[RE-wrenches] Inverter AM Interference - The Basics

[Wind-sun.com]

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/
..................................................................................................
----- Original Message ----- 
From: "Exeltech" <exeltech at yahoo.com>
To: "RE-wrenches" <re-wrenches at 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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