Excellent article that sums it all up.
The one thing I would add is that ferrite filters don't actively "do"
anything - they just act like a high resistance to EMI/RFI and swamp it out,
so you can put as many as you want, anywhere, and it won't hurt anything
(but it may also not do anything) - limited only by your budget.
We have used them for years to reduce AM interference from 12 volt
flourescent lights.
..................................................................................................
Northern Arizona Wind & Sun - Electricity From The Sun Since 1979
Solar Discussion Forum: http://www.wind-sun.com/ForumVB/
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----- Original Message -----
From: "Exeltech" <exelt...@yahoo.com>
To: "RE-wrenches" <re-wrenches@lists.re-wrenches.org>
Sent: Thursday, July 16, 2009 10:02 AM
Subject: [RE-wrenches] Inverter AM Interference - The Basics
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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