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Wattmeter: 5 easy tests



Original poster: Pat Williamson <williamson_p-at-yahoo-dot-com> 

Dear Gary,

PAT'S WATTMETER "HONESTY" TESTS
-------------------------------

To overstate the obvious, first test your meter attached
to a resistive load against standards known to be good.

That drudgery done, the second simple test would be to hook
a purely reactive load to your meter and observe a near-zero
reading. If you have some motor-run capacitors, that would be
a good choice. Each 22 uF would load your meter by 1 ampere.

The dual would be to hook a neon transformer with a
shorted secondary to your meter. These transformers have
some small loss and a certain amount of current-waveform
distortion so the results will not be quite as ideal
as those from the capacitor experiment. Regardless,
the results will definately show you whether your meter
is measuring watts or VA. As with the capacitors, you
would get a more reasonable needle deflection if you
ganged several neon-transformer primaries in parallel.
Each 15 kV, 30 mA neon-transformer will draw about
3.75 Amperes reactive but an order-of-magnitude less
resistive real current--a difference easily seen
on your wattmeter.

My guess? You'll find that your meter distinguishes
between VA and Watts and that you made a wise purchase!
Perhaps you should stop experimenting at this point! LOL

My fourth test would be for sensitivity to current-
waveform distortion. (Another term used in the industry
is "crest factor.") I would use a hotplate or cheap toaster
(i.e. sans electronics) as a load. You can use light-bulbs,
but they change resistance with current more dramatically
than hotplates and thus complicate this procedure. Observe
the meter reading with the pure sine of current. Then take
a lamp-dimmer rated adequately for the load. With the
oscilloscope, set the firing-angle of the dimmer for
50% duty cycle. The absolute-value-of-the-average is 50%,
but the RMS is 70.7% of the smooth-sine current. Keep
your measurement times short so that the resistance
change due to temperature rise in the load can be
disregarded. Keep in mind that these measurements
require more squinting at the scale than in the prior
experiments. If you started out with a midscale reading
you are now distinguishing readings in the area 10%
of full-scale--more likely less. I would not be surprised
if you observe a reading somewhere between the 50% and
70.7% values. More stringent wattmeter tests can be done
in a similar manner with the dimmer firing at later
than 90 degrees.

For the fifth test remove the dimmer and replace it
with a diode. You should observe, again, an RMS reading
of 70.7% the deflection observed with a sine input. Reverse
the direction of the diode and repeat this test. This test
is similar to the test immediately above, but it induces
a DC component into the line current which may fool the
meter and cause a lower-than-expected reading. In most
Tesla coil applications, the results from this particular
test are not important because the line is equally
loaded on both polarities of the sine wave.

Pat Williamson

 > Subject: Watt meters

 > Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com>

 > I found a Weston analog Watt meter (0-2 & 0-4kW) at a ham fest
 > some time ago and have yet to try it.  There are a pair of
 > terminals for load voltage and another pair for load current
 > sensing.  My question is-will this read true watts with a
 > reactive (i.e. Tesla coil) load?

 > Thanks, Gary Lau
 > MA, USA