difference b/w VA and Watts

[Badar]

hi
i wonder if somebody tells me what is the difference between VA and watt.Are they same things if not then P=VI, Does this power has units as watt or VA.
Does VA stand for Volt Ampere?

 

[ericgibbs]

hi,

The VA is volt-ampere and its usually used for AC, alternating current, to
indicate 'apparent power'. VA= V * A [* phase angle]

The Watt is used for DC, direct current.
W= V * I

Eric

 

[audioguru]

A load can be rated in Watts because it uses power. A transformer is rated in VA because it passes, not uses power.

VA is also something to do with a capacitive or an inductive load where the voltage and current do not have the same phase and therefore you can't simply multiply them to calculate the power used.

 

[Hero999]

If you connect a 120W 120V bulb to a 120V 50Hz power supply both the real and apparent power will be 120W or VA and the power factor will be 1.

If you connect the same 120W bulb to a 240V 50Hz power supply in series with a gigantic 26.53:mu:F capacitor the apparent power will be 240VA but the power factor will only be 0.5 and the real power will be only 120W.

The impedance of the capacitor is:
[latex]X_C = \frac{1}{2 \pi \times 50 \times 26.53 \times 10^{-6}} = 120 \Omega[/latex]

The resistance of the bulb is:
[latex]R = \frac{V^2}{P} = \frac{120^2}{120} = 120\Omega[/latex]

Therefore the bulb will only get half the voltage; the capacitor will get the rest. Capacitors don't disipate any energy though, all they do is store it, therefore the current becomes out of phase with the voltage.

Wikipedia explains the rest better than I do.
https://en.wikipedia.org/wiki/Power_factor

 

[RODALCO]

Good points raised above.

To add.

Transformers and generators are rated in VA, Because of a phase angle shift between voltage and current during inductive or capacitive loads.

It is important to bear that in mind.

When the power factor is low, the full load current from the TX is drawn earlier than at Unity PF under a resistive load

Most loads are inductive, eg coils in motors, other TX downstream etc.

 

[RODALCO]

Plenty of good hits under Google.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powfac.html

 

[Hero999]

Be ware of using heavilly capacive loads with transoformers; if the capacitor forms a resonant circuit with the transformer's secondary near the mains frequency or a harmonic then very high voltages can be generated possibly leading to smoke and fire.