EASA / CAR Part-66 · Module 4 · 4.3 Servomechanisms · Cat B1/B2
The TX rotor sets a magnetic field in the CT stator. The CT rotor output is E ∝ sin(error): zero when the two shafts agree, and its phase tells the servo which way to turn. Close the loop and the CT chases the TX to null.
A synchro is a rotating transformer. The rotor carries one winding, excited from the aircraft 26 V, 400 Hz reference. The stator carries three windings 120° apart. Because the coupling to each stator winding follows the cosine of the angle between it and the rotor, the three stator voltages are all at 400 Hz and all in phase (or exactly 180° out) with the reference, and only their amplitude and polarity change with position. Three numbers fix the angle uniquely. Read the line voltages at top left as you turn the TX: the largest possible line-to-line value is 11.8 V (the 0.454 transformation ratio on 26 V). Electrical zero is the rotor position where the S1–S3 voltage is zero.
In a control system the TX stator is wired S1–S1, S2–S2, S3–S3 to the stator of a control transformer (CT). The currents that flow in the three wires rebuild, inside the CT, a magnetic field pointing the same way as the TX rotor. The CT rotor is not excited; it is purely the output winding. Its voltage is E = Emax sin(error), where the error is the angle between the two rotors. When the CT rotor winding lies at 90° to the field (drawn here as the flux sliding along the flat coil), nothing links and the output is zero: that is the null.
The output is a 400 Hz signal whose amplitude gives the size of the error and whose phase gives its direction: in phase with the reference for one sign, 180° out for the other (watch the amber trace flip). A phase-sensitive demodulator and amplifier turn that into a signed drive for the servo motor, which turns the load and the CT shaft until the error is nulled. That is follow-up, closing the loop. A control transformer never drives a load itself; it only senses the difference. Remember the contrast with a torque system (TX to torque receiver), where the receiver is free-turning and develops its own torque instead.
Two things to watch for in the exam. First, the error is zero twice a turn, but only one null is stable: at 180° apart the torque points away from the null on both sides, so any disturbance sends the shaft round to the true null (use TX → 180°, then Kick). Second, hunting: the motor and load have inertia, so with too little damping the CT overshoots, reverses and oscillates about the null. Damping, usually by velocity (tachogenerator) feedback, removes it. Too much damping and the system becomes sluggish, creeping slowly to the answer.
The two buttons marked in red are the classic wiring faults. S1 and S3 interchanged mirrors the system: the CT turns the wrong way round, and only agrees with the TX at 0° and 180°. R1 and R2 interchanged reverses the reference polarity, so everything is shifted by 180°: the CT settles half a turn from where it should be. Both together give a mirror, offset by 180°. The reference (rotor excitation) has to be present for any of it to work: with it lost, every stator voltage and the CT output are zero.