1. 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