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Why is VA used instead of Watts?

Discussion in 'Electronic Basics' started by Eric R Snow, Nov 20, 2004.

  1. Eric R Snow

    Eric R Snow Guest

    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
     
  2. Bob Myers

    Bob Myers Guest

    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.
     
  3. 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.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  4. john jardine

    john jardine Guest

    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
     
  5. John Fields

    John Fields Guest

    ---
    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
    transformer can supply to a load and stay within its (the
    transformer's) temperature rise limitations. Since loads are
    frequently reactive, the transformer will be required to supply curent
    into the load out of phase with its output voltage, and since



    P = EI cos(phi)


    if the phase angle between current and voltage is 90°, P (power
    dissipated by the load, in watts) will be equal to zero no matter what
    E (the voltage across the load) and I (the current in the load) happen
    to be. However, current will still be flowing into and out of the
    transformer's winding resistances, so even if the load is dissipating
    no power at all, the transformer will be getting hot and, if the
    current is more than it's specified to be able to handle, its lifetime
    will be shortened.

    There is one case, though, where the transformer's rating, in
    VoltAmperes will be equal to watts, and that's when the load is
    resistive and cos(phi) will be equal to 1 because the voltage across
    it and the current through it will be in phase.
     
  6. jsmith

    jsmith Guest

    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.
     
  7. hotkey

    hotkey Guest

    the difference between w and va is the phase
    for dc w=va
    for ac w=va x (cos of the phase)
     
  8. 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.
     
  9. 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.
     
  10. 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.
     
  11. Ray

    Ray Guest

    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.
     
  12. Bob Myers

    Bob Myers Guest

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

    Bob M.
     
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