[Boatanchors] Need magnetic shield for shaded pole motor

[james.liles at comcast.net]

Hi John,

Both good suggestions but need 25 cuft/minute to cool a pair of 8122's. 
Probably over kill but the 8122's need 6 cuft/min each.  Hard to get a 
muffin fan with that volume that is quiet.  The DC motor is a good 
suggestion but can't exceed a body depth without shaft of 2 inches and will 
have to produce 1/150Hp to 1/100Hp at 3000rpm to drive the wheel in transmit 
mode.

I suppose an alternative would be to mount the blower outside the radio and 
connect with a hose but would truly like to retain the blower location only 
because I'm fussy about these glow in the dark historical place holders.

Thanks again for the suggestions.

Kindest regards Jim K9AXN



-----Original Message----- 
From: J. Forster
Sent: Thursday, January 02, 2014 2:44 PM
To: james.liles at comcast.net
Cc: boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Need magnetic shield for shaded pole motor

Can you replace the fan with a muffin style unit, possibly a brushless DC
version?

-John

==============


> Good afternoon Al, Charlie, Bob ... and thanks for all the replies.
>
> Here is my problem:  I am trying to mitigate a minor HUM problem with a
> transceiver that uses a shaded pole blower.  Worked on the Hum problem for
> far too many hours and found it to be the blower motor  ----- Who would
> guess????
>
> The shaded pole motor has an incredible amount of 60 cycle magnetic
> leakage extending out to about 3 inches.  Problem is the 60 cycle magnetic
> field easily passes through the aluminum chassis to influence the audio
> preamp circuits that are just below the blower.  To make it worse the
> audio preamp tubes are directly adjacent to the motor.  It is definitely
> the Gaussian not electrostatic effect because leaving the 115v hot line to
> the blower live is not a problem.  Was a design error that was never
> important enough to divert resources apparently.
>
> I have tried the mu metal shield for a 3” scope tube, strips of
> transformer laminate, and that incredibly expensive mu metal that we use
> to shield RF stages.   The most effective material is the transformer E
> sections but would like to try a solid sheet that I could solder into a
> congruent loop completely around the motor to short circuit the
> inconsistent magnetic fields.
>
> Mu metal/soft iron is only effective to short circuit two poles of a
> magnet.  If you expose a single pole to mu metal it will easily pass
> through and be felt on the other side.  Believe that to be the problem
> with shaded pole motors --- inconsistent field patterns.
>
> This is actually only a problem for a very fussy listener with a perfectly
> noise free antenna.  It was identified by Bill at Electrovoice 47 years
> ago when testing the radio for the Hallicrafters gang.  He detected it
> when listening to very weak CW signals.
>
> Thanks for the suggestions ---- Kindest regards Jim K9AXN
>
>
>
>
>
>
>
>
>
>
>
> ---------------  ORIGINAL MESSAGE -------------------------------
>
> Message: 7
> Date: Thu, 02 Jan 2014 14:33:36 -0500
> From: Al Klase <ark at ar88.net>
> To: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Need magnetic shield for shaded pole motor
> Message-ID: <52C5BF10.1000009 at ar88.net>
> Content-Type: text/plain; charset=windows-1252; format=flowed
>
> Jim,
>
> Non-ferrous metals, copper etc., will do little to attenuate a magnetic
> field. There is an alloy, know an mu-metal, that was designed just for
> such applications, but it's expensive and hard to come by. Google same.
> Start here:
> http://www.pmlindia.com/hi-perm/Magnetic-Shield.htm?gclid=CJ3ghvWc4LsCFTNp7AodLUsAmA
>
> However, mu-metal only has a slight edge over soft iron or even steel.
> The coffee can may be a little too thin. The wreckage of a LARGE
> transformer may be the place to start.
>
> Al
>
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