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Question about transformer specs


K Wind

Jan 1, 1970
What is the difference between "volt amps" and watts?


Fred Abse

Jan 1, 1970
What is the difference between "volt amps" and watts?


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.

John Popelish

Jan 1, 1970
K said:
What is the difference between "volt amps" and watts?


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.