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Construction and working

  • A single phase induction motor is similar to a 3-ɸ squirrel-cage induction motor in physical appearance. Its rotor is essentially the same as that used in 3-ɸ induction motors, Except for shaded pole motors, the stator is also very similar, There is a uniform air-gap between the stator and rotor but no electrical connection between them, It can he wound for any even number of poles, two, four and six being most common. Adjacent poles have opposite magnetic property and synchronous speed equation,



  • The stator windings differ in the following two aspects:

–        Firstly single phase motors are usually provided with concentric coils,

–        Secondly, these motors normally have two stator windings. In motors that operate with both windings energised, the winding with the heaviest wire is known as the main winding and the other is called the auxiliary winding. If the motor runs with auxiliary winding open, these windings are usually referred as running ,and starting:

–        In most of motors the main winding is placed at the bottom of the slots and the starting winding on top but shifted 900 from the running winding.


When the stator winding of a single phase induction motor is connected to single phase AC. supply, a magnetic field is developed, whose axis is always along the axis of stator coils. The magnetic field produced by the stator coils is pulsating, varying sinusoidally with time, Currents are induced in the rotor conductors by transformer action, these currents being in such a direction as to oppose the stator e.m.f Then the axis of the rotor e.m.f wave coincides with that of the stator field, the torque angle is, therefore, zero and no torque is developed on starting. However, if the rotor is given a push by hand or by other means in either direction, it will pick-up the speed and continue to rotate in the same direction developing operating torque. Thus a single phase induction motor is not inherently self starting and requires some special means for starting.

The above mentioned behaviour of this type of motor can be explained by anyone of the following theories:

  1. Double revolving field theory
  2. Cross-field theory.

The results given by both the theories are approximately same,

Double revolving field theory is described below:

The magnetic field produced by the stator coils is pulsating, varying sinusoidally with time. Ferrari pointed out that such a field can be resolved into two equal fields but rotating in opposite directions with equal angular velocities, The maximum value of each component is equal to half the maximum of the pulsating field.

If the initial time is such that the rotating vectors of the two component fields are along the Y-axis in the positive direction, the two component waves ɸ1 and ɸ2 coincide. The resultant of these