Question

Explain phase shifting (production of rotating magnetic field) in shaded pole induction motor with neat diagram.

Answer 1

### Phase Shifting in Shaded Pole Induction Motor (Rotating Magnetic Field)

A shaded pole induction motor is a type of single-phase induction motor. It operates based on the principle of a rotating magnetic field. In a shaded pole motor, phase shifting is achieved by using shading coils to create a time delay in the magnetic field, which is key to producing the rotating magnetic field.

Key Components of a Shaded Pole Induction Motor:

1. Stator: Contains the main poles and shading coils.
   

1. Shading Coils: Small copper coils placed around part of each pole.
   

1. Rotor: A simple squirrel-cage rotor that follows the rotating magnetic field created by the stator.
   

Working Principle:

1. Single-phase AC Supply: When a single-phase AC voltage is applied to the motor, it creates a magnetic field in the stator.
   

1. Magnetic Field: A rotating magnetic field (RMF) is needed to rotate the rotor. However, a single-phase supply does not produce a rotating magnetic field directly.
   

   
   - Without phase shifting, a single-phase AC supply would only create a pulsating magnetic field that does not rotate, and the rotor would not rotate effectively.

1. Shading Coil Action (Phase Shifting):
   

   - Each pole of the stator is divided into two parts: the shaded part (where the shading coil is placed) and the unshaded part.
   - The shading coil introduces a time delay in the magnetic field's behavior. This delay is because the current in the shaded coil takes a bit longer to build up due to the inductance of the coil.
   - This time difference causes the magnetic flux in the shaded part of the pole to lag behind the unshaded part. The result is that the magnetic field produced by the stator does not remain in one place but shifts across the pole.
   

1. Rotating Magnetic Field:
   

   - The combination of the delayed magnetic field in the shaded area and the unshaded area produces a weak rotating magnetic field.
   - This rotating field is what makes the rotor turn. The rotor tries to follow the changing magnetic field, and due to the time delay, the magnetic field continuously shifts around the pole, giving the impression of a rotating field.

Diagram (Illustration of the Phase Shifting Process in a Shaded Pole Motor):

`
       [     Stator Pole    ]
      /       (Main Pole)     \
     |   -------------------   |   
     |   |    Shaded Part    |  |    <-- Shading Coil (Copper Coil)
     |   |  (with Time Delay)|  |
     |   -------------------   |
     |        Unshaded Part    |   
      \------------------------/

    (Time Delay Creates a Phase Shift Between the Shaded and Unshaded Parts)
`

Explanation of the Diagram:

1. The stator pole is shown, with shading coils placed over part of the pole.
   

1. When the AC supply is applied, the magnetic flux in the unshaded portion reaches its peak earlier than the shaded portion due to the inductive delay caused by the shading coil.
   

1. This delay causes a phase shift between the two parts of the pole, creating a weak rotating magnetic field.
   

Summary:

1. Phase shifting is achieved by the shading coils that introduce a time delay in the magnetic field in the shaded portion of the pole.
   

1. This time delay makes the magnetic field appear to rotate, which causes the rotor to rotate.
   

1. The shaded pole induction motor is simple but not as efficient as other types of motors, as it produces a weak rotating magnetic field and has low starting torque.
   

This is the basic working of the shaded pole induction motor and how the rotating magnetic field is produced through phase shifting.