synchro n : a system consisting of a generator and a motor so connected that the motor will assume the same relative position as the generator; the generator and the motor are synchronized [syn: selsyn]

A synchro or "selsyn" is a type of rotary electrical transformer that is used for measuring the angle of a rotating machine such as an antenna platform. The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current, which by electromagnetic induction causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator. 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 an electric motor that will rotate in unison with the synchro. In the latter case, the whole device is also called a selsyn (a portmanteau of self and synchronizing).

Synchro systems were first used in the control system of the Panama Canal, to transmit lock gate and valve stem positions, and water levels, to the control desks.

Fire-control system designs developed during World War II used syncros 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, 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 have replaced syncros in most other applications.

Synchros designed for terrestrial use tend to be driven at 50 or 60 hertz (the mains 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 driven by the engines).

Selsyn motors were widely used in motion picture equipment to synchronize movie cameras and sound recording equipment, before the advent of crystal oscillators and microelectronics.

On a practical level, selsyns resemble motors, in that there is a rotor, stator, and a shaft which rotates or can be rotated. 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.

Single phase units have five wires: two for an excitor winding (typically line voltage) and three for the output/input. These three are bussed to the other selsyns in the system, and provide the power and information to precisely align by rotation all the shafts in the receivers. Be sure to match the voltages when building a selsyn interlock system. Different makes of selsyns have different output voltages.

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

In all cases; 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.

References

See also

• Amplidyne
• Rotary encoder
• Resolver
• RVDT