In the previous module, containing six lessons (23-28), mainly, the study of the single-
phase two-winding Transformers – a static machine, fed from ac supply, has been
presented. In this module, containing six lessons (29-34), mainly, the study of Induction
motors, fed from balanced three-phase supply, will be described.
In this (first) lesson of this module, the formation of rotating magnetic field in the air
gap of an induction motor, is described, when the three-phase balanced winding of the
stator is supplied with three-phase balanced voltage. The balanced winding is of the same
type, as given in lesson no. 18, for a three-phase ac generator.
Induction motor, rotating magnetic field, three-phase balanced winding, and
After going through this lesson, the students will be able to answer the following
How a rotating magnetic field is formed in the air gap of a three-phase Induction
motor, when the balanced winding of the stator is fed from a balanced supply?
Why does the magnitude of the magnetic field remain constant, and also what is the
speed of rotation of the magnetic field, so formed? Also what is meant by the term
Three-phase Induction Motor
A three-phase balanced winding in the stator of the Induction motor (IM) is shown in
Fig. 29.1 (schematic form). In a three-phase balanced winding, the number of turns in
three windings, is equal, with the angle between the adjacent phases, say R & Y, is
(electrical). Same angle of
(elec.) is also between the phases, Y & B.
A three-phase balanced voltage, with the phase sequence as R-Y-B, is applied to the
above winding. In a balanced voltage, the magnitude of the voltage in each phase,
assumed to be in star in this case, is equal
, with the phase angle of the voltage between
Version 2 EE IIT, Kharagpur