Why an Induction Motor is almost constant Speed Motor?

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. 

Slip Speed in an Induction Motor

A rotating magnetic field is produced when three phase supply is fed to stator of an induction motor. The rotating magnetic field has constant magnitude