Asynchronous motors

The asynchronous or induction motors work thanks to Faraday's law of induction (the induced electromotive force in a winding is proportional to the rate of change of its magnetic flux linkage) and Lenz's principle that the related induced current flowing in the winding produces a magnetic field which opposes the initial changing of the magnetic field. In the case of co-axial cylindrical electromagnetic structures, if a rotor is equipped with a short-circuited polyphase winding and the stator produces a revolving magnetic field in the airgap, the induced currents in the rotor winding will produce a torque. The torque tends to synchronize the rotor with the rotating field in order to nullify the variations of the flux linkage with the rotor winding. In jargon, the principle of induction motor operation can be described by saying that the rotor is “a follower” of the stator's rotating magnetic field. Even if this description is just qualitative, it is sufficient to understand that the asynchronous machine can produce torque at any speed (i.e. including the null speed) with the exception of the so-called “synchronous speed”: the mechanical speed of the magnetic field wave rotating in the airgap. In fact, at that synchronous speed there is no relative movement between the revolving wave and the rotor winding, as well as there is no time variation in its flux linkage. It is important to add that for isotropic ac electrical machines an average torque during a revolution can be obtained only if the stator and rotor magnetic fields have the same number of magnetic poles and that these two magnetizations rotate at the same speed with respect to the stator reference system.