There are various types of motors that use slip rings, which are electromechanical devices designed to facilitate signal transmission between stationary and rotating parts. The definition of slip rings brings clarity to its function in different electric motors, which often feature parts rotating up to 360 degrees. A slip ring transmits power (electricity), or data signals from a stationary component, usually made of carbon graphite brushes, to a rotating metal alloy ring. Since many applications require this kind of transmission, manufacturers provide various types of slip rings and each features unique properties. If you are in search of a slip ring for your application, there are multiple aspects to review before making your order. Below is a brief overview of slip rings and why they are used in different electric motors.

Slip ring design

There are several types of slip rings on the market. Although these standard designs are prevalent in electric motors, most slip rings are highly customized to suit unique applications. As such, you will not find too many standard models. Those looking for a slip ring are required to provide full specification instructions so the vendor can offer models and properties customized to the machinery and operating environment. The design of a slip ring is primarily dependent on its application. For instance, if you intend to transmit data signals, you need a slip ring with better electromagnetic interference (EMI) mitigation and higher bandwidth, which aren’t all that necessary when transmitting power. Similarly, if the device will be used outdoors or in extreme operating conditions and temperatures, it should be waterproof, weatherproof, insulation and customized with other properties to meet the demands of its application. Nonetheless, the basic components of any slip ring are stationary brushes and rotating rings, although a complete assembly may include end caps and bearing, among other structural features.

Slip ring in motors

Slip rings are used in motors that have rotating parts with an unlimited number of revolutions. If the rotating component has fixed revolutions, wires would be feasible. However, when the rotations are infinite, wires become impractical and slip rings replace them as the ideal way to transmit power and data signals between stationary and rotating parts. However, each slip ring plays a different role, which may not always be transmitting signals. For instance, the wound rotor motor, which is the slip ring in the AC induction motor, is used to add resistance to the rotor windings rather than to transfer power. To understand why we use slip rings in motors, it is crucial to define motors. In electrical engineering, a motor is a machine used to convert electrical energy into mechanical energy. The motor uses wire winding to join its magnetic field to the electric current, which in turn generates mechanical force by rotating a shaft. Slip rings are merely joints that are used to help transfer electrical signals from stationary to rotating parts of the motor. However, in a standard wound rotor motor, which features three rings made of copper, copper alloys and graphite metal brushes, the slip ring is primarily used to insert resistance. Slip rings are used to control signals in both analog and digital systems such as radio telescope data transmission, beacons, aerodrome, power shovel and heliostats.

Merits of using slip rings in motors

Slip rings are rotary joints designed to transfer power or data signals between stationary and rotating interfaces. However, this is far from the role it plays in most AC motors. These electromechanical devices insert resistors into the windings. By adding resistance, the slip rings ensure rotor and stator currents are more in phase, slightly improving the efficiency of the asynchronous motor. However, this is only useful when starting the motor. Once it reaches operating speed, the brushes lose contact with the winding resulting in low torque and reduced efficiency. Nonetheless, slip rings are still inseparable from motor systems and are instrumental for the initial torque requirement when the motor is starting. Because of this, slip rings have found a way into motors that require high power at low RPM (rotations per minute), typical in systems that move heavyweight. The unique pros and cons of slip rings in motors include:

Pros:

  • Provides high torque with low current required for starting the motor
  • Offers smooth acceleration ideal for moving a heavy load
  • Eliminates abnormal heating when the engine is starting
  • Speed is adjustable
  • Optimal running characteristics once the rotor resistance is cut off at full operating speed

Cons:

  • Poor speed regulation when external rotor resistance is still on
  • Slip ring motors require higher initial maintenance cost compared to squirrel cage motors

Choosing the right slip ring for your motor

Slip ring motors can offer incredible torque even at zero RPM. However, their merits in induction motors do not imply all offers are equal. There are several things to consider when selecting a slip ring for your motor or any other machinery. The first step is to analyze your needs and outline all specifications required for the motor. Slip rings have different designs and specifications depending on their applications, so it is important to order custom specifications and properties based on your motor needs. If you have the old or damaged model, you can use it as a canvas to find ideal replicas. It is also recommendable to review existing alternatives and emerging models, which tend to boast feature advancements. Although it could be a small improvement in brush coating technology or material that reduces mechanical abrasion, you will enjoy extended longevity.

Conclusion

Slip rings are used in a myriad of applications that require transferring power and data signals from a stationary part to a rotating ring. The existing models and specifications are built to suit unique applications, machinery and operating environments. Some feature weatherproofing and waterproofing while others are thick and insulated to withstand harsh environments. As such, it is essential to identify the unique properties you seek in a slip ring before installing it on your motor. Review all mechanical and electrical specifications, including the material used, current and circuits, size and dimensions as well as maintenance. You should also choose reputable vendors and top brands in the market.

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