NEETS Module 15 − Principles of Synchros, Servos, and Gyros
This type of winding is frequently used in both transmitters and receivers.
The drum or wound rotor has
coils wound in slots in a laminated core as shown in figure 1-6. This type of rotor is used in most synchro
control transformers and differential units, and occasionally in torque transmitters. It may be wound continuously
with a single length of wire or may have a group of coils connected in series. The single continuous winding
provides a distributed winding effect for use in transmitters. When the rotor is wound with a group of coils
connected in series, a concentrated winding effect is provided for use in control transformers. When used in
differential units, the rotor is wound with three coils so their magnetic axes are 120º apart.
Figure 1-6. - Drum or wound rotor.
Both types of synchro rotors have their coils wound on laminated cores that are rigidly mounted on a shaft. To
enable the excitation voltage to be applied to the rotor winding, two slip rings are mounted on one end of the
shaft and insulated from the shaft to prevent shorting. An insulated terminal board, mounted on the end of the
cylindrical frame, houses the brushes, which ride on the slip rings. These brushes provide continuous electrical
contact to the rotor during its rotation. Also mounted on the rotor shaft are low-friction ball bearings, which
permit the rotor to turn easily.
The stator of a synchro is a
cylindrical structure of slotted laminations on which three Y-connected coils are wound with their axes 120º
apart. In figure 1-7, view a shows a typical stator assembly consisting of the laminated stator, stator windings,
and cylindrical frame; view B shows the stator lamination and the slots in which the windings are placed. Some
synchros are constructed so both the stator and the rotor may be turned. Electrical connections to this type of
stator are made through slip rings and brushes.
Figure 1-7A. - Typical stator.
Figure 1-7B. - Stator lamination.
Now, refer to figure 1-4 for a view of a completed synchro assembly. The rotor has been placed in the stator
assembly, and a terminal board has been added to provide a point at which internal and external connections can be
Q-9. What are the two major components of a synchro?
Q-10. Which of the two main
types of rotors can have either a single winding or three Y-connected windings?
Q-11. How does
the stator receive its voltage?
Q-12. Where are the external connections made on standard
Synchro characteristics play a very important part in synchro troubleshooting and maintenance. By closely
observing these characteristics, you can generally tell if a synchro or synchro system is working properly. Low
torque, overheating, and improper operating voltages are just a few of the abnormal
characteristics found in synchro systems. In general, the load capacity of a synchro system is limited
by the number and types of receiver units loading the transmitter, the loads on these receiver units, and the
Torque is simply a measure of how much load a machine can turn. In torque
synchros, only small loads are turned; therefore, only a small amount of torque is required. The measure of torque
is the product of the applied force and the distance between the point of application and the center of rotation.
For instance, if a 3 ounce weight is suspended from a synchro pulley having a radius of 2 inches, the torque
required to move the weight is 6 ounce-inches. In heavy machinery, torque may be expressed in pound-feet, but
torque synchro measurements are in ounce-inches.
Note: The unit of torque is the pound-foot or ounce-inch.
Do not confuse this with foot-pounds, which is the measurement of work. Many times in referring to torque, tools
are marked in foot-pounds. While this use of foot-pounds is technically incorrect, common usage has made it
The torque developed in a synchro receiver results from the tendency of two electromagnets to
align themselves. Since the rotor can be turned and the stator usually cannot, the stator must exert a force
(torque) tending to pull the rotor into a position where the primary and secondary magnetic fields are in line.
The strength of the magnetic field produced by the stator determines the torque. The field strength depends on the
current through the stator coils. As the current through the stator is increased, the field strength increases and
more torque is developed.
Q-13. What major factors determine the load capacity of a torque-synchro
Q-14. Define the term "torque."
Q-15. What unit of measurement refers to the
torque of a synchro transmitter?
OPERATING VoltageS and FREQUENCIES
standard and Navy prestandard synchros are designed to operate on either 115 volts or 26 volts. Synchros used in
shipboard equipment are designed predominately for 115 volts, while most aircraft synchros operate on 26 volts.
Synchros are also designed to operate on a 60- or 400-Hz frequency. But like transformers, they are more
efficient at the higher frequency. Operating a synchro at a higher frequency also permits it to be made
physically smaller. This is because the lines of flux produced by the 400-Hz excitation voltage are much more
concentrated than those produced by the 60-Hz excitation voltage. Hence, the core of the 400- Hz synchro can be
made smaller than the core of the 60-Hz synchro. However, some 400-Hz synchro
units are identical in size to
their 60-Hz counterparts. This is done so that 60- and 400-Hz units can be physically interchanged without special
mounting provisions. The operating voltage and frequency of each synchro is marked on its nameplate.
The use of the smaller size synchro permits the construction of smaller and more compact equipment. The most widely used frequency for airborne equipment is 400 Hz. It is being used increasingly in shipboard equipment as well. The
newer gun and missile fire-control systems use 400-Hz synchros almost exclusively.
A synchro designed for
60-Hz operation may occasionally be used with a 400-Hz supply. There may be considerable loss of accuracy, but the
synchro will not be damaged. This should be done only in the case of an emergency when the specified replacement
is not available, and system accuracy is not critical.
NEVER connect a 400-Hz synchro to 60-Hz voltage. The
reduced impedance results in excessive current flow and the windings quickly burn out.
Q-16. What type of equipment normally uses 26-volt 400-hertz synchros?
TEMPERATURES and SPEEDS
Standard synchros are designed to withstand surrounding temperatures
ranging from -67º F to +257º F (-55º C to +125º C) at the terminal board. Prestandard synchros operate in a range
of -13º F to +185º F (-25º C to +85º C). When a synchro is energized and not loaded, its temperature should stay
within prescribed limits. Loading an energized synchro causes it to generate more heat. Similarly, overloading
causes a synchro to generate much more heat than it would under normal loading conditions and could possibly
result in permanent synchro damage. To meet military specifications, all standard synchros must be capable of
continuous operation for 1,000 hours at 1,200 revolutions per minute (rpm) without a load.
synchro has one of two specifications, depending upon its use in a data transmission system. Low-speed prestandard
synchros must be capable of continuous operation for 500 hours at 300 rpm without a load. Low-speed prestandard
synchros must be capable of continuous operation for 1,500 hours at 1200 rpm without a load.
When will a synchro generate more heat than it is designed to handle?
THEORY of Operation
Synchros, as stated earlier, are simply variable transformers. They differ from conventional transformers by
having one primary winding (the rotor), which may be rotated through 360º and three stationary secondary windings
(the stator) spaced 120º apart. It follows that the magnetic field within the synchro may also be rotated through
360º. If an iron bar or an electromagnet were placed in this field and allowed to turn freely, it would always
tend to line up in the direction of the magnetic field. This is the basic principle underlying all synchro
We will begin the discussion of synchro operation with a few basic points on electromagnets.
Look at figure 1-8. In this figure, a simple electromagnet is shown with a bar magnet pivoted in the
electromagnet's field. In view A, the bar is forced to assume the position shown, since the basic law of magnetism
states that like poles of magnets repel and unlike poles attract. Also notice that when the bar is aligned with
the field, the magnetic lines of force are shortest. If the bar magnet is turned from this position and held as
shown in view B, the flux is distorted and the magnetic lines of force are lengthened. In this condition, a force
(torque) is exerted on the bar magnet. When the bar magnet is released, it snaps back to its original position.
When the polarity of the electromagnet is reversed, as shown in view C, the field reverses and the bar magnet is
rotated 180º from its original position.
Figure 1-8. - Operation of an electromagnet with a bar-magnet rotor.
Keeping in mind these basic points, consider how the bar magnet reacts to three electromagnets spaced 120º
apart as illustrated in figure 1-9. In this figure, stator coils S1 and S3, connected in parallel, together have
the same field strength as stator coil S2. The magnetic field is determined by current flow through the coils. The
strongest magnetic field is set up by stator coil S2, since it has twice the current and field strength as either
S1 or S3 alone. a resultant magnetic field is developed by the combined effects of the three stator fields. Coil
S2 has the strongest field, and thus, the greatest effect on the resultant field, causing the field to align in
the direction shown by the vector in view B of the figure. The iron-bar rotor aligns itself within the resultant
field at the point of greatest flux density. By convention, this position is known as the zero-degree position.
The rotor can be turned from this position to any number of positions by applying the proper combination of
voltages to the three coils, as illustrated in figure 1-10, view (A), view (B), view (C), view (D), view (E), view
Figure 1-9. - Operation of three electromagnets spaced 120º apart.
Figure 1-10A. - Positioning of a bar magnet with three electromagnets.
Figure 1-10B. - Positioning of a bar magnet with three electromagnets.
Figure 1-10C. - Positioning of a bar magnet with three electromagnets.
Figure 1-10D. - Positioning of a bar magnet with three electromagnets.
Figure 1-10E. - Positioning of a bar magnet with three electromagnets.
Figure 1-10F. - Positioning of a bar magnet with three electromagnets.
Notice in figure 1-10, in views a C, and E, that the rotor positions are achieved by shifting the total
current through different stator windings (S1, S2, and S3). This causes the rotor to move toward the coil with the
strongest magnetic field. To obtain the rotor positions in views B, D, and F, it was necessary only to reverse the
battery connections. This causes the direction of current flow to reverse and in turn reverses the direction of
the magnetic field. Since the rotor follows the magnetic field the rotor also changes direction. By looking
closely at these last three rotor positions, you will notice that they are exactly opposite the first three
positions we discussed. This is caused by the change in the direction of current flow. You can now see that by
varying the voltages to the three stator coils, we can change the current in these coils and cause the rotor to
assume any position we desire.
In the previous examples, dc voltages were applied to the coils. Since synchros operate on ac rather than dc,
consider what happens when ac is applied to the electromagnet in figure 1-11. During one complete cycle of the
alternating current, the polarity reverses twice.
Figure 1-11. - Operation of an electromagnet with ac voltage.
Therefore, the number of times the polarity reverses each second is twice the excitation frequency, or 120
times a second when a 60-Hz frequency is applied. Since the magnetic field of the electromagnet follows this
alternating current, the bar magnet is attracted in one direction during one-half cycle (view A) and in the other
direction during the next half cycle (view B). Because of its inertia, the bar magnet cannot turn rapidly enough
to follow the changing magnetic field and may line up with either end toward the coil (view C). This condition
also causes weak rotor torque. For these reasons, the iron-bar rotor is not practical for ac applications.
Therefore, it must be replaced by an electromagnetic rotor as illustrated in figure 1-12.
Figure 1-12. - Operation of fixed and moveable electromagnets with ac voltage.
ource. During the positive alternation (view A), the polarities are as shown and the top of the rotor is
attracted to the bottom of the stationary coil. During the negative alternation (view B), the polarities of both
coils reverse, thus keeping the rotor aligned in the same position. In summary, since both magnetic fields change
direction at the same time when following the 60-Hz ac supply voltage, the electromagnetic rotor does not change
position because it is always aligned with the stationary magnetic field.
Q-18. How do
synchros differ from conventional transformers?
Q-19. Describe the zero-position of a synchro
SYNCHRO TORQUE Transmitter
The synchro transmitter converts the angular position of its
rotor (mechanical input) into an electrical output signal.
When a 115-volt ac excitation voltage is
applied to the rotor of a synchro transmitter, such as the one shown in figure 1-13, the resultant current
produces an ac magnetic field around the rotor winding. The lines of force cut through the turns of the three
stator windings and, by transformer action, induce voltage into the stator coils. The effective voltage induced in
any stator coil depends upon the angular position of that coil's axis with respect to the rotor axis. When the
maximum effective coil voltage is known, the effective voltage induced into a stator coil at any angular
displacement can be determined.
Figure 1-13. - Synchro transmitter.
Figure 1-14 illustrates a cross section of a synchro transmitter and shows the effective voltage induced in one
stator coil as the rotor is turned to different positions. The turns ratios in synchros may vary widely, depending
upon design and application, but there is commonly a 2.2:1 stepdown between the rotor and a single coil. Thus,
when 115 volts is applied to the rotor, the highest value of effective voltage induced in any one stator coil is
52 volts. The maximum induced voltage occurs each time there is maximum magnetic coupling between the rotor and
the stator coil (views A, C, and E). The effective voltage induced in the secondary winding is approximately equal
to the product of the effective voltage on the primary, the secondary-to-primary turns ratio, and the magnetic
coupling between primary and secondary. Therefore, because the primary voltage and the turns ratio are constant,
it is commonly said that the secondary voltage varies with the angle between the rotor and the stator.
Figure 1-14. - Stator voltage vs. rotor position.
When stator voltages are measured, reference is always made to terminal-to-terminal voltages (voltage induced
between two stator terminals) instead of to a single coil's voltage. This is because the
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