Single Phase Induction Motor - A Complete Guide for SSC JE EE Exams

Introduction

A single-phase induction motor is structurally similar to a 3-phase induction motor, except for a few key differences:

  • It operates using a single-phase AC supply.
  • It includes an additional centrifugal switch for starting purposes.

The stator winding is of distributed type, and the rotor is typically a squirrel-cage type. The alternating current produces an alternating flux in one spatial direction only, which cannot generate a rotating magnetic field. This makes single-phase induction motors inherently non-self-starting.

Why Single-Phase Induction Motors Are Not Self-Starting

  • A single-phase induction motor is not self-starting because the alternating current (AC) supplied to the stator winding produces an alternating magnetic field, not a rotating one.
  • When this alternating magnetic field interacts with the rotor, it induces equal and opposite torques in both directions of rotation. These torques cancel each other out, resulting in zero net starting torque. Therefore, the motor cannot start on its own.
➤ Key Reasons:
  • Alternating Field, Not Rotating Field
    • The stator winding produces an alternating magnetic flux that oscillates in one direction only, instead of rotating around the stator.

  • No Net Torque at Standstill
    • Due to the symmetrical induction of clockwise and counterclockwise torque, the rotor experiences zero net torque when stationary.

  • Lenz’s Law & Double Revolving Field Theory
    • According to the Double Revolving Field Theory, the alternating flux can be resolved into two rotating magnetic fields rotating in opposite directions. These produce equal and opposite torques at start → resulting in no motion.

How the Single Phase Induction Motor is Made Self-Starting:

  • To overcome this limitation, an additional starting or auxiliary winding is placed 90° electrically apart from the main winding. By adding a capacitor or resistance in series, a phase difference is created between the two windings. This generates a rotating magnetic field at startup, enabling the motor to develop sufficient starting torque.

Types of Single-Phase Induction Motors

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