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McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



an electric machine that makes it possible to produce an angular displacement of a shaft in a device or mechanism corresponding to the angular displacement of another shaft when the two shafts are not mechanically connected. In its operating principle a selsyn constitutes a rotary transformer, in which the rotation of the rotor causes a continuous change of the mutual inductance between the windings of the transformer, that is, between the single-phase primary winding (excitation winding) and the three-phase secondary winding (synchronization winding).

Selsyns are classified, according to the function they fulfill in “transmitting” angles, as selsyn transmitters, selsyn receivers, and differential selsyns. The rotor of a selsyn transmitter is mechanically connected to a rotating shaft; in the selsyn receiver, which is electrically connected to the transmitter, the rotation of the rotor exactly duplicates the rotation of the transmitter rotor in time and phase. Differential selsyns are used for algebraic summations of angular displacements of two shafts that are not mechanically connected.

In one of the simplest arrangements, both the selsyn transmitter and the selsyn receiver have a single-phase winding located on the rotor and a Y-connected three-phase winding located in grooves in the stator. The single-phase windings are connected in parallel to a common AC main, and the two three-phase windings are connected to each other. If the rotors of the selsyn transmitter and receiver are in such positions that emf’s arise in the synchronization windings equal in magnitude but opposite in direction, there will be no current in the synchronization circuit and no torques acting upon the rotors. If the rotor of the selsyn transmitter is then turned, some nonzero totals of emf and current will arise in the synchronization circuit. In each of the machines, torques will be generated as a result of the interaction of the magnetic fluxes in the excitation windings with the currents in the synchronization windings. In a selsyn receiver this torque tends to turn the rotor by an angle equal to the rotation angle of the selsyn transmitter; that is, the torque tends to move the rotor into such a position that the emf’s induced in the synchronization winding are again equalized.

In a differential selsyn both windings are three-phase windings. One of them is connected to the three-phase winding of one selsyn transmitter; the other is connected to the three-phase winding of a second selsyn transmitter. If the rotors of the differential selsyn and one of the selsyn transmitters are connected to primary shafts, then the rotation angle of the rotor of the second selsyn transmitter will be equal to the sum of the rotation angles of both primary shafts. If, on the other hand, the rotors of the selsyn transmitters are connected with the primary shafts, then the rotation angle of the differential selsyn will be equal to the difference of the rotation angles of the selsyn transmitters.

Selsyns may be contact or contactless. In contact selsyns one of the windings is located on the rotor, which is equipped with slip rings in order to provide a connection to the other windings. In contactless selsyns both windings are located on the stator, with the axis of the excitation winding perpendicular to the axis of the synchronization winding. A moveable magnetic circuit is used to couple the excitation flux with the synchronization winding. This magnetic circuit (rotor) has a special shape that makes it possible to change the direction of magnetic flux within a limit of 90°.

Selsyns are used for control and guidance in servomechanisms and for long-range transmission of instrument readings.


Svecharnik, D. V. Distantsionnye peredachi, 3rd ed. Moscow-Leningrad, 1974.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.