NEETS Module 15 — Principles of Synchros, Servos, and Gyros
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You should now have a good working knowledge of synchro systems. For further study and assistance in applying
this knowledge to synchro troubleshooting and alignment, consult the following references:
1. Applicable maintenance instruction manuals.
2. Military Handbook, Synchros, Description and Operation, MIL-HDBK-225A (March 91).
Now that you have completed this chapter, a short review is in order. The following review will refresh your
memory of synchros, their principles of operation, and how they are tied together to form synchro systems.
SYNCHRO resembles a small electric motor in size and appearance and operates like a variable
Synchros are used primarily for the rapid and accurate transmission of data. They are also used as control devices
in servo systems.
A SYNCHRO SYSTEM consists of two or more synchros interconnected electrically.
TORQUE SYNCHRO SYSTEMS are systems that use torque synchros to move light loads, such as dials
CONTROL SYNCHRO SYSTEMS are systems that use control synchros to control
servo systems. The servo system, in conjunction with the control synchro system, is used to move heavy loads such
as gun directors, radar antennas, and missile launchers.
MILITARY STANDARD SYNCHROS
are synchros that conform to specifications that are uniform throughout the Armed Services. They replace the
prestandard Navy synchros. A typical example of a military standard synchro designation code is 18TR6A. This code
has the following interpretation:
18—Synchro diameter of 1.71 to 1.80 inches
PRESTANDARD NAVY SYNCHROS are synchros
designed to meet Navy, rather than service- wide, specifications. A typical example of a prestandard Navy synchro
designation code is 5DG. This code has the following interpretation:
5—Synchro diameter of 3 3/8 to 3 5/8
1/2 inches, weight 5 lbs.
SCHEMATIC SYMBOLS for synchros are drawn in two different forms. Two of the five
standard military symbols are drawn to show only the external connections to the synchro. The other three symbols
are drawn to show both the external connections and the internal relationship between the rotor and the stator.
SYNCHROS ARE CONSTRUCTED like motors. Each contains a rotor, similar in appearance to the
armature in a motor, and a stator which corresponds to the field in a motor.
A SYNCHRO ROTOR is composed of either a single coil of wire wound on a laminated core or a
group of coils wound in slots in a laminated core. The laminated core is rigidly mounted on a shaft that is free
to turn inside the stator. Two slip rings are mounted on one end of the shaft to supply excitation voltage to the
rotor. There are two common types of synchro rotors - the salient-pole rotor and the drum or wound rotor.
The SALIENT-POLE ROTOR has a single coil of wire wound on a laminated core, shaped like a
dumbbell or the letter "H." This type of rotor is frequently used in transmitters and receivers.
The DRUM OR WOUND ROTOR may be wound continuously with a single length of wire or may have a
group of coils connected in series. This type of rotor is used in most synchro control
transformers and differential units, and occasionally in torque transmitters. When used in differential
units, the rotor is wound with three coils so their magnetic axes are 120 degrees apart.
A SYNCHRO STATOR is a cylindrical structure of slotted lamination on which three Y-connected
coils are wound with their axes 120º apart.
TORQUE is simply a measure of how much load a machine can turn. In heavy machinery, it is
expressed in pound-feet and in torque synchro systems, it is expressed in ounce-inches.
OPERATING VOLTAGES AND FREQUENCIES vary with different equipment. Synchros are designed for use on either
a 115-volt or a 26-volt power source. They also operate on either a 60- or 400-Hz frequency.
ELECTROMAGNETIC THEORY forms the basis for understanding all synchro operations.
The RESULTANT MAGNETIC FIELD in a synchro is the result of the combined effects of three
stator fields spaced 120º apart. The stator coil with the strongest field has the greatest effect on the
position of the resultant field.
The ZERO-DEGREE POSITION of a synchro, transmitter is the position where the rotor and the
S2 stator winding are parallel.
The SYNCHRO TRANSMITTER (TX) converts a mechanical input, which is the angular position of the
rotor, into an electrical output signal. The output is taken from the stator windings and is used by a TDX, a TDR,
or a TR to move light loads, such as dials and pointers.
MAXIMUM INDUCED STATOR VOLTAGE
occurs in a single synchro stator coil each time there is maximum magnetic coupling between the rotor and the
stator coil. This voltage is approximately equal to the product of the effective voltage on the primary, the
secondary-to-primary turns ratio, and the magnetic coupling between the rotor and the stator coil.
The SYNCHRO RECEIVER (TR) is electrically identical to the synchro transmitter. The
receiver, however, uses some form of rotor damping that is not present in the transmitter. This real difference
between a synchro transmitter and a synchro receiver lies in their applications. The receiver converts the
electrical data, supplied to its stator from the transmitter, back to a mechanical angular output through the
movement of its rotor.
DAMPING is a method used in synchro receivers to prevent the rotor from oscillating or spinning.
There are two types of damping methods - ELECTRICAL and MECHANICAL. The electrical method is commonly used in
small synchros, while the mechanical method is more effective in larger synchros.
A TORQUE SYNCHRO SYSTEM consists of a torque transmitter (TX) electrically connected to a
torque receiver (TR). In this system, the mechanical input to the TX is transmitted electrically to the TR. The TR
reproduces the signal from the TX and positions either a dial or a pointer to indicate the transmitted
CORRESPONDENCE is the term given to the positions of the rotors of a synchro transmitter and a
synchro receiver when both rotors are on 0º or displaced from 0º by the same angle.
SIGNAL is defined as the angle through which a transmitter rotor is mechanically turned. This
term has the same meaning as "transmitter's mechanical input."
may be in a direction opposite to that desired. When it is necessary to reverse receiver rotation, reverse the S1
and S3 connections on either the synchro transmitter or the synchro receiver. The causes both synchro rotors to
turn through the same angle but in opposite directions.
REVERSED ROTOR CONNECTIONS are common problems in new or modified synchro systems and should
not be confused with the deliberate reversal of the stator connections. The reversal of the R1 and R2 connections
on a synchro rotor causes a 180º error between the synchro transmitter and the synchro receiver, but the direction
of rotor rotation still remains the same.
A TORQUE DIFFERENTIAL SYNCHRO SYSTEM consists either of a TX, a TDX, (torque
differential transmitter), and a TR; or two TXs and one TDR (torque differential receiver). The system is used in
applications where it is necessary to compare two signals, add or subtract the signals, and finish an output
proportional to the sum of or difference between the two signals.
The TORQUE DIFFERENTIAL
has one electrical input to the stator and one mechanical input to the rotor. The TDX either adds or subtracts
these inputs, depending upon how it is connected in the system, and provides an electrical output from its rotor
proportional to the sum of or difference between the two signals.
The TORQUE DIFFERENTIAL RECEIVER (TDR) is electrically identical to the TDX. The only
difference in their construction is that the TDR has some form of damping. The real difference between the two
differentials lies in their applications. The TDR has two electrical inputs, one to the rotor and the other to the
stator. The output is the mechanical position of the rotor. As is the case with the TDX, the addition or
subtraction function of the TDR depends upon how it is connected in the system.
Introduction to Matter, Energy, and Direct Current,
to Alternating Current and Transformers, Introduction to Circuit Protection,
Control, and Measurement
, Introduction to Electrical Conductors, Wiring Techniques,
and Schematic Reading
, Introduction to Generators and Motors
Introduction to Electronic Emission, Tubes, and Power Supplies,
Introduction to Solid-State Devices and Power Supplies
Introduction to Amplifiers, Introduction to
Wave-Generation and Wave-Shaping Circuits
, Introduction to Wave Propagation, Transmission
Lines, and Antennas
, Microwave Principles,
, Introduction to Number Systems and Logic Circuits, Introduction
to Microelectronics, Principles of Synchros, Servos, and Gyros
Introduction to Test Equipment
, Radar Principles,
The Technician's Handbook,
Master Glossary, Test Methods and Practices,
Introduction to Digital Computers,
Magnetic Recording, Introduction to Fiber Optics