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Servo Amplifier
diodes, connected back to back, act as nonlinear
resistances. When a high voltage appears across the
The servo amplifier is a 10-watt plug-in amplifier
diodes, it appears as a low resistance or a short circuit.
with a push-pull output stage that feeds the semomotor
When a low voltage appears across the diodes, it appears
control winding. The servo amplifier drives the servo-
as a high resistance or an open circuit.
motor, which, in turn, repositions the control
When control transformer error voltages are small,
transformer rotors to null the error voltage to the servo
diodes CR12A, 13A, 14, and 15 act as a high resistance
amplifier. The amplifier has an internal power supply
and block the low speed (1X) signal from the servo
operating from 115 volts ac. It provides 12 volts dc and
amplifier. Diodes CR 16 and 17 act as a high resistance
unfiltered 40 volts dc for the amplifier stages. In
and allow the lightspeed (36X) signal to pass to the
addition, the power supply power transformer supplies
servo amplifier.
reference voltage for the servomotor and antistickoff
When the error voltages are high, diodes CR12A,
13A, 14, and 15 act as a low resistance and pass the
The amplifiers for 60- and 400-Hz units are similar
low-speed signal to the servo amplifier. Diodes CR16
except for the power transformers and capacitors.
and 17 act as a low resistance and short the high-speed
Gear train oscillation, or hunting, is caused by over-
signal before it reaches the servo amplifier. Resistors
shoot as the servo reaches its null. To prevent this,
R12, 13, and 14 are current-limiting resistors.
clamping circuits introduce a stabilizing voltage at the
amplifier input. This stabilizing voltage is proportional
to acceleration or deceleration of the unit.
Alarm Circuit
The low-speed control transformer output winding
connects in series with a 2.7-volt winding of the power
The alarm circuits in the synchro signal amplifier
transformer. This small, constant voltage (called the
monitor the 60- and 400-Hz output excitation, servo
antistickoff voltage) is added to the output voltage of the
excitation, and follow-up error. With all power sources
low-speed control transformer. It, in effect, shifts the
present and a follow-up error of less than 2 1/2, the four
angular position of the control transformer null, or
indicator lights on the access door will light. If one of
position of zero output. The antistickoff voltage is either
these conditions fails, the appropriate light will go out,
in phase or 180 out of phase with the low-speed control
indicating the problem area, and an alarm will sound.
transformer output.
With the equipment normally energized and the
If the high- and low-speed control transformers
alarm switch in the ON position, the alarm circuit will
were set to electrical zero at the same position, there
be open. A loss of any of the three power sources, a
would be a point at 0 and 180 where the error voltage
follow-up error of more than 2 1/2, or putting the alarm
would equal zero. Within 2 1/2 of the 180 point, the
switch in the OFF position will close the alarm circuit,
36-speed error signal would drive the servomotor to
causing an alarm to sound.
synchronize the control transformers at the 180 point.
The control transformers would also synchronize at the
180 point if the synchro signal amplifier were
Gear Train
energized when the control transformers were within
2 1/2 of the 180 point.
The gear train consists of a series of tine pitch,
precision, spur gears. They link together the rotors of the
To remove the chance of synchronization of the
two control transformers, four output synchros, and the
control transformers at the 180 point, the low-speed
control transformer is rotated 2 1/2 from corres-
pondence with the high-speed control transformer null,
or zero, position. An antistickoff voltage of constant
magnitude and phase is added to the single-speed
control transformer output. The resultant voltage is now
The synchro signal amplifier should require little
zero at the 185 point instead of the 180 point. At either
attention in service, there being few parts inside the
side of the 185 point, both the 36-speed and
amplifier unit or the synchros that need lubrication or
single-speed voltage tend to drive the synchro
replacement under normal operating conditions.
transmitters toward true zero.

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