Eric R Snow said:
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
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