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Re: [RFI] Honda Generator RFI

To: "Jim P" <jvpoll@dallas.net>
Subject: Re: [RFI] Honda Generator RFI
From: wng@daimlerchrysler.com
Date: Wed, 23 Mar 2005 17:54:24 -0500
List-post: <mailto:rfi@contesting.com>
Gentlemen,

There are two distinct issues here:  Differential Mode noise and Common 
Mode noise.

Differential mode exists from one conductor to another in the same bundle 
or harness. It is probable generated intentionally within the device in 
question, or some artifact of that generation.

Common mode exists on ALL of the conductors in a bundle or harness with 
the same voltage and phase relationship to some external reference.

With respect to AC power generation or any circuit carrying high DC 
current, the filter must take this current into account in its design.

A common mode filter passes ALL of the CURRENT-CARRYING conductors through 
the series element, but NOT the non current-carrying conductors (i.e.: 
safety or shield ground).  There must be NO SERIES IMPEDANCE in these 
conductors; it defeats their purpose and may create unsafe conditions.

The higher the frequency, the more likely the noise is mostly common mode. 
 By 20 or 30 MHz noise is usually nearly all common mode.  Below 2 or 3 
MHz, differential mode becomes prevalent.  By 100 kHz noise is almost all 
differential.

Common mode noise is most likely picked up through the antenna input.

Common mode filters for AC lines must pass all of the phase conductors 
through the single series element.  In the case of a ferrite toroid, L1, 
L2 and neutral contain a near zero sum of the powerline current, but 
effectively choke all of the common mode current.  A set of shunt elements 
(bypass capacitors) on the load side of the series element now provides a 
low impedance to ground, completing the L-section filter low-pass filter.

Differential mode noise is most likely to enter the device directly 
through the power supply connection.

Differential mode filters must act independently on each line 
independently (Although filtering just the ungrounded side usually works 
as well.), so the series element must provide low impedance at the power 
frequency and be large enough cross sectional area to avoid saturation 
(simple if it is DC, but saturation is more of an issue) and high 
impedance at the noise frequency.  This requires much more careful 
selection of the choke cores.  The shunt elements require the opposite 
characteristic in frequency domain:  low impedance at the noise frequency, 
high at the power frequency.  The circuit configuration is the same, with 
the shunt element on the load side of the series element for an L-section, 
low-pass filter.

All filtering should be done at the source if possible.  At the very least 
it will protect all nearby devices, and it always remains with the source. 
 Additional filtering of differential mode noise is sometimes required at 
the receiver.

Note that if the power source is three phase, L1, L2 and L3 all must pass 
through the single series element in a three-wire delta system, but L1, 
L2, L3 AND neutral fro a four-wire wye system.

Twisting follows the same rule:  single phase:  L1, L2 and neutral, three 
phase:  L1, L2 and L3 in a three-wire delta system, and L1, L2, L3 and 
neutral fro a four-wire wye system.  Do not use separate twisted pairs, 
The ground wire may be included in this section of the line. and is often 
the messenger or support wire in overhead systems. 

Remember:  No series impedance in any ground conductor!

One more note on grounding:  There should be only one system ground. Codes 
usually require this to be at the entrance panel and the bond to neutral 
must be only here.

The NEC has recently been amended for remote generators.  Newer generators 
have a screw that can be removed to isolate the local neutral from the 
case/frame, which still must be grounded!

In the case of inadequate (ground noise issues) receptacle grounds, a 
separate driven or other ground system may be employed.  This must be 
permanently marked by using orange service devices (receptacles).

For reference, I am the author of the DCX two-way radio installation guide 
(available from dealers as a service bulletin or from the ARRL website 
under RFI) and the corporate grounding design standard.  I am also a 
member of the SAE subcommittee on electrostatic discharge in fuel systems.

Bill Gilmore  WB8FPQ
Electromagnetic Compatibility Engineer
Core EMC, Electrical/Electronic Engineering
Powertrain Component EMC / RWD, SRT & HEV Platform EMC
DaimlerChrysler Corporation
Auburn Hills, MI





"Jim P" <jvpoll@dallas.net> 
Sent by: rfi-bounces@contesting.com
03/23/2005 01:50 PM

To
"Jim Brown" <jim@audiosystemsgroup.com>, "RFI List" <rfi@contesting.com>
cc

Subject
Re: [RFI] Honda Generator RFI






You brought it up Jim; and I can see how, given 
the prescription for looping the neutral and just 
one phase through 'the magnetic' back at the 
source where this saturation problem that you 
spoke of can become a problem; I simply gave 
the cirumstances under which this can occur

Quoting J. Brown from earlier:

   As to cores -- they are likely to saturate under 
   load if you put them on individual conductors, so 
   plan on putting them on the paired cable so that 
   the total field is small (that is, the hot is cancelled 
   by the return). Used this way, the cores will 
   attenuate common mode radiation from the line. 

Simply placing a 'core' on either phase plus neutral 
back at the source (we _were_ talking about suppres-
sing gen noise IIRC) results in a _full_ magnetic field 
contributed by the single phase, during balance, with 
little or no 'countering' magnetic is produced owing to 
little or no current on the neutral at/near the gen. This 
is really pretty basic stuff ...

A better location looks to be placement around both 
hot-conductors, such that at least partial cancellation 
of the field produced by the 60 Hz line current while
suppying common mode rejection to noise coming
from the gen would be achieved.

My choice? I'd buy a commercially rated filter and 
escape all this napkin-based 'ballpark engineering'.

Jim P  /  WB5WPA  /


---- Original Message ----- 
From: "Jim Brown" <jim@audiosystemsgroup.com>
To: "RFI List" <rfi@contesting.com>
Sent: Wednesday, March 23, 2005 11:07 AM
Subject: Re: [RFI] Honda Generator RFI


> On Wed, 23 Mar 2005 10:19:21 -0600, Jim P wrote:
> 
> >It does/it can, if you saturate the particular magnetics
> >selected, by improper application, which was what I 
> >was pointing out
> 
> What magnetics are being saturated, and how?  If all the current-
> carrying conductors are going through the ferrite choke, the net 
> field is limited to the common-mode current, which ought to be the 
> noise, unless something is broken (or shorted to a conductor that 
> doesn't go through the choke). 
> 
> I wouldn't expect a power transformer to generate RF trash upon 
> saturation. What else might I be overlooking? 
> 
> Jim
> 
> 
> 
> 
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