2. (10 points) In an AC induction motor, what is the synchronous speed? Why is the speed of the rotor always somewhat less than the synchronous speed? What is the difference called? What determines the amount of slip in an induction motor? Load. Solution 1. In an AC induction motor, When three phase supply is given to stator coils, it forms a rotating magnetic field in air gap between stator and rotr. The speed of that rotating magnetic speed is called the synchronus speed of the motor Ns = 120f/p f - frequency p - no: of poles 2. Rotor coil is shorted inside. As the stator magnetic flux cuts the rotor coils, according to faradays law , current is induced in rotor coil and direction of current is determioned by lenz law. This rotr current inturn produces a flux that inturn lags the stator flux as there are rotor lossed involved. hence, its speed is less that synchronus speed always 3. the difference is call the slip speed slip = (Ns-N)/Ns Ns - synchronus speed N - speed of rotor flux 4. As slip depends on synchronus speed, it is dependent on frequency of input voltage and also poles of motor .