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.
Here's why...
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.
Basic rules:
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 .. they're heavy.
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 grounded.
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.
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 listenable.
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.
Dan Lepinski
Sr. Engineer
Exeltech
Hunting and smothering RFI for more than 40 years ...
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