A technical question about inverters.

Hopefully someone can help.
I have a pure sine wave inverter and it runs everything I throw at it fine. However I have a few questions. I live in Australia, 240vac @ 50hz, and I
have a small multimeter that has a HZ measuring function. when I put the test probe in the power outlet at home, it shows a rock steady 50 hz. When I put it in the inverter, is show something like 60 khz.
When I have an AC/DC adapter (for a laptop) attached to the power point, and I put the Hz probe on the 12 volt output it shows 50 hz?? (Even though this is the DC end of the adapter)
But when I use the AC/DC adapter on the inverter, the DC output also shows 60 Khz.
I would love someone with technical knowledge to explain this to me... when I test the inverter on an oscilloscope, it shows a nice clean 50 hz sine wave, even though the multimeter show 60 khz.
I emailed the manufacturer of the inverter and this is his reply...
HI ROB
THE SINE WAVE PRODUCED IN THESE TYPES OF INVERTERS IS PRODUCED USING HIGH FREQUENCY PULSE WIDTH MODULATION (PWM) TECHNIQUE. IT IS NOT 100 % SINEWAVE BUT HAS APPROX. 3% THD. I AM NOT SURE HOW YOUR FREQUENCY METER WORKS. IT IS LIKELY THAT IT IS READING THE HIGH FREQUENCY PWM CONTENT. THE 50 HZ WAVEFORM MAY BE SEEN USING AN OSCILLOSCOPE.
BEST REGARDS
Now I have two more questions, even though everything seems to work well on the inverter, is there anyway that it is producing sub standard AC, and is there any way the 60khz coming through the dc end of the adapter could damage anything??
One final question, does anyone know a brand of pure sine wave inverter that will definately produce AC power completely identical to a normal power point outlet? (IE something that would show up as 50 hz when tested with my multi meter)
Thanks in advance for any advice.
Rob
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So what you probably have is a 50Hz size wave, which is modulated by a 3% 60kHz waveform. Your scope is probably sync'ed on the 50Hz, and so the small 60kHz component probably only has the visible effect of defocussing the line. If your scope has the ability to filter the 50Hz out from the trigger circuit, and you speed up the timebase by a factor of 10 and increase the y axis gain, you might see the 60kHz signal. (Might see it unstable even if you can't filter the trigger circuit.)
As for why your meter changes reported frequency when the unit is on or off load... There's probably a 60kHz filter on the output of the inverter to help reduce the 60kHz THD. If part of this consists of a line inductor, that's only going to work where there's a load.

I'm sure there is some way, but it's probably not very likely.

It's going to be a much less efficient inverter I suspect.
--
Andrew Gabriel

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well on

is
power
with my

Thanks Andrew, I have nothing more than a hobbyists understanding of all this, but your explanation helped. I did test it under load and it does indeed show 50 hz..
Cheers.
Rob
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60Khz is most likely the switching noise that is riding on the 50Hz fundamental sinewave & it should be quite easy to filter out with a small LC (low-pass) type filter. In fact, just placing a few microfarad (250VAC rated) capacitor across the inverter output should filter most of the 60KHz noise. Best thing is to look at the output waveform with a scope. If the horizonal scale is set at 5 millseconds per division, you should see 2 or 3 full cycles of AC on the screen. If that waveform is sinusoidal without any spikes or distortions, it cannot do damage to any device. But at 60KHz switching speed, you might get EMI/RFI interference (both conducted & radiated) so that radio might pickup hissing sounds, TV might show "snow", etc.
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Thanks for that explanation, It does produce a nice clean sine wave on the scope, and there is no percieved noise when used on radios or tvs, so I guess its ok.
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