Why is VA used instead of Watts?

Volts times amps = watts. So it would seem to me that the spec. on a transformer would be watts. But I see VA specified. So I'm thinking that VA does not have to be equal to watts. Is this correct? If so, why? Thank You, Eric R Snow

Reply to
Eric R Snow
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That IS correct, and the reason it is is that loads don't have to be purely resistive. As noted in another thread in this group, reactive loads (i.e., capacitances and inductances) make for a condition in which the current waveform and voltage waveform are out of phase. Only the resistive part of the load actually "consumes power", and that power is in watts. But if you multiply together voltage and current that are not in phase, you get a quantity that has both magnitude and phase, and therefore both a "real" and an "imaginary" component. The "real" part of this value is the "resistive" power, the power actually being consumed; the "imaginary" part is the "reactive" power, and actually represents energy that is being taken out of the circuit by the reactive elements only to be returned later.

The point of all this is that volt-amps can have a peak value that's greater than the resistive part alone, even though it's the only part of power that is actually being "consumed." Hence, transformers and other elements in an AC power system have to be rated in VA and not just watts.

For more information, look up "power factor" and the reasons for "power factor correction."

Bob M.

Reply to
Bob Myers

If you say something is rated for X watts, you generally mean that the thing is going to dissipate that amount of power.

However, the transformer isn't doing that. It's simply transporting the power, and the circuit connected to the secondary is doing the dissipation. Thus, they use VA, which has the units of power. The transformer is not going to generate the heat indicated by its VA rating.

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Regards,
   Robert Monsen
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Reply to
Robert Monsen

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The transformer's spec is based on how hot it's allowed to get, and
that spec spells out how much voltage and how much current the
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Reply to
John Fields

It's actually more technically correct to mark a "VA" rating on a transformer instead of an identical "Watts" rating. It's common practice though just to mark the transformer power rating as "Watts" (just look through any electronic parts catalogue). Everyone seems more familiar with a "Watt" rather than the cumbersome, pedantic "Volt-amp". The VA-Watts distinction is only noticed when a transformer is connected to none-resistor loads. Example ... A transformer (say 50Hz) with a 10V secondary winding capable of

2amps, could equally as well be marked as "20Watts" or "20VA".

Hang a 5ohm resistor across the secondary and 2amp flows from the 10V winding. the resistor gets hot as it is dissipating 20Watts of heat. The transformer is now at its spec' limit of 10V at 2amps. Change the load to a 2.5ohm resistor and the tranformer would overload and cook as the windings now try to supply 4amps at 10V. (It's the amps that's the killer)

Now just hang a 650u capacitor on the secondary. 10V still feeds the capacitor and 2amps of current will again flow and yet again the transformer is its spec' limit. Trouble is, no Watts power is being used. The cap' just borrows current on one half cycle and generously returns it the next. The transformer is technically supplying a 0.0 Watt load but sure as hell notices its full rated load of 2amps.

Hang a 1000u cap on the secondary and the transformer is well overloaded and will start cooking, as way too much current is being taken out of the windings. The capacitor though is still consuming 0.0 Watts. The transformer still supplying 0.0 Watts.

In these cases (all cases!) it's maybe better to rate the transformer as capable of supplying a max of "20Volt-amps". The Volt-amps rating can thus technically apply to any kind of load, whereas a Watt rating implies just resistor loads.

The peversity of tradition has therefore decreed that "Watts" will be marked on a transformer but we must remember that it isn't really Watts but a Volt-amp rereading of the same number.

(Also ... by marking in "Watts", transformer manufacturers can take benefit from a cop-out against customers 'misusing' their transformers in real world applications. )

regards john

Reply to
john jardine

VA vs Watts Simply stated, it refers to whether or not the current wave form and the voltage wave form are running concurrently (or in phase) with each other. If they are, we speak of power as Watts, if not, we refer to power as Volt Amperes. It all depends on the characteristics of the load that the power is looking into.

If the load is a resistor, the current and voltage wave forms run together so we have Watts. As soon as inductance or capacitance is added to the load, then we are dealing with Volt Amperes because the current will either lag or lead the voltage. This is where power factor becomes an issue. As the power factor deteriorates, so does the efficiency of the power supply.

Reply to
jsmith

the difference between w and va is the phase for dc w=va for ac w=va x (cos of the phase)

Reply to
hotkey

I've seen this answer posted more than once in this thread and I don't understand why these posters assume that there has to be a phase angle involved. Electricians talk about volt-amps reactive or VARs when they have a power factor of less than 1. But VAs don't have to have any power factor, which can be assumed as 1.

Reply to
Watson A.Name - "Watt Sun, th

I don't understand what you mean by, "which can be assumed as 1". A VA measurement is the product of an RMS volt measurement and an independent RMS current measurement. It contains no information about the phase angle between voltage and current or the power factor. Watts are constrained by a VA reading to be anywhere between the VA and the negative of VA, including zero.

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John Popelish
Reply to
John Popelish

don't

angle

they

In other words, the PF is unity, the phase angles between V and I is zero, and the load is resistive.

That's what I meant, but seems to have got snipped. Other people keep answering the VA question with this additional cosine of the phase angle, which isn't applicable.

Reply to
Watson A.Name - "Watt Sun, th

My 2cents Power factor. Power factor of 1 means all the power being supplied (by generator or transformer...) is being dissipated by the load. This is the most desired effect. Power factor less than 1, not all of the energy being supplied is being used, wasted energy or "stored" energy. Power factor greater than one = Bill Gates will be cleaning your home... it won't happen.

Algebraically on a cartesian plane VA=Watts+-j VAR

If none of these posts mean anything or are just confusing, just think of it as watts in AC.

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-Ray
Reply to
Ray

being

Mathematically, it CAN'T happen; the maximum value of cos(x) is exactly 1.000...

Bob M.

Reply to
Bob Myers

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