What kind of wattmeter? A digital meter expecting pure sine will
almost certainly be fooled. And don't the circulating current
wattmeters installed by the power company only measure current?
Alien Steve
the power company meters have a aluminium disk and 2 coils, one for voltage and one for current.
(i reckon a 3 phase unit has 6 coils).
this ensures that really the power is measured. the fields somehow multiply to generate the torque.
(i don't know the details just now but there are coils for voltage and current).
at least this is how the european power meters work.
There certainly can't be any power returned to the supply as there
would be with a capacitive lead or inductive lag load.
exactly what puzzles me.
I thought the definition of reactive power only requires the phase shift.
the problem is if you imagine the first sine approx. of the current it would still be positive
after the voltage is already negative (around the zero crossing).
this is the part where energy is delivered back normally. well, this is only showing at the
first sine approximation, not the real current, the real current always is same polarity as voltage.
I think around this point it the problem, maybe you can not use the sine approximation.
In hardware it would mean to add a filter, containing a capacitor. then it is explained,
the capacitor takes energy during the off state of the triac and gives it back to the source
during the little opposite polarity time.
This is the only explanation i have, there is phase offset but no reactive power.
ST
