`Sensorless´ speed measurement of inverter driven squirrel cage induction motors

A traditional method of measuring the rotational speed of the rotor of a motor involves the use of a shaft mounted transducer, such as a tachometer. Because of transducer wear and mounting problems, optical speed measurement methods such as a light beam interrupted by a slotted disc have been used. In each case the transducer presents problems of cost, mounting and robustness; particularly in harsh industrial environments. The authors examine a noninvasive method of measuring the rotor speed of an induction motor which involves the detection of phase current harmonics. In the case of an inverter drive, the speed signal is derived from the harmonics in the phase current which is reconstructed from the DC link current. The speed associated harmonics in the phase current are a result of air-gap eccentricity and unbalanced magnetic pull