10A1. General description. Self-synchronous
transmitters and indicator units are used in
the following I.C. systems:
Motor order telegraph system
Rudder angle indicator system
Bow and stern plane angle indicating systems
Underwater log system
Shaft revolution indicator system
Gyrocompass repeater system
Various fire control systems
Self-synchronous units are manufactured
by various companies which refer to them by
trade names such as Selsyns, Synchros, Auto
syns, and Telmotors. The theory of operation
is the same for all the various types of units.
For brevity, they will be referred to as selsyns
in the following text.
Thee are 2 basic designs of selsyn units,
One employs a 3-phase motor and a single phase
stator, the other employs a single-phase rotor
and a 3-phase stator. Both are energized by 115
volts, 60 cycle, single-phase, alternating current.
They will operate efficiently on a voltage and
frequency variation of +- 10 percent. That is,
they will operate in a voltage range of 103.5
volts to 126.5 volts and a frequency range of 54
cycles to 66 cycles. The 3-phase rotor, single-phase
stator design is most commonly used in
The indicators (motors) differ only mechanically
from the generators (transmitters) in that
a mechanism for dampening oscillations is
mounted on the rotor shaft. Electrically, they
The type designations that the Navy uses
for selsyn units indicate principally the size of
the unit. Common types used in submarines
Types "A" and "B" transmitters (generators).
Type "B" is larger than type "A."
Types "N" and "M" indicators (motors).
Type "N" is larger than type "M."
Figure 10-1. Sectional view of type "A" transmitter.
Figure 10-2. Sectional view of type "M" indicator.
10B1. Principle of operation. The following
discussion is based on the 3-phase rotor,
single-phase stator design (Figure 10-4).
The stator or fields of the transmitter and
all indicators in the circuit are connected to the
same supply. The 3-phase rotors are connected
electrically. When the supply circuit is closed, a
single-phase alternating current is impressed on
the interconnected stators. The single-phase current
in the stator windings, or primary circuits,
induces voltages in the rotor windings, or secondary
circuits. If the indicator rotors are in
exact correspondence with the transmitter rotor,
the voltage in phase 1 (R1) of the indicators is
equal to the voltage in phase 1 of the transmitter.
The same condition applies to phases 2 and
3 (R2 and R3). Because these voltages are balanced,
no current will flow in these circuits. If
the transmitter rotor is moved, the balance is
destroyed. Currents then flow in the rotor
circuits, setting up a torque. If the indicator rotor
is free to turn, the reaction is a restoring of the
original balance which results in the indicator
rotor being, brought into agreement with the
10B2. Maintenance. a. General. The brushes
and slip rings should be cleaned periodically
with an approved solvent. Even though the silver
slip rings and contacts may become discolored,
the conductivity is not necessarily reduced.
The ball bearings should be lubricated periodically
with a small amount of light mineral oil.
Sticky bearings, which may cause the indicators
to move sluggishly or prevent them from following the
transmitter accurately, should be repaired at once
or the unit will burn out. The
greater the displacement between transmitters
and indicators, the greater the current flow. The
dampener mechanism of the indicators should
be inspected periodically to insure that its
Figure 10-3. Mechanical analogy of selsyn transmitter and indicator.
Figure 10-4. Elementary wiring diagram of selsyn transmitter and indicators.
action is not sluggish. All connections should be
kept tight and the circuits kept clear of grounds.
Should it become necessary to replace a unit,
care must be taken to insure that all electrical
circuits are connected in their proper relation.
The following procedure will insure proper
performance of instruments at all times:
1. If trouble develops in a system, always
check the fuses and external wiring first. Inspect
for short circuits, grounds, or opens before
tampering with the instruments or selsyn units.
2. Check selsyn units for erratic or sticky
3. See that all gears are tight on their
shafts, and also that they are meshing properly.
4. Examine all bearings for dirt and free
operation. Clean, and then lubricate them with
a drop of fine, light oil.
5. Check the multipronged plug, jackboard, and
the terminal bar solder connections
6. Instruments that must be opened to replace
lamps or for other purposes should not be
left open any longer than necessary. Take care
that the ring light surfaces are not scratched.
When replacing covers, make certain that the
handle and rheostat shafts are properly aligned.
The drain valve, which is at the lowest point of
the instrument case, can be opened to remove
moisture which may have condensed in the instrument.
Opening of this drain valve will also
equalize pressures during compartment air tests.
7. To replace self-synchronous motive
power units, it is necessary to remove the entire
interior mechanism and the jackboard internal
wiring to the selsyn units.
CAUTION.When replacing a selsyn unit,
the electrical zero marks on the unit must be
aligned with the zero marks on the mounting
plate and gears. (See Section 10B2c.)
b. Trouble indications. On page 137 is a
table of conditions that may be encountered in
the operation of selsyn units due to circuit
derangements and mechanical troubles.
Figure 10-5. Elementary wiring diagram showing connections between selsyn transmitter and indicator.
Indicator pointers are 180 degrees out of position with
the transmitter pointer.
Reversed stator leads.
Indicators follow accurately as long as transmitter is
moved along slowly. If transmitter is moved rapidly, indicators get out
Open stator lead.
Indicator moves erratically over dial.
Open rotor lead.
Indicator is 120 degrees out to the right and rotates
opposite to transmitter.
Reversed R1 - R2
Indicator rotates opposite to transmitter rotation.
Reversed R1 - R3
Indicator oscillates back and forth before coming to
Indicator does not follow accurately and has a pronounced
Sticky bearings, bent shaft, or other misalignment.
Rotor operates as an induction motor
The three rotor leads are shorted or connected together.
Such a condition will quickly burn out the rotor winding.
c. Electrical zero setting. When a transmitter or an
indicator is removed from an instrument and replaced,
it is necessary to reset the
arrow. The instructions given here assume that
circuit MB is being set. The procedure is as
1. Check to see that the transmitter arrow
is at the center of the STOP position.
2. Install the new unit in position mechanically.
3. Cross connect S1 with R2 and S2 with
R1 and R3 (see Figure 10-6).
4. Connect 115-volt, 60-cycle, single-phase,
alternating current to leads marked MB and
MBB. The rotor will now assume its electrical
5. Loosen the setscrew that holds the arrow
in position on the arrow collar and loosen the
screws that hold the clamping collar.
6. Move the arrow to the center of the
STOP position on the dial. Tighten the screws
in the clamping collar, then tighten the setscrew
that secures the arrow to the collar.
7. Replace the original external wires. The
unit should now synchronize with the other units
in the system.
NOTE. The electrical zero setting for instruments
used in angle indicating systems
should be made to center the indicating arrow
on the 0 or zero position.
Figure 10-6. Selsyn connections for electrical zero.