What is Rotor?

Rotors are a fundamental part of electrical machines and play an important role in their operation. Rotor is the rotating part of an induction motor. It is connected to the mechanical load through the shaft. There are various designs of rotors available on the basis of application and type of machine, such as squirrel cage or wound type.

In this article we will go through the definition of the Rotor with its construction in detail with its diagram, we will also go through the classification of Rotors and difference between stator and rotor, At last we will conclude our article with some of its applications and FAQs.

Rotor has a hollow laminated core having slots on its outer periphery. It is placed inside the stator (stationary part of the machine) and is responsible for electromagnetic processes which lead to energy conversion (either electrical or mechanical). The rotor receives energy via windings. This energy creates a magnetic field which results in torque generation. This torque generator causes the rotor to spin and is used to drive external machinery.

Whether in generators or in motors, the type of rotor defines the efficiency, power output and operational requirements. A rotor generally consists of Rotor Core, Rotor Windings, and Rotor Shaft. Let us consider each part in the upcoming sections as well as discuss the working and applications of rotors in different machines.

A rotor of an electrical machine such as motor consists of following important parts:

• Rotor Core
• Rotor Shaft
• Rotor Windings
• Slip Rings
• Brushes
• Rotor Conductors
• Rotor Core: It is the central part of the rotor assembly. It is made up of thin steel laminated to reduce eddy current losses. Rotor core provides a path for magnetic flux generated via stator windings. The parameters such as efficiency, performance vary with the designing of the core.
• Rotor Shaft: It is connected with the rotor core. It provides support and motion to the rotor assembly. It is generally a steel cylindrical shaft made to withhold the mechanical stress and load during the operation of machine. Proper alignment and balance are necessary for proper functioning of a rotor.
• Rotor Windings: Rotor windings are the coils/wire wounded around the rotor core. These windings are necessary as they interact with magnetic field generated by the stator windings which in turn produces torque in the machine. The windings vary as per the type of rotor used in the machine.
• Slip Rings: Slip rings are an essential part of rotor construction. They help in transferring electrical signals between rotor windings and stator windings. They are an integral part of wound type of rotor.
• Brushes: Brushes also play an integral part in wound type rotor construction. They help in transferring electrical or power signals between rotating and stationary parts of the machine.
• Rotor Conductors: They help in conduction of electrical current. They have an integral role in construction of wound type rotor machine.

Given Below is the working of the Rotor

• A suitable supply is given to the stator windings. This creates a rotating magnetic field, which rotates around the stator at synchronous speed (Ns).
• The rotating field sweeps past through the air gap between rotor and stator, cutting through the rotor conductors, which are stationary.
• Due to relative speed between rotating flux and stationary rotor, Electromotive force (EMF) is induced in the rotor conductors.
• This induced emf in rotor, leads to torque generation in the rotor.
• Depending on the type of electrical machine used (motor or generator), energy conversion process takes place. For example, in motor electrical energy gets converted into mechanical energy whereas in a generator the mechanical energy is converted to electrical energy.
• As the rotor circuit is short-circuited (in case of Squirrel Cage rotors) or externally wounded (in case of Wound Type rotors), current starts flowing in the rotor conductors.
• The synchronization of rotor with the stator’s rotating magnetic field, induces currents in the rotor and generates torque.
• Thus, the speed of rotor is maintained along with synchronization of stator’s field.

Based on the winding placed in these slots, (called rotor windings), Rotors are of four types:

• Squirrel Cage Type
• Wound Type/ Slip Ring Type/ Phase Wound Type
• Salient Pole Rotor
• Non-salient Pole/Cylindrical [Pole Rotor]

Given Below is the construction and advantages of the Squirrel Cage Type Rotors

Construction of Squirrel Cage Type Rotors

• It consists of a laminated cylindrical core having parallel slots on its outer periphery.
• One copper or aluminum bar is placed in each slot.
• Such bars are connected at each end by metal rings known as end rings.
• This forms a permanently short-circuited winding (which is indestructible).
• The entire construction resembles a squirrel cage and hence the name.

Advantages of Squirrel Cage Type Rotors

• Simpler Construction
• Used in most of the three phase machines
• Very Durable
• Easy Maintenance and Cost Effective
• Efficient speed operation

Given Below is the construction and advantages of the Wound Type Rotors

Construction of Wound Type

• It consists of a laminated cylindrical core and carries a 3 phase winding (similar to stator).
• Rotor windings are uniformly distributed in the slots and are generally star- connected.
• The open ends of rotor windings are connected to three insulated slip rings on the rotor shaft with one brush on each slip ring.
• Initially, the external resistances are included in the rotor circuit to give a large starting torque.

Advantages of Wound Type

• External Resistances can be added as rotor bars are permanently short-circuited.
• Provides high starting torque
• Power factor can be easily improved.
• Efficient speed control

Given Below is the construction and advantages of the Salient Pole Rotor

Construction of the Salient Pole Rotor

• The core is made of hollow cylinder with stacked laminations of steel, to minimize eddy current losses.
• Salient poles project outwards from the core, thus the name- Salient Pole Rotor.
• Each salient pole has field winding wrapped around in it.
• A shaft connects core to the prime mover or the load and transmits the mechanical torque generated by the rotor.

Advantages of the Salient Pole Rotor

• Used in low and medium speed machines.
• Lower Windage losses.
• Delivers high starting torque.
• Offers improved power factor.

Cylindrical Pole Rotor

Given Below is the construction and advantages of the Cylindrical Pole Rotor

Construction of Cylindrical Pole Rotor

• The core is made of solid steel cylinder forming a smooth, round shape.
• Parallel slots are observed along the outer surface of the cylinder, running lengthwise.
• These parallel slots contains rotor windings, which are copper conductors, insulating in nature.
• At ends of rotors, large end rings are connected, which are generally made of steel.
• The shaft connects the rotor core to external load to transmit generated torque.

Advantages of Cylindrical Pole Rotor

• Efficient operation at high speeds.
• Minimum air resistance or windage losses.
• Simpler construction and maintenance.
• Doesn’t require additional damper windings for stabilization.

Given Below is the Difference Between Stator and Rotor

With the large starting torque, there is an increase in efficiency and can be used in many electrical machines. Rotors are very useful in industrial applications, energy generation systems and efficiently convert electrical energy into mechanical energy through electromagnetic principles. Rotors have a wide range of applications including:

• Motors:rotors are important components in electric motors, providing the rotational force needed for various industrial and consumer applications.
• Generators:In power generation systems, rotors convert mechanical energy into electrical energy.
• Turbines:Rotors in turbines help convert fluid energy
• Compressors:Rotors in compressors increase the pressure of gases

Thus, we conclude that a motor consists of two main parts, rotor, which is the rotating part of the machine and stator which is the stationary part of the machine. The rotation of rotor magnetic field wrt to rotation of stator magnetic field decides whether the machine acts as a generator or a motor. The construction of rotor depends on the type of electrical motor, whereas the stator’s construction is similar in all types of motor.

What is the difference between stator and rotor?

Rotor consists of the rotating part of a motor whereas a stator constitutes of the stationary part of the motor.

Why is rotor speed less than stator speed?

This is because if rotor speed was excessive there would be no relative speed between stator field and rotor conductors. Since there will be no induced rotor currents, there will be no torque to drive the rotor. Hence rotor speed is always less than stator field speed.

What is rotor frequency?

It is the frequency of alternating current flowing in rotor winding.