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

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 used to couple electrical signals between two parts which rotate in relation to each other.

Slip ring

Slip ring

A slip ring is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, are commonly found in electric motors, electrical generators for AC...

s could be used for the same purpose, but these would be subject to friction

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

, wear

Wear

In materials science, wear is erosion or sideways displacement of material from its "derivative" and original position on a solid surface performed by the action of another surface....

, 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

Magnetic flux

Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

 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.

Uses

The most common use of a rotary transformer is within videocassette recorder

Videocassette recorder

The videocassette recorder , is a type of electro-mechanical device that uses removable videocassettes that contain magnetic tape for recording analog audio and analog video from broadcast television so that the images and sound can be played back at a more convenient time...

s. Signals must be coupled from the electronics of the VCR to the fast-moving tape head

Tape head

A tape head is a type of transducer used in tape recorders to convert electrical signals to magnetic fluctuations and vice versa.-Principles of operation:...

s 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 sensor

Torque sensor

A torque sensor or torque transducer or torquemeter is a device for measuring and recording the torque on a rotating system, such as an engine, crankshaft, gearbox, transmission, rotor, or a bicycle crank. Static torque is relatively easy to measure...

s installed on electric motors, to allow electronic control of motor speed and torque using feedback

Feedback

Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

.

Rotary transformers cannot be used in most DC motors instead of commutators

Commutator (electric)

A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...

 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.

See also

• Metadyne
  Metadyne
  A Metadyne is an electrical machine with three, or more, brushes. It can be used as an amplifier or rotary transformer. It is similar to a third brush dynamo but much more complex, having additional regulator or "variator" windings. It is also similar to an amplidyne except that the latter has a...
  
  
• Rotary Converter
  Rotary converter
  A rotary converter is a type of electrical machine which acts as a mechanical rectifier or inverter. It was used to convert AC to DC or DC to AC power before the advent of chemical or solid state power rectification...
  
  
• Variable frequency transformer
  Variable frequency transformer
  A variable frequency transformer is used to transmit electricity between two alternating current domains. The VFT is a relatively recent development...