Selsyn Units

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• describe the operation of a simple selsyn system.

• describe the operation of a differential selsyn system.

• list several advantages of selsyn units.

The word selsyn is an abbreviation of the words self-synchronous. Selsyn units are special ac motors used primarily in applications requiring remote control. Small selsyn units transmit meter readings or values of various types of electrical and physical quantities to distant points. E.g., the captain on the bridge of a ship may adjust the course and speed of the ship; at the same moment, the course and speed changes are trans mitted to the engine room by selsyn units. On the engine telegraph system, mechanical positioning of a control transmits electrical angular information to a receiving unit. Similarly, readings of mechanical and electrical conditions in other parts of the ship can be recorded on the bridge by selsyn units. These units are also referred to as synchros, and are known by various trade names.


A selsyn system consists of two three-phase induction motors. The normally stationary rotors of these induction motors are interconnected so that a manual shift in the rotor position of one machine is accompanied by an electrical rotor shift in the other machine in the same direction and of the same angular displacement as the first unit.

ill 1 shows a simple selsyn system for which the units at the transmitter and receiver are identical. The rotors of these units are two-pole and must be excited from the same ac source. The three-phase stator windings are connected to each other by three leads between the transmitter and the receiver units. The rotor of each machine is called the primary and the three-phase stator winding of each machine is called the secondary. A rotor for a typical selsyn unit's illustrated in 2.

When the primary excitation circuit's closed, an ac voltage is impressed on the transmitter and receiver primaries. If both rotors are in the same position with respect to their stators, no movement occurs. If the rotors are not in the same relative position, the freely movable receiver rotor will turn to assume the same position as the transmitter rotor.

ill. 25-1 Diagram of selsyn motors showing interconnected stator and rotor windings connected to excitation source.

ill. 25-2 Wound rotor with oscillation damper and slip rings for selsyn units.

If the transmitter rotor is turned, either manually or mechanically, the receiver rotor will follow at the same speed and in the same direction.

The self-synchronous alignment of the rotors is the result of voltages induced in the secondary windings. Both rotors induce voltages into the three windings of their stators. These voltages vary with the position of the rotors. If the two rotors are in the same relative position, the voltages induced in the transmitter and receiver secondaries will be equal and opposite. In this condition, current won't exist in any part of the secondary circuit.

If the transmitter rotor is moved to another position, the induced voltages of the secondaries are no longer equal and opposite and currents are present in the windings. These currents establish a torque which tends to return the rotors to a synchronous position. Since the receiver rotor is free to move, it makes the adjustment. Any movement of the transmitter rotor is accompanied immediately by an identical movement of the receiver rotor.


ill 25-3 is a diagram of the connections of a differential selsyn system consisting of a transmitter, a receiver, and a differential unit. This system produces an angular indication of the receiver. The indication is either the sum or difference of the angles existing at the transmitter and differential selsyns. If two selsyn generators, connected through a differential selsyn, are moved manually to different angles, the differential selsyn will indicate the sum or difference of their angles.

A differential selsyn has a primary winding with three terminals. Otherwise, it closely resembles a standard selsyn unit. The three primary leads of the differential selsyn are brought out to collector rings. The unit has the appearance of a miniature wound-rotor, three-phase induction motor. The unit, however, normally operates as a single-phase transformer.

The voltage distribution in the primary winding of the differential selsyn is the same as that in the secondary winding of the selsyn exciter. If any one of the units is fixed in position and a second unit's displaced by a given angle, then the third unit which is free to rotate will turn through the same angle. The direction of rotation can be reversed by inter changing any pair of leads on either the rotor or stator winding of the differential selsyn.

If any two of the selsyns are rotated simultaneously, the third selsyn will turn through an angle equal to the algebraic sum of the movements of the two selsyns. The algebraic sign of this value depends on the direction of rotation of the rotors of the two selsyns, as well as the phase rotation of their windings.

The excitation current of the differential selsyn is supplied through connections to one or both of the standard selsyns to which the differential selsyn is connected. In general, the excitation current is supplied to the primary winding only. In this case, the selsyn connected to the differential stator supplies this current and must be able to carry the extra load without overheating. A particular type of selsyn, known as an exciter selsyn, is used to supply the current. The exciter selsyn can function in the system either as a transmitter or a receiver.


Selsyn units are compact and rugged and provide accurate and very reliable readings. Because of the comparatively high torque of the selsyn unit, the indicating pointer does not oscillate as it swings into position. Internal mechanical dampers are used in selsyn receivers to prevent oscillation during the synchronizing procedure. and to reduce any tendency of the receiver to operate as a rotor. The operation of the receiver is smooth and continuous and is in agreement with the transmitter. In addition, the response of the receiver to changes in position at the transmitter is very rapid.

ill. 3 A schematic diagram of differential selsyn connections

In the event of a power failure, the indicator of the receiver resets automatically with the transmitter when power is received. Calibration and time-consuming checks are unnecessary.

A number of advantages are offered by selsyn systems:

• The indicators are small and compact and can be located where needed.

• The simple installation requires running a few wires and bolting the selsyn units in place.

• Selsyn units can be used to indicate either angular or linear movement.

• Selsyn units control the motion of a device at a distant point by controlling its actuating mechanism.

• One transmitter may be used to operate several receivers simultaneously at several distant points.


The selsyn system is also referred to as a syncro system. The self-synchronous sys tem allows one rotor to act as a transmitter and another rotor to act as a receiver to follow the transmitter. There are several variations to allow the receiver to follow at some angle determined by a differential selsyn.


Select the correct answer for each of the following statements and place the corresponding letter in the space provided.

1. Selsyn transmitters and receivers resemble:

a. repulsion-induction motors.

b. three-phase, two-pole induction motors.

c. three-phase, four-pole induction motors.

d. synchronous machines.

2. When the primary excitation circuit's closed, ac voltage is impressed on the:

a. transmitter and receiver primaries.

b. transmitter rotor and the transmitter stator windings.

c. transmitter rotor and the receiver stator windings.

d. stator windings of both instruments.

3. A differential selsyn unit differs from a selsyn transmitter or receiver in that it requires

a. three-phase power for excitation.

b. an ac line connection to the stator winding.

c. dc on the rotor winding.

d. three connections to the rotor winding.

4. If the rotors of the two selsyn units in a selsyn indicating system are in exactly corresponding positions, the current in the secondary winding is _____.

a. within quadrature with the primary current.

b. in phase with the primary current.

c. zero.

d. less than the normal primary current.

5. Selsyn units are also referred to as:

a. synchros. c. wound-rotor motors.

b. induction motors. d. all of these.

6. The stator of the transmitter is directly connected to the stator of the receiver unit when a differential isn't used.

a. true b. false

7. In the transmitter and receiver system, the ac excitation is applied to the

a. stator winding. c. rotor winding only

b. stator and the rotor windings. d. none of these.

8. Cite several advantages of a selsyn system.


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