A sensorless speed acquisition method for induction machines

The speed of the rotor and the speed of the rotating magnetic field are different for induction machines. The difference is described by the slip which is normally unknown because it depends on the load. For motion control the speed must be known. This information is mostly taken by a separate rpm sensor. It is desirable to eliminate this sensor because the availability of the machine will be higher, working processes under demanding ambient conditions are easier feasible and the investment for the sensor will be saved, too. This paper explains the basic principle of a sensorless speed measurement which uses anisotropies to detect the rotor speed. Additionally a mathematical model describing the behaviour of the modified machine is presented.