[GreenKeys] order motor

[John Lawson]

On Fri, 21 Jul 2006, Don Merz wrote:

> Just curious--why 3 phase?



   Most simple selsyns (also called 'synchros') have a three-phase stator 
(fixed) winding, and a single-phase (rotating) rotor. One connects the 
stators together wire-for wire, at whatever distance between the units, 
and then excites the rotors with local single-phase AC, with the provision 
that the AC derives from the same source, ie. the AC on both rotors is 
in-phase.

  Then, torque applied to the shaft of one unit is reproduced at the shaft 
of the other.

   This is the simplest method of hook-up - it gets rapidly more complex 
from there - in fact these units make great analog computing elements - 
the huge Navy fire-control computers on the big destroyers were an amazing 
assembly of selsyns, servos, gearing, etc - they took in firing-table 
data, weapon and projectile info, target range and bearing, ship's motion 
(in real time) and then electrically trained the guns to put that shell on 
the target, by factoring in all those parameters (and more) .  Not bad for 
late-30s - early 40s analog computing tech.


   I have several pairs of these motors, BTW - was just playing around with 
a couple of the big brass units yesterday.  The classic Ham application 
was to couple one to the antenna rotor and have the other in the shack 
with a compass rose attached, or a little model of the Beam protruding 
thru a world-map, centered on one's QTH...


   Cheers

John  KB6SCO