1. Induction motors are also called asynchronous motor.
  2. Synchronous speed is given by the relation NS = 120f/p.
  3. Induction motors always run less than synchronous speed.
  4. More than 90% motors used in industries are three phase induction motors.
  5. Induction motors have simple and rugged construction, low cost, high efficiency, self starting torque and low cost maintenance.
  6. Induction motors have two main parts (i) Stator and (ii) Rotor
  7. Stator is divided into three parts (i) outer frame (ii) stator core and (iii) stator winding.
  8. Outer frame supports the stator core and to protect inner parts.
  9. For small machine it is casted and fabricated steel used for large machine.
  10. Stator core is made of laminated silicon steel.
  11. Core is laminated to reduce eddy current losses.
  12. The thickness of stampings usually varies from 0.3 to 0.5 mm.
  13. Silicon steel is reduced hysteresis losses.
  14. Enameled copper is used as stator winding.
  15. Rotor is rotating part of the induction motor. Rotor is divided into two types (i) squirrel cage rotor (ii) Slip ring or phase wound rotor.
  16. Squirrel cage rotor consists of a laminated cylindrical core having semi-closed circular slots at the outer periphery.
  17. The rotor winding is permanently short circuited with the help of end rings
  18. End rings are made of copper or aluminium.
  19. Rotor slots are skewed.

Skewing conductors have following advantages:-

  1. It reduces humming.
  2. It results smoother torque curves.
  3. It reduces the tendency of magnetic locking.
  4. It increases the rotor resistance.
  5. Slip ring rotors are also called phase wound rotor.
  6. This rotor made of laminated silicon steel and carries three phase winding.
  7. Slip rings are made of copper.
  8. Three phase slip ring induction motor uses three slip rings.
  9. Shaft is made of mild steel, and it is keyed to the rotor.
  10. The difference between synchronous speed and actual rotor speed is called slip.
  11. At standstill, the value of slip is one.
  12. Torque is directly proportional to the slip.
  13. Greater will be the slip, larger will be the torque developed.
  14. The value of at full load varies from about 6% small motors to about 2% for larger motors.
  15. Slip in an induction motor adjusts itself to such a value so as to meet the required driving torque under normal operation.
  16. The frequency of rotor currents depends upon the relative speed between rotor and stator field.
  17. When the stator is stationary, the frequency of rotor current is the same as the supply frequency.