K said:
What is the difference between "volt amps" and watts?
Ken
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
watts.
But if you are producing power watts are a better measure of what you
are making.