Electric motor Basics - Operating principles

In real life AC induction motors do not consist of magnets but of a physical rotor and stator.

The currents in the stator windings are generated by the phase voltages, which drive the induc-tion motor. These currents generate a rotating magnetic field, also referred to as stator field. The stator rotating magnetic field is determined by the winding currents and the number of phase windings.

The rotating magnetic field form the potential of the magnetic flux. The rotating magnetic field corresponds to electric voltage and the magnetic flux corresponds to electric current.

The stator rotating magnetic field rotates faster than the rotor to enable the induction of currents in the rotor conductor bars, thus creating a rotor magnetic field. The stator and rotor magnetic field generate their fluxes and these two fluxes will attract each other and create a torque, which makes the rotor rotate.

The operating principles of the induction motor are shown in the series of illustrations to your right.

Rotor and stator are thus, vital components in an AC induction motor. Stator and rotor are designed by sophisticated computer design programs. On the next pages, we will have a closer look at how stator and rotor are constructed.

                          Stator flux vs. rotor speed

Stator flux rotates
(i.e. 3000 min-1)

Rotor rotates slower
than the statorflux

i.e. 2900 min-1





The rotating stator flux is caused by the rotating sta-tor magnetic field which is formed by the currents in the different phase windings


                                      Generation of rotor flux

                     The rotor experiences that the stator flux rotates                                                                       at a speed of 3000 - 2900 = 100 min-1                                             

  The difference in speed causes currents to be induced into the rotor.
      These rotor currents generates a rotor flux.
The rotor flux is rotating at a speed of 3000 min^-1 (like the stator flux)


                        Generation of torque

The direction of the rotor flux generates two magnetic poles

The direction of the stator flux generates two magnetic poles.

The attraction of the rotor magnetic north pole towards the stator south pole and vice versa generates a force between stator and rotor. This force constitutes the motor torque that makes the rotor rotate.