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Question: Need more easy explanation of 3 phase induction motor?
Solution:
Construction of a Three Phase Induction Motor
An electrical motor is an electromechanical device which converts electrical energy into mechanical
energy. In the case of three phase AC (Alternating Current) operation, the most widely used motor is a
3 phase induction motor, as this type of motor does not require an additional starting device. These
types of motors are known as self-starting induction motors.
To get a good understanding of the working principle of a three phase induction motor, it is essential
to understand how these motors are constructed. A 3 phase induction motor consists of two major parts:
• Stator
• Rotor
Stator
The stator of a three phase induction motor is made up of numbers of slots to construct a 3 phase
winding circuit which we connect with the 3 phase AC source. We arrange the three-phase winding in
such a manner in the slots that they produce one rotating magnetic field when we switch on the three-
phase AC supply source.
Rotor
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The rotor of a three phase induction motor consists of a cylindrical laminated core with parallel slots
that can carry conductors. The conductors are heavy copper or aluminum bars fitted in each slot and
short-circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are
slotted a little skewed because this arrangement reduces the magnetic humming noise and can avoid
stalling of the motor. In general, the field effects of stator-rotor slot combinations are the common
causes for the magnetic humming noise in induction motors.
Working
Production of Rotating Magnetic Field
The stator of the motor consists of overlapping winding offset by an electrical angle of 120°. When we
connect the primary winding, or the stator to a 3 phase AC source, it establishes a 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 is either closed through an
external resistance or directly shorted by the end ring, it cuts the stator rotating magnetic field. Due to
this, 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 a 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 would be 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 is no electrical
connections need to be made to the rotor.
Thus the three phase induction motor is:
• Self-starting
• Having armature reaction and brush sparking because of the absence of commutators and
brushes that may cause sparks
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• Robust in construction
• Economical
• Easier to maintain
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