Optimal Selection of Rotor Bar Number in Multiphase Cage Induction Motors

Abstract

Rules for the selection of rotor bar numbers which minimize current and torque ripples are derived in this paper for a general symmetrical multiphase cage induction machine with prime phase number and integral slot winding. Analytically obtained expressions for optimal rotor bar number selection are validated by means of totally independent simulations, one based on a parameterized winding function (PWF) model of the induction machine and the other employing time-stepping finite-element analysis (TSFEA). As a case study, five-phase four-pole cage induction motors with forty stator slots and different number of rotor bars are comparatively analyzed. Results obtained from the PWF model are in excellent accordance with those independently obtained by TSFEA and both confirm the correctness of the proposed selection criteria. The practical motivation of the study is that an incorrect selection of rotor bar number can lead to parasitic torques of significant amplitude and, presently, there are no general rules available in the literature which may guide designers towards an optimal design choice for a general number of phases. INDEX TERMS Induction machines, multiphase stator winding, parasitic torques, rotor slot harmonics, skewing, winding function model.

Optimal Selection of Rotor Bar Number for Minimizing Torque and Current Pulsations Due to Rotor Slot Harmonics in Three-Phase Cage Induction Motors

“…This model has been validated against accurate time-stepping FE simulations in several previous works, e.g. [22]- [25]. An excellent agreement of results from the two totally independent methods has been always observed, both qualitatively and quantitatively, proving that the PWF model is a means for reliable motor simulation.…”

“…In a recently published paper, [25], general symmetrical m-phase, 2p-pole cage induction machine, where m is a prime number (m3), with an integer slot stator winding is studied through a rigorous mathematical analysis with the aim of finding rules for selecting the optimal numbers of rotor bars. Optimal numbers are defined as such numbers of rotor bars that do not lead to appearance of rotor slot harmonics in the stator current spectrum.…”

“…In the mentioned work, the  th harmonic of m-phase stator winding rotating flux-density wave is considered, [25],…”

“…The flux-density waves, produced by the cage rotor and responsible for rotor slot harmonics, were also analyzed in detail in [25], [26], [27],…”

“…where  U , s is slip and  is an integer, which defines the order of the RSHs (=1, 2, …). It was shown in [25] that, in order for RSHs not to exist in the stator current spectrum, the number of rotor bars must not be a divisor of either 2p(mz1) or 2pmz for any positive integer z. By limiting the number of rotor bars to the following range that is practically feasible, 0.5 1.5 S R S   , (5) R should then satisfy the following inequality,…”