What is a 3 Phase Induction Motor?

     

    A three-phase induction motor is an electric motor that operates on a three-phase AC power supply. It uses a rotating magnetic field generated by the three-phase current in the stator windings to induce a current in the rotor, which produces torque and causes the rotor to turn, converting electrical energy into mechanical energy.

    3 Phase Induction Motor Construction

     

    A 3 Phase induction motor has two main parts –

    • Stator
    • Rotor

    By a small air gap ranging from 0.5 mm to 4 mm the rotor and stator are separated depending on the power rating of the motor.

    1. Stator of Three Phase Induction Motor

    The stator is the stationary part of a three-phase induction motor. 

    1. Structure: The stator is made up of a steel frame that houses a hollow, cylindrical core made of thin layers of silicon steel. These layers reduce energy losses.

    2. Slots and Windings: The inner part of the core has evenly spaced slots. Insulated wires are placed in these slots and connected to form a three-phase winding, arranged in either a delta or star configuration.

    3. Poles and Speed: The windings are designed with a specific number of poles. Fewer poles result in higher motor speeds, while more poles result in lower speeds.

    4. Magnetic Field: When three-phase electricity is supplied to the stator windings, it creates a rotating magnetic field. This field induces currents in the rotor, causing it to turn and generate mechanical motion.

    2. Rotor of 3 Phase Induction Motor

     

    The rotor of an Induction motor is a laminated core hollow cylindrical, slots are constructed on its outer periphery. On these rotor slots, the rotor windings are placed.

    Depending upon the winding placement, the rotor of a 3-phase induction motor is of two types −

    1. Squirrel Cage Type Rotor
    2. Wound Type or Slip-Ring Type Rotor
    Squirrel Cage Type Rotor

     

    A squirrel cage rotor is a component of a three-phase induction motor.

    1. Structure: It has a cylindrical core made of laminated steel. The outer part has slots that hold uninsulated aluminum or copper bars.

    2. Short-Circuited Design: The bars are connected at both ends by thick rings of the same material, forming a structure that looks like a squirrel cage.

    3. Induction Principle: When the stator creates a rotating magnetic field, it induces currents in the rotor bars, causing the rotor to turn. The rotor isn’t directly connected to the power supply.

    4. Advantages:

      • Simple and robust design
      • Can operate in various environments
      • Commonly used in industries

    5. Disadvantages: It has low starting torque.

    6. Skewed Slots: The slots on the rotor are skewed (slightly angled) to:

      • Reduce noise
      • Provide smoother torque
      • Prevent magnetic locking (cogging) between rotor and stator teeth
     
    Wound Rotor or Slip Ring Rotor

    A slip ring rotor is another type of rotor used in three-phase induction motors. 

    1. Structure: It has a laminated cylindrical core with slots holding insulated wires. These wires form a three-phase winding, similar to the stator.

    2. Star Connection: The windings are connected in a star (Y) formation, with the open ends connected to slip rings mounted on the rotor shaft.

    3. Slip Rings and Brushes: Slip rings and brushes allow external resistors to be connected to the rotor circuit.

    4. Advantages:

      • External resistors can reduce starting current and increase starting torque
      • Speed control is possible by adjusting the resistors

    Both types of rotors are used in different applications based on their characteristics. The squirrel cage rotor is favored for its simplicity and durability, while the slip ring rotor is chosen for applications requiring better control of starting torque and speed.

    Working Principle of a 3-Phase Induction Motor

     

    A three-phase induction motor works on the principle of electromagnetic induction. 

    1. Electromagnetic Induction: When an electric conductor is placed in a rotating magnetic field, it generates an electromotive force (EMF). This principle is called electromagnetic induction.

    2. Main Parts:

      • Stator: The stationary part of the motor. It has windings arranged at 120° angles from each other.
      • Rotor: The rotating part that carries the main winding.

    3. Power Supply: The motor receives power through the stator windings only, making it a single-excited motor.

    4. Creating the Rotating Magnetic Field:

      • When a three-phase supply is given to the stator, it creates a rotating magnetic field (RMF) within the stator.
      • This RMF moves in an anticlockwise direction and varies with the positive and negative half-cycles of the power supply.

    5. Inducing EMF in the Rotor:

      • The stationary rotor conductors cut through the rotating magnetic field created by the stator.
      • This induces an EMF in the rotor due to electromagnetic induction.

    6. Rotor Current and Flux:

      • The induced EMF generates a current in the rotor conductors, which are short-circuited via end rings.
      • This current in the rotor creates its own magnetic flux.

    7. Interaction of Fluxes:

      • The rotor flux interacts with the stator flux. At one end of the rotor conductor, the fluxes cancel out, while at the other end, they reinforce each other.
      • This creates a difference in flux density, pushing the rotor towards the low-density flux region.

    8. Generating Torque:

      • The interaction between the rotor and stator fluxes produces a force (torque) on the rotor.
      • This torque causes the rotor to turn in the same direction as the rotating magnetic field.

    9. Speed of the Rotor:

      • The rotor never reaches the speed of the rotating magnetic field (synchronous speed) due to “slip.”
      • This slip is essential for the induction motor to function, making it also known as an asynchronous motor.

    Key Points

    • Single Excitation: Only the stator is connected to the power supply.
    • Rotating Magnetic Field: Created by the three-phase current in the stator.
    • Induced EMF: Generated in the rotor by cutting through the rotating magnetic field.
    • Torque Production: Resulting from the interaction of rotor and stator fluxes.
    • Asynchronous Operation: The rotor always runs at a speed less than the synchronous speed due to slip.

    Types of 3-Phase Induction Motors

     

    Three-phase induction motors are mainly classified into two types based on the rotor design:

    1. Squirrel Cage Induction Motor
    2. Slip-Ring (Wound Rotor) Induction Motor

    1. Squirrel Cage Induction Motor

     

    Named because the rotor resembles a squirrel cage, this type of motor is the most common due to its simple and rugged construction.

    Construction:

    • Rotor: Made of a cylindrical laminated core with slots on the outer periphery. These slots are skewed to prevent cogging, which ensures smooth operation and reduces noise.
    • Rotor Bars: Instead of windings, it has bars made of brass, copper, or aluminum. These bars are short-circuited by end rings, creating a closed circuit.
    • End Rings: The bars are welded to the end rings, providing mechanical support and ensuring a solid structure.

    Features:

    • No External Resistance: Due to the short-circuited rotor bars, external resistance cannot be added.
    • Simple and Robust: The construction is straightforward and durable, with no slip rings or brushes needed.

    Advantages:

    • Easy to maintain and operate.
    • Cost-effective due to simple construction.
    • High reliability in various industrial applications.

    Disadvantages:

    • Low starting torque.
    • Limited control over rotor current.

    2. Slip-Ring (Wound Rotor) Induction Motor

     

    This motor uses a wound rotor and is also known as a phase-wound motor. It allows for external resistance to be added, which can control starting torque and speed.

    Construction:

    • Rotor: Made of a laminated cylindrical core with semi-closed slots on the outer periphery, carrying a three-phase insulated winding.
    • Winding: The rotor winding has the same number of poles as the stator and is connected in a star (Y) configuration.
    • Slip Rings and Brushes: The ends of the winding are connected to three copper slip rings mounted on the rotor shaft. Brushes contact these slip rings to connect external resistors.

    Features:

    • External Resistance: External resistors can be connected through slip rings to control starting current and torque.
    • Speed Control: The resistors can also be used to control the motor’s speed.

    Advantages:

    • High starting torque.
    • Better control over starting current and speed.

    Disadvantages:

    • More complex and expensive due to slip rings and brushes.
    • Requires more maintenance.

     

    Applications:

    • Slip-ring motors are used in applications where high starting torque is needed, such as in cranes, elevators, and compressors.
    • Squirrel cage motors are used in general industrial applications due to their simplicity and reliability