Slip Ring

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What Is a Slip Ring?

A slip ring is defined as an electrochemical device that is used to transmit signals and power from a stationary structure to a rotating structure. A slip ring is known by other names, such as an electrical swivel, an electrical rotary joint or a rotating electrical connector. It is used in various electrical machines or electromechanical systems mainly to improve mechanical performance and simplify operations. A slip ring is used mostly in applications that require rotation while transmitting electric power.

This component is found and used in space satellites, gun turrets, electro-optic sensor gimbals, radar antennas, etc. Additionally, slip rings are also commonly found in electrical generators for alternating current (AC) systems and alternators, packaging machinery, wind turbines and cable reels.

Sometimes some people tend to use the word commutator in place of slip rings. However, commutators are basically different devices that are essentially designed for use on DC motors and generators for reversing the polarity of the current in the armature windings. Commutators are segmented, while slip rings are continuous. It should be noted that the terms are not interchangeable. It should be avoided.

Composition of Slip Ring

A slip ring comprises a metal ring and brush contact, which is made up of graphite and usually runs on the outer diameter of the metal ring. The brushes can be made of phosphor bronze as well, but graphite is cheaper. The electric current that is established passes from the metal brush to the metal ring. If there is a need for one or more electrical circuits, the additional brush is assembled along the rotating axis. The number of rings and brushes is decided as per the application and design of the machine. Meanwhile, either rings or brushes are stationary. The other components will be rotating.

The brushes are usually fixed with rotating rings depending on the rotations per minute (RPM).  Also, rings can be rotated with fixed brushes. In these setups, the brushes will maintain contact with the ring by the pressure exerted by the springs. Rings are mostly mounted on the rotor, and it is rotating. Brushes, on the other hand, are fixed and placed on the brush house. Rings are often made up of metal like brass and silver, and they are insulated from each other with the help of plastic or nylon.

Types of Slip Rings

Slip rings are constructed in different sizes and types. Let us look at some types of slip rings below.

• Wireless Slip Rings: They do not depend on methods adopted using carbon contact brush and friction-based metal in slip rings. Instead, they transfer data and power wirelessly using the electromagnetic field that is produced by the coils placed in the rotating receiver and stationary transmitter. These wireless slip rings are considered a great replacement for traditional slip rings. It not only reduces mechanical contact, but this slip ring is easy to maintain and can even be used in harsh and tough environments. One disadvantage of this slip ring is that only limited power can be transmitted between the coil.

• Mercury Wetted Slip Rings: As the name suggests, mercury contact is used as conducting media. Here, current and electrical signals are transferred by liquid metal. So the sliding brush contact is replaced by a pool of liquid metal that is bounded to the contacts. During rotation, the liquid metal establishes a connection between rotating and stationary contacts. Mercury slip rings are noted for their stable connection and their low resistance. Caution has to take to prevent any leakage of mercury, as it could lead to contamination, which could lead to extremely serious conditions. Nonetheless, this type of slip ring device is also limited by temperature. Mercury freezes and becomes solid at around -40 °C.

• Pancake Slip Rings: In a pancake slip ring, conductors are placed on the flat disc. They are the type of concentric rings that are placed at the interior of a rotating shaft. This configuration possesses a high weight and higher capacitance and collects wear debris on its vertical axis efficiently.  In some cases, it has reduced axial length, which is proved to be convenient for some applications.
• Ethernet Slip Ring: This type of slip ring is used to transfer the Ethernet protocol through a rotary system. Three very crucial parameters are considered for choosing an Ethernet slip ring for communication. These include return loss, insertion loss and crosstalk. Ethernet slip rings help in reducing losses, balancing impedance and controlling crosstalk.
• Fibre Optic Slip Ring: These are used for transferring large amounts of data and passing signals across rotating interfaces. A fibre optic slip ring is also called a fibre optic joint or fibre-electric rotary joint. This type of slip ring is found in devices such as HD video transmission systems, radar, video monitoring systems, medical equipment, etc.
• Miniature Slip Rings: As the name suggests, this type of slip ring is very small in size. As for its application, it is mainly designed for use in small devices to transfer signals or power from a rotating device.
• Through Hole Slip Rings: There is a hole present in the centre of the slip ring. The purpose of this slip ring is to transmit power or signal 360˚. It is usually installed with a flange on a sleeve bracket. This slip ring offers low noise, and less maintenance is required.

Today, slip rings are virtually used in any application that includes a rotating base or platform. In addition to the traditional application of transmitting power, they can also be used to transmit analogue and digital signals from different devices.

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Slip rings &#; also referred to as rotary electrical joints, electric swivels, and collector rings &#; are devices that can transmit power, electrical signals, or data between a stationary component and a rotating component. The design of a slip ring will depend on its application &#; transmitting data, for example, requires a slip ring with higher bandwidth and better EMI (electromagnetic interference) mitigation than one that transmits power &#; but the basic components are a rotating ring and stationary brushes.

If the rotation of one component involves a fixed number of revolutions, it may be possible to use spools with sufficient cable length and rotating speed to allow for the required revolutions, although cable management in this setup can be quite complex. But if one component rotates continuously, using cables to transmit signals between the rotating and stationary components isn&#;t practical or reliable in many cases. 

Slip rings in AC motors

In a version of the AC induction motor referred to as a wound rotor motor, slip rings are used not for transferring power, but for inserting resistance into the rotor windings. A wound rotor motor uses three slip rings &#; typically made of copper or a copper alloy &#; mounted to (but insulated from) the motor shaft. Each slip ring is connected to one of the three phases of rotor windings. The slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat. As the slip rings turn with the rotor, the brushes maintain constant contact with the rings and transfer the resistance to the rotor windings.

Adding resistance to the rotor windings brings the rotor current more in-phase with the stator current. (Recall that wound rotor motors are a type of asynchronous motor, in which the rotor and stator electrical fields rotate at different speeds) The result is higher torque production with relatively low current. The slip rings are only used at start-up, however, due to their lower efficiency and drop-off of torque at full running speed. As the motor reaches its operating speed, the slip rings are shorted out and the brushes lose contact, so the motor then acts like a standard AC induction (aka &#;squirrel cage&#;) motor.

Slip ring or commutator?

You may have noticed that the design and function of a slip ring sounds very similar to that of a commutator. While there are similarities between the two, there are critical distinctions between slip rings and commutators. Physically, a slip ring is a continuous ring, whereas a commutator is segmented. Functionally, slip rings provide a continuous transfer of power, signals, or data. Specifically, in AC motors, they transfer resistance to the rotor windings.

Commutators, on the other hand, are used in DC motors to reverse the polarity of current in the armature windings. The ends of each armature coil are connected to commutator bars located 180 degrees apart. As the armature spins, brushes supply current to opposing segments of the commutator and, therefore, to opposing armature coils.

Slip rings are used in virtually any application that includes a rotating base or platform, from industrial equipment such as index tables, winders, and automated welders, to wind turbines, medical imaging machines (CT, MRI), and even amusement park rides that have a turntable-style operation. Although the traditional application for slip rings was to transmit power, they can also transmit analog and digital signals from devices such as temperature sensors or strain gauges, and even data via Ethernet or other bus networks.

Feature image credit: Rotary Systems Inc.

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