[Boatanchors] Need magnetic shield for shaded pole motor

[J. Forster]

The air capacity is easy:  eBay  310310219077, for example.

-John

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


> 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
>>
>> ______________________________________________________________
>> Boatanchors mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/boatanchors
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:Boatanchors at mailman.qth.net
>>
>> List Administrator: Duane Fischer, W8DBF
>> ** For Assistance: dfischer at usol.com **
>>
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>>
>
>