Chapter 2 Poly phase Induction Motor Prepared by:- Jaydeepsingh Chauhan Electrical Department SVBIT Gandhinagar

induction motor 2 Induction Motor Most popular motor today in the low and medium horsepower range Very robust in construction Speed easily controllable using V/f or Field Oriented Controllers Have replaced DC Motors in areas where traditional DC Motors cannot be used such as mining or explosive environments Of two types depending on motor construction: Squirrel Cage or Slip Ring Only Disadvantage: Most of them run with a lagging power factor

induction motor 3 Stator Squirrel Cage Rotor Slip ring Rotor Cut section Of Induction Motor Construction of Induction Motor

induction motor 4 AC Machine Stator ‘a’ phase axis ‘b’ phase axis ‘c’ phase axis 120 0

induction motor 5 Squirrel Cage Rotor

induction motor 6 Slip Ring Rotor The rotor contains windings similar to stator. The connections from rotor are brought out using slip rings that are rotating with the rotor and carbon brushes that are static.

induction motor 7 1 Cycle Amp time t0t0 t1t1 t2t2 t3t3 t4t4 t 01 t 12 Currents in different phases of AC Machine

induction motor 8 aFcFc a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c FbFb FaFa F FbFb FcFc F FaFa F FbFb FcFc FcFc FbFb F Space angle (  ) in degrees F FaFa FcFc FbFb t = t 0 = t 4 t = t 1 t = t 2 t = t 3 t = t 0 = t 4 RMF(Rotating Magnetic Field)

induction motor 9 Torque Production in an Induction Motor In a conventional DC machine field is stationary and the current carrying conductors rotate. We can obtain similar results if we make field structure rotating and current carrying conductor stationary. In an induction motor the conventional 3-phase winding sets up the rotating magnetic field(RMF) and the rotor carries the current carrying conductors. An EMF and hence current is induced in the rotor due to the speed difference between the RMF and the rotor, similar to that in a DC motor. This current produces a torque such that the speed difference between the RMF and rotor is reduced.

induction motor 10 Slip in Induction Motor However, this speed difference cannot become zero because that would stop generation of the torque producing current itself. The parameter slip ‘s’ is a measure of this relative speed difference where n s,  s,f 1 are the speeds of the RMF in RPM,rad./sec and supply frequency respectively n,  are the speeds of the motor in RPM and rad./sec respectively The angular slip frequency and the slip frequency at which voltage is induced in the rotor is given by 