NATURAL VARIABLE MODELING AND PERFORMANCE OF INTERIOR PERMANENT MAGNET MOTOR WITH CONCENTRATED AND DISTRIBUTED WINDINGS

ABSTRACT

Interior Permanent Magnet (IPM) motor is widely used for many industrial applications and has relatively high torque ripple generated by reluctance torque. Since the configuration of the  stator  has  great  influence  on reluctance  torque,  different  stator configuration is necessary to improve the torque performance of IPM motor. Natural variable modeling and performance comparison of Interior Permanent Magnet Motor with Concentrated winding (CW), Short pitched and Full pitched distributed winding (DW) is presented in this project report. Three phase Interior Permanent Magnet Motor with identical rotor dimensions, air gap length, series turn number, stator outer radius, and  axial  length  was  studied  with  different  stator  winding  configuration.  Basic parameters and machine performance, such as inductances, copper losses, power density, efficiency at  high and low speed, torque ripple, rotor speed with load torque, phase currents, electromagnetic torque, controllability and demagnetization tolerance are compared. As a means of supplementing analysis of the IPM motor, winding function theory (WFT) is used to analyze the motor. Winding function theory has enjoyed success with induction, synchronous, and even switched reluctance machines in the past. It is shown that this method is capable of analyzing IPM motor with different stator configuration and the simulations were carried out by using Embedded MATLAB function. It was observed that, the concentrated winding IPM motor has a lower copper loss of 0.3 kw and 3.7 kw at low and high speed respectively and 133 Nm high peak torque developed, pull out power of 58 kw, torque ripple of 96 Nm, average torque of

142 Nm, demagnetization tolerance of 60%, amplitude of the fundamental winding is

26.45 and efficiency of 89. the short pitched distributed winding IPM motor has a lower copper loss of 0.35 kw and 3.6 kw at low and high speed respectively and 116 Nm high peak torque developed, pull out power of 57 kw, torque ripple of 71 Nm, average torque of 185 Nm, demagnetization tolerance of 78%, amplitude of the fundamental winding is

27.53 and efficiency of 87. As for full pitched distributed winding IPM motor has a lower copper loss of 0.35 kw and 3.6 kw at low and high speed respectively and 116 Nm high peak torque developed, pull out power of 56 kw, torque ripple of 71 Nm, average torque of 185 Nm, demagnetization tolerance of 78%, amplitude of the fundamental winding is 29.3 and efficiency of 88.

CHAPTER ONE

1.0 Introduction

1.1 Overview

Over  the  years,  the  application  of  electric  motors  has  replaced  vast  numbers  of mechanical rotating devices. From tiny motors used in wristwatches, to very large motors used for ship propulsion and wind turbines. There are numerous types of electric motors available for present-day applications, of which the AC types are most commonly used in high performance applications due to  its increased efficiency and  excellent  dynamic performance [1]. The classifications of common types of AC motors are shown in Figure

1.1[2].

Figure 1.1: Classification of AC machine types

The Induction, Surface Permanent Magnet (SPM), Inset Permanent Magnet Machine, and Interior Permanent Magnet (IPM) machine types have already been applied to present day drive systems. Induction, SPM and inset PM machines usually have a lower power rating compared to the IPM machine and are most commonly applied as an Integrated

Motor Assist (IMA) system, where the main driver of the vehicle is the internal combustion engine while the electric motor assists. On the other hand, the IPM machine itself produces up to 73kW or more of power and can be driven in full electric mode, producing zero emissions. [2]

This project report will focus on the IPM machine type, which is generally preferred due to three main reasons: Firstly, the buried magnets make the rotor structurally stronger, which make it  more capable of withstanding higher speeds. Secondly, the additional useful reluctance torque, resulting from the salient pole structure, thus giving the motor greater field-weakening capabilities. Additionally, this saliency allows sensorless control, properties which the SPM does not offer [3]. Lastly, the possibility of changing the geometry of buried magnets in the rotor makes it possible to employ flux concentration, and provides the possibility of saliency ratio optimisation [4].

With the availability of high energy permanent magnet materials and advanced power electronics, the fields in which IPM machines can be applied to are rapidly broadening. They include aerospace, nautical, automobile, rail transportation, medical, generation and industrial process automation [5]. Common magnet geometries include single-piece/pole, rectangular shaped magnet design (Figure 1.2a), segmented magnet design (Figure 1.2b), v-shaped magnet design (Figure 1.2c), and the multi-barrier design (Figure 1.2d) [6].

Each  of  these  designs  has  its  advantages  and  disadvantages:  The  single-piece/pole magnet design, for example, is the easiest to manufacture, but has larger magnet losses compared to the other designs due to the larger magnet pole surface [7]. The segmented magnet design has lower magnet losses and better field-weakening capability but requires more magnet pieces. It also results in decreased magnet flux density due to the leakage

flux in the iron bridges [8]. The v-shaped magnet design provides flux concentration but also requires more magnet pieces compared to the single-piece/pole design.

(a) rectangular single-piece/pole magnets                                                   (b) segmented magnets

(c) v-shaped magnets                                                                                  (d) multi-barrier magnets

Figure 1.2: Various IPM rotor geometries

The multi-barrier magnet design creates a very high saliency ratio, but consequently results in an increased amount of structural stress on the rotor steel [6]. In practice, there is no single rotor that can satisfy all applications. The pros and cons of each design as well as more specific magnet type and shape have to be altered to meet desired specifications. As magnets are very brittle, there are also practical limits to the manufacturability of the magnets.

Before the 21st century, the majority of IPM machines were designed with distributed stator windings (DW). The use of concentrated winding (CW) was not popular due to a poor torque to magnetomotive force (MMF) ratio. However in the early 21st century, Cros and Viarouge [9], Magnussen and Sadarangani [10] proved that by an appropriate choice of slot and pole combination, the winding factor can be significantly increased, thus increasing output torque. Additionally it was also shown that with appropriate slot and pole combination, cogging torque can also be reduced.

Stator windings can either be single-layer or double-layer. The choice depends on the desired machine performance characteristics. Single-layer CW creates high self- inductance and low mutual-inductance which leads to better fault-tolerant capability. On the other hand, double-layer CW has lower airgap MMF harmonic components, thereby resulting  in  smaller  torque ripples and  lower  magnet  eddy current  losses [11].  The winding layouts for single-layer and double-layer DW are shown in (Figure1.3a) and (Figure 1.3b), while the layouts for single-layer and double-layer CW are shown in (Figure1.3c) and (Figure 1.3d) respectively [12].

Finally, the study is primarily concerned with the natural variable modeling and performance comparison of Interior permanent magnet (IPM) motors with a short pitched distributed winding, full-pitched distributed winding and concentrated winding. An Interior  permanent  magnet  (IPM)  motor  has  many  advantages  such  as  high  power density, efficiency and wide speed operation. These merits make it particularly suitable for automotive and other applications where space and energy savings are critical. However, IPM motor has relatively high torque ripple generated by reluctance torque which results in noise and vibration [13].

(a) Single-layer distributed windings                                           (b) Double-layer distributed windings

(c) Single-layer concentrated windings                                    (d) Double-layer concentrated windings

Figure 1.3: Various stator winding layouts

Torque ripple in IPM motor is often a major concern in applications where speed and position accuracy are great important. Since the component configuration such as a stator has  great  impact  on reluctance torque, different  stator configuration is  necessary to improve the performance of IPM motor [14]. Most previous works to obtain optimal design for torque ripple reduction have been restricted to size optimization in which design parameters are known in priori and fixed throughout the optimization process [15]. The size design variables include slot opening, depth of rotor yoke, angle of one pole magnet, thickness of permanent magnet and so on.

1.2 Research Objectives

The general objective of this project report is firstly to present a feasibility study on the IPM motor with different stator configuration. Subsequently to carryout a natural variable modeling and performance comparison on a three phase IPM motor with short pitched distributed winding, full-pitched distributed winding and concentrated winding. Despite the fact that the machines are now widely in use, there has been considerable interest in a three phase Interior Permanent Magnet Motor. Finally, the study will help clarify the advantages and disadvantages of implementing the different stator configuration in IPM motor, as well as its prospects in industrial applications requiring high efficiency.

1.3. Thesis Outline

In order to conduct the stated project objectives, this project report is outlined as following:

Chapter I covers overview and some backgrounds on interior permanent magnet motor. In this chapter the main objectives and motivations of this project, thesis outline and study limitation are introduced.

Chapter  II,  this  chapter  gives  the  literature  reviews  on  Interior  Permanent  Magnet machine technology, permanent  magnet  materials, winding function theory and  why winding function theory.

Chapter III gives the analysis of Interior Permanent Magnet Motor with winding function theory and  its  modifications  which  include  basic  winding  function theory,  winding function theory for machines with salient air gaps, winding function theory applied to magnetic devices,  verification  of a  single  phase  per  rotor  and  Torque  calculated  from

inductance. It also include the clock diagram of IPM motor, Total Harmonic Distortion (THD), winding factor (kw), Slot-fill Factor, the Voltage Equations, Torque   Ripple   and Losses in IPM motor which includes the core loss, magnet loss, stator winding loss and mechanical losses

Chapter IV is a set performance, simulation and results. Chapter V is a Conclusions and Recommendations.

1.4 Study Limitations

A major limitation encountered in this project report thesis occurred during the modeling of the different stator configuration of the machines and only the first harmonics was used since third harmonic would be eliminated if the motor is connected in star. Eventually, certain assumptions deemed necessary.

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