AIR‐GAP FIELD ANALYSIS AND DETERMINATION OF TORQUE OSCILLATIONS IN THE ASYNCHRONOUS MACHINE

The torque harmonics generated by asynchronous machine slotting are essentially linked to air‐gap permeance variations due to the rotor rotation. These variations lead to machine inductance coefficient fluctuations which are at the begining of torque harmonics. So the problem consists to determine the permeance variation law as well as for its mean value that for its “harmonics”. The aim of this study is to end at analytical expressions which allow to calculate the torque harmonic magnitudes whatever the considered machine. In consequence the numerical methods (e.g. finite element method) are not appropriated because their implementation considering the rotation will be too long and, in the other hand, they only permit, from calculus results, to estimate this permeance variation law. To resolve this problem by analytical way this law must be characterized by a simple expression. That leads the authors to translate the complex phenomena, linked to the field line repartition in the slotting, by simple laws applied to a fictitious slot model. After presenting the selected model to characterize the slots, the authors introduce the angle per unit permeance notion which permits to make clear the air gap permeance variation law in function of rotor rotation. The experimental verifications which relate to inductance coefficients and torque harmonics permit to validate these permeance variation law and in consequence the selected fictitious slot model. The evolution of these quantities in function of the magnetic field magnitude inside the machine permits the authors to propose a fictitious slot model evolution law which can take the saturation into account.