Abstract
The paper develops a method to choose the number of rotor bars in order to eliminate rotor slot harmonics in stator current spectrum and pulsation torques that are their consequence. Mains-fed, three-phase cage induction motors with the most common number of pole pairs and number of stator slots, that result in integer slot winding, are analyzed. The analysis is based on the recently derived general rule for optimal selection of rotor bars, valid for symmetrical multiphase machine with prime number of phases and integer slot stator winding. As a tool for validation of analytically predicted results, parameterized winding function (PWF) model is used. Electromagnetic torque ripple factor is used as a measure of goodness of the number of rotor bar selection. The practical motivation of the study is an attempt to supersede the many existing rules for rotor bar number selection that, depending on the source, may be different, and provide a unified general approach to the problem. One of the main findings derived in the paper is ascertainment that increasing the number of pole pairs increases the degree of freedom in choosing the proper number of rotor bars. The same applies when the number of motor phases increases.
Highlights
A mains-fed three-phase cage induction motor still dominates in industrial applications worldwide more than a century after its invention
It should be noted that the existence of rotor slot harmonics (RSHs) in stator current spectrum may be desirable, for example in sensor-less speed control based on identifying the frequencies of such harmonics, [2]
This paper addresses the problem of determining the optimal number of rotor bars R of a three-phase induction motor to cancel current and torque ripples related to the RSHs of different orders
Summary
A mains-fed three-phase cage induction motor still dominates in industrial applications worldwide more than a century after its invention. As the induction motor is a significant consumer of electrical energy globally, more stringent criteria are regularly imposed on its manufacturers, primarily in terms of the efficiency and in terms of NVH (noise, vibration, harshness) requirements, [1]. One of the issues that, in the authors’ opinion, has not been exhaustively analyzed is the influence of the number of rotor bars on the occurrence of rotor slot harmonics (RSHs) in the stator current spectrum. These mostly unwanted highfrequency current components lead to additional Joule losses and directly affect motor efficiency. It should be noted that the existence of RSHs in stator current spectrum may be desirable, for example in sensor-less speed control based on identifying the frequencies of such harmonics, [2]
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