The stator is saturated by the magnetic field of the rotor in different axis, depending on the rotor position. The saturation results in a reduction of the motor winding inductance. The variation of the inductance can be detected with a high frequency voltage applied to the motor windings. The resulting high frequency currents are modulated by the varying winding inductance.
If the machine is connected to a pulse width modulated frequency inverter the PWM frequency works as a carrier frequency for the rotor position detection .
This sensorless method of detecting the rotor position works even in the standstill of the machine. Unfortunately this circuit is quite complex and therfore expensive - in at least two of the three motor phases the current has to be detected.
Using this sensorless detecting on a motor with a high power efficiency will not work correctly, because there is very low saturation in the iron.