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.
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
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).
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.
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 =
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
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
KW = KVA x pf
for three phase
KVA = EISQRT(3)/1000
KW = EISQRT(3)pf/1000
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.
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
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
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
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,
| 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.