Page 335 - CITS - Electronic Mechanic - TT - 2024

ELECTRONICS MECHANIC - CITS

Working of Three Phase Induction Motor Production of

Rotating Magnetic Field

The stator of the motor consists of overlapping winding offset by an electrical angle of 120o. When we connect
the primary winding, or the stator to a 3 phase AC source, it establishes rotating magnetic field which rotates at
the synchronous speed.

Secrets Behind the Rotation
According to Faraday’s law an emf induced in any circuit is due to the rate of change of magnetic flux linkage
through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or
directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar
and due to this emf a current flows through the rotor conductor.
Here the relative speed between the rotating flux and static rotor conductor is the cause of current generation;
hence as per Lenz’s law, the rotor will rotate in the same direction to reduce the cause, i.e., the relative velocity.
Thus from the working principle of three phase induction motor, it may be observed that the rotor speed should
not reach the synchronous speed produced by the stator. If the speeds become equal, there would be no such
relative speed, so no emf induced in the rotor, and no current would be flowing, and therefore no torque would
be generated. Consequently, the rotor cannot reach the synchronous speed. The difference between the stator
(synchronous speed) and rotor speeds is called the slip. The rotation of the magnetic field in an induction motor
has the advantage that no electrical connections need to be made to the rotor.
Thus the three phase induction motor is:
• Self-starting
• Less armature reaction and brush sparking because of the absence of commutators and brushes that may
cause sparks
• Robust in construction
• Economical
• Easier to maintain
Advantages

• Low Cost: Induction machines are very cheap when compared to synchronous and DC motors. This is due
to the modest design of the induction motor. Therefore, these motors are overwhelmingly preferred for fixed
speed applications in industrial applications and for commercial and domestic applications where AC line
power can be easily attached.

• Low Maintenance Cost: Induction motors are maintenance-free motors unlike dc motors and synchronous
motors. The construction of an induction motor is very simple and hence maintenance is also easy, resulting
in low maintenance cost.

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CITS : E & H - Electronics Mechanic - Lesson 205 - 222