Wikipedia
Rotary transformer 1.jpg
Rotary transformer 2.jpg
colspan=2|Left: The fixed portion of a 6 channel rotary transformer used in a six-head VCR. Two additional shorted turns improve the isolation of the two outermost windings from each other and from the other, innermost windings.
Right: The rotating portion of the rotary transformer showing three of the six tape heads
A rotary (rotatory) transformer is a specialized transformer used to couple electrical signals between two parts that rotate in relation to each other. They may be either cylindrical or 'pancake' shaped.
Slip rings can be used for the same purpose, but are subject to friction, wear, intermittent contact, and limitations on the rotational speed that can be accommodated without damage. Wear can be eliminated by using a pool of liquid mercury instead of a solid ring contact, but the toxicity and slow corrosion of mercury are problematic, and very high rotational speeds are again difficult to achieve. A rotary transformer has none of these limitations.
Rotary transformers are constructed by winding the primary and secondary windings into separate halves of a cup core; these concentric halves face each other, with each half mounted to one of the rotating parts. Magnetic flux provides the coupling from one half of the cup core to the other across an air gap, providing the mutual inductance that couples energy from the transformer's primary to its secondary.
In brushless synchros, typical rotary transformers (in pairs) provide longer life than slip rings. These rotary transformers have a cylindrical, rather than a disc-shaped, air gap between windings. The rotor winding is a spool-shaped ferromagnetic core, with the winding placed like thread on a spool. The flanges are the pole pieces. The stator winding is a ferromagnetic cylinder with the winding inside, and end poles that are discs with holes, like washers.