Stator line current spectrum content of a healthy cage rotor induction machine

The paper analyzes the stator current spectrum of a healthy squirrel cage induction machine. The knowledge of stator current spectrum of the healthy cage rotor induction machine is a starting point for diagnosis of different faulty regimes using a noninvasive diagnostic technique known as motor current signature analysis. Magnetomotive force (MMF) space harmonics, slot permeance harmonics as well as saturation of main magnetic flux path through the virtual air gap permeance variation are taken into analytical consideration. Air gap flux density waves were obtained by multiplying the corresponding MMF waves with air gap permeance waves. General rules which give the connection between the number of rotor bars and the existence of rotor slot harmonics in the stator current spectrum are derived, too. Their appearance as well as magnitude depends on the corresponding air gap flux density wave pole pair number. Predicted spectral components of the stator current are experimentally verified on two laboratory motors with different number of rotor bars.