Skewed Rotor Bars Research Articles

Towards the optimal ‘slot combination’ for steady-state torque ripple minimization: an eight-pole cage rotor induction motor case study

This paper analyses the impact of the number of stator slots to rotor bars, i.e. the slot combination, in a cage rotor induction motor (IM) on its electromagnetic torque ripple during steady-state conditions. The study is focused on a conventional low voltage, eight-pole, three-phase, mains-fed IM design. To adequately evaluate the torque ripple magnitude, a ‘torque ripple factor’ is introduced and numerically calculated. To this end, IM behaviour in two common eight-pole stator slot number geometries is examined using numerical models, where, for each examined stator geometry, operation with a range of different rotor bar numbers used in practical IM designs is evaluated. For the sake of completeness and cross-validation, two different modelling approaches are used: a recently reported low-computational intensity winding function-based model and a commercial finite element analysis electromagnetic modelling software package. The paper findings deliver a detailed insight into the impact of rotor bar number, whether skewed or otherwise, on the motor steady-state torque ripple level. The presented results are indicative of the fact that the number of rotor bars only, and not the slot combination, is the dominant factor that influences the electromagnetic torque ripple in IMs with skewed rotor bars.

FE analysis of coupled electromagnetic-thermal phenomena in the squirrel cage motor working at high ambient temperature

PurposeThis paper aims to elaborate the method and algorithm for the analysis of the influence of high temperature on electric and thermal properties of the materials, as well as thermal phenomena process.Design/methodology/approachThe paper presents specially author’s software for the transient finite element analysis of coupled electromagnetic-thermal problems in a squirrel cage induction motor. The numerical implementation is based on finite element method and step-by-step algorithm. The nonlinearity of a magnetic circuit, the dependence of electric and thermal parameters on temperature, the movement of a rotor and skewed rotor bars have been taken into account. To verify the developed algorithm and software, the influence of high ambient temperature on selected electromagnetic and thermal parameters of the induction motor was examined.FindingsThe results of simulations compared with measurements confirm the adequacy of this approach to the analysis of coupled electromagnetic-thermal problems.Research limitations/implications3D effects have only been taken into account when using quasi-3D techniques (e.g. the multi-slice for skewed rotor slots).Practical implicationsThe author’s software developed can be useful in the analysis and design of squirrel cage motors, especially motors working in high ambient temperature.Originality/valueThe paper offers appropriate author’s software for the transient and steady-state analysis of coupled electromagnetic and thermal problems in squirrel cage motors with skewed slots.

A New 2-D Multi-Slice Time-Stepping Finite Element Method and Its Application in Analyzing the Transient Characteristics of Induction Motors Under Symmetrical Sag Conditions

Voltage sag is one of the most common power quality disturbances in industry, which causes huge inrush current in the windings of induction motors, and adversely impacts the motor’s secure operation. For better understanding of the operation and protection of induction motors, there is a need for a detailed research of the transient characteristics of induction motors during sag events. In this paper, we first proposed a novel 2-D multi-slice field-circuit-motion coupling time-stepping finite element method (2-D multi-slice FCM coupling T-S FEM) for calculating the transient performance of the induction motor with skewed rotor bars. In the proposed method, the equations of field, circuit, and motion are expressed in a nodal matrix and coupled together. Based on this method, we have analyzed the electromagnetic properties of a 5.5-kW induction motor with skewed rotor bars under symmetrical voltage sag conditions. Then, the influences of voltage sag’s sag magnitude, phase-angle jump, and initial phase angle on the stator inrush peak currents of the 5.5 kW and a 55-kW induction motors is studied. Finally, the proposed method is compared with 3-D FEM, the traditional 2-D multi-slice FEM and the measured data. The results show that the proposed method can reduce the computation time significantly with high precision.

3-D Sub-Domain Analytical Model to Calculate Magnetic Flux Density in Induction Machines With Semiclosed Slots Under No-Load Condition

In this paper, a novel 3-D sub-domain analytical model is developed to determine magnetic flux distribution in single-cage induction machines (IMs) with skewed rotor bars under no-load condition in an effort to more detailed analysis of spatial harmonics. The proposed model, along with an optimization algorithm, is as an alternative solution to finite-element analysis (FEA) in optimizing the geometry of IMs. The analytical method is based on the resolution of 3-D Laplace and Poisson's equations in cylindrical coordinates using the separation of variables method to calculate the magnetic vector potential for corresponding sub-domain. The proposed model includes the effect of slotting and tooth tips for the stator and rotor slots, which is usually neglected in a 2-D analysis due to the complexity of differential equations. Also, the proposed 3-D model can be used for any slot-pole combination in addition to considering the asymmetrical effect in the axial direction, which is a source of noise, vibration, and excessive losses in IMs. To evaluate the performance of the proposed 3-D analytical model, calculated magnetic-field distribution is compared with the results obtained from the 3-D FEA.

A Design Method for Cage Induction Motors With Non-Skewed Rotor Bars

To overcome the influence of the skewed rotor bars on the operating performance of the cage induction motors, this paper presents a design method for non-skewed small and medium cage induction motor based on the time-stepping finite element method, in which the approaches of asymmetrical rotor slots, closed slots, reasonable slot combination, and air-gap length are adopted and studied systematically. With this method, the cage induction motors with non-skewed rotor bars can be realized. Simultaneously, the slot harmonic, which is caused by slot opening, can be eliminated effectively and the starting performance can also be improved. The design example on a 5.5 kW cage induction motor is presented and the experimental validation is also performed.

A new method of characteristic analysis for three‐phase induction motors

AbstractGenerally speaking, the characteristics of induction motors (IM) are analyzed by means of the equivalent circuit with d–q transformation. Although this method is simple, the magnetic flux distributions in the stator and rotor are defined as sinusoidal waves. Therefore, the equivalent circuits become relatively complex for the high‐frequency components and transient states. For detailed analysis, the finite element method (FEM) is used. The FEM can take account of the influence of skewed rotor bars and slots, and of the skin coefficient. However, the calculation time is very long for high‐frequency components and transient analysis. We therefore propose a new method of characteristic analysis for IM. The proposed method analyzes the magnetic flux simply based on the construction of the IM. It can analyze transient and steady states and high‐frequency components regardless of the type of source. The influence of skewed rotor bars and slots can also be treated simply. This paper describes the method of analysis, presents the results of the proposed method and compares them with experimental results, and describes the d–q transition. © 2001 Scripta Technica, Electr Eng Jpn, 135(4): 64–75, 2001

Design and analysis of practical induction motors

A method to estimate the flux-linkages and parameters of induction motors with skewed rotor bars is presented. The proposed method is based directly on a multi-slice time stepping finite element model, hence the effects of saturation, eddy-current, skewed rotor bars and high-order harmonic fields, can all be included readily in the solution. Special formulas for calculating rotor currents, rotor flux-linkages and air-gap flux in quadrature-phase winding system are deduced. The computed currents and flux-linkages from the finite element model are used directly to compute the parameters of the lumped circuit model with the help of the least squares method. The advantages of the proposed method are its simple physical concept, high accuracy and versatility. The method is suitable to study virtually all modes of operating conditions, including dynamic operations, of induction motors.

A time-stepped 2D-3D finite element method for induction motors with skewed slots modeling

To model an induction motor with skewed rotor bars, we propose a hybrid 3D approach using the 2D invariant hypothesis. Our method is coupled with the electric circuit equations for both the stator and the rotor. We take into account the magnetic materials nonlinearity. The mechanical rotation is modeled, using the slipping surface technique. The obtained results are compared with measurements and results of the 2D standard model, which was applied to a similar motor but with an unskewed rotor.

Generation and rotation of 3-D finite element mesh for skewed rotor induction motors using extrusion technique

A simple method to generate and rotate 3-D finite element meshes for skewed rotor induction motors using extrusion techniques is presented. Special techniques to consider the geometrical structure of skewed rotor bars are described. With the proposed method, a change in the topology of the meshes at different rotor positions needs minor modifications only. The 3-D mesh for the rotor can thus be rotated with minimal extra computing time. Here the 2-D multi-slice mesh is used as the base-planes for extruding the 3-D mesh and the results of the 2-D multi-slice model can thus be used in the 3-D model. The techniques reported in this paper greatly simplifies the 3-D mesh generation, resulting in a considerable reduction in the computing time of the associated 3-D time stepping model. The generated meshes have been used successfully in constructing the 3-D time stepping finite element model for studying the electromagnetic field of induction motors.

Analysis of axially non-uniform loss distribution in 3-phase induction motor considering skew effect

This paper discusses the phenomena of the axially nonuniform distribution of magnetic flux densities and losses in a three-phase squirrel cage induction motor of which the rotor bars are skewed. A 2-D complex finite element method, taking account of the effects of the skewed rotor bars, is utilized for the analysis of characteristics such as copper and iron losses and the loss distributions are examined. The summing up values of nonuniform losses resulted from the finite element analysis are compared with measurements.

Direct modeling of the starting process of skewed rotor induction motors using a multi-slice technique

Both the starting current and the starting torque of induction motors cannot be evaluated and measured accurately and readily. In particular, the geometrical features of the skewed rotor bars are very difficult to study if a general 2-D finite element method is used. This paper presents an approach in using a multi-slice, time stepping 2-D eddy-current finite element method to study the starting processes of skewed rotor induction machines. The fields of the multi-slices are being solved en bloc simultaneously, and thus the eddy current effects can be taken into account directly. New forms of the governing equations for the multi-slice model are derived so as to allow the meshes of the multi-slices to be taken as one 2-D mesh. The resultant algorithm is then very similar to that of general 2-D problems. Special time stepping techniques for studying the starting process of motors are also presented. The performances of motors with skewed and nonskewed rotors are shown. It was found that the simulation results correlated very well with test data.

A comprehensive approach to the solution of direct-coupled multislice model of skewed rotor induction motors using time-stepping eddy-current finite element method

Normally a complicated three-dimensional (3-D) approach is needed to study the field pattern of induction machines with skewed rotor bars. In this paper, a time-stepping two-dimensional (2-D) eddy-current finite element method, based on multislice technique, is described to study the steady-state operation and the starting process of skewed rotor induction machines. The fields of the multislices are being solved en bloc simultaneously, and thus, the effects of the eddy current and saturation can be taken into account directly. New forms of the governing equations for the multislice model are derived, which allow the meshes of multislices to be taken as one 2-D mesh so that the algorithm is very similar to that of general 2-D problems. Special techniques required for the mesh generation in the multislice model and the salient structures of the software are also described. The results obtained by using the program being developed have very good correlation with test data.