Radio frequency Interfereence with SW inverter [RE-wrenches]

Steve Willey, Backwoods Solar steve at backwoodssolar.com
Tue Jun 5 08:47:27 PDT 2001


> Hi Steve
> So, what was the final outcome, did you succeed in making a "quieter radio
> environment"? Did you try the ProSine 3.0, and if so was the noise
> different?  I'd appreciate knowing more details of your visit as I know
you
> will have done a very thorough  assessment and investigation.
>
> Thanks in advance,
>
> Ezra Auerbach


Yes, sorry bout that, I was interrupted in writing that message and
minimized it to finish that evening, but it got sent meanwhile in unfinished
form. here is the complete version:

I recently drove out to the Olympic peninsula to resolve a customer
complaint about excessive radio noise from a SW4024.  He had complained to
me and to the factory for a year, wanting one of us to come witness the
problem. I went with an identical replacement inverter as well as a Prosine
3.0 to swap in for comparison. This customer was a ham radio operator with
oscilloscope on site, so we looked at waveform and harmonics listened on
several short wave receivers, and inspected the grounding and wiring of the
system.

The grounding was well done, up to code with all components close together.
The received interference from the inverter seemed to be radiated from the
AC wiring of the house, as it would go down with wiring removed from the
inverter output or with inverter turned off.  House wiring was standard
romex not in conduit. Metal conduit might have restricted some of the
interference level, but had not been used in this now finished small home.

Most of the noise was 60 Hz hash sound, appearing in bands on HF radios, 3.3
to 3.7MHz, 4.1 to 4.5 MHz and like that through up the spectrum, fading away
in the higher radio bands. I do not recall the frequency above which he
considered it acceptable, but this customer was irate and of the opinion
that it exceeded the FCC radiation limits.  I had no way to conform that
speculation.

The oscilloscope showed the waveform as would be expected for SW4024.  The
customer had noted some instances of a step in each quarter wave that seemed
to be disconnected from the next voltage step more than the others and had
suggested that was a defective FET, however all the steps were pretty equal
and I saw no specific differences from other SW oscilloscope patterns I have
observed. We confirmed this by patching in a new SW4024 I had brought and
observing no change at all. So the conclusion was that the customer did not
have a defective individual unit.

For comparison we then patched in the Prosine 3.0/24 I had brought, and the
noise was, as expected, completely different.  Prosine has high frequency
components that produce warbling tones throughout the amateur radio bands.
It had none of the hash, but the pattern of tones that sounded like what is
often called "birdie" oscillations was repeated across most of the lower
amateur radio bands.  The customer judged that to be more of a problem and
elected not to swap inverter types.

Since all his amateur radio equipment other than his computer operated off a
12 volt battery, my best suggestion was to turn the inverter off when
operating on SSB and AM modes.  This was a very small house so the equipment
upstairs could not be more than about 35 feet from the inverter. He did have
good outside antennas for the radio receivers. There seemed to be no
practical retrofit that would have helped short of twisting and conduit for
all the AC distribution, and that was not possible in the finished home. I
think he is satisfied that his problem has been investigated, but he is
still of the opinion that RFI emissions from the equipment exceeds FCC
limits.


Steve Willey   steve at backwoodssolar.com   www.backwoodssolar.com
Backwoods Solar Electric Systems
Phone(208)263-4290  Fax (208) 265-4788  in Idaho, USA



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