Asymmetric Design in Electric Motors: Enhancing Performance and Efficiency

Introduction 

The invention of electric motors (E-motors) has drastically transformed and revolutionized every aspect of our daily lives. The ever-growing demand for E-motors has expanded to cover a wide range of applications such as home appliances, medical devices, robotic arms, space shuttles, etc. Moreover, the environmental crises alongside the increasing price of fuel gases have accelerated the transition of the global market to the Electric Vehicle (EV) industry. 

The figure below shows the increasing global sales of EVs from 2012 to 2021 hence, demonstrating the importance of the EV industry in coming years. 

Global Sales of EVs in Recent Years [1] 

Given the fact that the performance of the E-motor plays a crucial role in determining the efficiency and capability of the propulsion system of the EV, a considerable amount of recent research has been conducted to effectively improve and ultimately optimize the E-motor’s structure. Both academia and industry are working collaboratively to achieve this goal and eventually provide a sustainable solution for the transportation system. To help the engineers and researchers in their endeavors, EMWorks has dedicated a comprehensive package for E-motors analysis; making it possible to design and analyze a plethora of topologies and achieve the optimal solutions for EV and other applications. 

 E-Motor Design using MotorWizard Tool

This article presents a technical summary of one the novel design methods called “Asymmetric Design” to deliver a better performance of E-motors such as higher power density, lower torque ripple, and an optimal utilization of materials like permanent magnets. For this purpose, we will start by defining the concept of “asymmetric design” and how a motor can be asymmetric in terms of shape and structure. Next, we will dive into the analysis and comparison of the E-motor’s performance before and after applying asymmetricity through the design procedure and show how the asymmetric design improves the performance of the machine from an electromagnetic perspective. We will conclude with a summary of the findings of present work as well as some potential venues for future research.  

Definition of Asymmetric Design 

The general design procedure of E-motors consists of several steps. Different design parameters such as inner and outer diameter, the width and height of stator teeth, and magnet dimensions must be determined for the initial design. After that, through an iterative calculation process, the design parameters change to reach an optimum design matching with design requirements. The figure below illustrates the main aspects of the design and optimization of E-motors in one frame. 

 Main Aspects for the Design and Optimization of E-Motors [2] 

According to this figure, choosing a proper motor type and its topology is a crucial step toward achieving the best possible design based on pre-defined constraints. Adopting an “Asymmetric” shape for the E-motor design can occur at this very first step. The figures (a) and (b) compare the differences between two interior permanent magnet synchronous motors (IPMSMs) as an example of symmetric and asymmetric designs. 

IPMSM Cross-Section with (a) Asymmetric Design (b) Symmetric Design [3] 

As shown in the symmetric and conventional design, the magnets and the barriers follow a symmetric pattern. However, the presence of additional flux barriers in the asymmetric design caused asymmetry in the cross-section leading to new design capabilities. Induction motor (IM) with an axially skewed structure is a well-known example of asymmetric E-motor designs.   

Construction of an IM’s Squirrel Cage Rotor with a Skew Structure [4] 

Moreover, the growing interest in PM-based motors due to their distinguished performance has attracted researchers to explore new venues to implement asymmetric design approaches in PM-based machines. The figure below exhibits a recent categorization of IPMSMs based on different asymmetric design methods.

Categorization Method of Asymmetric IPMSMs [5] 

According to this categorization, the asymmetry can be applied in 4 shapes. In Group 1, the asymmetry has been employed in the axial direction dividing the PMs and the iron core into two separate parts. In other groups, the asymmetry changes over the cross-sectional view with respect to PMs and the rotor core. Looking at the above examples, the asymmetric design of E-motors can be referred to as any geometrical asymmetry applied to E-motor’s construction with aim of having better performance. In the next section, we will discuss how the asymmetric design helps improve and optimize the performance of E-motors and opens new opportunities for versatile industries and applications.

Effects on the E-motor’s Performance 

There are diverse ways to adopt asymmetric design techniques during the E-motors development and optimization process. However, each technique can target a specific performance characteristic to improve. We will study PM-machines to investigate the effect of asymmetric design on the E-motor’s performance. Important characteristics such as average torque, torque ripple, and cogging torque will be discussed. We will also show how the asymmetric design affects these performance criteria. 

Average Torque 

The output electromagnetic torque of a conventional IPMSM machine in a dq-frame can be written as: 

Where ψPM, is, Ld, and Lq, are the PM flux linkage, the stator current, and the dq-inductances. β is the current angle in figure (a). However, in case of having a shifted magnetic flux due to asymmetric construction, the PM flux linkage will gain a phase shift (α)  against D-axis as it is shown in figure (b).

Vector Diagrams in d–q-Axis Coordinates. (a) Symmetric IPMSM (b) Asymmetric IPMSM [5] 

Therefore, the electromagnetic torque of asymmetric IPMSM will be written as: 

In this regard, the shift applied to the PM flux linkage vector causes a shift in the position of PM torque as displayed in the graphs below. By increasing α through an asymmetric design approach, it is possible to reduce the distance between maximum reluctance and PM torque along current angle axis. As a result, higher torque can be achieved. 

Torque–Current Angle Characteristics. (a) Symmetric IPMSM (b) Asymmetric IPMSM [5] 

Cogging Torque 

High cogging torque is one of the main issues of IPMSMs that should be considered during the design and optimization process. This phenomenon is caused by the attraction force between stator teeth and the PM located on the rotor. One of the common methods to tackle this issue is to step-skew the rotor structure which is basically an asymmetric design technique. The figure below shows a step-skewed structure of an IPMSM with 3 segments. 

A Conventional IPMSM with a Step-Skew [6] 

Another asymmetric solution to reduce the cogging torque is to adopt cross-sectional asymmetry in the same way shown in [5].  In the figure below, the proposed asymmetric pole shape has led to significantly reduced cogging torque. This reduction is caused by modifying the flux path passing through the magnets and the stator teeth. The results show how effective the asymmetric design can be in the case of cogging torque reduction of PM machines.  

Cogging Torque Comparison between the Symmetric (Conventional) and Asymmetric (Proposed) IPMSM [7]

Torque Ripple 

The torque ripple is one of the important performance criteria which is desired to be low to ensure a smooth rotation and torque generation of an E-motor. Air-gap field harmonics, the stator’s current time-harmonics, and the core saturation are some of the most influential factors that can determine the ripple amplitude of the output torque. In this regard, applying cross-sectional asymmetries in the structure of the machine can help to noticeably reduce the torque ripple, the same as the cogging torque shown in [7].   

In addition to the cross-sectional asymmetries, the axial asymmetric design approach in the categorization methods (group 1) can also be a significantly effective method to reduce the torque ripple to the machine. In [8], the details of torque ripple compensation by the method of axial asymmetric design are discussed. The figure below shows the comparison of torque waveforms resulting from symmetric and asymmetric PM-SynRM designs. As can be seen, the torque ripple rate has been considerably reduced through the proposed asymmetric design.

Comparison between Torque Ripple of Symmetric and Asymmetric PM-SynRMs [8] 

Limitations

One of the main issues with the asymmetry structure is that it can force the motor to perform in one direction either clockwise or counterclockwise. For instance, while the torque ripple reduces in clockwise rotation, it might increase in the counterclockwise direction. The other problem the high manufacturing cost due to the complexity of the asymmetric motor geometry. Both problems must be properly addressed during the design process. Consequently, by creating a reasonable trade-off between the benefits and drawbacks of the asymmetric design, it is possible to achieve better E-motor performances for desired applications. 
 

In a nutshell, there is great potential to use the asymmetric design method to achieve better performances with better material utilization. The average torque, the cogging torque, and the steady-state torque ripple have been analyzed showing the benefits of this design technique. In addition, the possible drawbacks of the asymmetric structures were discussed.   

References