Excellent Dan! The first thing I would do in most cases is your #5.
relatively low level RFI. As an amateur operator (KB5HMU) I sometimes
have to deal with close RFI at 160 meters, 1.8 to 2kHz. I have a
frequency, which increases gain, the RFI can be eliminated. So try
improving the AM reception first. For sure it will make an improvement.
Post by Exeltech
Interference from inverters will always be an issue. It's a
difficult topic for many to understand .. and equally difficult to
reduce. Note you can reduce .. but not eliminate the interference.
To complicate things, the farther a radio is from the transmitter,
the more difficult this issue will be to resolve.
To achieve the highest efficiency possible, inverter power circuits
today transition from off to on in an extremely short time, as in
totally off to totally on in microseconds .. or even nanoseconds.
Internally, within the inverter, even "sine" wave models use square
waves at various points. Why? Solid state devices operate with the
least energy loss when they're completely off .. or turned on in a
strongly "saturated" mode - meaning turned on to their maximum
possible level with the least possible resistance. The transition
from off to on is commonly done in one step, from zero to max ..
then back again.
Micro-processor clocks also operate in this fashion, as do the
signals within the processor, and any related communications circuits.
Square waves are a composite of a sine wave, plus all odd harmonics
(odd integer multiples) of the original sine wave frequency. To
create a 100 kHz square wave, we start with a 100 kHz sine wave and
add sine waves of 300 kHz, 500 kHz, 700 kHz .. and so forth, up into
the many Mhz region. The number of harmonics added is astonishing.
As a consequence, these harmonics radiate into the AM broadcast
band .. and far beyond. To make matters worse, the circuits in
inverters are not "linear", which is to say they don't faithfully
reproduce the exact waveform put into them. It's done by intent,
but with a side-effect. This non-linearity turns the circuits into
"mixers". Mixers are a part of every radio and television. We use
mixer circuits to combine two frequencies and obtain others. When
non-linear circuits are fed a large number of signals, they add and
subtract all the various combinations of signals to create still
other frequencies .. and so it goes.
Radio frequency interference ("RFI") originates from many different
aspects of an inverter. If the inverter is battery-based, you'll
have many hundreds of amps being switched on and off very rapidly by
the inverter "front end". To handle the hundreds of amps, the input
resistance ("impedance") of the inverter must be very low .. on the
order of a few milliohms.
String inverters connected to a series array of PV operate on the
same principals, but at lower currents and higher voltages than
their battery-based counterparts.
RFI filters work on the basis of a voltage divider, posing a very
high impedance to the interference (blocking it), but a very low
impedance to the DC that must flow, minimizing loss at DC. This is
a very difficult challenge due to the high amperages involved.
The same is true of inverter AC output circuits. AC output is more
easily addressed because the current is much lower than the DC input
(battery based systems only). Conversely, inverters connect to AC
circuits in the home, turning every inch of the house wiring into an
antenna that radiates the interference.
As mentioned earlier in this thread, it's best to reduce the
interference at the source - in this case, the inverter.
First step is to try to determine where the bulk of the interference
is originating. The DC leads? AC leads? Inverter case? All the
above? Each has its own set of possible steps to reduce RFI. Leads
are the most likely culprit. A battery-operated shortwave radio
with a signal strength indicator can be an invaluable tool here. If
you have one, you're ahead of the game. If you consider buying
one .. ensure it also receives the AM broadcast band. Most do.
1) Keep the DC leads from the battery to the inverter as short as practicable.
2) Twist the DC leads together if possible. If not possible, keep
them as close together as you can. The goal is to have the RFI
magnetic energy from each lead cancel the RFI magnetic energy in the
other. As was also pointed out, it may be helpful to run each DC
leg in metal conduit and then GROUND the conduit to an earth ground
- the shorter the better. Failure to ground the conduit will simply
turn the conduit into another antenna. An RFI ground is separate
from the earth "protective" ground. If you use the AC "ground", it
too becomes an antenna unless it's kept short, and you've got a good
connection to the grounding electrode conductor with highly
conductive earth. It's tough to achieve all three together, but it
can be done in some locations.
Someone suggested a "filter capacitor" be connected across the DC
leads. This won't hurt, but isn't likely to be effective given the
very low impedance of inverter input circuits.
3) Ferrite cores may be slipped over the length of each cable, and
placed at the point where the cables exit the inverter. Toroid
cores or similar may be of help, but you'll need many of them, and
they'll need to extend at least two to three feet starting at the
inverter. More is better, and keep in mind .. when many are used ..
Do not install them at the battery end. Installing at the battery
end, and leaving some cable exposed at the inverter allows the
exposed conductors at the inverter to act as antennas.
Select the proper type of ferrite. Surprisingly, various
formulations of ferrite react differently depending on the frequency
range in which they're used. For example, some ferrites are good
for 100-500 Mhz, and would not do a good job blocking RFI that
interferes with AM radio. For AM radio RFI, select ferrite that's
rated to work from 250 kHz up to 2 Mhz or more.
4) AC EMI/RFI filters are also available, and may be installed on
the AC output circuit at the inverter. These are made by Corcom,
Tyco, and others. Select a unit rated for the output voltage AND
current of the inverter. RFI filters will be UL/ETL/CSA
recognized. If you find some that aren't .. don't buy them.
5) As was suggested, a radio with external antenna may help,
especially if the antenna is fed with coaxial cable, which can act
as a shield until the cable is well away from the house and/or
inverter. Keep the radio antenna as far from the inverter and house
wiring as you can.
6) A battery-operated radio is also an option. This too was
mentioned earlier in this thread. Even well-filtered inverter AC
output always carries with it some level of interference. A weak
radio signal will still be affected by a weak source of interference.
7) Ground the inverter housing in accordance with the manufacturer's
instructions. All inverters today are required to meet certain
levels of FCC interference criteria. Actions of internal RFI
filtering circuits may be improved if the inverter is properly
8) Ever drive into a parking garage while listening to the radio,
and the radio station gets very weak or disappears altogether??
Same thing happens when we drive through long highway tunnels.
We can make use of that trait. It's caused by the reinforcing steel
bars ("re-bar") acting to block the radio signals from getting to
the antenna on your vehicle. The same characteristic that keeps
signals from getting to your radio, also works to keep interference
In addition to all the above, you may have to construct a screen
around the entire inverter, then connect the screen itself to earth
ground. This screen should NOT come into contact with the inverter
housing. To do so would defeat the purpose of the screen. However,
properly filtered DC and AC leads may pass through it.
In this case, you'll be constructing a "Faraday shield", which will
keep interference inside. Surprisingly, this can be ferrous or non-
ferrous metal. I'd recommend ferrous (such as chicken wire with
small openings), for ease of soldering. Build a "box" around the
inverter, including the back of the inverter. To do this, you'll
need a board or other means to keep the inverter enclosure from
contacting the wire.
Once you've constructed the box .. connect the box to its own "RFI"
earth ground. This will be similar to a standard protective ground.
Next, add a bond wire from the RFI ground to the system protective
earth for the system. This RFI-ground to safety ground bond wire
should be outside if you can .. and buried in the soil if at all
possible. Adding this bond wire avoids opportunity for AC ground
loops or other issues. Keeping it in the soil also slightly reduces
the opportunity it will become an antenna for the interference. If
all the above are done properly, they won't impact the effectiveness
of the box you've just constructed.
Reducing radio frequency interference is, at best, a snipe hunt.
The strength of the radio/TV station signal itself can and will
vary, and is dependant on a variety of variables. This can give the
impression something you've done had an effect on the interference
level from the inverter, where in fact you didn't change a thing.
The weaker the radio signal, the more difficult it will be to reduce
the interference from the inverter to make the radio signal
The best thing to do is keep the inverter and all of its wiring as
far from the radios as you can. If this simply isn't possible ..
see steps 1-8.
I wish you well.
Hunting and smothering RFI for more than 40 years ...
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