Stator and Rotor in Electrical Machines – Definition, Types, Construction, and Functions

An electrical machine is an energy conversion device that can convert electrical energy into mechanical energy or mechanical energy into electrical energy. A machine that converts electrical energy into mechanical energy is called electric motor. An electrical machine that converts mechanical energy into electrical energy is called electric generator. Both electric generator and motor has almost same construction and mechanical parts. The two essential parts of any electrical machine (generator or motor) are stator and rotor. The stator is a stationary part of the machine, whereas the rotor is a rotating part of the machine.

In this article, we will explore the basics of stator and rotor of an electrical machine. Also, we will learn construction and characteristics of different types of rotors of electrical machines.

What is a Stator in Electrical Machines?

As the name implies, the stator is the stationary part of an electrical machine, motor or generator. It is generally the outer part of the electrical machine having three main components namely, stator frame, stator core, and stator winding.

The stator frame is the outermost cover of the stator. It protects the internal parts like windings from mechanical and environmental damages.

The stator core is a hollow cylinder having slots on the inner periphery to insert stator windings. The stator of an electrical machine is made up of high-grade silicon steel laminations.

The stator windings are the coils made up of copper or aluminum wires. These windings are connected to the external electrical system. In the case of electric motor, the stator winding accepts electrical energy from the supply. In the case of electric generator, the stator winding supply electrical energy to the external electrical loads.

Functions of Stator in Electrical Macines

The stator of an electrical machine performs the following functions −

• In an electric motor, the stator acts as the magnetic field system and produces the main field flux required for working of the motor.

• In an electric generator, the stator acts as the armature. It consists of stator or armature windings in which the emf and current is induced that is then supplied to the external electrical loads.

• It provides mechanical covering to the internal assembly of electrical machines.

Hence, this all about stator of an electrical machine. Overall, the stator of all electrical machine (motor or generator) has almost same construction. However, it performs different functions in motor and generators.

What is a Rotor in Electrical Machines?

In electrical machines, the rotor is a moving or rotating part that can freely move on its own axis. The rotor of any electrical machine (motor or generator) consists of three main parts namely, rotor core, rotor winding, and rotor shaft.

The rotor core is a cylindrical shaped component made up of high-grade silicon steel laminations. It has slots on the outer periphery to insert rotor windings.

The rotor winding can be a coil or conductor made up of copper or aluminum. The rotor windings are wound on the rotor core.

The rotor shaft is a long rod to which the mechanical load or turbine is attached. The rotor core is mounted on the shaft. Hence, when a torque is produced in the rotor core, it also rotates the shaft.

Functions of Rotor in Electrical Machines

The rotor of electrical machines performs the following functions −

• In electric motors, the rotor acts as the component that transforms electrical energy into rotational mechanical energy to drive the mechanical load like a fan. In other words, the rotor acts as the armature in electric motors and develops the working torque in the machine.

• In electric generators, the rotor acts as the magnetic field system and produces a rotating magnetic field required to induce an emf in the armature.

• In both types of electrical machines, motor and generator, the rotor in the moving part that rotates within the machine.

Types of Rotors in Electrical Machines

Different types of electrical machines have different types of rotor constructions and designs. The common types of rotors used in electrical machines are given below −

• Squirrel Cage Rotor

• Wound Rotor or Slip-Ring Rotor

• Salient Pole Rotor

• Cylindrical Rotor

Let us discuss each type of rotor in detail along with its features.

Squirrel Cage Rotor

The squirrel cage rotor is a type of rotor commonly used in induction motors. It consists of a rotor core made up of thin silicon steel laminations. It has copper or aluminum conductors embedded in the surface of the core. These conductor bars are short-circuited at both ends with the help of metal end rings.

It is also important to note about the squirrel cage rotors that the conductor bars are slightly skewed along their length to reduce the fluctuation in the torque and the noise.

Advantages of Squirrel Case Rotor

The following are some key advantages of squirrel cage rotors −

• These rotors can be used in a wide range of electric motors from a few watts to tens of megawatts.

• Squirrel cage rotors are simple to design.

• They are very strong in construction.

• Squirrel cage rotors can provide fairly constant speed operation from light load to full load.

• Squirrel cage rotors are easy to replace.

Applications of Squirrel Cage Rotor

Squirrel cage rotors are mainly employed in induction motors that are used in applications where self-starting and constant speed is desired. Some common applications of squirrel cage rotors are given as follows −

• In motors used to drive centrifugal pumps.

• In motors used to drive industrial loads such as conveyors, pumps, compressors, etc.

• In electric motors used in fans and blowers, etc.

Wound Rotor or Slip-Ring Rotor

The wound rotor is another type of rotor used in three-phase induction motors. It is also called slip-ring rotor because the rotor windings are connected to external resistance through slip rings.

The wound rotor consists of a rotor core having slots on the outer periphery to insert rotor winding. These rotor windings are connected to the slip-rings wound on the rotor shaft. Where, the slip-rings are further connected to an external resistance.

Advantages of Wound Rotor

The following are some main advantages of wound rotors −

• Wound rotor draws less starting current.

• Wound rotor can develop high starting torque.

• Wound rotors also have high overload capacity.

• Wound rotors provide easy speed control.

• The power factor of wound rotor can be improved easily.

Applications of Wound Rotor

The slip-ring or wound rotors are employed in induction motors that are used in applications where high starting torque and variable speed is required. Some common applications of wound rotors are given below −

• In induction motors used in electric trains and locomotives.

• In motors used in cranes, lifts, and hoists.

Salient-Pole Rotor

A salient-pole rotor is a type of rotor used in synchronous motor and generators. This type of rotor has salient or projected poles mounted on a hub.

The salient-poles of the rotor are made up of thin steel laminations. The rotor windings are provided on these projected poles.

This type of rotor is larger in diameter and shorter in axial length. The salient-pole rotors are used in low-speed electrical machines. Also, they have non-uniform flux distribution in the air gap of the machine.

Advantages of Salient Pole Rotor

Some key benefits of using salient pole rotors in electrical machines are given below −

• Salient pole rotors can be used with slow speed prime movers such as hydro-turbines.

• Salient pole rotors reduce the windage losses in the machine.

• Salient pole rotors also reduce the noise levels of the machines.

• These rotors reduce the bearing loads.

• Salient pole rotor provides good ventilation in the machine.

Applications of Salient Pole Rotor

Salient-pole rotors are mainly used in slow speed synchronous machines. Some practical applications of salient pole rotors are given below −

• In alternators driven by water turbines.

• Salient pole rotors are also used in car alternators.

• These rotors are also used in diesel generators.

• Salient pole rotors are also used in synchronous motors operating at a speed between 100 RPM to 400 RPM.

Cylindrical Rotor

A cylindrical rotor is a type of rotor having a smooth cylindrical construction. It is made up of a solid forged steel having slots on the outer periphery to place rotor windings.

The cylindrical rotor is also known as non-salient pole rotor as it has no projected or salient poles. Since, this rotor has a smooth cylindrical construction, hence it creates a uniformly distributed air-gap between the stator and rotor inside the machine.

The cylindrical rotor has smaller diameter and larger axial length. Due to solid cylindrical design, this type of rotor has higher mechanical strength.

Advantages of Cylindrical Rotor

The following are key benefits of cylindrical rotors −

• Cylindrical rotor has minimum windage losses due to smooth air gap.

• These rotors create a uniform air gap between stator and rotor in the machine.

• Cylindrical rotors produce less noise.

• There is no need of damper winding in the case of cylindrical rotor.

Applications of Cylindrical Rotor

Cylindrical rotors are used in electrical machines that operate at high speeds around 1500 RPM to 3000 RPM. Some practical applications of cylindrical rotors are given below −

• High speed synchronous motors.

• Alternators driven by steam turbines, gas turbines, etc.

Conclusion

In conclusion, stator and rotor are two main components of any electrical machine. Where, the stator is the stationary part whereas the rotor is the moving part of the machine. The stator of all electrical machines has almost same construction. Whereas, different electrical machines have different types of rotors depending on their construction and requirements. In this detailed article on the stator and rotor of electrical machines, I have explained the construction and applications of stator and different types of rotors used in electrical machines.