Mechanical Design of Electric Motors

737 Pages·2014·48.19 MB·English

by  Wei Tong

Mechanical Design
of
Electric Motors

Mechanical Design
of
Electric Motors

Wei Tong
Kollmorgen Corporation, Danaher Corporation
Radford, Virginia
CRC Press
Taylor & Francis Group
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Boca Raton, FL 33487-2742
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Version Date: 20140131
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To my wife, Zhangqing Zhuo, and daughter, Winnie Tong,
for their love, support, and patience

Contents
Preface ...........................................................................................................................................xxi
Author ..........................................................................................................................................xxv
List of Symbols .........................................................................................................................xxvii
1. Introduction to Electric Motors ...........................................................................................1
1.1 History of Electric Motors ............................................................................................1
1.2 Motor Design Characteristics ......................................................................................2
1.2.1 Motor Torque ....................................................................................................2
1.2.1.1 Static and Dynamic Torque ............................................................2
1.2.1.2 Motor Torque in Motor-Load System ............................................4
1.2.1.3 Continuous Torque ...........................................................................6
1.2.1.4 Peak Torque .......................................................................................6
1.2.1.5 Stall Torque........................................................................................7
1.2.1.6 Cogging Torque and Reduction Methods ....................................8
1.2.1.7 Torque Ripple ..................................................................................13
1.2.2 Motor Speed ....................................................................................................14
1.2.2.1 Continuous Speed ..........................................................................15
1.2.2.2 Peak Speed ......................................................................................15
1.2.2.3 Speed Ripple ...................................................................................15
1.2.3 Torque Density ...............................................................................................15
1.2.4 Motor Power and Power Factor ....................................................................16
1.2.5 Torque–Speed Characteristics ......................................................................17
1.2.6 Mechanical Resonance and Resonant Frequency .....................................20
1.2.7 Load-to-Motor Inertia Ratio .........................................................................25
1.2.8 Duty Cycle ......................................................................................................26
1.2.9 Motor Efficiency .............................................................................................29
1.2.9.1 Definition of Motor Efficiency ......................................................29
1.2.9.2 IEC Standards on Efficiency Classes of AC Electric Motors ......30
1.2.10 Motor Insulation ............................................................................................31
1.2.11 Motor Operation Reliability .........................................................................36
1.3 Classifications of Electric Motors ..............................................................................36
1.3.1 DC and AC Motors ........................................................................................38
1.3.2 Single-Phase and Three-Phase Motors .......................................................39
1.3.3 Induction and Permanent Magnet Motors .................................................40
1.3.4 Synchronous and Asynchronous Motors ...................................................44
1.3.5 Servo and Stepper Motors ............................................................................45
1.3.6 Gear Drive and Direct Drive Motors ..........................................................49
1.3.7 Brush and Brushless Motors ........................................................................53
1.3.8 Reluctance Motors .........................................................................................54
1.3.9 Radial-Flux and Axial-Flux Motors ............................................................55
1.3.10 Rotary and Linear Motors ............................................................................56
1.3.11 Open and Enclosed Motors ..........................................................................57
1.3.12 Motor Classification according to Power Rating .......................................57
vii
viii Contents
1.4 Motor Design and Operation Parameters ...............................................................58
1.4.1 Back EMF Constant, K ..................................................................................58
e
1.4.2 Torque Constant, K........................................................................................59
t
1.4.3 Velocity Constant, K .....................................................................................60
v
1.4.4 Motor Constant, K ........................................................................................60
m
1.4.5 Mechanical Time Constant, τ .....................................................................61
m
1.4.6 Electrical Time Constant, τ ..........................................................................63
e
1.4.7 Thermal Time Constant, τ ..........................................................................64
th
1.4.8 Viscous Damping, K ....................................................................................65
vd
1.5 Sizing Equations ..........................................................................................................66
1.6 Motor Design Process and Considerations .............................................................67
1.6.1 Design Process ...............................................................................................68
1.6.2 Design Integration .........................................................................................73
1.6.3 Mechatronics ..................................................................................................74
1.7 Motor Failure Modes ..................................................................................................74
1.8 IP Code .........................................................................................................................76
References ...............................................................................................................................78
2. Rotor Design ..........................................................................................................................83
2.1 Rotor in Induction Motor ...........................................................................................83
2.1.1 Wound Rotor ..................................................................................................83
2.1.2 Squirrel Cage Rotor .......................................................................................85
2.1.2.1 Factors Impacting Resistance of Squirrel Cage Rotor ...............86
2.1.2.2 Double-Cage Rotor .........................................................................87
2.1.2.3 Casting Squirrel Cage Rotor .........................................................88
2.1.2.4 Skin Effect ........................................................................................91
2.1.3 Induction Motor Design Types and Their Performing
Characteristics ...............................................................................................91
2.2 Permanent Magnet Rotor ...........................................................................................92
2.2.1 Discovery of Phenomenon of Magnetism ..................................................92
2.2.2 Permanent Magnet Characteristics .............................................................93
2.2.3 Permanent Magnet Materials .......................................................................95
2.2.3.1 Ferrite Permanent Magnets ..........................................................96
2.2.3.2 Rare-Earth Permanent Magnets ...................................................96
2.2.3.3 New Developments of Permanent Magnets ...............................97
2.2.4 Magnetization ................................................................................................98
2.2.5 Factors Causing Demagnetization ..............................................................99
2.2.6 Maximum Operation Temperature ...........................................................102
2.2.7 Permanent Magnet Mounting and Retention Methods .........................102
2.2.8 Corrosion Protection of Permanent Magnets ..........................................107
2.3 Rotor Manufacturing Process .................................................................................109
2.3.1 Lamination Materials ..................................................................................109
2.3.2 Lamination Cutting .....................................................................................111
2.3.3 Lamination Surface Insulation ..................................................................112
2.3.4 Lamination Annealing ................................................................................113
2.3.5 Lamination Stacking ...................................................................................113
2.3.6 Rotor Casting for Squirrel Cage Motor .....................................................113
2.3.7 Heat Treatment of Casted Rotor ................................................................114
2.3.8 Rotor Assembly ............................................................................................115
Contents ix
2.3.9 Rotor Machining and Runout Measurement ...........................................115
2.3.10 Rotor Balancing ............................................................................................115
2.3.10.1 Types of Unbalance ......................................................................118
2.3.10.2 Rotor Balancing Machine ............................................................120
2.3.10.3 Balancing Operation ....................................................................121
2.4 Interference Fit ...........................................................................................................122
2.4.1 Press Fit .........................................................................................................124
2.4.2 Shrink Fit .......................................................................................................128
2.4.3 Serration Fit ...................................................................................................131
2.4.4 Fitting with Adjustable Ringfeder® Locking Devices ............................132
2.4.5 Fitting with Tolerance Rings ......................................................................132
2.5 Stress Analysis of Rotor ...........................................................................................135
2.6 Rotordynamic Analysis ............................................................................................137
2.6.1 Rotor Inertia ..................................................................................................137
2.6.2 Motor Critical Speed and Resonance ........................................................139
2.7 Rotor Burst Containment Analysis ........................................................................143
2.7.1 Rotor Burst Speed ........................................................................................143
2.7.2 Energy in Rotating Rotor ............................................................................145
2.7.2.1 Kinetic Energy in Rotor ...............................................................145
2.7.2.2 Elastic Potential Energy in Rotor ...............................................146
2.7.2.3 Ratio of Potential Energy to Kinetic Energy of Rotor .............147
2.7.3 Rotor Burst Containment Design ..............................................................147
References .............................................................................................................................150
3. Shaft Design ........................................................................................................................155
3.1 Shaft Materials ...........................................................................................................155
3.2 Shaft Loads.................................................................................................................159
3.3 Shaft Design Methods ..............................................................................................160
3.3.1 Macaulay’s Method ......................................................................................160
3.3.2 Area–Moment Method ................................................................................161
3.3.3 Castigliano’s Method ...................................................................................162
3.3.4 Graphical Method ........................................................................................163
3.4 Engineering Calculations ........................................................................................163
3.4.1 Normal Stress for Shaft Subjected to Axial Force ...................................164
3.4.2 Bending Stress for Shaft Subjected to Bending Moment .......................165
3.4.3 Torsional Shear Stress and Torsional Deflection .....................................168
3.4.4 Lateral Deflection of Shaft ..........................................................................172
3.4.4.1 Lateral Deflection due to Bending Moment .............................173
3.4.4.2 Lateral Deflection due to Transverse Force ..............................174
3.4.4.3 Lateral Deflection due to Shear Force ........................................176
3.5 Shaft Design Issues ...................................................................................................176
3.5.1 Shaft Design Considerations ......................................................................176
3.5.2 Shaft Rigidity ................................................................................................178
3.5.3 Critical Shaft Speed .....................................................................................178
3.5.3.1 Shaft with Uniform Diameter ....................................................179
3.5.3.2 Stepped Shaft ................................................................................179
3.5.4 Dimensional Tolerance ................................................................................179
3.5.5 Dynamic Shaft Runout ...............................................................................180
3.5.6 Shaft Eccentricity .........................................................................................180