Control Synchros system

Control Synchros


Control synchros differ from torque synchros, in that their function is to produce an error voltage signal in the receiving element, as opposed to the production of a rotor torque. Typical uses of control synchros are in servoed altimeters and airspeed indicators which operate in conjunction with central air data computers.

The interconnection of the two elements of a control synchro system is shown in Figure 4.3.13. By convention, the transmitter is designated as CX, and the receiver designated as a control transformer (CT). The CX is similar to a torque transmitter, and from the diagram it will be noted that the a.c. supply is connected to the CX rotor only. The CT rotor is not energized since it acts merely as an inductive winding for detecting the phase and magnitude of error signal voltages which are supplied to an amplifier. The amplified signals are then fed to a two-phase motor which


is mechanically coupled to the CT rotor. Another difference to be noted is that a control synchro system is at electrical zero when the rotor of CT is at 90° with respect to the CX rotor.

Figure 4.3.13: Control synchrc system

If the rotor of CX is rotated through a certain angle, the resultant flux in the CT stator will be displaced from its datum point by the same angle, and relative to the CT rotor position at that instant. An error voltage is therefore induced in the rotor, the phase and magnitude of the voltage depending on the direction of CX rotor rotation, and on the degree of misalignment between it and the CT rotor. The error voltage is then amplified and fed to the control phase of the motor, the other phase (reference phase) being continuously supplied with alternating current. Since the control phase voltage of a two-phase motor can either lead or lag the reference phase voltage, then the phase of the error voltage will determine the direction in which the motor will rotate, and its magnitude will determine its speed of rotation. As the motor rotates, it turns the rotor of the CT in the appropriate direction, thereby reducing its displacement relative to the CX rotor. Rotation continues until both rotors are in alignment (bearing in mind, of course, that the electrical zero points are at 90° from each other) at which position no further error voltage is induced.