This is a synchronous motor that does not require a special start-up auxiliary motor. The  rotor  consists  of  stout  copper  (or aluminium)  conductors  arranged  in  the form of a cylindrical cage (commonly known as a 'squirrel cage' rotor). These are laid in slots in a soft iron core and all the bars are electrically connected up together at each end by copper  (or aluminium) rings. Three stator windings arranged at 120° to each other around the rotor are energised by the three phases of an ac supply and this creates a magnetic field that  rotates at the frequency of the supply.

With the rotor stationary, the rotating magnetic field induces an emf in the cage that in turn drives a current through its conductors (an 'eddy'  current).This current reacts against the magnetic field to produce a torque that causes the rotor to turn  in   the direction of the  rotating magnetic field. Note that if the rotor were ever able to  'catch up' with the rotating magnetic field, then the conductors of the rotor cage would not then experience any changing magnetic field., no emf would be induced  in  the  rotor  and  therefore  no current (and therefore no torque either) in the rotor. Some torque will always be needed to overcome mechanical losses (friction, air resistance etc). Therefore in practice the rotor always turns more slowly than the rotating magnetic field, how much depending   on the  amount  of torque required  by the  motor to  overcome both the mechanical losses and the mechanical load applied to the motor. The fractional difference in speed is called the 'slip'.

Slip = synchronous speed - rotor speed

             Synchronous speed

The larger the torque applied to the motor,the greater the slip required to produce the torque needed. Note that because of the slip, the frequency of the induced currents in the rotor is less than that of the applied stator voltage. (The induced voltage is proportional to the rate of change of the magnetic field strength as ‘seen’ by the rotating armature. Hence if the slip is small, the frequency of the currents flowing in the rotor is low and so the effect of any inductance of the rotor is  negligible. (Z=jωL). In this case, only the resistance of the rotor limits the current in  the rotor (and hence the torque produced by the motor).

Torque    =  K.S/R


where K is a constant for a given machine.Usually R is made very small (hence the stout copper or aluminium rotor cage) to allow a high torque output.

Advantages:  no brushes or slip rings are required – relatively easy and cheap to make. Reliable. Smooth torque output.

Disadvantages:  operates at one speed (determined by the frequency of the threephase ac supply used). Needs electronic controllers to produce variable frequency supplies if required to operate at variable speeds. Normally needs three-phase supplies (it is possible to use single phase supplies with special designs) Applications: aircraft fuel pumps, (immersed in fuel to aid cooling), fans, conveyer belt drives, pumps etc.