ure: 800-336-2112 ext. 279 540-552-3011
For sales assistance: 800-577-8685 ext. 222 828-837-5115 or 800-336-2112 ext. 197
For technical application assistance: 800-577-8685 ext. 256 828-837-5115
www.litton-ps.com email: info@litton-ps.com
Synchro Application Guide
As a circuit element, the synchro is essentially a variable-cou-
pling transformer; the magnitude of the magnetic coupling be-
tween the primary and secondary, and hence the magnitude of
the output voltage, varies according to the position of the rotat-
able element. In function, the synchro is an electromechanical
transducer. A mechanical input such as a shaft rotation is
converted to a unique set of output voltages, or a set of input
voltages is used to turn a synchro rotor to a desired position.
Synchros can be classified in two overlapping groups: torque
synchros and control synchros.
Torque synchros include transmitters (CG), differentials (CD)
and receivers (CR).
Control synchros include transmitters (CG), differentials (CD)
control transformers (CT), resolvers (CS), linear transformers
(LT), and the two hybrid units transolvers (CSD), and differen-
tial resolvers (CDS).
Synchro Fundamentals
Synchro Types
TRANSMITTER
The synchro transmitter (CG) consists of a single-phase, sa-
lient-pole (dumbbell-shaped) rotor and a three-phase, Y-con-
nected stator.* The primary or input winding is usually the
rotor; the stator is usually the secondary or output element.
The rotor is excited through a pair of slip rings with an AC
voltage. The field produced by the input voltage induces a
voltage into each of the stator phases. The magnitude of the
induced phase-voltage depends on the angle between the ro-
tor field and the resultant axis of the coils forming that stator
phase. Since the axes of the three stator phases are 120°
apart, the magnitudes of the stator output voltages can be writ-
ten as:
V
s1-3
=k V
r2-1
sin



V
s3-2
=k V
r2-1
sin (



+120)
V
s2-1
=k V
r2-1
sin (



+240)
Where k is the maximum coupling transformation
ratio (TR)
Which is further defined as TR=Vout (max.) and is a
scalar quantity.
Vin
is the rotor position angle.
V
s1-3
is the voltage from the S1 terminal to the S3
terminal, and all other voltages are similarly defined
here and throughout this discussion.
These stator voltages are either approximately in time-phase or
approximately 180° out-of-time-phase with the applied voltage.
The amount by which the output voltages differ from the exact 0°
or 180° time-phase relationship with the input voltage is known
as the synchro (time) phase shift. For a synchro operated at 400
Hz working into an open circuit, the output voltage will always
lead the input voltage by a few degrees (8° to 20° for small sizes;
2° to 8° for larger sizes).
From the above transmitter equations it can readily be seen that
nowhere over the entire 360° rotation of the rotor will the same
set of stator voltages appear. The transmitter thus supplies
information about the rotor position angle as a set of three output
voltages. To make use of this information, however, it is neces-
sary to find an instrument which will measure the magnitude of
these voltages, examine their time-phase relationships and
return them to their original form a shaft position. Such an
instrument is the synchro receiver (CR). These two units the
transmitter and the receiver form the most basic synchro
system.
RECEIVER
In construction, the receiver is electrically identical to the trans-
mitter. The output voltages vary with rotor position in the identi-
cal manner as that given for the transmitter. In use, the receiver
is connected back-to-back with a transmitter; i.e., like-numbered
terminals are connected together (See Figure 1) and the rotors
are excited in parallel. At the instant the system is energized, if
the rotors of each unit are not at the exact same angle relative to
the stator phases, voltage differences exist across each pair of
stator windings causing current to flow in both stators. This
stator current produces a torque on each rotor.
FIGURE 1
Since the CG rotor is constrained from turning, the resultant
torque acts on the CR rotor in such a direction as to align itself
with the transmitter. When alignment occurs, the voltages at
each stator terminal are equal and opposite, and no current
flows. Perfect synchronization is never achieved in practice
because of the internal friction (due to bearings and brushes)
of the receiver. To minimize this error, the receiver is designed
to have a maximum starting friction of 2700 mg-mm.
Turning the transmitter rotor from the equilibrium position will
again exert a force on the receiver rotor. As soon as this devel-
oped force exceeds the internal friction of the receiver, the CR
will track the CG to its new position. The torque developed on
the receiver shaft is proportional to the angle between the two
rotors and is usually expressed in mg-mm/deg. Methods for
measuring the torque produced by a transmitter-receiver pair
are to be found in Society of Automotive Engineers Specification
ARP-461.
For literature: 800-336-2112 ext. 279 540-552-3011
For sales assistance: 800-577-8685 ext. 222
828-837-5115 or 800-336-2112 ext. 197
For technical applications assistance:
800-577-8685 ext. 256 828-837-5115
*In this guide, the use of the word phase will always indicate a space-phase
relationship, unless a time-phase relationship is specifically referenced.
A Subsidiary of Northrop Grumman
w w w.litton-ps.com
V
in
R2
R1
R2
R1
CG
CR
S1
S2
S3
S1
S2
S3 For literature: 800-336-2112 ext. 279 540-552-3011
For sales assistance: 800-577-8685 ext. 222 828-837-5115 or 800-336-2112 ext. 197
For technical application assistance: 800-577-8685 ext. 256 828-837-5115
www.litton-ps.com email: info@litton-ps.com
Receivers are constructed to minimize oscillations and over-
shoot or spinning when the rotor is turning to a new position.
The time required for the rotor to reach and stabilize at its new
rest position is called the damping or synchronizing time. This
time varies with the size of the receiver, the inertia of the load,
and the system torque. By special receiver construction, the
damping time can be reduced if required by system consider-
ations.
The CG-CR system is used to transmit angular information from
one point to another without mechanical linkages. The standard
transmission accuracy for such a system is 30 arc minutes. The
information can be sent to more than one location by paralleling
more than one receiver across the transmitter. The more
receivers used, however, the less accurate the system, and the
larger the power draw from the source.
DIFFERENTIAL
A third type of synchro may be added to our basic torque system
the differential (CD). The differential stator is three-phase, Y-
connected and is usually the primary element; the rotor is cylin-
drical and is also wound with three Y-connected phases. The
output voltages of the CD depend not only on the input voltages
but also on the rotor shaft position. As shown in figure 2, the
differential stator is normally excited from a transmitter stator,
and the differential rotor is connected to the receiver stator. The
output voltages of the differential are dependent now on both
the transmitter rotor position ( CG
) and its own rotor position
( CD
). The receiver rotor will seek a position ( CR
) which is either
the sum or difference of the transmitter and differential rotor
angles ( CR
= CG
± CD
), depending on how the CG and CD
stators are interconnected. CG CD CR
FIGURE 2
1
CD
2
FIGURE 3
The differential may also be energized between two transmit-
ters as shown in Figure 3. In this system, each transmitter is
turned to its desired angle, and the differential rotor is forced to
assume a position which is either the sum or the difference of
the angles between the transmitter rotors ( CD
= 1
± 2
). In this
application, the differential is sometimes called a differential
receiver and is especially constructed with an extremely low
starting friction (5000 mg-mm) to minimize system errors. An
accuracy of 1° is standard.
All synchro systems are subject to one serious drawback
torque levels typically run around 3000 mg-mm per degree of
receiver displacement. This is sufficient to turn a dial or a
pointer but nothing larger without increasing system errors.
When higher torques are required, synchros are used to con-
trol other devices which will provide these torques. An integral
part of these control systems is the synchro control transformer.
(CT).
CONTROL TRANSFORMER
The CT consists of a three-phase, Y-connected stator and a
single-phase drum (cylindrical) rotor. In normal usage, the
stator is the primary element, the rotor is the secondary, and
the unit is connected as shown in Figure 4. From the sche-
matic of Figure 4, it can be seen that the transmitter sets up a
voltage field in the CT stator whose direction is exactly that of
the transmitter and whose magnitude is directly proportional.
As the transmitter rotor turns with the CT rotor stationary, the
magnitude of the CT stator field remains constant, and its
direction exactly matches that of the transmitter. The field cut-
ting across the CT rotor induces a voltage in the rotor. The
magnitude of this voltage depends on the sine of the angle
between the axis of the rotor winding and the stator flux vector;
the time phase of the CT output voltage is either approximately
in time-phase or 180° out-of-time-phase with the exciting volt-
age on t