Why an Induction Motor is almost constant Speed Motor?
An induction motor always runs at speeds just below its synchronous speed. At no load the speed is closest to its synchronous speed. This running speed reduces further when loaded towards its full load capacity, to about 5% for a moderately rated machine tojust more than 5% for smaller rated machine. The decreased speed of the motor with an increase of loading is insignificant, and, an induction motor can be called as almost constant speed motors
The induction motor is almost a constant speed motor. The speed the rotor flux slightly lags the stator flux and there exist a relative speed between stator and the rotor. The difference of the speed motor changes slightly with increase of loading on the motor. At no s called the slip speed. At no load, the slip is very less and and the torque produced by the motor meets the no load losses and frictional losses. Hence, at no load, the motor rotates almost at the synchronous speed( Ns= 120f/P).
When the load increases, the motor tries to deliver the load to mechanical load and , in this process the speed of the motor decrease. The decreased speed of the motor cause increased slip and, the motor draws large rotor current to deliver the torque which tends to bring the speed of the motor to its original speed.