Torque Synchros

Torque Synchros


These are the simplest form of synchro and are used for the transmission of angular position information by means of induced signals, and for the reproduction of this information by the position of a shaft at an output or receiver element. A typical application of torque synchros is in flight instrument systems.

A torque synchro system comprises two electrically similar units interconnected as shown in Figure 4.3.12, and by convention one is designated the transmitter (TX) and the other the receiver (TR).

Each unit consists of a rotor carrying a winding, and concentrically mounted in a stator carrying three windings the axes of which are 120° apart. The principal physical differences between the TX and the TR are that the rotor of the TX is mechanically coupled to an input shaft, while the TR rotor is free to rotate. The rotor windings are connected to a source of single-phase a.c. supply, and the corresponding stator connections are joined together by transmission lines. The similarity between these connection arrangements and a conventional transformer may also be noted; the rotors correspond to primary windings and the stators to secondary windings.

When the rotors are aligned with their respective stators in the position indicated they are said to be at `electrical zero'; this refers to the reference angle standardized for synchros at which a given set of stator voltages will be produced, and by this convention enables replacement synchros to be matched to each other.

With power applied to the rotors, due to transformer action a certain voltage will be induced in the stator coils the value of which will be governed, as in any transformer, by the ratio of the number of turns of the rotor (primary) and stator (secondary) coils.

When the rotors of TX and TR occupy the same angular positions and power is applied, equal and opposite voltages will be produced and hence no current can flow in the stator coils. The system (and any other type of synchro) is then said to be at `null'.

When the rotors occupy different angular positions, for example when the TX rotor is at the 30° position and the TR rotor is at electrical zero, an unbalance occurs between stator coil voltages causing current to flow in the lines and stator coils. The currents are greatest in the circuits where voltage unbalance is greatest and their effect is to produce magnetic fields which exert torques to turn the TR rotor to the same position as that of the TX.


As the TR rotor continues to turn, the misalignment, voltage unbalance and currents decrease until the 30° position is reached and no further torque is exerted on the rotor.

In considering this synchronizing action one might assume that, since currents are also flowing in the stator coils of TX, its rotor would be returned to `null'. This is a reasonable assumption, because in fact a torque is set up tending to turn the rotor in a clockwise direction. However, it must be remembered that the rotor is being actuated by some prime mover which exerts loads too great to be overcome by the rotor torques.