Lecture 8 synchros - theory of operation

ICE 3015: CONTROL SYSTEM
COMPONENTS
Class 8: Synchros – Theory of Operation
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
Control System Components (ICE 3015)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2018

Theory of Operation:
• 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.

• Consider how the bar magnet reacts to three electromagnets
spaced 120º apart as illustrated in figure.

• 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, view (A), view (B), view (C),
view (D), view (E), view (F).
• Notice 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 above 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.

• During one complete cycle of the alternating current, the polarity
reverses twice.
• 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.

• In this figure, both stationary and rotating coils are connected to
the same 60-Hz source.
• 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.

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, 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 in next slide 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.

• 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 voltage induced in one stator winding cannot
be measured because the common connection between the stator
coils is not physically accessible.
• In summary, the synchro transmitter converts the angular position
of its rotor into electrical stator signals, which are sent through
interconnecting wires to other synchro devices.

References:
• Navy Electricity and Electronics Training Series (NEETS)
Module 15—Principles of Synchros, Servos, and Gyros
http://www.rfcafe.com/references/electrical/NEETS-
Modules/NEETS-Module-15-1-1-1-10.htm
• M.D. Desai, “Control System Components”, Prentice Hall India
Learning Private Limited, 1st edition, 2008.

Lecture 8 synchros - theory of operation

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