| Prior to the development of the rotary transformer, a slip-ring pickup was used, though this was prone to developing signal noise due to corrosion of the slip rings. | |
A rotary (rotatory) transformer is a specialized transformer used to couple electrical signals between two parts which rotate in relation to each other.
Slip rings could be used for the same purpose, but these would be subject to friction, wear, intermittent contact, and limitations on the rotational speed that can be accommodated without damage. By comparison, 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, 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 that are more commonly used. These 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 hardware washers.
The most common use of a rotary transformer is within videocassette recorders. Signals must be coupled from the electronics of the VCR to the fast-moving tape heads carried on the rotating head drum; a rotary transformer is ideal for this purpose. Most VCR designs require coupling more than one signal to the head drum. In this case, the cup core has more than one concentric winding isolated by individual raised portions of the core; the transformer used with the head drum shown to the right couples six individual channels.
Another use is to transmit the signals from rotary torque sensors installed on electric motors, to allow electronic control of motor speed and torque using feedback.
Rotary transformers cannot be used in most DC motors instead of commutators as transformers can only transfer AC current.
The so-called "brushless DC motors" as used in some washing machines are actually AC motors. House current is first rectified to DC, which is then converted again in a power control module that generates a variable-frequency, variable-voltage AC signal for the motor.