Analytical method for accurate determination of nonlinear magnetization curve of induction motor

Abstract

This work proposes an analytical method for determining nonlinear magnetization curve and magnetizing inductance of an induction motor (IM) from lamination design details and core material data i.e the B-H curve. The operating point of an IM on nonlinear B-H curve varies with load on the motor. Due to the nonlinear nature of the B-H curve, the magnetizing inductance of an IM should not remain constant and vary with load variation. Generally, the magnetizing inductance is regarded as constant for all motor load conditions. Considering a constant magnetizing inductance implies violating the nonlinear nature of the B-H curve. The proposed analytical method calculates the ampere-turn (AT) required to establish air-gap flux (ϕ) through its path i.e. through different motor parts. For a given value of air-gap flux, flux density (B) is calculated with known motor geometry size. The magnetizing field intensity (H) for the calculated flux density is determined from the B-H curve. AT is calculated using the determined value of magnetizing field intensity and the lamination size. The total AT thus calculated gives the magnetizing current required to establish the given value of air-gap flux. Further, using winding details the flux linkage (λ) is determined. Subsequently the same is repeated for all possible values of air-gap flux to get the complete nonlinear magnetization curve. Nonlinear magnetization curve obtained from proposed analytical method is compared with that obtained experimentally as well as using finite element method, the result matches with both comparisons.