What is a kVa?

[AdrianW]

VAr is the measure of complex power (proportional to the current though).

 

No it isn't. VAr is Volt-Amperes-Reactive. It it the unit of Reactive power. The unit of complex power is the VA.

[blip2]

No it isn't. VAr is Volt-Amperes-Reactive. It it the unit of Reactive power. The unit of complex power is the VA.

 

Technically it's still a complex power (since it exists in the imaginary plane) but yes more specifically it's the reactive power :P

[powersolve]

Sorry if this is really obvious, but I have been looking at hiring generators online (music-video we are thinking of shooting) and some of them have power outputs specified in watts, and some are specified in kVa (Kilovolt Amps I'm guessing). Obviously I am perfectly familiar with Watts, but I have never come across kVa before http://www.blue-room.org.uk/public/style_emoticons/default/huh.gif

 

I know that Power = Current * Voltage, so therefore surely a kVa is just another form of Watts (KIlovolts * Current). If this is the case then why bother with them at all? What is the difference between them?

 

I'm sure this is something really obvious, and I did do a quick google but the responses didn't quite make sense, hence why I am asking here. http://www.blue-room.org.uk/public/style_emoticons/default/wink.gif

 

Thanks for your help,

Charlie

 

In the context of generators, the difference between the gen's kVA and kW ratings is fairly simple in practice:-

- The kW rating is the rated power output of the actual engine (the prime-mover)

- The kVA rating is essentially the current rating of the alternator that's attached to the engine. For single-phase, I=kVA/kV. That's the max current that the alternator can carry for a sustained period without overheating its windings.

 

For most hire gens, the alternator kVA rating is designed to be around 1.25 times the engine's kW output, meaning that the alternator can handle any elextrical load up to the engine's kW rating and with a power-factor down to 0.8 - which is pretty low, but is not unusual for an induction motor load such as a bouncy-castle blower, for example. In reality, you'd rarely choose to load a gen beyond around 80% of its engine's kW rating, which means that you also stay well within the alternator's current rating, too.

 

When looking at hiring gens, then, I'd suggest that one gen described as 1kW is essentially the same beast as another described as 1.25kVA.

 

Hope this helps.