An electrical motor is such an
electromechanical device which converts electrical energy into a mechanical
energy. In case of three phase AC operation, most widely used motor is Three
phase induction motor as this type of motor does not require any starting
device or we can say they are self starting induction motor.For better
understanding the principle of three phase induction motor, the basic
constructional feature of this motor must be known to us. This Motor consists
of two major parts:
Stator:
Stator of three phase induction motor is
made up of numbers of slots to construct a 3 phase winding circuit which is
connected to 3 phase AC source. The three phase winding are arranged in such a
manner in the slots that they produce a rotating magnetic field after 3Ph. AC
supply is given to them.
Rotor:
Rotor of three phase induction motor
consists of cylindrical laminated core with parallel slots that can carry
conductors. Conductors are heavy copper or aluminum bars which fits in each
slots & they are 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 magnetic humming noise & can avoid
stalling of motor.
Working of Three Phase
Induction Motor:
The stator of the motor consists of
overlapping winding offset by an electrical angle of 120°. When the primary
winding or the stator is connected 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 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 observed that the rotor speed should not reach the
synchronous speed produced by the stator. If the speeds equals, there would be
no such relative speed, so no emf induced in the rotor, & no current would
be flowing, and therefore no torque would be generated. Consequently the rotor
can not 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:
Ø Self-starting.
Ø Less armature reaction and brush sparking
because of the absence of commutators and brushes that may cause sparks.
Ø Robust in construction.
Ø Economical.
Ø Easier to maintain.
