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Re: [TowerTalk] Looking for a pair of selsyns for an old Telrex rotor

To: Andy Bazar <>,,
Subject: Re: [TowerTalk] Looking for a pair of selsyns for an old Telrex rotor
From: Jim Lux <>
Date: Tue, 30 Aug 2005 06:52:29 -0700
List-post: <>
At 04:40 PM 8/29/2005, Andy Bazar wrote:
>Hello Charles Johnson,
>I found a wooden box full of these items while walking
>my dogs today along the street.  There are model
>2J1G1, Selsyn Control Transformers and 2J1F1 Selsyn
>Generators of various condition in the box.
>I came across your note when looking for information
>on how to connect these units and use them for
>something practical.
>I will trade you a couple units plus shipping cost for
>some information on how to use them and how to deal
>with the fact that they are 400 cycle units if you are
>interested.  I was going to see what happened using
>lower voltage 60 cycle on a set but won't do it till I
>hear from you first.  I suspect that might result in a
>funeral party for both of them.

You can run them at a lower voltage (like 60/400ths the rated voltage). Try 
something like 6.3V.

They'll buzz a bit, but it will work.

BTW, these aren't a real high power device, so cobbling together a 400Hz 
supply to run them isn't all that tough.  Precision of frequency isn't all 
that important, so something like a 555 will work, along with some suitable 
driver transistors.  Or, if you want a "rack and stack" solution, use an 
audio oscillator (a cheap guitar tuner will work) and a cheap car stereo amp.

>I presume that one way to use them is to connect all
>the same terminals of one unit to the other unit and
>then apply 57.5 volts ac at 400 cycles across the R1
>and R2 terminals and find that turning the shaft of
>unit #1 will turn the shaft of unit #2 a corresponding

That's how it works... R = rotor, S = stator.
The voltage on the single phase rotor of the sending unit creates a 3 phase 
signal in the stator, which has a standard 3 phase winding.  That creates a 
magnetic field in the receiver that is aligned the same way, and the rotor 
of the receiver tries to adjust itself to match.

In practice the devices are bidirectional.  You could hook generators to 
generators or receivers to receivers.  I think (but am not sure) that the 
windings are a bit different, but that's probably only relevant if you are 
looking for ultimate performance.

>If you are interested, let me know.  I did find some
>application information on them but nothing very
>detailed and nothing which got into the reason why
>these particular units were 400 cycle.  I suspect that
>would improve their resolving accuracy but that's
>purely a guess.

Almost certainly they are from an airborne application, and 400 Hz 
magnetics weigh a lot less than 60 Hz. The frequency doesn't change the 
resolution all that much.

>I thought that I might be able to use them as a wind
>direction indicator or perhaps a rudder angle
>indicator for my sailboat.

Should work just fine for that.  If you're lucky, they're weather sealed 
units.  In any event, they are rugged devices.  They were very, very common 
as war surplus until they were replaced by other methods.  Resolvers (a 
high resolution synchro with high quality bearings and good construction 
precision) are still used for absolute angle sensing, but these days, you 
run the 3 stator signals into a nifty module or chip from DDC and turn it 
into a 16 bit number.


See:  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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