In other words, the rotating flux field appears to the stator as the north and south poles of a magnet rotating about the stator.
The speed of induction motors can however be varied over a limited range by varying the rotor resistance as noted in the section on slip but only by using wound rotor designs negating many of the advantages of the induction motor.Since motor speed depends on the speed of the rotating field, speed control can be effected by changing the frequency of the AC power supplied to the motor.Similar to the DC machine, the torque in an induction motor T is proportional to the flux density B and the induced rotor current I.Thus T = k is a constant depending on the number of stator turns, the number of phases and the configuration of the magnetic circuit.The motor speed will settle at an equilibrium speed when the motor torque equals the load torque.
This occurs when the slip provides just enough current to deliver the required torque.
An induction motor is therefore an asynchronous machine The relative motion between the rotating field and the rotating rotor is called the slip and is given by: S = is the rotor speed.
Since the rotor current is proportional to the relative motion between the rotating field and the rotor speed, the rotor current and hence the torque are both directly proportional to the slip.
p must be an even integer since for every north pole there is a corresponding south pole.
The following table shows motors speeds for motors with different numbers of poles working with different AC supply frequencies.
Once current is flowing in the rotor windings, the motor action due to the Lorentz force on the conductors comes into effect.