> My question is this.  Do I have to measure phase, via current and
> voltage, in order to get a usable value?  And if I do have to have both,
> I would prefer to use the CT for current and a galvanic field sense
> method to 'see' the voltage.  I would not get an accurate absolute
> voltage, but I would get an accurate phase lead/lag.  And if I have the
> phase angle and current, how would one then calculate the KVA's?  I once
> knew some of this stuff from school, but you know, don't use it and you
> loose it.

In theory, the electric company charges you for the instantaneous voltage
times current integrated over the billing period.  This is also the measure
of the amount of energy delivered.  So, if you wanted to perform the same
measurement, you would have to sample voltage and current "often" compared
to the 60Hz line frequency, then multiply and filter the result to get live
power useage, or accumulate the result to get energy used.

However, if you just want a quick idea of where the power is going, you
probably don't need to be this sophisticated.  Just measuring current will
probably give you a pretty good idea what is going on.  If the current is
low, then the power drain on that line has to be low too.  A high current
could in theory indicate either high power consumption or a large phase
angle.  If both current and voltage are sinusoids, then the power drain is
the RMS current times the RMS voltage times the cosine of the phase angle.
Note that even at a 45 degree phase angle, the power is only 30% less than
indicated by measuring the current alone.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, olin@cognivis.com, http://www.cognivis.com

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