Slip ring

Sketch of a cross-section of slip rings for an electric motor. In this example, the slip rings have a brush-lifting device and a sliding contact bar, allowing the slip-rings to be short-circuited when no longer required. This can be used in starting a slip-ring induction motor, for example.

A slip ring is a device that allows the transmission of media such as power, electrical signals, energy, optical signals, gases or liquids from a fixed structure to a rotating structure. A slip ring can be used in any electromechanical system that requires rotation while transmitting power or signals. It can improve mechanical performance, simplify system operation and eliminate damage-prone wires dangling from movable joints.

Also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels, or electrical rotary joints, these rings are commonly found in slip ring motors, electrical generators for alternating current (AC) systems and alternators and in packaging machinery, cable reels, and wind turbines. They can be used on any rotating object to transfer power, control circuits, or analog or digital signals including data such as those found on aerodrome beacons, rotating tanks, power shovels, radio telescopes, telemetry systems, heliostats or ferris wheels.

A slip ring (in electrical engineering terms) is a method of making an electrical connection through a rotating assembly. Formally, it is an electric transmission device that allows energy flow between two electrical rotating parts, such as in a motor.

Slip rings on a hydroelectric generator;
A - stationary spring-loaded graphite brushes,
B - rotating steel contact ring,
C - insulated connections to generator field winding,
D - top end of generator shaft.

Composition

Typically, a slip ring consists of a stationary graphite or metal contact (brush) which rubs on the outside diameter of a rotating metal ring. As the metal ring turns, the electric current or signal is conducted through the stationary brush to the metal ring making the connection. Additional ring/brush assemblies are stacked along the rotating axis if more than one electrical circuit is needed. Either the brushes or the rings are stationary and the other component rotates. ^{[1] [2]} This simple design has been used for decades as a rudimentary method of passing current into a rotating device.

Alternative names and uses

Some other names used for slip ring are collector ring, rotary electrical contact and electrical slip ring. Some people also use the term commutator; however, commutators are somewhat different and are specialized for use on DC motors and generators. While commutators are segmented, slip rings are continuous, and the terms are not interchangeable. Rotary transformers are often used instead of slip rings in high-speed or low-friction environments. ^[3]

A slip ring can be used within a rotary union to function concurrently with the device, commonly referred to as a rotary joint. Slip rings do the same for electrical power and signal that rotary unions do for fluid media. They are often integrated into rotary unions to send power and data to and from rotating machinery in conjunction with the media that the rotary union provides. ^[4]

Slip ring types

The two most common types of slip ring

Slip rings are made in various types and sizes; one device made for theatrical stage lighting, for example, had 100 conductors. ^[5] The slip ring allows for unlimited rotations of the connected object, whereas a slack cable can only be twisted a few times before it will bind up and restrict rotation.

Mercury-wetted slip rings

Mercury-wetted slip rings, noted for their low resistance and stable connection use a different principle which replaces the sliding brush contact with a pool of liquid metal molecularly bonded to the contacts. ^[6] During rotation the liquid metal maintains the electrical connection between the stationary and rotating contacts. However, the use of mercury can pose safety concerns if not properly handled, as it is a toxic substance. The slip ring device is also limited by temperature, as mercury solidifies at approximately -40 °C. ^[7]

Pancake slip rings

A pancake slip ring ^[8] has the conductors arranged on a flat disc as concentric rings centered on the rotating shaft. This configuration has greater weight and volume for the same circuits, greater capacitance and crosstalk, greater brush wear and more readily collects wear debris on its vertical axis. However, a pancake offers reduced axial length for the number of circuits, and so may be appropriate in some applications.

Wireless slip rings

Main article: Rotary transformer

Wireless slip rings do not rely on the typical friction-based metal and carbon brush contact methods that have been employed by slip rings since their invention, such as those explored above. Instead, they transfer both power and data wirelessly via a magnetic field, which is created by the coils that are placed in the rotating receiver, and the stationary transmitter. ^[9] Wireless slip rings are considered an upgrade from — or alternative to — traditional slip rings, as their lack of standard mechanical rotating parts means they are typically more resilient in harsh operating environments and require less maintenance. However, the amount of power that can be transmitted between coils is limited; typically a traditional contact-type slip ring can transmit orders of magnitude more power in the same volume.

Optical slip rings

An optical slip ring, also known as a fiber optic rotary joint or optical rotary joint, is a device used in various applications to transmit optical signals (light) from a stationary component to a rotating component without any interference or signal loss. It is the optical equivalent of an electrical slip ring, which transmits electrical signals across rotating interfaces. ^[10]

The primary purpose of an optical slip ring is to maintain continuous and uninterrupted communication between devices or systems that are rotating or in relative motion. This is commonly seen in applications such as: optical fiber communications, remote sensing and instrumentation, military equipment, wind turbines and other fields..

Through bore Slip Ring

A through-hole slip ring, also known as a hollow shaft slip ring, is a specialized electrical component used to transmit electrical signals, power or data from a stationary structure to a rotating structure. The term "through-hole" refers to the central opening or hollow shaft through the center of the slip ring that allows other components or structures to pass through. ^[11]

References

1. "Cobham plc :: Aerospace and Security, Aerospace Communications, Annemasse". Cobham.com. 2011-02-13. Archived from the original on 2013-05-27. Retrieved 2011-09-02.
2. "how slip rings work". Uea-inc.com. Retrieved 2011-09-02.
3. "Slip Ring Glossary". PeterLaper. Retrieved 2013-01-24.
4. "Slip Ring definition" . Retrieved 2013-03-01.
5. Alan Hendrickson, Colin Buckhurst Mechanical design for the stage Focal Press, 2008 ISBN   0-240-80631-X, page 379 with an illustration of pancake and drum-type slip rings.
6. "mercotac".
7. "Frequently Asked Slip Ring Questions". Moog.com. 2009-06-23. Retrieved 2011-09-02.
8. Peter W. Fortescue, John Stark, Graham Swinerd Spacecraft systems engineering John Wiley and Sons, 2003 ISBN   0-470-85102-3 page 521
9. "Slip Rings | How Does a Slip Ring Work • PowerbyProxi". PowerbyProxi. Archived from the original on 2016-11-24. Retrieved 2016-11-24.
10. "Fiber Optic Rotary Joints". Moog, Inc. - Precision motion control products, systems, servovalves, actuators.
11. "Slip Rings with Through Bores". Moog, Inc. - Precision motion control products, systems, servovalves, actuators.

External links

• Video depicting the interior of a slip ring in motion