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Selection criteria of induction machines for speed-sensorless drive applications

Induction motors, both three and single phase, are used extensively for adjustable speed drive applications. These machines are structurally very robust and are primary source of motive power and speed control where DC machines cannot be used. For closed loop control of these machines, sensorless speed estimation is usually preferred. Among the current estimation techniques available for speed-sensorless induction motor drives, speed measurement based on rotor slot related harmonic detection in machine line current happens to be a prominent one. While these harmonics can be strong in certain kinds of machine, some other machines may exhibit very weak rotor slot harmonics that can be obscured by noise. Skewing, slot shapes and types, structural unbalances etc. also have a prominent effect on the detectability of these harmonics. The present paper attempts to investigate this problem based on the interaction of pole pairs, number of rotor bars and stator winding. Though the analysis and experimental results have been mainly provided for three phase squirrel cage induction motors, single phase and slip ring induction motors have also been addressed. Further, it has been shown that eccentricity related fault detection could also be easily accommodated with this kind of speed detection technique at no or negligible extra cost when certain motors are selected.