A synchro is a type of rotary electrical 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...

 that is used for measuring the angle of a rotating machine such as an antenna platform. In its general physical construction, it is much like an electric motor. The primary winding of the transformer, fixed to the rotor

Rotor (electric)

The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...

, is excited by an alternating current

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

, which by electromagnetic induction

Electromagnetic induction

Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

, causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator

Stator

The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...

. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive a receiver synchro that will rotate in unison with the synchro transmitter. In the latter case, the whole device may be called a selsyn (a portmanteau of self and synchronizing).

Uses

Synchro systems were first used in the control system of the Panama Canal

Panama Canal

The Panama Canal is a ship canal in Panama that joins the Atlantic Ocean and the Pacific Ocean and is a key conduit for international maritime trade. Built from 1904 to 1914, the canal has seen annual traffic rise from about 1,000 ships early on to 14,702 vessels measuring a total of 309.6...

 in the early 1900's to transmit lock gate and valve stem positions, and water levels, to the control desks.
Fire-control system

Fire-control system

A fire-control system is a number of components working together, usually a gun data computer, a director, and radar, which is designed to assist a weapon system in hitting its target. It performs the same task as a human gunner firing a weapon, but attempts to do so faster and more...

 designs developed during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 used synchros extensively, to transmit angular information from guns and sights to an analog fire control computer, and to transmit the desired gun position back to the gun location. Early systems just moved indicator dials, but with the advent of the amplidyne

Amplidyne

An amplidyne is an electromechanical amplifier invented during World War II by Ernst Alexanderson. It is usually an AC motor driving a DC generator with modifications to increase the power gain available...

, as well as motor-driven high-powered hydraulic servos, the fire control system could directly control the positions of heavy guns.
Smaller synchros are still used to remotely drive indicator gauges and as rotary position sensors for aircraft control surfaces, where the reliability of these rugged devices is needed.
Digital devices such as the rotary encoder

Rotary encoder

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...

 have replaced synchros in most other applications.

Selsyn motors were widely used in motion picture equipment to synchronize movie camera

Movie camera

The movie camera is a type of photographic camera which takes a rapid sequence of photographs on strips of film which was very popular for private use in the last century until its successor, the video camera, replaced it...

s and sound recording equipment, before the advent of crystal oscillators and microelectronics

Microelectronics

Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...

.

Large synchros were used on naval warships, such as destroyers, to operate the steering gear from the wheel on the bridge.

Operation

On a practical level, synchros resemble motors, in that there is a rotor, stator, and a shaft. Ordinarily, slip rings and brushes connect the rotor to external power. A synchro transmitter's shaft is rotated by the mechanism that sends information, while the synchro receiver's shaft rotates a dial, or operates a light mechanical load. Single and three-phase units are common in use, and will follow the other's rotation when connected properly. One transmitter can turn several receivers; if torque is a factor, the transmitter must be physically larger to source the additional current. In a motion picture interlock system, a large motor-driven distributor can drive as many as 20 machines, sound dubbers, footage counters, and projectors.

Synchros designed for terrestrial use tend to be driven at 50 or 60 hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

 (the mains

Mains

Mains may refer to:* Mains electricity * Mains power around the world* Electricity transmission* Public utility, "mains services", including electricity, natural gas, water, and sewage disposal...

 frequency

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

 in most countries), while those for marine or aeronautical use tend to operate at 400 hertz (the frequency of the on-board electrical generator

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 driven by the engines).

Single phase units have five wires: two for an exciter winding (typically line voltage) and three for the output/input. These three are bussed to the other synchros in the system, and provide the power and information to precisely align by rotation all the shafts in the receivers. Synchro transmitters and receivers must be powered by the same branch circuit, so to speak; the mains excitation voltage sources must match in voltage and phase. The safest approach is to bus the five or six lines from transmitters and receivers at a common point. Different makes of selsyns, used in interlock systems, have different output voltages. In all cases,

Three-phase systems will handle more power and operate a bit more smoothly. The excitation is often 208/240 V 3-phase mains power.

Synchro transmitters are as described, but 50 and 60-Hz synchro receivers require rotary dampers to keep their shafts from oscillating when not loaded (as with dials) or lightly loaded in high-accuracy applications.

A different type of receiver, called a control transformer (CT), is part of a position servo that includes a servo amplifier and servo motor. The motor is geared to the CT rotor, and when the transmitter's rotor moves, the servo motor turns the CT's rotor and the mechanical load to match the new position. CTs have high-impedance stators and draw much less current than ordinary synchro receivers when not correctly positioned.

Synchro transmitters can also feed synchro to digital converters, which provide a digital representation of the shaft angle.

Synchro variants

So called 'brushless synchros' use rotary transformer

Rotary transformer

A rotary transformer is a specialized transformer used to couple electrical signals between two parts which rotate in relation to each other....

s (that have no magnetic interaction with the usual rotor and stator) to feed power to the rotor. These transformers have stationary primaries, and rotating secondaries. The secondary is somewhat like a spool wound with magnet wire, the axis of the spool concentric with the rotor's axis. The "spool" is the secondary winding's core, its flanges are the poles, and its coupling does not vary significantly with rotor position. The primary winding is similar, surrounded by its magnetic core, and its end pieces are like thick washers. The holes in those end pieces align with the rotating secondary poles.

For high accuracy in gun fire control and aerospace work, so called multi-speed synchro data links were used. For instance, a two-speed link had two transmitters, one rotating for one turn over the full range (such as a gun's bearing), while the other rotated one turn for every 10 degrees of bearing. The latter was called a 36-speed synchro. Of course, the gear trains were made accurately. At the receiver, the magnitude of the 1X channel's error determined whether the "fast" channel was to be used instead. A small 1X error meant that the 36x channel's data was unambiguous. Once the receiver servo settled, the fine channel normally retained control.

For very critical applications, three-speed synchro systems have been used.

So called multispeed synchros have stators with many poles, so that their output voltages go through several cycles for one physical revolution. For two-speed systems, these do not require gearing between the shafts.

Differential synchros are another category. They have three-lead rotors and stators like the stator described above, and can be transmitters or receivers. A differential transmitter is connected between a synchro transmitter and a receiver, and its shaft's position adds to (or subtracts from, depending upon definition) the angle defined by the transmitter. A differential receiver is connected between two transmitters, and shows the sum (or difference, again as defined) between the shaft positions of the two transmitters. There are synchro-like devices called transolvers, somewhat like differential synchros, but with three-lead rotors and four-lead stators.

A resolver

Resolver (electrical)

A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog device, and has a digital counterpart, the rotary encoder.-Description:...

 is similar to a synchro, but has a stator with four leads, the windings being 90 degrees apart physically instead of 120 degrees. Its rotor might be synchro-like, or have two sets of windings 90 degrees apart. Although a pair of resolvers could theoretically operate like a pair of synchros, resolvers are used for computation.

A special T-connected transformer arrangement invented by Scott ("Scott T"

Scott-T transformer

A Scott-T transformer is a type of circuit used to derive two-phase current from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source....

) interfaces between resolver and synchro data formats; it was invented to interconnect two-phase AC power with three-phase power, but can also be used for precision applications.

See also

• Amplidyne
  Amplidyne
  An amplidyne is an electromechanical amplifier invented during World War II by Ernst Alexanderson. It is usually an AC motor driving a DC generator with modifications to increase the power gain available...
  
  
• Rotary encoder
  Rotary encoder
  A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed...
  
  
• Resolver
  Resolver (electrical)
  A resolver is a type of rotary electrical transformer used for measuring degrees of rotation. It is considered an analog device, and has a digital counterpart, the rotary encoder.-Description:...
  
  
• RVDT