DIYbanter - View Single Post - Electric power consumption measuring device

In article ,
writes:
Andrew Gabriel wrote:
In article ,
writes:
Andrew Gabriel wrote:

I bought one of the older ones Maplin stocked a few years ago,
and that's hopeless on non-resistive loads (out by a factor of
3 on SMPSU's -- too high, which is a puzzling way for it to be
in error).

If it does not sample fast enough, then it may well overestimate
current consumption.

Yes, I guess if the spike width at the voltage peaks is small
compared with the sampling rate (hello Mr Nyquist;-) and the
sampling is somehow always catching it (prehaps due to synching
with the voltage waveform), then that would account for it.

Isn't it as much to with low power factor as anything. The
cheap/simple power meters simply measure average (and I mean average,
not RMS) voltage and current, then multiply the two together and apply

If it did this, it would under estimate rather than 3 times
over estimating. That's why I said the form of the error was
puzzling.

No it wouldn't. It could have lots of amps flowing at 240 volts but
with (say) 10% power factor the Volts*Amps would be way more than the
power.

The example was a SMPSU, not a capacitive or inductive load.
Averaging a SMPSU current pulse or failing to take into account
it only happens when mains voltage is around 340V would give
an under reading. I can't imagine what mistaken assumption
they've made which results in such a load 3 times over reading.

--
Andrew Gabriel