[RFI] Honda Generator RFI

Wed Mar 23 20:41:33 EST 2005

Nice summary, I hope everyone takes it 
to heart; 'been there and done that' a 
few times over, having come from a rather
'hostile' power environment involving 
numerous direct off-line muti-ten-kilowatt 
'switchers' and line-type 'pulsers'.

By comparison, consumer electronics is
quite tame. Even ham radio RFI is tame
compared to a lot of EM environs out in 
the free world, where arc furnaces, 
induction heaters, hundred horsepower
DC motors are cycled and pulsed hundreds
to thousands of times a second for speed 
and torque control.

We, as hams, actually have it fairly easy!

The filering of numerous direct off-line 
muti-ten-kilowatt 'switchers' and line-type 
modulators delivering HV 100 KW pulses to 
the cathodes of Magnetrons is quite a 
messy affair; it should be mere child's 
play to 'quiet down' a simple rotary 60 Hz 
machine but the simplicity may end there; 
the control or excitation for the rotor may 
involve any of the more modern switching 
techniques to control voltage levels, I don't 
know what these smaller power plants use for 
regulation as I have only studied some of the 
larger power plants that generate power commer-
cially.

Speaking of those military transmitters again
I was involved with earlier-on in my life,
both 400 Hz facility powered units as well 
as airborne-powered 400 Hz units:  

   Noise, what noise? What you're seeing in 
   the way of those 200 V spikes at the transmit 
   rep-rate looks normal ... that EMI filter at 
   the input will clean all that up ... <grin>

Jim P  /  WB5WPA  / 

  ----- Original Message ----- 
  From: wng at daimlerchrysler.com 
  To: Jim P 
  Cc: Jim Brown ; RFI List ; rfi-bounces at contesting.com 
  Sent: Wednesday, March 23, 2005 4:54 PM
  Subject: Re: [RFI] Honda Generator RFI

  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 at dallas.net> 
        Sent by: rfi-bounces at contesting.com 
        03/23/2005 01:50 PM 
       To "Jim Brown" <jim at audiosystemsgroup.com>, "RFI List" <rfi at 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 at audiosystemsgroup.com>
  To: "RFI List" <rfi at 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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