Why do we use slip rings in motors? Electric motors in electrical engineering are machines used to convert electrical energy into mechanical energy. Electric motors use windings to combine their magnetic field and current to generate force through a rotating shaft.

There are different types of motors depending on the power supply, structural design, use and type of output movement. The most common types are squirrel cage motors and slip ring motors.

 Slip rings are electromechanical devices in the form of joints. It is used for the transmission of electrical signals, power or data and rotation in fixed components.

The reason why slip rings are used in motors is to insert resistance in the rotor windings. Loops control circuits in digital and analog systems. Data transmission in radio telescopes, shovels, airports, beacons or heliostats.

The design of the slip ring depends on its application. For data transmission, you need a higher rate bandwidth and EMI (electromagnetic interference) than the rate of transmission. The basic design of a slip ring includes a rotating ring and a brush.

Physical design of slip ring motor

The slip ring motor has a phase winding rotor with a double-layer winding coil of a three-phase alternator. Each slip ring in the motor is connected to one of the three phases of the rotor winding.

The rotating core is made of slotted steel laminations to accommodate three-phase windings. The windings are set 120 degrees apart. The winding starts from the inside and on the outside, its ends are connected to an insulating slip ring mounted on the rotor shaft. The wound end uses a carbon brush to contact the slip ring in the form of a carbon brush.

A wound rotor motor has three slip rings made of copper or its alloy.

The slip ring is mounted directly on the motor shaft, but is insulated. The brush used with the slip ring is graphite and is connected to the rheostat. The slip ring rotates with the rotor, while the brushes remain in contact with the ring, transferring resistance to the rotor windings.

The windings of the rotor poles must be equal to the number of poles in the stator. The winding is usually three-phase, but it can also be wound into two-phase.

Features of slip ring motors

1. Inductive resistance generated by using slip ring rotor circuit

2. Uses external resistors for current limiting components to stabilize impedance

3. Use reduced startup current

4. Increased starting force

5. Easy customization based on required start speed and required torque

6. Speed of slip ring can be adjusted by adding external resistance

The torque level depends on the amount of pull out of the speed range using different resistance levels.

Working of slip ring motor

The carbon brush connected to the rheostat initiates three-phase current, while the slip ring adds external resistance to the rotor circuit. Resistance is necessary, especially in the beginning to generate more torque.

After starting, it runs and reaches normal conditions, and then slip ring short circuit occurs. The metal collar is pushed along the shaft, creating contact on the brush. The brush is lifted from the slip ring and eliminates contact, thereby reducing losses, wear and tear due to friction.

Why increasing external resistance is important

If the slip ring motor has additional external resistance, the rotor resistance will be very high at startup, which will reduce the rotor current and increase the torque to the maximum.

In order to generate a maximum torque that is proportional to the rotor resistance, slip is required. With slippage and increased external resistance, it is possible to achieve pull-out torque at low speeds. This is why slip ring motors work best when high inertia loads require pull-out torque at almost zero speed.

Effect of starting a slip ring motor as a normal motor

If a slip ring motor is started when the rotor terminal or slip ring is short-circuited, it is bound to withstand a high locked rotor current with a very low rotor torque.

Advantages of slip ring motors

1. Controls the speed of induction motors and makes it easier than other types of motors

2. In slip ring motors, pull-out torque is achieved even at zero revolutions per minute (RPM)

3. Compared to squirrel-cage induction motors, the starting torque is much higher.

Compared with squirrel-cage motors, slip-ring motors have a smaller current range at startup, from 250% to 350%, while squirrel-cage motors have 600% to 700% of full load current.

Other types of slip ring motor types:

Pancake slip ring: 

its conductors are arranged on a flat disc centered on a rotating shaft. This design uses a shorter axial direction. This design has the following disadvantages:

1. For the same circuit requirements, a complete motor has more weight and volume than other motors

2. With greater capacitance and crosstalk

3. High brush wear rate

Due to the high brush wear rate, debris can accumulate on the vertical axis due to wear

Mercury wetted slip ring: 

It replaces brush contacts with liquid metal bonded to the contacts. Liquid metal (mercury) maintains electrical contact during rotation between fixed and rotating contacts.

Designed with low resistance and stable connection. The main disadvantage is that mercury is toxic. Mercury wetted slip ring motors are restricted in industries where food, health or pollution poses a serious threat.

Wireless slip rings: 

They transmit without actual contact or friction and contact brushes. Data and power are transmitted through magnetic fields generated by coils placed in rotating receivers and fixed transmitters.

Wireless slip ring motors can operate in harsh environments. They also require less maintenance and upkeep. The only limiting factor is the low power transfer compared to other contact slip rings.

in conclusion 

Why we use slip rings in electric motors is to reduce starting current and increase torque. The slip ring motor has this function. Slip rings in electric motors help improve the functionality of many inventions.

The use of slip rings in lifting and pumping enables people to energize and use electric motors in their daily activities. Examples include elevators, cranes, water pumps, etc.