A three-phase induction motor with a caged rotor, also known as a squirrel cage rotor, is a robust and widely used AC electric motor. Here’s a breakdown of its construction:
Main Components:
- Stator: The stationary outer part of the motor that houses the magnetic field generating components.
- Rotor: The rotating inner part of the motor that converts electrical energy into mechanical energy.
Stator Construction:
- Stator Frame: A rigid steel frame that supports and protects the stator core and windings.
- Stator Core: Made of laminated electrical steel to minimize eddy current losses. The laminations are typically slotted to accommodate the stator windings.
- Stator Windings: Three sets of insulated copper wires distributed around the stator core. These windings are connected to create a rotating magnetic field when supplied with three-phase AC power. The number of poles created by the windings depends on the winding configuration and the desired motor speed.
Rotor Construction (Squirrel Cage Rotor):
- Rotor Core: Similar to the stator core, it’s made of laminated electrical steel to minimize eddy current losses. The rotor core also has slots on its periphery.
- Rotor Bars: Heavy copper or aluminum bars inserted into the rotor core slots. These bars are usually brazed or welded to end rings at both ends of the rotor. The end rings are also made of copper or aluminum and short-circuit all the rotor bars together.
Operating Principle:
- When three-phase AC power is supplied to the stator windings, a rotating magnetic field is generated.
- The rotating magnetic field induces an electric current in the rotor bars due to the principle of electromagnetic induction.
- The induced current in the rotor bars interacts with the rotating magnetic field, creating a force that tries to rotate the rotor in the same direction as the magnetic field.
- As the rotor starts to rotate, due to Lenz’s Law, the induced current in the rotor bars tries to oppose the change in magnetic field. This creates a torque that keeps the rotor speed slightly less than the speed of the rotating magnetic field, a condition known as slip.
Advantages of Squirrel Cage Rotor:
- Simple and robust construction: The caged rotor design is very reliable and requires minimal maintenance.
- Low cost: Due to its simple design, squirrel cage rotors are generally less expensive to manufacture compared to wound rotor motors.
- High starting torque: The robust rotor construction allows for good starting torque capabilities.
Disadvantages of Squirrel Cage Rotor:
- Limited speed control: Squirrel cage motors typically have limited speed control options compared to wound rotor motors. Their speed is primarily determined by the supply frequency and the number of poles in the stator windings.
- Lower efficiency at lower loads: Squirrel cage motors might have lower efficiency when operating at partial loads.
This description provides a basic overview of the construction and operation of a three-phase induction motor with a caged rotor. The specific design details and materials used might vary depending on the motor’s application and power rating.