[Boatanchors] Need magnetic shield for shaded pole motor

[james.liles at comcast.net]

Good evening Brian etal,

Haven’t heard from you for a while.

The problem is with the design of a shaded pole motor which is an incredible Gaussian mess by nature.  The problem is with the 60 cycle H field that surrounds the motor which can be felt with an E core up to 6 inches away.  The aluminum chassis and the use of a copper shield is essentially invisible to the H field and does nothing to contain it.  

The mu metal or high permeability metals have been used to contain an H field but with limitations.  It can be used very successfully as a magnetic short circuit.  If you expose a sheet of mu metal to a north or south pole individually, you will find it on the other side with little attenuation.  The point is, mu metal or high permeability metals are great to short circuit a north and south pole but have little effect on a single pole.

The only way that I have seen non magnetic materials do low frequency H shielding is with super conductive ceramics.

I have pretty much solved the problem using a simplistic and perhaps silly method.  I turned the motor so the coil is toward the top cover.  This moved it away from the audio preamp tube and circuits.  About a 70% fix.  Next I coupled the H field through an E core to the area of the audio tube and circuits finding a path that coupled an out of phase field to the effected area.  Voila, now 100% all is well.

Now I can tune a –140dbm 90% modulated AM signal with a 3db S/S+N ratio, distinguish the sidebands, and not detect hum.  I am finished with this thing.

Your have been very generous with your ideas and council and enjoy the friendship of the list.          

Kindest regards Jim K9AXN

From: Brian Clarke 
Sent: Friday, January 03, 2014 5:42 PM
To: rbethman ; mailto:Boatanchors at mailman.qth.net 
Cc: james.liles at comcast.net 
Subject: Re: [Boatanchors] Need magnetic shield for shaded pole motor

Silver-plated copper is good for E-field attenuation, but approximately useless for H-field attenuation. For H-field attenuation, you need high permeability steel of a thickness dependent on the frequency and the amount of attenuation desired. See any standard text on EM shielding, eg Ott. 
73 de Brian, VK2GCE.

Brian Clarke
BE, MBA, PhD, CPEng, FIEAust
MD, Clarke & Associates P/L



  ----- Original Message -----
  From:
  "rbethman" <rbethman at comcast.net>

  To:
  "Boatanchors @ Mailman" <Boatanchors at mailman.qth.net>

  Cc:
  <james.liles at comcast.net>

  Sent:
  Fri, 03 Jan 2014 15:26:20 -0500

  Subject:
  Re: [Boatanchors] Need magnetic shield for shaded pole motor


  On 1/2/2014 9:23 AM, james.liles at comcast.net wrote:
  > Any ideas for an effective Gaussian shield for an open frame shaded 
  > pole motor or a source for very high permeability transformer core 
  > uncut sheets.
  >
  > Thanks for any ideas--- Kindest regards Jim K9AXN
  Jim,

  I see your address in QRZ. I have what may be precisely what you need.

  Would two sheets of silver plated copper expanded mesh, 13" x 13 " x 
  0.009" do the encapsulation of this shaded pole motor?

  <snip>

  Bob - N0DGN



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