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Question about transformer specs

Discussion in 'Electronic Basics' started by K Wind, Aug 10, 2003.

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  1. K Wind

    K Wind Guest

    What is the difference between "volt amps" and watts?

  2. Brett

    Brett Guest

  3. Fred Abse

    Fred Abse Guest

    I've posted about this not long ago, but I suppose I'll have to do it
    again :-(

    Volt amps is just that, the product of voltage and current without paying
    any attention to phase angle. Usually used in transformer ratings, since
    the transformer manufacturer has no control over the power factor of the
    load the user will put on it. It isn't the same thing a power (watts).

    Watts are _power_, that is to say energy dissipated or generated per unit
    time. If the load has an impedance which is a pure resistance, then
    watts=volt amperes, ie volts times amps. If the load or generator has an
    impedance which is partly resistive and partly reactive (power factor <1),
    then watts do not equal volts times amps any more. True power is the
    product of voltage and current in the _resistive_ component only, the
    energy due to the voltage and current in the reactive component is
    returned to the supply every half cycle. This used to be called "wattless
    power", but is more correctly called "reactive volt-amperes"

    Supply companies are particularly keen to ensure that large customers
    maintain the power factor (equal to the cosine of the phase angle) at
    almost (but not quite, since you can't generate at exactly unity power
    factor) unity. This is usually achieved by placing capacitors in parallel
    with loads which have an inductive component (which most idustrial loads
    do) to balance out the reactance. Fluorescent lamps which employ inductive
    ballast invariably have a PF correction capacitor built in.

    In a nutshell (and simplifying):

    Watts = volt-amperes good.
    Watts <> volt-amperes bad.
  4. watts are joules per second, so they represent average energy flow in
    a given direction. If the case involves DC, you can solve for watts
    by just multiplying volts across some circuit by the current passing
    through it, But energy can pass in either direction, so if the
    current is changing direction, the energy flow can be positive or
    negative, depending on the signs of both voltage and current.

    In an AC circuit, watts are the average power flow in a given
    direction, which may actually include parts of the cycle when power is
    positive and parts when it is negative. In the parts of the cycle
    where current and voltage have the same sign (as would be the case if
    the load were pure resistance) the power is positive, but when the
    current and voltage have opposite signs, it means that energy stored
    in the load is being returned to the AC source, so this lowers the
    average power being delivered to the load. Watts takes all this into
    account. To measure AC watts you need to average the product of the
    instantaneous volts and the instantaneous amps (including signs) over
    an AC cycle.

    Volt amps measures the movement of energy, regardless of which way it
    is going. It is the RMS voltage times the RMS amperes. The RMS deals
    only with magnitude (RMS stands for Root Mean Squared, or square root
    of the average of the instantaneously squared magnitude), and amps or
    volts squared looses information about whether the sign was positive
    or negative). Volt amps is easier to measure in an AC circuit, (just
    takes a volt meter and an ammeter, and you to multiply their readings)
    and is a useful measurement if you are dealing with power
    transformers. The volts are mostly what cause core losses. The
    amperes are mostly what cause copper losses. Transformers get hot
    even if they drive purely reactive loads that consume no average

    But if you are producing power watts are a better measure of what you
    are making.
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