Re: Re: How to work out what type of power generator?

[Ant <ant@xxxxxxx>]

On Saturday 13 September 2003 17:57, Stephen wrote:

> Perhaps a silly question, but why is generator output measured in kVA
> rather than kW? Is there a difference?

This ought to be in the FAQ. In fact, there ought to be a FAQ ;)

The Watt (or kW) is a unit of power that happens to have the same SI
dimensions as the product of voltage and current, thus for a DC circuit, we
can say that:

P (power) = V x I

This is also true for an AC circuit, IF AND ONLY IF 1) we use RMS values
for
the continually varying voltage and current waveforms, and 2) there is no
REACTANCE (capacitance or inductance) in the circuit.

Reactances in an AC circuit do not themselves dissipate power (ideally),
but
they do cause a phase-shift between the voltage and the current waveforms.
This means that the maths gets harder: you can no longer calculate the
power
by multiplying the voltage and the current. If you did, the unit of your
result would be a Watt, it'd be a "volt-amp" or VA.

The "real power" in the circuit's resistance can only be found by
multiplying
the RMS voltage across it with the proportion of the RMS current flowing
through it which has *exactly the same phase*, ie

P(real) = V(rms) x I(rms) x cos PHI

Where PHI is the phase angle between the voltage and the current, and
"cos
PHI" is commonly known as the Power Factor.

VA figures are generally higher than "real" W figures for AC,
because the VA
figure includes power which is stored in reactive components during one
half
of the AC cycle and returned in the other, thus cancelling out.



Well, you did ask!

cheers

ant