Slip rings—also known as rotary electrical joints, electric rotary joints, and slip rings—are devices that can transfer electrical power, electrical signals, or data between stationary and rotating parts. The design of a slip ring will depend on its application; for example, data transmission requires slip rings with higher bandwidth and better EMI (electromagnetic interference) mitigation than power transmission. However, its basic components are a rotating ring and a stationary brush.
▲A complete slip ring assembly includes end caps, bearings, and other structural features. However, the basic components of a slip ring are the ring and the brush.
If the rotation of a component involves a fixed number of revolutions, a spool with a sufficiently long cable and rotational speed can be used to achieve the required number of revolutions, although cable management in this setup can be very complex. However, if a component rotates continuously, using cables to transmit signals between rotating and stationary components is impractical or unreliable in many cases.
slip rings in AC motors
In a form of AC induction motor called a wound-rotor motor, slip rings are not used to transmit power; instead, they are used to insert resistance into the rotor windings. Wound-rotor motors use three slip rings—typically made of copper or a copper alloy—mounted on the motor shaft (but insulated). Each slip ring is connected to one of the three-phase rotor windings. Slip ring brushes, made of graphite, are connected to a resistive device, such as a rheostat. As the slip rings rotate with the rotor, the brushes maintain stable contact with the slip rings and transfer resistance to the rotor windings.
▲The slip ring on a wound rotor AC motor. Once the motor reaches its operating speed, the brushes are lifted by the springs, and the slip ring is short-circuited through the sliding contact rod.
Increasing the resistance of the rotor windings causes the rotor current to be in phase with the stator current. The result is higher torque at relatively low current. However, slip rings are only used during startup because they are less efficient and torque decreases at full operating speed. Once the motor reaches its operating speed, the slip rings short-circuit, the brushes lose contact, and the motor behaves like a standard AC induction (also known as a "squirrel-cage") motor.
▲The slip rings in a wound-rotor motor form a secondary external circuit. Inserting a resistor into this circuit allows the motor to generate very high torque during startup, which is necessary for moving loads with high inertia.
Slip ring or commutator?
You may have noticed that the design and function of slip rings sound very similar to commutators. While there are similarities, there are important differences between slip rings and commutators. Physically, slip rings are continuous rings, while commutators are segmented. Functionally, slip rings provide a continuous transmission of power, signals, or data. Especially in AC motors, they transfer resistance to the rotor windings.
On the other hand, commutators are used in DC motors to reverse the polarity of the current in the armature windings. The end of each armature coil is connected to a commutator bar spaced 180 degrees apart. As the armature rotates, brushes supply current to the opposite section of the commutator, and thus to the opposite armature coil.
Application of slip rings
Slip rings can be used in virtually any application, including rotating bases or platforms, from industrial equipment such as indexing tables, winding machines, and automated welding machines, to wind turbines, medical imaging machines (CT, MRI), and even turntable operations on amusement park rides. While the traditional application of slip rings is power transmission, they can also transmit analog and digital signals from devices such as temperature sensors or strain gauges, and even transmit data over Ethernet or other bus networks.
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