Download lesson topic 8.1 synchros and control

Synchros and Control Transformers
Lesson Topic
8.1
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
SYNCHRO AND CONTROL TRANSFORMER
Synchro Identification
Type of function
C = CONTROL
T = TORQUE
Frequency
4 = 400hz
6 = 60hz
16TR6a
Diameter
in inches and tenths of
inches (1.51” to 1.6”)
Modification
a or no letter = Original
Specific function
D=Differential
R=Receiver
T=Transformer
X=Transmitter
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
SPECIFIC FUNCTION
• LETTER
•
D
•
R
•
T
•
X
DESIGNATION
DIFFERENTIAL
RECEIVER
TRANSFORMER
TRANSMITTER
If the letter b follows the specific function designation the
synchro has a rotatable stator. Ex. 1 6 T R b 6 a
SEVEN BASIC FUNCTIONAL CLASSES
1. Torque Transmitter = TX
2. Torque Differential Transmitter = TDX
3. Torque Receiver = TR or TRX
4. Torque Differential Receiver = TDR
5. Control Transmitter = CX
6. Control Differential Transmitter = CDX
7. Control Transformer = CT
All torque systems will start with a “T”, transmitter have an “X” and receivers an “R”.
All control systems will have a “C” for control, transmitter have an “X”.
Torque and control synchro systems may NOT be interchangeable.
Military Standard Synchro Code
• Identification marking synchros.
–Example:18CT6A
Military Standard Synchro Code
18CT6A
-Diameter
-Represented in inches.
– ex) 18 = 1.8 “
Military Standard Synchro Code
18CT6A
• First Letter
–Synchro General Function
»C – Control
Military Standard Synchro Code
18CT6A
• Second Letter
– Specific Function
» T – Transformer
»
Military Standard Synchro Code
18CT6A
• Frequency
• Indicated by the last number.
• 6 – 60HZ
18CT6A
• Modification
• Upper-case letter
• Indicates how many times a synchro has been factory
modified.
• ex) A - Original synchro
B - First modification etc.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS, THEORY OF OPERATION
Synchros are rotary, electromechanical, position
sensing devices, resembling
a motor, that are used to
convert mechanical signals
to electrical or vice versa.
SYNCHRO AND CONTROL TRANSFORMER
•
CHARACTERISTICS
(a) Physical
Synchros are simply variable
transformer .
Rotor – primary winding w/c
maybe rotated 360 degrees.
Stator – three stationary winding
spaced 120 electrical degrees
apart.
SYNCHRO CONSTRUCTION
A typical synchro has 2 major components:
1. Rotor – composed of a single winding (R1, R2) receiving the excitation voltage
from an external source.
Salient-pole rotor
Drum or wound rotor
Just like motors and generators slip
rings are attached to the rotor shaft, and
the excitation voltage is applied via
brushes.
2. The stator is composed of three wye-connected windings (S1,S2,S3).
Stators are very similar to the stator of
a motor or generator.
It is cylindrical in construction made of
laminated material on which the stator
windings are wound.
SYNCHRO FUNDAMENTAL
The synchro transmitter resembles a small bipolar 3-phase motor.
The stator is wound with a 3
circuit, Y-connected winding.
The rotor is wound with a single
circuit winding.
R-Rotor
S-Stator
BLOCK AND SCHEMATIC DIAGRAM OF A
SYNCHRO
The schematic symbols for the synchro transmitter and
receiver are the SAME.
Synchro Schematic Symbols
On the synchro schematic symbol the position of the arrow indicates the angular
displacement of the rotor in the figures C, D and E below the displacement is zero
degrees.
Synchro Fundamentals
• 2 types of synchros.
– Torque
– Control
Synchro Fundamentals
• Torque synchros provide enough torque to position
light loads.
– Dial
– Pointer
• Control synchros provide a small signal that is used
to position heavy loads.
– gun turrets
– missile launchers
– Numerous synchro receivers
SYNCHRO AND CONTROL TRANSFORMER
(TX)
(TR)
BASIC SYNCHRO SYSTEM CONFIGURATION
BASIC SYNCHRO SYSTEM CONFIGURATION
The Stator of TX will mirror
its induced magnetic field in
the stator of TR
Rotor is usually
geared to a manual
or mechanical
input.
BASIC SYNCHRO SYSTEM CONFIGURATION
BASIC SYNCHRO SYSTEM CONFIGURATION
CONTROL TRANSFORMER
CONTROL TRANSFORMER
SYNCHRO AND CONTROL TRANSFORMER
SYNCHRO AND CONTROL
TRANSFORMER
SYNCHRO AND CONTROL TRANSFORMER
SYNCHRO AND CONTROL TRANSFORMER
Electrically Zeroing
• Synchros must be electrically zeroed at
installation in order to function in
correspondence.
• There are three methods.
– Voltmeter
– Electric Lock
– Synchro Tester
Voltmeter method
•
•
•
•
Safest
Most accurate
Most common method
Three Steps
– Mechanical Zero (Step 1)
• Entails aligning rotor to zero mark.
– Coarse Zero (Step 2)
• Ensures fine zero step is not 180 deg out of phase.
– False Null.
– Fine Zero (Step 3)
• Ensures exact zero.
Voltmeter Method (Step 1)
• Mechanical Zero Procedure
– Deenergize System.
– Place dial pointer to the zero position.
– Tape dial pointer down.
– Disconnect Stator leads.
Voltmeter Method (Step 2)
• Course Zero Procedure
– Connect a jumper between R-2 and S-3.
– Connect meter leads between R-1 and S-2.
– Loosen captive screws around synchro housing.
• Only enough to allow movement of housing.
–
–
–
–
–
Set meter to volts AC scale.
Don PPE.
Energize System.
Turn housing until 37VAC reading is obtained.
Disconnect all meter leads and jumper.
Voltmeter Method (Step 3)
• Fine Zero Procedure
– Deenergize System.
– Connect meter leads between S-1 and S-3.
– Set meter to volts AC scale.
– Don PPE.
Voltmeter Method (Step 3)
– Energize System.
– Turn housing until null reading is obtained.
• Maximum reading 2.5 VAC.
– De-energize System.
– Return system to normal configuration.
– Conduct system test.
Electrically Zeroing
• Electric Lock
– Fastest method
– Rotors must be free to turn
– Caution: do not have system energized for more
than 2 minutes.
Electrically Zeroing
• Synchro Tester
– Primary function of synchro tester is to detect
faulty synchros.
• Can be used to zero synchros
• Less accurate since the dial is graduated in ten degree
increments.
Common System Faults
• Most synchro problems are related to wiring or
alignment.
– Poor installation.
– Loose connections.
– Broken leads or frayed cable.
• Fault symptoms will be present either in all TRs
or a single TR.
– All TRs
• Fault is in TX.
– Single TR
• Fault is in that TR.
Seven Synchro Faults
• Rotor Faults
– Shorted Rotor
– Open Rotor
– Reversed rotor
• Stator Faults
–
–
–
–
Shorted Stator
Open Stator
Reversed Stator
3 Reversed Stator Pairs
Rotor Faults
• Short
– Zero Torque
– Blown Fuses
• Open
– Low Torque
– May be 180 degrees out of correspondence.
• Reverse
– Normal torque
– Always 180 degrees out of correspondence.
Stator Faults
• Short
– High torque
– Rotor is locked across good winding
– Will affect all synchros in a system
• Open
– Normal torque
– Rotor will oscillate between good windings
• Note: always check synchro symptoms slowly to ensure accurate
indication.
Stator Faults
• Reverse
– Normal torque
– Opposite rotation
– Will correspond on good winding
• 3 Reverse Stators
– Normal torque
– Same direction
– Will lead or lag 120 out of correspondence.
Troubleshooting
Six step trouble shooting technique.
• A strategic approach to troubleshooting
–
–
–
–
–
–
(1) Symptom recognition
(2) Symptom elaboration
(3) List probable faulty function
(4) Localize faulty function
(5) Localize faulty circuit
(6) Failure analysis
Troubleshooting
• (1) Symptom recognition
– Consult tech manual on proper operation to determine
if there is a fault.
• Many times system is functioning properly
• (2) Symptom Elaboration
– Record specifics of symptom.
– i.e. What, where, how many etc.
• (3) List probable faulty function
– Decide based on first 2 steps where the problem is.
– TX or TR?
Troubleshooting
• (4) Localize the faulty function
– Take readings and prove previous findings.
• (5) Localize the faulty circuit
– Using readings, determine exact point of fault.
• (6) Failure Analysis
– Always be able to explain how you fixed it to your
chain of command or supervisor.
Corrective Maintenance
• Common practices when working with
synchros.
– Never disassemble a synchro.
– Always replace defective synchros
– Never lubricate a synchro
– Never force a synchro into place.
Synchros and Control Transformers
• Summary
QUESTIONS????
Synchros and Control Transformers
Lesson Topic
8.1
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• Synchros play a very important role in the operation of
Navy equipment. Synchros are found in just about every
weapon system, communication system, underwater
detection system, and navigation system used in the Navy.
The importance of synchros is sometimes taken lightly
because of their low failure rate. However, the technician
who understands the theory of operation and the
alignment procedures for synchros is well ahead of the
problem when a malfunction does occur. The term
"synchro" is an abbreviation of the word "synchronous." It
is the name given to a variety of rotary, electromechanical,
position-sensing devices.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• A synchro resembles a small electrical motor in size and
appearance and operates like a variable transformer. The
synchro, like the transformer, uses the principle of
electromagnetic induction. Synchros are used primarily for
the rapid and accurate transmission of information
between equipment and stations. Examples of such
information are changes in course, speed, and range of
targets or missiles; angular displacement (position) of the
ship's rudder; and changes in the speed and depth of
torpedoes. This information must be transmitted quickly
and accurately. Synchros can provide this speed and
accuracy. They are reliable, adaptable, and compact.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
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 operations. We will
begin the discussion of synchro operation with a few
basic points on electromagnets
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• 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.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• 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.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• Keeping in mind these basic points,
consider how the bar magnet reacts
to three electromagnets spaced
120º apart as illustrated in figure 19. 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.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• 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 A 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 zerodegree 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 (F).
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• 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.
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.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• Since the magnetic field of
the electromagnet follows
this alternating current, the
bar magnet is attracted in
one direction during onehalf 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).
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• This condition also causes
weak rotor torque.
• For these reasons, the ironbar rotor is not practical for
ac applications. Therefore, it
must be replaced by an
electromagnetic rotor as
illustrated in figure 1-12. In
this figure, both stationary
and rotating coils are
connected to the same 60Hz source.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• 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 CLASSIFICATION, STANDARD MARKING AND SYMBOLS
There are two general classifications of synchro systems:
1. Torque systems – designed to move light loads.
Dials or pointers for remote indicators like engine order telegraph, remote
compass indicators or wind direction indicators.
2. Control systems – designed to move heavy loads.
Gun mounts, Radar antenna and missile launchers.
The load dictates the type of synchro system.
Synchros are designed to operate at one of two frequencies, 60 or 400 Hertz and
have excitation voltages of 26 volts or 115 volts.
CAUTION: Never connect a 400 Hz synchro to a 60 Hz voltage. They will burn!
Torque receivers are electrically identical and physically the same size as torque
transmitters. The difference is the addition of some form of damping.
(DAMPING – used to prevent oscillations in, or the spinning of receiver rotors.
This is accomplished either electrically or mechanically.)
Synchro receivers are sensitive to sudden motion or oscillations.
BASIC SYNCHRO SYSTEM OPERATION
1. In a torque synchro system when the TX rotor is initially turned the receiver will
NOT be in correspondence.
2. The magnetic field around the TX rotor follows the movement of the rotor.
3. The magnetic coupling between the TX rotor and individual stator windings
change.
4. The imbalance in stator voltages between TX and TR causes current flow.
5. Current flow in the receiver produces a resultant magnetic field in the receiver
identical to that of the transmitter.
a. Torque is now applied to the receiver rotor.
b. Receiver rotor changes to the same angle as transmitter rotor.
c. The stator voltages of TX and TR began to equal out.
6. The torque receiver function is to convert the electrical data supplied to its stator
(from the transmitter), back to a mechanical angular position through the
movement of its rotor.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• a. Synchros
• (1) Synchro Identification - Example 16TR6A
• (a) Diameter-The first two digits indicate the
diameter of the synchro in tenths of an inch, to the
next higher tenth.
• (b) Usage-The first letter indicates the general
function of the synchro-C for control and T for
torque. The next letter indicates the specific
function of the synchro, as follows:
LESSON TOPIC 8.1 SYNCHROS AND CONTROL TRANSFORMERS,
THEORY OF OPERATION
• (d) the last number in the designation indicates the
operating frequency - 6 for 60Hz and 4 for 400Hz
• (e) The lower case letter following the frequency
indicator is the modification designation.
• 1) The letter "a" indicates that the synchro design is
original.
• 2) The first modification indicated by the letter "b"
etc.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• 2) Characteristics
• (a) Physical
• 1) Synchros are simply variable transformers, with one
primary winding (the rotor) which may be rotated 360
degrees and three stationary secondary windings (the
stator) spaced 120 electrical degrees apart.
• 2) Receivers are electrically identical to transmitters, except
for the addition of some form of dampening.
• a) The transmitter is the unit whose rotor is turned
mechanically by gears.
• b) The receiver is the unit whose rotor follows the
movement of the transmitters rotor.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
•
•
•
•
•
•
•
•
(b) Electrical.
1) A synchro works on the same basis as a transformer.
2) The rotor is the primary and the stator is the secondary.
3) When an AC excitation voltage is applied to the rotor of a
synchro transmitter; the resultant current produces an AC
magnetic field around the rotor windings
4) This magnetic field is induced into the stator windings.
5) The magnitude of the voltage induced into the stator
windings is dependent upon:
a) The angular position of the coil with respect to the rotor.
b) The amount of AC excitation voltage on the rotor.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (3) System Operation.
• (a) A simple synchro system (TX-TR) consists of a
transmitter (TX) electrically connected to a
receiver (TR).
• (b) Both rotors will be connected in parallel to the
same voltage source.
• (c) The mechanical input to the TX is electrically
transmitted to the TR.
• (d) The Tr converts this electrical signal into a
mechanical output, which positions either a dial or
pointer to indicate the transmitted information.
Synchro Systems
• Two or more Synchros connected in parallel form a
synchro system.
– Transmitter (TX)
• receives mechanical input
• example: Helm is turned by helmsman.
– Receiver (TR)
• Provides mechanical output.
• example: Ships rudder responds to helm.
– TXs and TRs will rotate in “correspondence.”
• Same angle
• Same direction
• Same torque
Synchro Fundamentals
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (e) Synchros in correspondence
• 1) At correspondence, the voltage in each receiver
stator equals that of the corresponding transmitter
stator coil.
• 2) The receiver's coil voltages oppose the
transmitter's coil voltages, each producing equal
voltages but the voltages are equal and opposite in
magnitude and polarity.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• 3) At this instant no current can flow in the
transmitter through S1 and S2 or S3 and S2
windings because the receivers equal but opposite
voltages across S1 and S2, and across S3 and S2
oppose the current flow. (voltage is zero)
• 4) With no current flow to establish a magnetic
field, no force is exerted on the rotors.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (f) Synchros out of correspondence.
• 1) The coupling across the S3 winding has increased.
• 2) Thus, the balance between the transmitter and receiver
stator voltages is upset.
• 3) Current flows in the stator circuit in direct proportion to
the voltage imbalance existing in the circuit.
• 4) The magnetic polarities established in the system at that
particular instant cause each stator winding in the receiver
to act as an electromagnet which causes the rotor to turn.
• 5) Once the rotors are aligned, stator voltages are equal
and opposite and current flow stops and the magnetic
fields collapse.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• b. Control Transformers
• (1) Definition
• (a) Synchro device that compares two signals, the electrical signal
applied to its stator and the mechanical signal applied to its rotor. The
output is electrical and is taken from the rotor.
• (2) CT identification - Same ID system as synchros
• (3) Characteristics
• (a) Physical
• 1) A control transformer has one rotor coil which is capable of
rotating among three (3) secondary windings, which are 120
electrical degrees apart.
• a) Similar to a synchro transmitter
• 2) Rotor is positioned perpendicular to the S2 winding when at
electrical zero.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (b) Electrical
• 1) The stator windings (inputs) are excited from a
synchro transmitter (typically a CX) or similar
device.
• 2) Through mutual induction an error voltage is
produced in the rotor (electrical output).
• 3) The rotor is never connected to an AC supply, it
is the output. The stator is considered the primary
winding and the rotor is considered the secondary
winding of the transformer.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• 4) Rotor position is derived from an external
mechanical force.
• 5) Control Transformers produce NO TORQUE on
their own, but they produce a voltage output from
their rotor terminals (R1, R2).
• (4) Principles of Operation
• (a) Excitation is supplied to the stator from a
transmitter.
• (b) A voltage is induced into the rotor winding.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• 1) The amplitude and phase of the induced voltage
depends on the angular displacement of the CT
rotor in respect to the stator's magnetic field.
• 2) When the rotor position is perpendicular to the
stator field, the voltage across the rotor is
minimum.
• 3) The maximum output voltage from the rotor of
the CT is approximately 55 volts , with a 115 VAC
system.
• This is found when the rotor is parallel to the stators
magnetic field.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (c) The rotor of the CT will usually be positioned by
an external force to null the error voltage. The rotor
is then perpendicular to the stator's magnetic field.
• (d) The voltage output from the rotor of the CT
never becomes zero, but generally at
correspondence falls as low as 50-125 millivolts
when used in a 115 VAC system.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• c. Simple Servo Loops
• (1) CT follow-up servo loop.
• (a) Turning the synchro rotor shifts the stator field
of the CT, causing an error voltage to be induced
into the CT rotor.
• (b) The phase and magnitude of the error voltage is
determined by the direction and amount that the
synchro rotor is turned in relation to the CT rotor.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• (c) The CT rotor error voltage is fed to an amplifier,
which in turn drives a servo motor.
• (d) The servo motor, through gearing, usually
positions a dial or counter and also positions the CT
rotor back in angular correspondence with the
transmitter's rotor.
• This causes the CT rotor error voltage to decrease to
zero.
• (e) The servo amplifier will then have a zero input
and the servo motor will stop.
LESSON TOPIC 8.1 SYNCHROS AND CONTROL
TRANSFORMERS, THEORY OF OPERATION
• d. Servomotor
• (1) Servomotors are manufactured AC or DC.
• (a) The AC servomotor is usually used to drive light
loads at constant speed.
• (b) The DC servomotor is usually used to drive
heavy loads at variable speeds.
SYNCHRO AND CONTROL TRANSFORMER
SYNCHRO CONSTRUCTION
SYNCHRO AND CONTROL TRANSFORMER