The speed variation of an asynchronous motor a basic aspect of the great majority of induction motors.
Presumably you are a student in this forum asking a class related question?
I'm trying to understand why you're not getting the answer you need from your text book that should be covering this principal in detail. Does your text book not cover this? Or is my assumption you're a student in a class wrong?
An asynchronous motor means that the rotor (or shaft) doesn't turn at a fixed speed locked to the power supply line frequency (like a synchronous motor). Instead, the rotor lags the synchronous magnetic field created by the line frequency current in the motor's stator by a percentage called "slip". As the rotor is loaded with more mechanical load the rotor is slowed. As it slows the slip increases because the speed difference between the rotor and the synchronous speed of the stator gets greater.
As the slip increases the back-emf (ElectroMagnetic-Force) is reduced. That causes more current to flow thru the stator. That causes a larger magnetic force to pull the slipping rotor back towards the stator's synchronous rotating field, restoring some of the increased slip.
The adding of shaft load:
1) Increases the slip
2) The back emf reduces
3) More motor current is drawn
4) The stator magnetic field increases
5) The rotor speeds back up.
Note: The more the load the the slower the motor turns the more the slip. However without the emf feedback aspect the motor would slow rapidly.
The amount of motor current can only reach the motor's nameplate amount or the motor will be damaged.
