Construction and Working Principle of 3 phase Induction Motor

Construction and Working Principle of a 3-Phase Induction Motor

Construction

A 3-phase induction motor consists of two main parts: the stator and the rotor.

• Stator:

  • The stator is the stationary part of the motor.
  • It is constructed from a steel frame that encloses a hollow, cylindrical core made of thin laminations of silicon steel to minimize hysteresis and eddy current losses.
  • Slots are evenly spaced on the inner periphery of the laminations to house the stator winding.
  • The stator winding is a 3-phase winding that is wound for a specific number of poles to achieve the desired speed.
  • The stator winding is connected to a 3-phase power supply, which creates a rotating magnetic field.

• Rotor:

  • The rotor is the rotating part of the motor.
  • It is separated from the stator by a small air gap, typically ranging from 0.4 mm to 4 mm.
  • There are two main types of rotors: squirrel cage and wound rotor.
    • Squirrel cage rotor:
      • This is the most common type of rotor due to its simplicity and robustness.
      • It consists of a laminated cylindrical core with conductive bars (typically aluminum or copper) embedded in the slots.
      • The bars are short-circuited at both ends by end rings, forming a cage-like structure.
    • Wound rotor:
      • This type of rotor has a 3-phase winding similar to the stator winding.
      • The rotor winding is typically star-connected, and its open ends are connected to three slip rings mounted on the rotor shaft.
      • Brushes resting on the slip rings allow for external connections, typically to a 3-phase star-connected rheostat.

Working Principle

The operation of a 3-phase induction motor relies on the principle of electromagnetic induction.

1. Rotating Magnetic Field:

   • When 3-phase power is supplied to the stator winding, a rotating magnetic field is created within the stator.
   • This field rotates at a constant magnitude and speed, known as the synchronous speed (Ns), which is determined by the number of poles and the frequency of the power supply: Ns = 120f/P.

2. Induction of Rotor Currents:

   • The rotating magnetic field passes through the air gap and cuts the rotor conductors.
   • This relative motion between the rotating field and the stationary rotor conductors induces an electromotive force (EMF) in the rotor conductors.
   • Since the rotor circuit is short-circuited (either by the end rings in a squirrel cage rotor or externally in a wound rotor), currents start flowing in the rotor conductors.

3. Torque Development:

   • The current-carrying rotor conductors experience a force due to the interaction with the stator’s magnetic field.
   • This force produces a torque on the rotor, causing it to rotate in the same direction as the rotating magnetic field.

4. Slip:

   • The rotor of an induction motor never reaches the synchronous speed.
   • The difference between the synchronous speed (Ns) and the actual rotor speed (N) is called the slip (s): s = (Ns - N)/Ns.
   • Slip is necessary for the induction of rotor currents and, consequently, for torque development.
   • The slip increases with increasing load, resulting in a slightly lower rotor speed.

5. Rotor Frequency:

   • The frequency of the induced currents in the rotor is directly proportional to the slip.
   • At standstill (s = 1), the rotor frequency is equal to the stator frequency.
   • As the rotor accelerates, the slip decreases, and so does the rotor frequency.

The 3-phase induction motor is a robust and versatile machine widely used in various industrial applications. Understanding its construction and working principle provides insights into its performance characteristics and control methods.