What is a synchro and what are its types

Synchros are electromagnetic devices that are used to transmit positional data electrically from one location to another. It can also be used to compute the sum of two rotations, or the difference in angle between them and they are also used in applications that require low output torques. It can also be called as an electromagnetic transducer which can convert the angular position of the shaft into an electrical signal. Mostly AC transmission system is known as synchros because of their synchronous action in reproducing the angular movement of a shaft.

Principle operation of synchro

Synchro is a form of a rotating transformer that resembles a small AC motor and they are widely used as an element of measuring and control systems involving rotatable shafts. The primary coil is wound on the rotor and couples magnetically to the secondary coils on the stator and the connections to the external terminals are through slip rings. Synchros mostly have three secondary windings arranged 120 degrees apart to give the varying voltage ratios as the primary winding is rotated. The rotor energizing voltage is an AC reference voltage at 50 Hz and this induces a voltage across each stator winding which is dependent on the instantaneous angle between the rotor and stator coil axes. So the voltage across any pair of stator terminals is, therefore, the sum or difference, depending on the phase of the individual stator winding voltages. It can also be called as an electromechanical transducer. The magnitude of the magnetic coupling varies between the primary and secondary according to the position of the rotating element. This, in turn, varies the magnitude of the output voltage

How synchro is used in servo system  

Control synchro system

The control synchro system is the most common of all synchros and is extensively used in aircraft and navigation systems. Control synchros are similar to the torque synchros but the signal from the receiver is going to be amplified to drive output and the impedance of the windings is much higher so there is no danger of the system burning out. The control transformer can act as a null detector and can be used in servo systems. Control synchros can be used in electromechanical servo and shaft positioning.

Classification of synchros

Synchro’s can be classified into two overlapping groups they are torque synchros and control synchros.

Torque synchros – transmitters, differentials, and receivers

Control synchros – transmitters, differentials, control transformers, resolvers, linear transformer, and the two-hybrid units: transolvers and differential resolvers

Torque transmitter

It is used to generate an electrical signal corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator is kept stationary. The electrical signal is derived from the position of the rotor relative to the stator. These transmitters are generally used in a remote position indicating system as a transmitting element.

Torque receiver

The rotor of the receiver moves to a position-dependent on the electrical angular information received from its connected torque transmitter or torque differential transmitter. They are mostly used as a receiving element in a remote position indicating system.

Torque differential receiver

Differential receiver is electrically connected to two torque transmitters, the rotor of the torque differential receiver, which is free to move aligns with the stator field and the position of the stator field depends on the input from the two transmitters and the way in which they are interconnected, torque differential receiver can be used to indicate the sum of the transmitter inputs, or the difference between them by using suitable connection.

Torque differential transmitter

It has a transmitter which receives electrical positional information from a torque transmitter and a rotor which is mechanically positioned. Because of this, it can transmit electrical information corresponding to the sum or difference between the electrical input and its own rotor angle.

Control transmitter

It is used to generate an electrical signal which is corresponding to the angular position of a mechanical component. The rotor is connected to the component and the stator is kept stationary and the electrical signal is derived from the position of the rotor relative to the stator. It is used as a position transmitting element in a remote position control system.

Control transformer

A control transformer is electrically connected to the control transmitter and then it is used to produce an electrical signal for driving a servo system. The electrical signal produced is an AC voltage with an amplitude and phase-dependent on the position of the rotor relative to the stator.

Control differential transmitter

Control differential transmitter receives electrical information from a control transmitter and has a rotor that can be mechanically moved and it will be helpful to transmit an electrical signal which is proportional to the sum or difference in angle between the electrical input and its own rotor position.


Transolvers are a control transformer with a secondary rotor winding wound in a space quadrature to the main winding. If it is used as a control transformer then the secondary rotor winding of the transolver will be dummy loaded symmetrically to avoid unbalance and when it is used as a transmitter then the rotor winding will be shorted to ground.


A resolver is a variant of synchro and it has two stator windings displaced by 90 degrees, it can be used in a similar way as synchros to transmit data in a control chain using a resolver transmitter at one end and resolver control transformer at the other. Resolvers are used in robot control and radar system.

What is the use of synchro

It can be used for electromechanical servo and shaft positioning and control synchros are used in aircraft instruments and navigation systems. Synchros are used for data transmission and it is used in radar antenna. Synchros can be used in measurement and control applications

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