Editorial Feature

What is a Selsyn?

Selsyn or synchro is an electro-mechanical device used for the easy and precise transmission of angular data between two or more remote points. The readings may be provided in terms of mechanical position or as an electrical signal.

Selsyn is a portmanteau of ‘self-synchronous’. In the broad sense, selsyn systems can refer to a variety of rotary, electromechanical, position-sensing devices. In the early 1900s, selsyn systems were initially used in the control system of the Panama Canal to transmit lock gate and valve stem positions as well as water levels to control desks.

Advancement in the smaller sized selsyns has allowed the construction of smaller and more compact equipment.

Types of Selsyns

Selsyn or synchro systems consist of two or more selsyns interconnected electrically. There are two types of selsyn systems:

  • Torque system - The selsyns in this system provide a low-power mechanical output, which is sufficient to position an indicating device, trigger a sensitive switch, or move light loads without the need for power amplification. Accuracy on the order of one degree is attainable by this system. In the torque system, only small loads are turned and hence only a small amount of torque is required. Some torque systems can be used as control systems; however, control systems cannot replace torque units.
  • Control-type system – This  particular system is used in applications such as follow-up links and error detectors in servo and automatic control systems. For this system, the transmitted signal manipulates the power source.

Working Principle

A selsyn system requires the use of at least two synchro machines. The selsyn motors resemble two synchronous motors. They are connected to each other with three wires.The system uses the principle of electromagnetic induction.

When the rotors of the two machines are in corresponding positions, the voltages of the two secondaries at that given time are equal in magnitude and opposite in direction. In this case, there is no current flow in the rotor circuits, and no torque is formed in either machines; thus the machines remain in equilibrium.

When the two rotors are not in corresponding positions, the voltages of the two secondaries do not neutralize each other and a current is produced in the two secondaries. This current produces torque in the machines, which in turn acts upon the rotors, causing them to move in corresponding positions.

Basically, the selsyn machine falls into these functional categories - torque transmitter (TX), control transmitter (CX), torque differential transmitter (TDX), control differential transmitter (CDX), torque receiver (TR), torque differential receiver (TDR), torque receiver-transmitter (TRX), and control transformer (CT).


The following are the key application areas of selsyn:

  • Military and navy – for an underwater detection system, navigation system, and communication system
  • In aircrafts
  • Manufacturing and processing plants – for indicating the position of generator rheostats and water-wheel governors, and turntables
  • Municipal utilities
  • Steel mills
  • In shipboard equipment
  • In remote signaling systems such as signaling from switchboard to generator room; marine signals between engine room and bridge; and steel-mill furnace to blower room
  • For automatic or remote position control such as synchronizing of incoming generators; water-wheel governors; motor drives system frequency; gates or valves; and color screens on lights in theaters
  • In the operation of two machines so as to maintain a definite time-position relation or a definite speed relation, such as lift bridges, hoists, kiln drives, elevators, unit printing presses, and conveyors.


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