Single-Phase Induction Motor

 

Single-Phase Induction Motor

1. Construction

A single-phase induction motor mainly consists of two parts:

1. Stator (Stationary Part)

  • Laminated steel core with slots.

  • Single-phase winding placed in the slots.

  • Connected to AC single-phase supply.

  • Produces an alternating (pulsating) magnetic field.

2. Rotor (Rotating Part)

  • Usually squirrel cage type rotor.

  • Made of aluminium or copper bars short-circuited by end rings.

  • Placed inside the stator with a small air gap.

Simple Construction Diagram

      Single Phase Supply
            ~
            │
   ┌─────────────────────┐
   │        STATOR       │
   │   (Stator Winding)  │
   │                     │
   │      ┌────────┐     │
   │      │ ROTOR  │     │
   │      │(S.C.)  │     │
   │      └────────┘     │
   │                     │
   └─────────────────────┘

2. Principle of Operation

When single-phase AC supply is given to the stator:

  • An alternating magnetic field is produced.

  • This field induces emf in the rotor conductors.

  • Rotor current flows due to electromagnetic induction.

However, a single-phase induction motor is not self-starting because the pulsating magnetic field produces equal torque in opposite directions at standstill.

To explain its operation, we use the Double Revolving Field Theory.

3. Double Revolving Field Theory

The theory states that:

A pulsating magnetic field can be resolved into two rotating magnetic fields of equal magnitude rotating in opposite directions at synchronous speed.

  • One field rotates in the forward direction.

  • The other rotates in the backward direction.

  • Each field has half the magnitude of the original field.

Diagram of Double Revolving Fields

            Resultant Pulsating Field
                    │
                    ▼
               ─────────
              /        \
             /          \
            ▼            ▼

     Forward Rotating      Backward Rotating
          Field                 Field
      (Clockwise)           (Anticlockwise)

        Speed = Ns           Speed = Ns

4. Working Based on Double Revolving Field Theory

At Standstill

  • Forward field produces forward torque (Tf).

  • Backward field produces backward torque (Tb).

  • Both torques are equal and opposite.

Therefore,

So the motor does not start by itself.

When Rotor Starts Rotating

If the rotor is given an initial push:

  • Slip with forward field decreases.

  • Slip with backward field increases.

  • Forward torque becomes greater than backward torque.

Therefore,

Tnet=Tf−Tb>0

The motor continues to rotate in the forward direction.

5. Important Points

  • Single-phase induction motor is not self-starting.

  • Uses double revolving field theory to explain operation.

  • Two fields rotate at synchronous speed in opposite directions.

  • Net torque appears only when rotor starts moving.

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