An ideal transformer load for the power company will make the whole system look like a purely resistive load, which is pretty much a pipe dream.

Vermin,

I believe you are also asking why transformers are rated 600VA instead of 120VAC, 5A, and that relates back to manufacturing. In the days of E-I silicon steel, 60Hz, linear transformers, once you knew the VA you knew the approximate size, the approximate weight (how much was steel and how much was copper), and the approximate cost. You would also know which winding machines to use and which department would assemble it, etc. For the most part (there were a few exceptions) it really didn't matter whether you had a 10V,10A transformer or a 100V, 1A one. I can remember my then boss yelling at me because I always estimated shipping weights in VA per pound and that made no sense to him.

However, the package should still be marked 120VAC, 5A.

Tom

if the transformer is rated for 1000 KVA and you run it on a highly reactive load that draws 1000 Watts, you may well end up with a 1000 watt pure load and a 500 watt reactive load = 1500 watts of heat in a thng made for 1000 watts = fire and destruction...maybe.

The maker wants you to know the limits of the devive.

OK. I think my problem here is that I don't know what a "reactive load" or a "pure load" means. Also, wasn't the term "active load" also used? So what's that.

Sorry guys, I guess I have some catching up to do.

Oh, well that certainly won't help.

Pure load = resistive load = unity power factor, power factor = 1

Reactive load = inductive load = lagging power factor, power factor <1.0. Also, the words "active load" used are likely to mean the same thing (although I would not call a resistive load "passive").

Capacitive load = leading power factor, power factor >1.0 (far less common than reactive and resistive loads).

Inductive load is by far the most common form of load. All motors are inductive, and the power grid network works on an inductive load.

OK, so here is a guy installing a backup diesel generator for an entire building. The generator he is installing a 50,000 KVA generator. How does he come about with the figure of 50,000 KVA? How does he measure the building? Or how does he assess their requirements and picks the right size generator?

If one parameter (either Volts or Amps) are furnished, does he calculate the other? And how does he know it's right for the building?

You can know the voltage and the peak current consumed by the building. In addition, you can measure the cos(φ) but you will not need it. Multiply the voltage with the current, that will be a VA or kVA value, add at least 25-30 % for future use and now you can chose the proper generator.

A 50,000 KVA diesel generator ?
1hp = 0.736KW (KVA) so 50,000 / 0.736 = an 80,000 hp diesel ... wooooowwwww ...

please send me some pics of this "monster"

As for the 50,000 kVA, we all know our dear friend double dipped the k of kVA and meant 50 kVA.

As for rating of equipment, ratings can be cultural as well as practical. We use kW in USA for generator ratings and most everybody else uses kVA. kW goes towards engine sizing limits, the most important thing in generator sizing. kVA goes to amperage capability for the generator, so in either case one must also know the power factor to understand the limits of each machine. Transformers overload via amps, not watts, so they will always be rated in kVA which reflects total amps (real power and reactive power . . . reactive power is kind-of a recirculating magnetism that doesn't do real work but does heat up things, so think of kVA as the heat load before toasting itself). Motors do work, that's why we size them and buy them to fit the 'work' load. So work is expressed in kW, not kVA, remembering that kVA includes this component of recirculating magnetism (reactive) that does no work, but heats up stuff.

Well, anyway, this is a simple non scientific explanation.

Thanks! I think I'm starting to get it.

Yeah, but here's the thing... Everyone here says that KW ≠ KVA!!! So it's big, but not quite as big as you'd imagine.

Yes, but 50,000kVA is the same as 50MVA which is a rather large generator (and not very portable). As an example, a 250kVA (0.25MVA) diesel gen-set fits nicely into a 20 foot container.

Yes, I think you're right. Probably more like 50 KVA (500 KVA?). The generator and diesel engine were about 20' long (roughly), and about 8' wide. Lots of dials, gauges, and LED displays, too.

Also, when he was testing it, he hooked it to this 6' X 6' X 6' box that he said was basically nothing more than a giant toaster.

He also said that during a bad blackout in the winter, he had a bigger one on a truck at a building site. He drove it home and hooked the entire block up with power until the blackout was over!

<power factor >1.0>

There seems to be some confusion.

The power factor of the load = [the work done and the heat dissipated by the secondary circuit] / [the applied voltage multiplied by the current drawn]. It is numerically equal to the cosine of the angle between the voltage waveform and the current waveform, and the cosine function cannot give a result greater than 1.0.

So capacitance loads have a power factor <1.0 too. On AC, with a pure theoretical capacitance load, the current leads the voltage by 90deg as the capacitor charges up and discharges. Cos(90deg) = 0 ; no overall work is done, and no heat is dissipated, and the power factor is zero.

With a pure theoretical inductance load, the current lags behind the voltage by 90deg. Cos(-90deg) = 0 ; no overall work is done and no heat is dissipated, and the power factor is zero.

In most real-life situations there will be an element of work done or heat disspated in the secondary circuit, and so the power factor will therefore be greater than 0 and less-than-or-equal-to 1.0, depending on what is actually connected and what it is doing.

Transformers are rated in VA or KVA (or even MVA), because this number takes into reckoning the ability of the transformer to supply a capacitive and/or inductive load of this combination. If the figure were given in KW, then there is the potential to confuse this figure with the heat dissipated by the transformer itself, which will be considerably lower than the power transferred to the secondary circuit.

Gah, I meant leading power factor for capacitive loads.

I think I will put that one down to the cold cup of tea on my desk yesterday.

OK. I think I'm getting it. It has a lot to do with the power being in the form of AC and not DC. Almost all of my high-power experience has been involved with storing fairly high amounts of DC, and then dumping it all at once. So I work lots with KW/s = Joules.

So if the power was somehow delivered in DC, you wouldn't really need KVA. However, since you are using AC, both inductive and reactive loading have to be taken into account. That's why for some large diesel generator installations I've seen, a very large and heavy choke was put in place with the generator. Am I right?

Vermin,

Help an old man in power generation to understand what you mean by 'choke' in layman terms. I think I know, but tell me anyway (this is how I speak when I really don't have a clue but somehow I feel it saves me). Depending on your answer, I'll try to explain power factor and generator reactance ratings.

It sounds like he's talking about the neutral grounding reactor. On larger units the generator neutral is often connected to earth through a large reactor or resistor to limit phase to ground fault current.

Probably so.

A big induction coil.

<So if the power was somehow delivered in DC, you wouldn't really need KVA>

Actually, you wouldn't have any kVA at all, because you wouldn't have a transformer connected to DC!

Think of it as

• kVA being the power passed through it to somewhere else, and
• kW being the heat dissipated from it regardless of what is being passed through it to somewhere else.

You still need to keep within its kVA rating even if all that was connected to it was a huge bank of capacitance, for example; there would be current to the load (A) without any dissipation (W) in the load. Enough capacitance and the transformer could still be overloaded, even if no heat is being dissipated within the load in this case.

Now, look, it's raining now, and I need to go back in my cardboard box until it stops. That's not unreasonable, is it? I'm getting soaked!

In simple terms transformer ratings for KVA tells you how much amperage can be run through it before it burns up. KW tells you how much it will consume. usually under normal running conditions.

In a power generation - transmission - consumption/conversion paradigm, components in the generation have less control over power factor. The transmission components have NO control over power factor while the consumption side components such as a motor or even a bulb have A LOT OF CONTROL over power factor. Also the motors and bulbs deliver the consumed electrical power in "some other form" such as mechanical or light energy. Unlike electrical circuits, power is "defined" to be always real in mechanical and light world. So it is "easier" to define kW to such components.

However for transmission components such as transformers, the manufacturer has NO idea what power factor the transformer is going to operate because it is largely decided by the load. THis is one of the main reasons why a motor is specified in kW and a transformer in KVA.

There are exceptions to my comments and I have traded details to convey the concept.

Hope it helps.

Hello Shikus, I think you've got the best answer of your question from Codemaster.

the power transformer is always rated in KVA because the operating power factor is dedicated by the load (specially in Island operation). So, simply:

A 1000 KVA transformer can supply 1000 KW resistive load, and can supply less than 1000 KW inductive or capacitive load according to the power factor.

Such a power factor is unknown to the manufacturer and should be a free parameter for the user.

As a result, if you have a bottle neck transformer (i.e. fully loaded) while the operating power factor is less than 1 (inductive), you can improve this power factor (approching unity) by using capacitors for example that is just to increase the transformer power capability, i.e. to connect additional load to the transformer.

You'll find the same thing on a nameplate of any generator.

Finnaly, you'll never find a power factor greater than one as mentioned by "Jack of all Trades" Cos φ ≤ 1

Regards... SAMAK

Hello Samak

you'll never find a power factor greater than one as mentioned by "Jack of all Trades" Cos φ ≤ 1

How about cos i = cos √-1 = 1.543, for those who don't live in the real world?

Codey

Hello Codemaster

"you'll never find a power factor greater than one as mentioned by "Jack of all Trades" Cos φ ≤ 1"

The angle between a voltage space phasor and a current one is always treated as a real electrical quantity of which doesn't live in the imaginary world. Such an angle is called a power factor, that's what we are talking about.

Regards... Samak

Thanks, Samak, but it was a joke, about complex number theory.

cos (i.x) is a real number, = cosh x = (e^x + e^-x)/2 and can be as large as you like if x is big enough.

Cheers......Codey

Thank you Codemaster,

May you reply to forum thread: "Voltage rating of power cables?"

Also,I want to invite our forum colleague to help in the above thread.

Regards... Samak

The manufacturer of transformers has no influence on what will be connected to the transformer. Inductive or capacitive loads (expressed by the factor "cos phi") lead to reactive power consumption which can be in sum significantly bigger than active power and so kVA is the dimension. Motors cannot be loades with "reactive load" because torque is always active - so here the right dimension is kW.

The electrical parameters of an AC motor will still contain a reactive component due to the inductive nature of the motor windings. However, motors will still be rated in KW or horsepower because that's the value that matters when selecting a motor. The only purpose for a motor is to do real work by rotating the shaft.

When designing a electrically-driven mechanical system, such as a pump, one first must look at how much flow or pressure the pump must provide. Pump curves can be generated to determine the real power (KW/HP) rating of the motor. Then design the electrical supply to the motor to handle the real power plus the reactive requirements of the motor itself.

This is a convention. as said above, the load is not always resistance feature, a little power will be returned back to generator, this power dont distribute to load only flow in transmit media, like wire etc. this will be tansmitted into heat and waste.

this is we called power factor less than 1. the part wasted is called useless power, generally expressed by VAR while useful power expressed in W, kw, or MW etc.

VAR+W=VA.

they all express power.

if you use V@A expression, popeole can know more detail of specification. but when you hope to lknow power, you hve to calculate again.

core loss dependent on voltage&copper loss dependent on current. so the transformer is measured in KVA AND dependent on the load we r using on the secodary i.e, may be resistive or inductive or capactive