# Exact meaning of KVA??

• posted

I see KVA on transformers. What does this really mean? I know KiloVoltAmps. Is it rated this way because there are no 'KiloWatts' consumed?

I don't understand KWH either. Doesn't watts already have time in it? Joules/Sec Seems like it is redundant in time.

I see portable generators on wheels rated in KW. (12 KW generator) How come the generator is in KW.

• posted

No, it's the voltage of the transformer times its current rating. This would be the same as watts, except that the current and voltage waveforms aren't always perfect sine waves or are in phase with each other.

Right. Joules/Sec * Sec = Joules. A KWH meter measures Joules used.

It shouldn't be. It too should be rated in KVA. If you look at the specs you'll likely see some disclaimer about "power factor" (which is essentially KW/KVA).

• posted

kVA and kW are units of power. kWh (and kVAh) are units of energy.

1 joule = 1 watt·second

--s falke

• posted

are

The kVA rating on transformers does not have to do with 'no kW consumed'. I'm not sure how much you know about electricity. Let's say that the kVA rating lets you determine how many amps the transformer can carry, without worrying whether those amps are associated with real power (Watts (W)) or so-called reactive power (VoltAmperesReactive (VAR)). The transformer doesn't care.

time

kWh is 'energy', kW is 'energy per unit time'. Run a 100W lightbulb for 100 days. How much power does it draw? 100W. How much energy does it consume? 0.1kW * 2400h = 240kWh.

You can convert this to Joules if you like ... 1kWh *

1000W/1kW * 3600s/h = 3,600,000 Watt seconds, or 3,600,000 J. So, 1kWh = 3.6 MJ. The lightbulb consumed 240*3.6M = 864MJ.

They are specifying that the most real power it can supply is 12kW. How many kVA can it supply? Well, that depends on the nature of the load. As such, maybe they are happier describing its output capability in kW because it is a firm number not subject to many factors. You could probably find out from documentation or the manufacturer more about its kVA ability. People familiar with generators know a couple rules of thumb that do a pretty good job of estimating kVA ability.

j
• posted

KVA ratings are used becasue the power factor is usually not known for the load at the time of manufacturer. For a power factor of one KVA equals KW but for any other power factor KW is less than KVA. for single phase KVA = EI / 1000 KW = EIpf/1000 or KW = KVA x pf

for three phase KVA = EISQRT(3)/1000 KW = EISQRT(3)pf/1000 or KW = KVA x pf

pf is power factor and can vary from 0 to 1 (is a cosine of an angle) but is usually about .7 to .8 lagging for highly inductive loads and about .9 leading for capacitive loads. Most industrial loads are inductive with lots of motors and transformers, however, synchronous motors give a capacitive load.

• posted

As others have pointed out....

Transformers ratings are typically their current capabilities and the design voltage. Of course, these vary from the primary side to the secondary side by the turns ratio. But a convenient thing to notice is that while the voltage may step down by the turns ratio, the current capability steps up. So the VA rating is the same for the high-voltage side and the low-voltage side. So rather than list each winding's voltage and ampacity, often just supply voltage ratio and VA. To keep numbers easy, for larger units list in kVA or MVA.

Since a Watt is a Joule/sec, if you integrate the power over time you get the energy that has 'flowed' over time period. If the power of the load is constant, the integration is easy, just multiply the power times the time. Traditionally, the amount of electric energy used has been stated in Watt-hours, although Watt-seconds is also used. For larger amounts, kiloWatt-hours is often used. It is somewhat 'redundant' since you are taking a unit that is "unit of energy per unit time" and multiplying by "time" to get back to "unit of energy", but it has been with us a long time now, and not likely to be replaced by kiloJoule anytime soon.

Ratings of generators are actually quite complicated. The sales literature of portable units leaves a lot to be desired ;-)

Commercial generators actually have a whole set of curves (called appropriately, "capability curves"). The main windings are limited in the total current they can carry continuously. The nominal voltage is fixed by insulation, terminal spacing, electronic regulators etc... So, combining the nominal voltage and current of the armature winding, one rating is kVA or MVA.

Another rating of generators is the amount of current the field winding they can handle. This limits how poor a power factor (lagging) that a unit can operate with rated kVA and still maintain nominal voltage. If the unit must supply a load with a lower pf, the total armature current capability must be reduced or terminal voltage suffers. If the unit is to supply a leading pf load, the capability is reduced because of severe iron-end heating of the rotor.

If it is a three phase unit, the total imbalance of the three line currents must be limited to avoid iron heating of the stator caused by negative-phase-sequence currents.

Notice, that I haven't mentioned kW (what you found some portable generators rated in). This is because the kVA is the true *generator* limit. Of course, if a *generator* has a rating of 10kVA, then it cannot safely handle more than 10kW, assuming the load has a unity power factor. But the maximum kW of a genset is more a function of the prime mover (engine, turbine, windmill, whatever makes it go 'roundy 'roundy) than the generator.

You can hook a 20kVA generator up to a 5kW engine. Although the generator

*could* handle a 20kW load (if pf is unity), the engine will stall out long before you reach 20kW.

Conversely, you can hook a 20kVA generator up to a 4000 hp engine. Load it up with a few 100kW and the engine won't even blink, just keep right on spinning. But the windings of the generator will start to smoke in no time at all because you have exceeded the generator's kVA limit.

Small portable units for retail sale are rated based on a few assumptions. That the load you will connect is 'typical' with some lights and light-duty motors (pf of between 1.0 and .8 lagging). So manufacturers build/buy an engine for 5kW, and a generator for 5kW / 0.8 = 6.25kVA and call it a '5kW' unit. Keeps the average consumer's life simple.

But some manufacturers would like to cut corners and boost the sales 'hype'. So some might build a 5kVA unit (slightly cheaper) and *assume* the pf is

1.0 and call it a 5kW unit (sells better than a 4kW unit). And the engine might only be able to supply 5kW for a short time, not continuously. So, BUYER BEWARE...

daestrom

• posted

At least you were the first to correct K to k. K=1024 k=1000

• posted

| I see KVA on transformers. What does this really mean? | I know KiloVoltAmps. Is it rated this way because there are | no 'KiloWatts' consumed?

In simpler terms, a kVA rating (or just VA rating) is a measure of the ability to carry power. A kW rating would be actual power consumed (or generated). Reactive power (sometimes inappropriately referred to as imaginary power) is power drawn, and then given back (over and over). Both real power and reactive power produce real heat loss in transmission lines and transformers.

| I see portable generators on wheels rated in KW. (12 KW | generator) How come the generator is in KW.

Some have both kW and kVA ratings, and the ratings can be different based on the generator's ability to carry reactive power, and its ability to load its source (diesel engine, turbine, electric motor, hand crank, etc). Some generator manufacturers, such as Generac or Kohler, have some nice PDF files providing lots of specs for their units.

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