Unlike the direct current motor, in an induction motor there is no electrical connection to the rotor, circuit and therefore, the same condition exists as in the DC motor i.e., the rotor conducts carry current in the stator magnetic field and thereby have a force exerted upon them tending to move them at right angle to the field.
- When 3 phase stator winding is energized from a 3 phase, a rotating magnetic field set up which rotates around the stator at synchronous speed (NS=120F/P).
- The rotating filed passes through the air-gap and cuts the rotor conductor, which as yet, are stationary. Due to the relative speed between the rotating flux and stationary rotor,
emfsare induced in the rotor conductor. since the rotor circuit is short-circuited current start flowing in the rotor conductors.
- The current carrying rotor conductors are placed in the magnetic field produced by the stator. consequently,
mechanicalforce acts on the rotor conductor. the sum of the mechanical force on all the rotor conductor produces a torque which tends to move the rotor in the same direction as the rotating filed.
- The fact that
rotoris urged to follow the stator filed (i.e., rotor moves in the direction of stator filed) can be explained by Lenz’s law.
- According to this law, the direction of the rotor current will be such that they tend to oppose the cause producing them.
- Now, the cause producing the rotor current is the relative speed between the rotating field and the stationary rotor conductor. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator filed and tries to catch it.
- In case the relative motion is zero i.e., the rotor runs at synchronous speed, there would be no induced emf and no current in the rotor conductors, no rotor filed and hence no torque. so, an induction motor cannot run at synchronous speed.
- An induction motor running at no load will have a speed very close to the synchronous speed and therefore,
emfsin the rotor winding will be very small. This small emf gives a small current. producing a torque just sufficient to overcome the losses such as due to friction and maintain the rotor in motion.
- As the motor slows down, the relative motion. Between the magnetic field and the rotor increased. This result the greater rotor currents and greater developed torque.
In respect of speed-load characteristics, a 3-phase induction motor is quite similar to a dc shunt motor.
Also Read- Principle of operation of DC Generator
Also Read- Types of DC generators