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A technical question about inverters.

R

Rob

Jan 1, 1970
0
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
 
A

Andrew Gabriel

Jan 1, 1970
0
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

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.
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??

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

It's going to be a much less efficient inverter I suspect.
 
R

Rob

Jan 1, 1970
0
Nam Paik said:
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.

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.
 
R

Rob

Jan 1, 1970
0
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.

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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