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•Three phase induction motors are very popular, because of their simple, robust
construction and high reliability.
•It consist of mainly two parts- Stator and Rotor
Stator- Outer Part is called as frame whereas inner part of it is called as
stator core. The core is made up of number of stampings, in which different slots
are cut to receive three phase winding, which is connected to three phase AC
• Three phase windings are arranged in such a manner in the slots that they
produces rotating magnetic field after AC supply is given to them.
• The windings are wound for a different number of poles, depending upon the
• Speed is inversely proportional to number of poles, given by the formula
Where, NS = Synchronous speed, f= frequency, and P= number of poles.
Rotor- It consist of cylindrical laminated core with parallel slots that carry
Conductors: Are heavy copper or aluminum bars which fit in each slot.
These conductor are brushed to the short circuiting and ring
• The slots are not exactly made parallel to the axis of the shaft but
are slotted a little skewed for the following reason-
• They reduce magnetic hum or noise.
• They avoid stalling of motor.
• When three phase supply is given to the motor, the resulting current generate a
magnetic flux ф.
• Due to the switching sequence of three phase current in R Y B phases, The generated
flux rotates around the “rotor conductor.”
• According to the faraday’s law which state that “ An EMF get induces in a
conductor when a changing magnetic filed link through it. Here as you can
see, two conductors are facing rotating magnetic filed produced by the
• EMF is induced in the copper bar and due to this current flows in the rotor.
• The direction of current can be given by “lenz law” which state that the direction of
induced current will be in opposite of the motion causing it.
• Here the relative velocity between the rotating flux and static rotor conductor is a cause
of current generation.
• Hence the rotor will rotate in same direction to reduce the cause i.e. relative velocity thus
rotating the rotor of induction motor.
• Therefore in this process poles are induces on the surface of the rotor and these poles
always try to catch opposite poles which are rotating on the stator periphery.
• Thus rotor rotates in the direction of rotating magnetic flux.
• Rotor poles always try to catch the rotating poles on the stator frame but they can not
achieve the same speed because if rotor is able to rotate at same speed their will be no
relative motion between the stator poles and rotor poles and hence there is no change
in flux experience by the rotor.
• In this case EMF induced on the rotor conductors will be zero.
• Therefore rotor always rotate in less speed than the speed of the rotating flux.
• This process continues until the supply is given to the stator winding.
• IN THIS WAY INDUCTION MOTOR WORKS