[Milsurplus] Navy Radar Servo Question

[J Forster]

This is a question about the design choices made by the Navy contractor for the
VC-1 PPI Repeater display.

Background:  The PPI Display (Plan Position Indicator) was routed to several
auxiliary repeaters about the ship so that the radar images were available to
different people. The slaved displays got the following signals: azimuth, video,
and trigger.  The azimuth signal was in the form of a three phase synchro stator
signal and the rotor excitation. Each display unit had a servo to slave its
magnetic deflection yoke position in that display to the antenna position
(either true or relative).

The servo was composed of a synchro Control Transformer coupled with gears to
the yoke and servo motor. The output of the CT went to an AC coupled servo power
amplifier...  essentially an audio amplifier for 60 Hz.,...  and then to the
brushed of a UNIVERSAL motor (with brushes). The field of the universal motor
was connected to the rotor excitation of the synchro Control Transmitter.

Now the question: Why did they use a universal motor with brushes, rather than
an induction motor in the servo? When I first saw this scheme Tuesday, it seemed
very, very odd. Clearly, in both types of motors, the direction of rotation is
determined by the relative phase of the current in the rotor and field windings
and the torque is proportional to the rotor current at constant field, so what's
the advantage of using a brush type motor? Is it just because GE made the thing?

Wondering,
-John

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