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PERMANENT MAGNET ASSISTED SYNCHRONOUS RELUCTANCE MOTOR DESIGN AND PERFORMANCE IMPROVEMENT A Dissertation by PEYMAN NIAZI Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment
PERMANENT MAGNET ASSISTED SYNCHRONOUS RELUCTANCE MOTOR DESIGN AND PERFORMANCE IMPROVEMENT A Dissertation by PEYMAN NIAZI Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY December 005 Major Subject: Electrical Engineering 005 PEYMAN NIAZI ALL RIGHTS RESERVED PERMANENT MAGNET ASSISTED SYNCHRONOUS RELUCTANCE MOTOR DESIGN AND PERFORMANCE IMPROVEMENT A Dissertation by PEYMAN NIAZI Submitted to the Office of Graduate Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Approved by: Chair of Committee, Committee Members, Head of Department, Hamid A. Toliyat Prasad Enjeti Shankar. P. Bhattacharyya Reza Langari Costas Georghiades December 005 Major Subject: Electrical Engineering iii ABSTRACT Permanent Magnet Assisted Synchronous Reluctance Motor Design and Performance Improvement. (December 005) Peyman Niazi, B.S., Isfahan University of Technology (IUT), Isfahan, Iran; M.S., Khaje Nassir Toosi University of Technology, Tehran, Iran Chair of Advisory Committee: Dr. Hamid A. Toliyat Recently, permanent magnet assisted (PMa)-synchronous reluctance motors (SynRM) have been considered as a possible alternative motor drive for high performance applications. In order to have an efficient motor drive, performing of three steps in design of the overall drive is not avoidable. These steps are design optimization of the motor, identification of the motor parameter and implementation of an advanced control system to ensure optimum operation. Therefore, this dissertation first deals with the design optimization of the Permanent Magnet Assisted Synchronous Reluctance Motor (PMa-SynRM). Various key points in the rotor design of a low cost PMa-SynRM are introduced and their effects are studied. Finite element approach has been utilized to show the effects of these parameters on the developed average electromagnetic torque and the total d-q inductances. As it can be inferred from the name of the motor, there are some permanent magnets mounted in the rotor core. One of the features considered in the design of this motor is the magnetization of the permanent magnets mounted in the rotor core using the stator windings to reduce the manufacturing cost. iv At the next step, identification of the motor parameters is discussed. Variation of motor parameters due to temperature and airgap flux has been reported in the literatures. Use of off-line models for estimating the motor parameters is known as a computationally intensive method, especially when the models include the effect of cross saturation. Therefore in practical applications, on-line parameter estimation is favored to achieve a high performance control system. In this dissertation, a simple practical method for parameter estimation of the PMa-SynRM is introduced. Last part of the dissertation presents one advanced control strategy which utilized the introduced parameter estimator. A practical Maximum Torque Per Ampere (MTPA) control scheme along with a simple parameter estimator for PMa-SynRM is introduced. This method is capable of maintaining the MTPA condition and stays robust against the variations of motor parameters. Effectiveness of the motor design procedure and the control strategy is validated by presenting simulation and experimental results of a 1.5 kw prototype PMa-SynRM, designed and manufactured through the introduced design method. v To my dear dad and mom for their continuous support and devotion. To Shermin for her priceless love. Peyman Niazi vi ACKNOWLEDGMENTS My deep appreciation is first given to almighty God for blessing me with success in my efforts and blessing me with the erudition of several people whose advice, assistance and encouragement helped me throughout the completion of this thesis. I would like to express my heartfelt appreciation to my advisor, Prof. Hamid A. Toliyat, for his support and continuous help. His knowledge, invaluable guidance, understanding and patience inspired the completion of this thesis. I am very grateful to work with such an insightful and caring professor. My sincere gratitude also goes to the members of my graduate study committee: Prof. Prasad Enjeti, Prof. Shankar Bhattacharyya, and Prof. Reza Langari for their valuable advice and help through the years I spent at Texas A&M University. I would like to acknowledge the Department of Electrical and Computer Engineering at Texas A&M University for providing an excellent academic environment. Special thanks go to Ms. Tammy Carda, Ms. Linda Currin, Ms. Gayle Travis and Prof. Huang for all their efforts. Grateful acknowledgment is extended to LG Electronics Co. for their cooperation during this work and providing us with part of the experimental setup. I would like to extend my sincere appreciation to my fellow colleagues and frien at Advanced Electric Machine and Power Electronics Laboratory, past and present: Dr. Mehdi Abolhasani, Dr. S. M. Madani, Dr. Leila Parsa, Dr. Masuod Hajiaghajani, Dr. Sang-Shin Kwak, Salih Baris Ozturk, Sheab Ahmed, Steven Campbell, Salman Talebi, Rahul Khopkar, Bilal Akin, Dr. Namhun Kim, Dr. Lei Hao and Dr. Tilak vii Gopalarathnam. I honor their frienhip and so many good memories throughout my time at Texas A&M University. Last but certainly not the least; I would like to thank my parents and my beloved Shermin, for their patience, care and endless devotion. I am very grateful to my dad for supporting me and teaching me to be strong. Also, I am deeply indebted to my mom for her patience and her prayers. I believe without them I would have been lost. I do not have the wor to express my gratitude to Shermin for her emotional support and priceless love she has brought into my life through these years. During these years, whenever I was exhausted, hopeless and tired of struggling with the obstacles in my work, my prayers to God and the encouraging wor of my loved ones were the only relief for me. No wor can express my heartfelt gratitude to them for their endless love, care and sacrifice. viii TABLE OF CONTENTS Page ABSTRACT... iii DEDICATION...v TABLE OF CONTENTS... viii LIST OF FIGURES...xii LIST OF TABLES...xvi CHAPTER I INTRODUCTION...1 A. Overview...1 B. Evolution of Synchronous Reluctance Motor Conventional design...8. Segmental design Double barrier design Axially-laminated design Transversally-laminated design Permanent magnet assisted SynRM...15 C. Modern Synchronous Drives...16 D. Research Objectives...18 E. Thesis Outline...0 II DESIGN OF A LOW COST PERMANENT MAGNET ASSISTED SYNCHRONOUS RELUCTANCE MOTOR... A. Introduction... B. Mathematical Model of SynRM The d-q equation of synchronous reluctance machine...5. The steady state equations for a synchronous reluctance motor Phasor equations for a synchronous reluctance machine...8 ix TABLE OF CONTENTS (Continued) CHAPTER Page 4. Torque expression for constant Volt/Hertz and constant current operation Maximum power factor...30 C. Design Criteria Computer aided design...36 i. Why we need computer aided design...36 ii. The nature of the design process Finite element approach...39 i. Energy functional...40 ii. Finite element formulation...43 iii. Boundary conditions...46 v. Solution techniques...47 vi. Parameter from field...49 D. Design Procedure Design strategy Design tool...53 i. Effect of the single flux barrier width...55 ii. Effect of the flux barrier location...58 iii. Effect of the flux barrier insulation ratio...61 iv. Effect of the pole span on the pole pitch ratio...63 v. Effect of the air-gap length...64 vi. Effect of the mechanical strutting...65 E. Proposed Motor...68 F. Experimental Reslts...77 G. Conclusion...80 III ON-LINE PARAMETER ESTIMATION OF PM-ASSISTED SYNCHRONOUS RELUCTANCE MOTOR...8 A. Introduction...8 B. Parameter Identification Algorithms...83 C. Parameter Estimation...84 D. Multiple Reference Frame...9 E. Modified Parameter Estimation Method Low pass filter...94 F. Simulation and Experimental Results...97 G. Conclusion...10 x TABLE OF CONTENTS (Continued) CHAPTER IV Page ROBUST MAXIMUM TORQUE PER AMPERE (MTPA) CONTROL OF PM-ASSISTED SYNCHRONOUS RELUCTANCE MOTOR A. Introduction B. Maximum Torque Per Amper Control C. MTPA Control System D. Simulation Study E. Experimental Results F. Conclusion...11 V CONCLUSION AND EXTENSION...1 A. Conclusion...1 B. Suggestions and Extensions...16 REFERENCES...18 VITA...136 xi LIST OF FIGURES FIGURE Page 1-1 Basic three phase, two pole reluctance variable motor, single saliency SynRM Basic three phase, two pole reluctance variable motor, double saliency switch reluctance motor Flux barrier type rotor of reluctance motor of the sixties Four-pole conventional salient pole design Four-pole isolated segmental rotor design Four-pole double-barrier rotor design Four-pole axially-laminated rotor design Four-pole transversally-laminated rotor design Four-pole transversally-laminated PM assisted rotor design Modern transversally laminated rotor for synchronous reluctance motors Axially laminated rotor for synchronous reluctance motors Two-pole synchronous reluctance motor Phasor diagram for synchronous reluctance machine Power factor vs. saliency ratio (K) of a synchronous reluctance motor when the motor is controlled with the maximum power factor control scheme Typical triangular finite element connected to other finite elements Mesh generated by a Maxwell Stator structure Illustration of design parameters Modification of one flux barrier width...55 xii LIST OF FIGURES (Continued) FIGURE Page - 11 The torque of a single flux barrier rotor as a function of the rotor angle barrier width The maximum, minimum and average normalized torque values as a function of flux barrier width The flux plots with flux barrier widths of a) mm, b) 8mm The direction of the flux barrier movement The torque of a single flux barrier rotor as a function of the rotor angle The maximum, minimum and average torque as a function of the flux barrier location Rotor with 3 barrier and different insulation ratio, a) W tot =0., b) W tot =0.4, c) W tot = The maximum, minimum and average torque as a function of the insulation ratio The rotor structure with a pole span caused by the q-axis cut-out The behavior of the torque as a function of the pole span ratio (τ p / τ) Behavior of output torque as a function of the rotor angle and airgap Behavior of output torque as a function of the rotor angle and radial rib width Behavior of output torque as a function of the rotor angle and tangential rib width Rotor flux barriers geometry of optimized SynRM Proposed PMaSynRM Magnetization of PM through the stator windings Air gap flux density and PM flux while stator has one turn winding....71 xiii LIST OF FIGURES (Continued) FIGURE Page - 8 Proposed stator Variation of d-q axes fluxes vs. stator current vector amplitude Calculated d-q axes inductances (L d -L q ) vs. current for PMa-SynRM and SynRM Saliency ratio (L d / L q ) vs. current Saturation effect due to the PM of the rotor Line-to-line back-emf in PMa-SynRM Torque-angle curves of the PMa-SynRM and SynRM Stator and rotor laminations of the proposed PMa-SynRM Actual back-emf line voltage at 1800 rpm Torque-angle curves of the PMa-SynRM A four pole PMa-SynRM rotor B-H characteristics of ferrite Sensitivity of estimated L q to the change of PM flux and stator resistor at 3600 rpm Sensitivity of estimated L d to the change of stator resistor at 3600 rpm Back-EMF due to permanent magnets in phase A Normalized harmonics of line-line back-emf due to PMs Block diagram of control system along the parameter estimator Block diagram of the parameter estimator...96 xiv LIST OF FIGURES (Continued) FIGURE Page 3-9 d-q axes inductances and (L d -L q ) vs. current Approximated permanent magnets back-emf used in the simulations On-line estimated parameters (L d, L q ) On-line estimated parameters (λ m ) PMa-SynRM speed control system kW prototype PMa-SynRM Back-EMF voltage at 1800 rpm Experimental results of inductance estimation, a) Measured i b) Measured i qs c) Estimated L d) Estimated L qs A four pole PMa-SynRM rotor Block diagram of MTPA control system along the parameter estimator Illustration of current vector swing to find the MTPA operating point Flowchart of MTPA procedure Approximated permanent magnets back-emf used in the simulations Calculated current phase angle (β) versus amplitude of the stator current vector in order to achieve MTPA Comparison of the output torque in the conventional MTPA control and the proposed one Stator voltage versus stator current at 1800 rpm under the MTPA control Block diagram of the PMa-SynRM test-bed xv LIST OF FIGURES (Continued) FIGURE Page 4-11 Laboratory experimental setup Experimental results of conventional MTPA, a) measured output torque b) encoder pulse indicating rotor d-axis c) filtered current of phase A d) current phase angle (β) Experimental results of proposed MTPA, a) measured output torque b) encoder pulse indicating rotor d-axis c) filtered current of phase A d) current phase angle (β)...10 xvi LIST OF TABLES TABLE Page - 1 Stator winding information Efficiency of proposed motor for T out =. N.m 1 CHAPTER I INTRODUCTION A. OVERVIEW This study is primarily concerned with the optimum design and robust maximum torque per ampere vector control of inverter fed permanent magnet assisted synchronous reluctance motors (PMa-SynRM) with a simple motor parameter estimator. The PM assisted synchronous reluctance machine is mainly a type of synchronous reluctance motors (SynRM) which is a family member of brushless AC machines consisting of the conventional dc permanent magnet machine, the permanent magnet synchronous machine and the cage induction machine. The members of this family have a standard three phase stator of induction machine with spatial sine wave rotating field. Generated torque is relatively smooth and as a result, the operation is quiet. A conventional three phase inverter can be used to drive the motors of this family if electronically controlled drive is desired. Most of the early work on the SynRM in 1960 s and 1970 s was related to the linestart machine. The requirement of a squirrel cage for line-start, along with some other manufacturing factors, compromised the rotor design and led to relatively poor performance compared to an induction machine. Because of this poor performance, SynRM was mainly ignored until late 1980 s. With the development of power transistor technology and vector control theory over This dissertation follows the style and format of IEEE Transactions on Industry Applications. the past decays, the performance of SynRM has been drastically improved and this motor started to be seriously considered as a possible alternative to the other brushless machines (particularly an induction motor) in the variable speed industrial applications. By controlling the machine via a transistor voltage inverter, line-start feature was no longer necessary for SynRM. Therefore, the starting cage was removed from the rotor and it was designed such that gives the maximum saliency ratio. The main motivations for the renewed interest in the SynRM are: 1. Improved saliency ratio makes the SynRM competitive with an induction machine, particularly in terms of power factor and inverter kva requirement.. Small to medium size high performance drives may have simpler control using the SynRM as compared to the field oriented controlled induction machine. 3. It can be operated stably down to zero speed at full load unlike an induction motor which may suffer overheating problems. In addition, SynRM appears to be more efficient at low speed than an induction machine. 4. By adding appropriate amount of magnet into the rotor core, efficiency improves without having significant back-emf and without necessary change in the stator design. Because of the existence of flux barriers, demagnetization is hard to occur if strong magnets are used. Demagnetization due to the machine overloading and high ambient temperature is a significant problem in IPMs. Before summarizing the main motivations for the work presented in this thesis, presenting the historical development of the machine can help the readers to have an 3 insight on the trend of SynRM evolution. Creating this background can highlight the major contribution of presented work in this thesis. The earliest reference on SynRM s that could be found was published in 193 [1]. Since then, various machine designs have been proposed in the literature by a number of authors. The main purpose of the previous works on the design of SynRM was to improve the overall efficiency of the motor. These designs are classified into several distinctive categories. The second part of this chapter attempts to give an overview of the machine evolution in chronological order. Each of the machine categories are separately discussed emphasizing important design aspects, main features, and performance limitations. This section finally merges to the state of the art PMa-SynRM drive and its numerous advantages over the other members of the brushless family. These merits are considered in more detail and represent the main motivations for studying this machine. Besides having an optimum design for the motor, having an optimal controller is also necessary to improve the performance of overall drive. The third section of this chapter reviews the trend of the modern SynRM drives and emphasizes the pros and cons of different industrial drives. Finally the fourth and fifth sections of the Chapter I present the objectives of this research and outline the thesis structure. 4 B. EVOLUTION OF SYNCHRONOUS RELUCTANCE MOTOR The principle of using the differences of reluctances to produce the torque has been known for over 160 years. Before the discovery of the rotating magnetic field by Tesla in 1887, the first reluctance motor was similar to the doubly salient synchronous reluctance motor, nowadays known as the switched reluctance motor. The first rotatingmagnetic-field synchronous motor was, however, introduced by Kostko not earlier than in 193 (Kostko 193). There are different designations for singly salient synchronous reluctance motors in the literature. The most popular names for this motor are: Reluctance motor (RM), Synchronous Reluctance Motor (SRM, Synchrel, SynRM) and Reluctance Synchronous Motor (RSM). In this thesis, Synchronous Reluctance Motor (SynRM) is used as the name and abbreviation for this motor. Figure 1-1 shows a cross-sectional view of a single saliency RM consisting of a non-salient stator and a two-pole salient rotor, both made of high-permeability magnetic material. This figure shows a three-phase stator winding although any number of phases is possible. Figure 1- shows the cross-sectional view of a three-phase double saliency RM. In principle, the SynRM is similar to the traditional salient pole synchronous motor but does not have an excitation winding in its rotor. In this case only the rotor is constructed with salient poles. The stator inner surface is cylindrical and typically retains many of the benefits of variable reluctance motors and at the same time eliminates its several disadvantages. Before the development of today s AC motor drives, in a variable speed drive, motor was supplied from a fixed frequency power source. In this case, it 5 was necessary that the SynRM includes a squirrel cage on the rotor to provide the starting torque for line-start. Otherwise, the rotor could not accelerate and synchronize with the supplying network. The squirrel cage was also needed as a damper winding in order to maintain synchronism under sudden load torques. The presence of a cage for line starting in the rotor structure was interfering with the requirements of the optimum rotor design. B+ C- Rotor d-axis A- Rotor A+ C+ Stator B- Rotor q-axis Figure 1-1 Basic three phase, two pole reluc
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