"Andrew Gabriel" wrote in message
...
Ah, things become clearer! So many new devices seem to run using
"pretend"
ac (for want of a better phrase). Makes you wonder what kind of trouble
we're heading for. The 3 phase supplies in some factories where I've
worked
have looked scary on a scope due to the effects of inverters and dc
drives
on the larger machinery.Then they take this crap to run computers and
office
equipment. Adding filtering on the input helps but surely they should
try to
stop the wave deformation in the first place. Thanks for the info Andy,

New EU regulations limit the harmonic Current distortion allowed
by appliances. Low power appliances are still allowed to be low
power factor, but higher powered appliances aren't. This is relatively
new though, and lots of existing appliances don't conform. Large
industrial users get charged a penalty for low power factor.

Yeah, but many of them get round the pf by switching in and out large banks
of capacitors which only adds to the interesting shape of what used to be
sine-waves. Spikes of up to 30kv seem pretty common (although of short
duration) but even low-ish values of 3-500v sat on the peak for 5mS can
bugger up some expensive gear.


Just found a snapshot of the mains voltage waveform I took in our
office a couple of years ago:
http://www.cucumber.demon.co.uk/bagmains.jpg
which is clearly badly distorted from a perfect sine wave.

--
Andrew Gabriel

Nice! I suppose it'll be handy if you ever need a square wave for testing
purposes ;-)
Regards, Richard