Abstract
In this work, the modelling and performance analysis of six-phase asynchronous motor under asymmetrical faults based on finite element analysis (FEA) is prioritised. Initially, an experimental test on a three-phase squirrel cage induction motor is performed to provide validation and enable the extraction of necessary machine data to be used in the study. From the extracted machine data, the three-phase induction motor is first modelled and analysed in FEA for proof of concept, after which the same motor is remodeled in six-phase to carry out the analysis under different asymmetrical fault conditions, in both single and double layer winding configurations. For the laboratory experiments, no-load, blocked rotor, load and retardation tests were carried out on a three-phase single layer winding induction motor prototype, and compared against FEA simulation results in ANSYS Maxwell 2D software. The use of a three-phase motor prototype is due to, among other things, the absence of a six-phase drive for operating a six-phase motor prototype. A good agreement between the test three-phase motor and the simulated three-phase motor is obtained in terms of the core loss which were measured at 57 W and 56.6 W, between simulation and experiments, respectively. Based on the comparison done between the three-phase and six-phase single layer winding motors in transient FEA, the percentage overshoot in speed and core loss are 4.95% and 3.72% for speed, 14.04% and 5.74% for core loss, respectively. The reduced percentage overshoot in speed and core loss at transient of the six-phase single layer winding motor is an indication of the six-phase single layer winding motor, consuming less energy than the three-phase single layer winding motor. On the other hand, the double layer winding layout of three-phase and six-phase motors when compared, gave respectively 4.73% and 3.79% overshoot in speed, and 14.02% and 4.45% overshoot in core loss, under transient performance condition. Again, the lower percentage overshoot on the six-phase motor is an indication of six-phase motor taking less energy at transient than the three-phase motor. Comparison of the six-phase motor in both single and double winding layer configurations, in term of percentage torque ripples, reduction in speed and reduction in core loss, under asymmetrical faults conditions of loss of phase A excitation voltage, indicates that 47.3%, 1.43% and 18.08% for single layer winding as against 3.42%, 1.31% and 18.08% for double layer winding. In terms of implementing the faults on phases A and B voltage excitations, the single layer winding six-phase motor recorded 38.9%, 4.16% and 8.48% while its double layer winding counterpart had 39.96%, 7.68% and 44.86% for torque ripple, speed reduction and core loss reduction, respectively. While implementing the fault analysis at phases A and D, the single layer winding had 32.1%, 7.53% and 8.48% as against 78.37%, 7.69% and 46.98% for torque ripple, speed reduction and core loss reduction, respectively. The performance analysis under these different fault conditions show that six-phase induction motor in both winding configurations displayed better functional capability despite loss of phases unlike the equivalent three-phase motor that malfunctions and stops operation due to loss of phase excitation. To this end, six-phase squirrel cage induction motor, investigated and compared with its equivalent three-phase motor in this work, is found suitable for use in critical operations such as submarine, due to its better performance at transient and fault tolerant capability.
TABLE OF CONTENTS
Title page i
Declaration ii
Certification iii
Dedication iv
Acknowledgment v
Table of contents vi
List of tables xii
List of figures xiii
List of plates xxvi
Appendices xxvii
List of symbols xxviii
Abstract xxxi
CHAPTER 1 INTRODUCTION 1
1.1 Background of Study 1
1.2 Statement of Problem 15
1.3 Aim and Objectives 17
1.4 Scope of Study 18
1.5 Significance of Study 18
1.6 Research Outline 19
CHAPTER 2 LITERATURE REVIEW 20
2.1 Three-phase Induction Motor 20
2.2 Modelling and Analysis of Three-phase Induction Motor 23
2.3 Three-phase and Multi-phase Induction Motor 24
2.3.1 Modelling and analysis of multi-phase motor 39
2.4 Six-phase Induction Motor 43
2.4.1 Modelling and analysis of six-phase induction motor 44
2.5 Faults on Induction Motor 56
2.6 Other Reviewed Papers 65
2.7 Research Gaps 77
CHAPTER 3 MATERIALS AND METHODS 78
3.1 Finite Element 78
3.2 Mathematical Principles of Maxwell 2D 79
3.3 Ansys Maxwell Software 79
3.3.1 Ansys RMxpt 80
3.3.2 Maxwell 2D 80
3.3.3 Maxwell 3D 81
3.4 Parameters of three-phase Induction Motor 81
3.5 Modeling of three-phase Single and Double Layer Winding Induction Motor in RMXPT Design and Ansys Maxwell 2D 83
3.5.1 Design and modelling of three-phase single layer winding
induction motor 83
3.5.1.1 Transfer of model to Maxwell 2D for modelling 86
3.5.1.2 Excitation of phases 88
3.5.2 Modelling of three-phase double layer winding induction motor 89
3.5.2.1 Excitation of phases 92
3.5.3 Boundary condition 94
3.5.4 Mesh operation 94
3.5.5 Motion setup 95
3.5.6 Solve setup 95
3.5.7 Test on simulated and actual three-phase motors 96
3.5.7.1 No load test 96
3.5.7.2 Blocked rotor test 99
3.5.7.3 Loading conditions 101
3.5.8 Result plots of three-phase single and double layer
winding motor 103
3.6 Laboratory Experiment on the Prototype Three-phase Single
Layer Winding Induction Motor 104
3.6.1 Modelling of three-phase induction motor 105
3.7 Modelling Six-phase Induction Motor 106
3.7.1 Electrical modelling 108
3.7.2 Mechanical modelling of three-phase induction motor 111
3.7.3 Data calculation and winding configuration 112
3.7.3.1 Calculation of number of slots per pole per phase
and other parameters from three-phase motor 112
3.7.3.2 Winding configuration for phases A, B, C, D, E and F 113
3.8 Modelling of Six-phase Single Layer Winding Induction Motor
in Maxwell 2D 115
3.8.1 Phases excitation 116
3.9 Modelling of Six-phase Double Layer Winding Induction Motor in Maxwell 2D 119
3.9.1 Ansys Maxwell model of six-phase double layer winding 119
3.10 Tests on Both Single and Double Layer Six-phase Induction Motors 124
CHAPTER 4 RESULTS AND DISCUSSION 125
4.1 Three-phase Single Layer Winding 125
4.1.1 No-load condition 125
4.1.2 Blocked rotor condition 128
4.1.3 Rated load condition 130
4.1.4 Asymmetrical fault at phase A 133
4.1.5 Asymmetrical faults at phases A and B 136
4.2 Three-phase Double Layer winding 139
4.2.1 No load condition 139
4.2.2 Blocked rotor 141
4.2.3 Rated load condition 144
4.2.4 Asymetrical fault at A 147
4.2.5 Asymetrical fault at phases A and B 150
4.3 Experimental Result from the Prototype Three-phase Motor 153
4.3.1 Losses calculation 153
4.3.2 Calculation of inertia 154
4.3.3 Validation of experimental and simulated results 154
4.4 Six-phase Single Layer Winding 155
4.4.1 Blocked rotor condition 155
4.4.2 No load to rated load condition 157
4.4.3 Asymetrical fault at phase A 160
4.4.4 Asymetrical fault at phases A and B 162
4.4.5 Asymetrical fault at phases A and D 165
4.5 Six-phase Double Layer Windig 167
4.5.1 Blocked rotor 167
4.5.2 No load to rated load 170
4.5.3 Asymetrical fault at phase A 172
4.5.4 Asymetrical fault at A and B 175
4.5.5 Asymetrical fault at A and D 177
4.6 Comparison of Three-phase Single and Three-phase Double Winding Induction Motor 180
4.6.1 No load 180
4.6.2 Rated load 182
4.7 Comparison of Three-phase Single and Six-phase Single Layer
Winding Induction Motor 185
4.7.1 No load 185
4.7.2 Rated load 187
4.8 Comparison of Three-phase Double and Six-phase Double Layer Winding Induction Motor 190
4.8.1 No load 190
4.8.2 Rated load 192
4.9 Comparison of Asymmetrical Fault Conditions in Six-phase Single and Six-phase Double Layer Winding 195
4.9.1 Asymmetrical fault at phase A 195
4.9.2 Asymmetrical fault at phases A and B 198
4.9.3 Asymmetrical fault at phases A and D 200
CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 206
5.1 Conclusion 206
5.2 Recommendations 208
5.3 Contribution to Knowledge 208
References 210
LIST OF TABLES
3.1: Three-phase motor data 82
3.2: No-load test readings 97
3.3: Blocked rotor test readings 101
3.4: Load readings 103
3.5: Coil arrangement in comparison with three-phase counterpart 114
4.1: No load test result 153
4.2: Comparison of three-phase single layer and double layer winding induction motor 184
4.3: Comparison of three-phase single layer and six-phase single layer winding induction motors 189
4.4: Comparison of three-phase double and six-phase double layer winding induction motor 194
4.5: Comparison of six-phase single and six-phase double layer winding induction motors with fault at phase A 197
4.6: Comparison of six-phase single and six-phase double layer winding induction motors with fault at phases A and B 199
4.7: Comparison of six-phase single and six-phase double layer winding induction motors with fault at phases A and D 202
LIST OF FIGURES
3.1: Stator frame of the three-phase single layer slot induction motor 84
3.2: Coil arrangement phase A of the three-phase single layer slot induction motor 84
3.3: Coil arrangement phase B of the three-phase single layer slot induction motor 85
3.4: Coil arrangement phase C of the three-phase single layer slot induction motor 85
3.5: Coil arrangement of the three-phase single layer slot induction motor 86
3.6: Quarter model of transferred three phase induction motor to 2D 87
3.7: Full model of the three-phase motor winding excitation 87
3.8: Phase A excitation 88
3.9: Phase B excitation 88
3.10: Phase C excitation 89
3.11: Phase A coil arrangement 90
3.12: Phase B coil arrangement 90
3.13: Phase C coil arrangement 91
3.14: Coil arrangement of the stator of the double layer slot three-phase induction motor 91
3.15: Full model of the three-phase double layer winding motor winding excitation 92
3.16: Phase A excitation 93
3.17: Phase B excitation 93
3.18: Phase C excitation 94
3.19: Motion setup/band 95
3.20: Approximate equivalent circuit for no-load test 96
3.21: Equivalent circuit for blocked rotor test 99
3.22: Per phase equivalent circuit of three-phase induction motor 104
3.23 : D-Q equivalent circuit of three-phase induction motor 105
3.24: D-Q equivalent circuit of a six-phase induction motor 108
3.25: Induction motor mechanical model 112
3.26: The clock diagram of stator winding 114
3.27: Full model of the six-phase single layer induction motor in Maxwell 2D 115
3.28: Phase A excitation 116
3.29: Phase B excitation 116
3.30: Phase C excitation 117
3.31: Phase D excitation 117
3.32: Phase E excitation 118
3.33: Phase F excitation 118
3.34: Clock diagram of double layer winding of six-phase induction 119
3.35: Full model of six-phase double layer winding induction motor 120
3.36: Phase A excitation 121
3.37: Phase B excitation 121
3.38: Phase C excitation 122
3.39: Phase D excitation 122
3.40: Phase E excitation 123
3.41: Phase F excitation 123
4.1: No load torque of three-phase single layer winding 125
4.2: No load input voltage of three-phase single layer winding 126
4.3: No load speed of three-phase single layer winding 126
4.4: No load current of three-phase single layer winding 127
4.5: No load core loss of three-phase single layer winding 127
4.6: Blocked rotor torque of three-phase single layer winding 128
4.7: Blocked rotor input voltage of three-phase single layer winding 128
4.8: Blocked rotor speed of three-phase single layer winding 129
4.9: Blocked rotor phase current of three-phase single layer winding 129
4.10: Blocked rotor core loss of three-phase single layer winding 130
4.11: Rated load torque of three-phase single layer winding 131
4.12: Rated load input voltage of three-phase single layer winding 131
4.13: Rated load speed three-phase single layer winding 132
4.14: Rated load phase A current of three-phase single layer winding 132
4.15: Rated load core loss of three-phase single layer winding 133
4.16: Torque of three-phase single layer winding with asymmetrical fault at phase A 134
4.17: Voltage of three-phase single layer winding with asymmetrical fault at phase A 134
4.18: Speed of three-phase single layer winding with asymmetrical fault at phase A 135
4.19: Phase A current of three-phase single layer winding with asymmetrical fault at phase A 135
4.20: Core loss of three-phase single layer winding with asymmetrical fault at phase A 136
4.21: Torque of three-phase single layer winding with asymmetrical fault at phases A and B 136
4.22: Input voltage of three-phase single layer winding with asymmetrical fault at phases A and B 137
4.23: Phase A and B currents of three-phase single layer winding with asymmetrical fault at phases A and B 138
4.24: Core loss of three-phase single layer winding with asymmetrical fault at phases A and B 138
4.25: No load torque of three-phase double layer winding 139
4.26: No load voltage of three-phase double layer winding 139
4.27: No load speed of three-phase double layer winding 140
4.28: No load current of three-phase double layer winding 140
4.29: No load core loss of three-phase double layer winding 141
4.30: Blocked rotor torque of three-phase double layer winding 142
4.31: Blocked rotor voltage of three-phase double layer winding 142
4.32: Blocked rotor Speed of three-phase double layer winding 143
4.33: Blocked rotor current of three-phase double layer winding 143
4.34: Blocked rotor core loss of three-phase double layer winding 144
4.35: Rated torque of three-phase double layer winding 145
4.36: Voltage of three-phase double layer winding on rated load 145
4.37: Rated speed of three-phase double layer winding 146
4.38: Currents of three-phase double layer winding on rated load 146
4.39: Core loss of three-phase double layer winding on rated load 147
4.40: Torque of three-phase double layer winding with asymmetrical fault on phase A 147
4.41: Phase A voltage three-phase double layer winding with asymmetrical fault on phase A 148
4.42: Speed of three-phase double layer winding with asymmetrical fault on phase A 148
4.43: Phase A current of three-phase double layer winding with asymmetrical fault on phase A 149
4.44: Core loss three-phase double layer winding with asymmetrical fault on phase A 149
4.45: Torque of three-phase double layer winding with asymmetrical fault on phase A and B 150
4.46: Phase A and B voltage of three-phase double layer winding with asymmetrical fault on phase A and B 151
4.47: Phase A and B current of three-phase double layer winding with asymmetrical fault on phase A and B 152
4.48: Core loss of three-phase double layer winding with asymmetrical fault on phase A and B 152
4.49: Plot of rotational losses Prot against square of voltage V2 154
4.50: Blocked rotor torque of the six-phase single layer winding 155
4.51: Blocked rotor input voltage of the six-phase single layer winding 155
4.52: Blocked rotor speed of the six-phase single layer winding 156
4.53: Blocked rotor current of the six-phase single layer winding 156
4.54: Blocked rotor core of the six-phase single layer winding 157
4.55: No load to rated load torque of the six-phase single layer winding 157
4.56: No load to rated load input voltage of the six-phase single layer winding 158
4.57: No load to rated load speed of the six-phase single layer winding 158
4.58: No load to rated load current of the six-phase single layer winding 159
4.59: No load to rated load core loss of the six-phase single layer winding 159
4.60: Torque of the six-phase single layer winding with asymmetrical fault at phase A 160
4.61: Input voltage of the six-phase single layer winding with asymmetrical fault at phase A 160
4.62: Speed of the six-phase single layer winding with asymmetrical fault at phase A 161
4.63: Phase A current of the six-phase single layer winding with asymmetrical fault at phase A 161
4.64: Core loss of the six-phase single layer winding with asymmetrical fault at phase A 162
4.65: Torque of the six-phase single layer winding with asymmetrical fault at phases A and B 162
4.66: Phase A and B voltage of the six-phase single layer winding with asymmetrical fault at phases A and B 163
4.67: Speed of the six-phase single layer winding with asymmetrical fault at phases A and B 163
4.68: Phase A and B current of the six-phase single layer winding with asymmetrical fault at phases A and B 164
4.69: Core loss of the six-phase single layer winding with asymmetrical fault at phases A and B 164
4.70: Torque of the six-phase single layer winding with asymmetrical fault at phases A and D 165
4.71: Phase A and D voltage of the six-phase single layer winding with asymmetrical fault at phases A and D 165
4.72: Speed of the six-phase single layer winding with
asymmetrical fault at phases A and D 166
4.73: Phase A and D current of the six-phase single layer
winding with asymmetrical fault at phases A and D 166
4.74: Core loss of the six-phase single layer winding with asymmetrical fault at phases A and D 167
4.75: Blocked rotor torque of the six-phase double layer winding 167
4.76: Blocked rotor input voltage of the six-phase double layer winding 168
4.77: Blocked rotor speed of the six-phase double layer winding 168
4.78: Blocked rotor current of the six-phase double layer winding 169
4.79: Blocked rotor core loss of the six-phase double layer winding 169
4.80: No load to rated load torque of the six-phase double layer winding 170
4.81: No load to rated load input voltage of the six-phase double layer winding 170
4.82: No load to rated load speed of the six-phase double layer winding 171
4.83: No load to rated load current of the six-phase double layer winding 171
4.84: No load to rated load core loss of the six-phase double layer winding 172
4.85: Torque of the six-phase double layer winding with asymmetrical fault at phase A 172
4.86: Phase A voltage of the six-phase double layer winding with asymmetrical fault at phase A 173
4.87: Speed of the six-phase double layer winding with asymmetrical fault at phase A 173
4.88: Phase A current of the six-phase double layer winding with asymmetrical fault at phase A 174
4.89: Core loss of the six-phase double layer winding with asymmetrical fault at phase A 174
4.90: Torque of the six-phase double layer winding with asymmetrical fault at phases A and B 175
4.91: Phase A and B voltage of the six-phase double layer winding with asymmetrical fault at phases A and B 175
4.92: Speed of the six-phase double layer winding with asymmetrical fault at phases A and B 176
4.93: Phase A and B current of the six-phase double layer winding with asymmetrical fault at phases A and B 176
4.94: Core loss of the six-phase double layer winding with asymmetrical fault at phases A and B 177
4.95: Torque of the six-phase double layer winding with asymmetrical fault at phases A and D 177
4.96: Phase A and D voltage of the six-phase double layer winding with asymmetrical fault at phases A and D 178
4.97: Speed of the six-phase double layer winding with asymmetrical fault at phases A and D 178
4.98: Phase A and D current of the six-phase double layer winding with asymmetrical fault at phases A and D 179
4.99: core loss of the six-phase double layer winding with asymmetrical fault at phases A and D 179
4.100: No-load torque of single layer and double layer winding of three-phase induction motor 180
4.101: No-load speed of single layer and double layer winding of three-phase induction motor 180
4.102: No-load core loss of single layer and double layer winding of three-phase induction motor 181
4.103: No-load to load torque of single layer and double layer winding of three-phase induction motor 182
4.104: No-load to load speed of single layer and double layer winding of three-phase induction motor 183
4.105: No-load to load core loss of single layer and double layer winding of three-phase induction motor 183
4.106: No-load torque of single layer winding of three-phase and six-phase induction motor 185
4.107: No-load speed of single layer winding of three-phase and six-phase induction motor 186
4.108: No-load core loss of single layer winding of three-phase and six-phase induction motor 186
4.109: No-load to load torque of single layer winding of three-phase and six-phase induction motor 187
4.110: No-load to load speed of single layer winding of three-phase and six-phase induction motor 188
4.111: No-load to load core loss of single layer winding of three-phase and six-phase induction motor 188
4.112: No-load torque of double layer winding of three-phase
and six-phase induction motor 190
4.113: No-load speed of double layer winding of three-phase and six-phase induction motor 191
4.114: No-load core loss of double layer winding of three-phase and six-phase induction motor 191
4.115: No-load to load torque of double layer winding of three-phase and six-phase induction motor 192
4.116: No-load to load speed of double layer windig of three-phase and six-phase induction motor 193
4.117: No-load to load core loss of double layer winding of three-phase and six-phase induction motor 193
4.118: Torque of six-phase single and six-phase double layer winding with asymmetrical fault at phase A 195
4.119: Speed of six-phase single and six-phase double layer winding with asymmetrical fault at phase A 196
4.120: Core loss of six-phase single and six-phase double layer winding with asymmetrical fault at phase A 196
4.121: Torque of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and B 198
4.122: Speed of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and B 198
4.123: Core loss of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and B 199
4.124: Torque of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and D 200
4.125: Speed of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and D 201
4.126: Core loss of six-phase single and six-phase double layer winding with asymmetrical fault at phase A and D 201
LIST OF PLATES
3.1: The test machine 81
3.2: Machine set-up for no-load Test 97
3.3: Machine set-up for blocked rotor test 100
3.4: The resistor box used for loading the machine 102
3.5: Measuring equipment during machine test 102
Appendices
1: Three-phase induction machine design – single layer winding- Rmxprt 219
2: Three-phase induction machine design – double layer winding- Rmxprt 228
3: Voltage and current curves measured from the oscilloscope 237
4: Cross section of stator showing the winding configuration of three-phase motor used for experiment in the laboratory 238
5: Dismembering of the three-phase motor used for experiment in the laboratory 239
6: Cross section of stator, rotor and shaft of three-phase motor used for experiment in the laboratory 238
7: Stator and rotor slots of the three-phase motor 239
8: Retardation test curve 240
9: IEEE conference paper from the research work 241
LIST OF ABBREVIATION/SYMBOLS
AC/ac alternating current
DC/dc direct current
PWM pulse width modulation
PMSM permanent magnet synchronous motor
MMF magno-motive force
EVs electric vehicles
HEVs hybrid electric vehicles
VSI voltage source inverter
HPO higher phase order
DTC direct torque control
DSP digital signal processor
SVPWM space vector pulse width modulation
DSIM dual stator induction motor
PI proportional integral
RFO rotor field oriented
IM induction motor
3-D three dimension
2-D two dimension
ASD adjustable speed drive
IFOC indirect field oriented control
SPWM sine pulse width modulation
PD proportional differential
HP/hp horse power
VI volt-current
SCR sillicon controlled rectifier
MOSFET metal-oxide-semiconductor field-effect transistor
CSI current source inverter
LCCSI load-commutated current source inverter
GTO gate turn off
TSCFE-SS time stepping coupled finite element state space
MFF motor field force
FE finite element
PSPICE personal(computer) simulation program with integrated circuit emphasis
THD total harmonic distortion
FEM finite element model
PMSMs permanent magnet synchronous motors
ZSVC zero sequence voltage component
SPIM six-phase induction motor
GA genetic algorithm
AWSP-IM asymmetrical wound six-phase induction motor
TPIM three-phase induction motor
SVWM space vector pulse width module
mABC modified artificial bee colony
FPA flower pollination algorithm
FEA finite element analysis
EMF electro motive force
BLDC brushless direct current
RMXpt rotatory machine expert
EM electro-magnet
B flux density
H field intensity
J current density
E electric field
ì magnetic permeability
σ electrical conductivity
D electric flux density/displacement vector
í magnetic reluctance
VA
,VB, VC, VD, VE and VF voltage excitations
dq0 direct-quadrature-zero transformation
dq direct-quadrature transformation
CHAPTER 1
INTRODUCTION
The research work is introduced in this chapter. A brief review is made to obtain background information on the subject. The set objectives to be achieved are highlighted in this chapter to guide the research proceedings.
1.1 BACKGROUND OF STUDY
Multi-phase is a phase number greater than three. Electricity is available in three-phase system, as it is generated in three phases, transmitted and distributed. This constitutes the most economical number of generator and transmission phases, as there is a balance between the complexity of the three-phase scheme and the power improvement. However, a power electronic system, which could be a voltage or current source inverter, is made to supply such three-phase motor. For the power electronic system, limit to their number of legs are not constrained. The number of phases of output in the system is similar to respective number of legs. Therefore, in putting additional leg to the power electronic system, leads to the increase of the number of phases. This freedom of phase increment at will, is the main interest in developing multi-phase electric machine with more than three phases (Haitham et al., 2012). Six-phase machines have been of interest and attracted attention in the research work, after their merits were realized in 1983. The multi-phase motor attracted much interest from research community after the advent of cost effectiveness and reliability of power efficient devices, as well as its powerful digital devices for signal processing. The main purpose of an electric drive system is controlling the energy conversion from both elctrical to mechanical in either way. It must be noted that, power drives with multi-phase machines are needed in a lot of technical areas, such as traction, hybrid / electric vehicles and propulsion for ships. For many decades, multi-phase motor, fed by inverter systems have been adapted as a promising technique in arriving at high power specification in conjunction with devices that limit the amount of voltage. Such structure is commonly found in a 3-level inverter supplying a 3-phase electric motor system (Marouani et al., 2007). The parallel circuit dual to multi-level system is essentially the base of the concept of the multi-phase inverter. For a multi-phase machine system, windings greater than three are wound in equivalent stator of the machine. The merits of multi-phase machine as likened to its equivalent three-phase system include;
- A torque density going higher in the same machine specification with torque pulsations reducing, windings that have higher number of phases producing fields, which have smaller harmonic content and made the remaining space harmonic field to be used for torque.
- Fault tolerance and machine reliability are enhanced, this is because if a phase loss occurs, the machine can still start and run. The difference is the case in three-phase counterpart, where a phase loss result in inabilty to start and massive torque pulsation, if it was running.
- Division of the controlled power on many inverter legs is possible. This will lead to the reduction in current stress of individual switches. Hence the choice of either parallel and/or series arrangement of semiconductor switches can be allowed or expunged from the power electronic device (Marouani et al., 2007).
Ac machines are simpler and robust to construct. In addition, they allow high-speed and gaurantee free maintenance operation. Advantages, such as power/weight ratio and low cost are common features of interest in squirrel cage induction machines. Rectifier with inverter combination, are widely being used in most industrial applications. The input of the inverter is mostly dc, fed through rectification that is not uncontrolled. Non sinusoidal ac is applied in inverter drive systems; where a six-step voltage waveform is frequently used. Nevertheless, in ac drive system that needs high-performance, it is preferable to use the pulse width modulated (PWM) waveforms. PWM techniques enhances control of drives when used. Design of the PWM schemes should remain paramount so as to do away with the unwanted dominant current harmonics. Sinusoidal PWM remains one of the most frequently used methods in motor drives. Other methods which include the step and triangular PWM are good for digital microcomputers in terms of implementation. Current-impressed scheme, gain prominence in some uses, where fast control of motor currents, instead of voltages is required. A number of investigations in terms of transient and steady-state evaluation of an induction motor supplied from variable frequency quasi-square wave sources were also seen in Ho and Sen, (1986).
Since the use of five-phase induction motor started in 1969, the machines have been providing suitable alternatives to three-phase motors. This for sure is right for high-power as well as safety concerned variable-speed considerations, in which a five-phase motor drive is made, using inverters with small power rating per leg with assured reliability of operation, even in redundancy. Fault-tolerant properties are especially important for applications related to the concept of “more electric” aircraft. In five-phase (and generally multi-phase) systems, the noise features are improved, a reduction in the losses of the stator winding, which leads to high efficiency and reduction in torque ripple are enhanced. There are many benefits and excellent characteristics of multi-phase motors in some latest quest for contemporary multi-phase induction motors. A vector control mechanism for a five-phase machine remains in its fundamental form, irrespective of machine type, common to the the mechanism used in a three-phase machine. However, since the vector control of an ac machine requires only two axis currents for decoupled flux and torque control, a higher torque density can be achieved in the six-phase machine by utilizing the remaining two degrees of freedom. Third harmonic injection of stator current makes the use of the third spatial harmonic of the field for the torque production, adding to the fundamental field harmonics. The merits of a two-six-phase motor drive system remains reducing the number of needed inverter outputs in comparing to the same two-motor three-phase system. This translates into increased reliability at component level due to a smaller number of components. It should be observed, however, that due to fault-tolerant capacity, one of the primary advantages of mult-phase systems remains enhanced machine efficiency (Levi et al., 2007). The induction machine has gained prominence in its usage in the industry. For its usage as load in a test set-up for experiments in gears as well as thermal combustion motors, a dynamic aspect of motor model is necessary, including main flux on saturation. In as much as ripples in torque causes severe vibrations of the system, which are developed by system resonance of the test set-up, a model is important, that focuses on torque ripple, made by the motor alone even at sinusoidal input and ripples of torque owing to current harmonics, as a result of inverter inputs (Rentschler and Binder, 2005).
Multi-phase induction motor has received increasing interest since the second half of 1990s due to several merits of multiphase and superior performance of PMSM system over the three-phase drive system. This increasing interest is because this machine can give reasonable improvements in different aspects of performance when compared with conventional three-phase motor and six–phase induction motors. From history, some technical points for using muti-phase machines are:
- In a given motor power output, multi-phase motor leads to the reduction of stator current per phase.
- Improved reliability is seen in using more than three-phase in induction motors.
- Reduction in pulsating torques production by harmonic components in the excitation waveforms in multi-phase machines.
- Enhancement of fault tolerant attributes.
- Higher degree of freedom in the inverter output.
For a given machine power, increase in number of phases brings about a reduction in power per phase, thus resulting in the use of smaller power, without the increase in phase voltage. The phase increase in multi-phase motor results in considerable achievement because of its reliability in operation, when one or more phases are out of excitation. Three-phase induction motor counterpart remains sensitive to different types of faults, which affect motor phases. In an event of fault in one phase, motor operation is disrupted for a unscheduled maintenance. The machine under fault is able to continue operation but it is not still self-starting. The design of fault tolerant motor systems is explained as a consequence of heavy costs incurred. On the other side, multi-phase machines continue to operate with unbalanced rotating composition and excitation in post-fault situation, generating a greater proportion of their produced torque with tiny pulsations relative to the three-phase motors.
Induction motors are known as the workhorse in industrial applications. The coming of multi-phase induction motor with the means of speed control, has made it popular and most applied in actualising a lot of applications nowadays. It has many merits than other types of machines, which include robustness, can adapt to hazardous location, low in maintenance and cheap in manufacturing. With the rapid growth in advancement in the industry and transportation sector, there is call for higher power per volume, efficiency and more reliabilty of machines as a needed solution to obtain a bigger, more reliable machines to drive high power and crucial load (Tomer and Dubey, 2013).
Multi-phase induction motors in higher phase order are viable solution to the need for larger and more reliable machines. Unlike the classical three-phase induction machines, higher phase order machines have stator consisting of a phase number of more than three. Six-phase induction machines are well documented in research articles stretching over the last five decades. Many surveys and reviews were conducted on the state-of - the-art multi-phase machines, as well as documentation on the reality of up to twelve-phase induction machines in higher phase order. More than three phase order machines have many merits over their three phase equivalent which has to do with reduction of torque ripple, smaller current in a phase with similar rating and most especially, fault tolerant capability. The ability of multi-phase phase order asynchronous machines working under open circuit fault of one or more phases are worked on and documented. This is very needful in trains and submarines (Venter et al., 2012).
This offers a better solution to transport sector, as it provides higher energy efficiency and reliability, lower emission and less maintenance costs. This idea had its first implementation in the aeronautical industrial application, where it implies on-board electrification facilities by electric, mechanical and bleed air/pneumatic devices are continually available. The idea has a merger with the historic hybrid propulsion systems, used in sea transport, in which diesel electric propulsion is free from the control of the shaft line.
The ground transport sector provides convergence of auxiliaries’ electrification and power train hybrid system, which produces a wide range of techno-economic challenges (Cavagnino and Tenconi, 2014).
The advances in modern power electronic devices and the development in the control system makes it possible to take the number of phases of motor drives as one of the design variables. As a follow up, the traditional three-phase solution is simply a particular case. The increment in the phase number result in the reduction of the stator current in a phase with same power rating, with evident benefits in terms of inverter power devices. In addition, the developed spatial distribution of magnet motive force (MMF) in the motor air gap, helps in the reduction of the rotor copper losses and the torque pulsation amplitude. It is also seen as the benefit of multi-phase machine over three-phase, the of degree of freedom that can be of advantage in several ways. This degree of freedom can be adequately utilized for the improvement in terms of fault-tolerant capacity of the motor. “This attributes stands for a crucial point for all the applications leading to a high degree of reliability” (Casadei, et al., 2009).
Industrial and transportation applications are exposed to a growing need for high-power electric motor capable of ensuring high static and dynamic performance. In high power applications, the high cost of the motor and the energy static conversion equipment allows the use of digital devices for control with more computational performance. The performance of motor has limitation owing to the low frequency of switching of electronic switches (Borghetti et al., 2008).
Several works on multi-phase induction-motor have been done since the beginning of the century providing attributes that are suitable for high industrial power applications. This is mainly applied in the areas of ship propulsion, traction which include electric and hybrid electric vehicles as well as the idea of more electric aircraft. These are restricted range of uses, where traditional three-phase motors are not provided off the shelf or are not suitable to the specifications, has several advantages, such as higher power per rms ampere ratio, low ripples of torque, reduction in acoustic noise and low vibration. As such, multi-phase systems also enhances additional degrees of freedom in comparison to their three-phase equivalent. The new subspace (term x-y) appears in multiphase machines using the vector space decomposition. This special feature has encouraged researchers to realize independent control of multi-phase motor drives by means of series and parallel connections, a free disturbance mode of operation for post fault conditions as well as higher torque density by injection of current harmonic in the concentrated-winding motors. The latter option has shown the applications sector excellent opportunities. . It is based on the interaction of the spatial and electrical harmonics of the same order, which generates a component rotating at the fundamental frequency. This component accounts for the torque enhancement, as well as giving a reasonable MMF shape that is useful for saturation to be avoided. The reduction of the torque ripples of the six-phase induction motor was affirmed and control schemes were made, but no study was carried out to analyze the impact of the harmonic injection on the stability of the machine. Consequently, it has been addressed whether torque enhancement also implies stability enhancement or whether the opposite situation occurs (Duran et al., 2008).
Using classic three-phase converters, induction motors have the less benefits of poor voltage and current properties. Thus, various PWM approach mainly on inverter of two-level, there are many techniques for circular flux vector and developing output power at the same time. Multi-level inverters have been of interest in the past few years as a promising one, which shows itself by changing the structure topology of inverter. Multi-phase system remains vital choice which aim is to modify the stator windings arrangement of induction motor. All the techniques highlighted above can be implemented combined with PWM techniques to obtain more perfect result. In the system of multi-phase machine drive, higher than three-phase windings are implemented in the equivalent stator. The most interesting is the six-phase induction motor, with two sets of windings spatially displaced by 30 electrical degrees with neutral line, isolated. Device which has such configuration, removes the sixth harmonic pulsating torque component and contributes immensely to the reduction of rotor losses as a result of the rotor current harmonic component reduction (Tiejun et al., 2007).
If the supply is from a six-step inverter, three-phase squirrel cage induction motors will have a pulsating component of torque, which equals six times the supply frequency. This is the reason for excess mechanical vibration as well as lower system performance capability, especially at low speed and high torque operation. If two three-phase inverters are used to supply a machine that has two three-phase stator winding sets, the amplitude of the pulsating torque will be substantially reduced and the frequency of the pulsating torque will move from 6 to 12 times the frequency of supply. For this reasonable torque wave shape to be acheived, the two three-phase stator winding sets must have to be shifted by 30⁰, which counters normal symmetrical displacement that is used in standard six-phase motors (Nelson and Krause, 1974).
An asymmetrical six-phase motor, that consisted two asymmetrical six-phase induction motors was tested and a symmetrical six-phase two-motor system that comprised a six-phase and a three-phase induction motors has also been examined in details (Mohapatra et al., 2005). Excellent decoupling of dynamics was also carried out in (Levi et al., 2005). A two-motor five-phase system, that comprised two-phase induction motors, was tested in an experiment by Iqbal et al. (2005), the demonstration showed that control of the two motors is indeed independent. It is founded that a two-motor five-phase and symmetrical six-phase motor is the highest option for real-world industrial application. Details of the dynamic modelling of these two two-motor transport schemes can also be obtained using overall electrical machine technique for the symmetrical six-phase motor and the respective two-motor five-phase motor (Levi et al., 2008).
Over a decade, interest in multiphase machines has substantially increased. The impetus behind this accelerated development can be found in three application areas, namely ship propulsion, more electric aircraft and traction (railways, Evs and HEVs). Multi-phase machines have two separate characteristics in their implementation areas that differentiate them from their three-phase equivalent. Firstly, the inverter's necessary power per-phase value is decreased, which is of specific significance in high-power systems. Secondly, “the regulation of multi-phase machines has extra degrees of freedom. These additional degrees of freedom can be used for two purposes ; torque production can be increased by injecting current harmonics of higher order or additional degrees of freedom can be used to improve the motor's fault tolerance and allow the motor to operate normally with the loss of one or more phases”. Since vector control of any ac device needs only two current components when only basic components are used to produce torque, the residual degree of liberty can be used to control other devices within a multi-machine group. “This constitutes the main idea behind the concept of series-connected multiphase machine systems, initially proposed and further developed for a two-motor five-phase drive in a two-motor six-phase drive” (Jones et al., 2006).
Industrial economic impediments lead to situation, where single supply is used for many machines. For traction systems, two of the dc motors are arranged in series, for only one chopper used. Independent control of torque is possible using field current control. For 3-phase ac machines that the supply is by a three-leg inverter, that is arranged in series, is unable to have such control independently. Independent control, which is connected in series, is possible for n-leg induction or synchronous machine (Semail et al., 2005).
Machines with multi-phases are commonly used in some variable speed drive applications due to the fact that they have higher efficiency, with lower magnitude of pulsating torques and be acoustically quieter than their three-phase equivalent. One additional merit is that multi-phase machines can tolerate fault, which leads to greater security in specialized applications. This is particularly suitable for potential marine applications as the primary propulsion motors. “Several researchers have addressed the issue of fault tolerance, focusing the large part on the performance of machines in which a single phase is open circuited and on compensation strategies for open-circuit faults” (Apsley and Williamson, 2005).
The introduction of multiphase motor concept many years ago has witnessed a new inflow of research interest in the few years owing to the technological advances of power electronic switching devices and also because of the recent development of the specialized applications, which include electric vehicle, aircraft, locomotive traction and naval electrical ship propulsion system, which high performance and reliability are criteria. High accuracy, fault tolerance and greater power density are the main advantages of the multi-phase motor system. A five-phase legs provides additional voltage vectors, when comparing with the three-phase converter, so the output of the modulator can have resolution. These specific properties provide considerable potential for further exploration in terms of performance improvement. Compared with the three-phase system, the five-phase poses more complexity and the methodology and some well-known fact in three-phase systems cannot be adapted to the five –phase system (Lu and Corzine, 2005).
The vector control of the five-phase induction motor is to produce the basic present waveform and the related third harmonic of this basic and properly mix these waveforms, thus creating the required adjacent triangular flux in the air gap. As a result an improvement in the power density and output torque of the five-phase induction motors can be achieved. Direct torque control of the five-phase induction motor is also of great importance (Xu et al., 2002a).
The induction machine, supplied by voltage source inverter, has several advantages such as a rough and low cost motor, elevated waveform fidelity capacity with PWM procedure, fairly high efficiency. However, because of the constraints on the ratings of the gate-turn-off form semiconductor power devices, their applications are still restricted to the lesser half of the high energy spectrum. To achieve high power ratings in such systems, multi-level inverters have been developed in the past decades as promising approach. Another strong contender in achieving high power is the multi-phase inverter fed multi-phase induction machine drive system. In addition to enhancing power rating, a multi-phase system also has the merit of high reliability at the system level. In particular, with the loss of one or more of the stator winding excitation sets, a multi-phase induction motor can continue to be operated with an asymmetrical winding structure and unbalanced excitation. The most commonly used analytical tool for the analysis of unbalanced operation of electric machines has been the well known symmetrical component method. In this method, a balanced structure is assumed even after the machine looses one or more of its phases. Although it has been used successfully in the steady state analysis on sinusoidal excitation, however, as far as the dynamics of the machine is concerned, the method looses its utility due to the fact that the interaction between the loss phases and the remainder of the machine windings no longer exist and thus drastically alter the dynamic behavior of the machine (Zhao and Lipo, 1996).
A three-phase, five-phase, six-phase as well as twelve phase induction machine is characterized with the spatial shift between phases of 120⁰, 72⁰, 60⁰, and 30⁰ electrical degrees. The rotor winding is, for the sake of generality, treated as an equivalent n-phase winding, of the same properties as the stator winding. It is assumed that the rotor winding, already transferred to stator winding, using winding transformation ratio, results in maximum value of the mutual stator to rotor inductance terms equal in value to the mutual inductance within the n-phase rotor winding (Dhillon et al., 2013).
The adjustable speed system, enables the induction motor to have higher performance, and also eliminate the impediment of the motor phase numbers to inject power for motor having any number of phases. As a result, the multi-phase induction motors have gained extensive application. Compared with the traditional three phase motors, multiphase motor possess many advantages such as reducing the amplitude and increasing the frequency of torque pulsation, reducing the harmonic currents, reducing the current per phase without increasing the voltage per phase, reducing the dc link current harmonics, and providing higher flux density, more output torque, and increased reliability. The developed resilient current control method ensures that the six-phase induction motor operates continuously and steadily without additional hardware connections in the event of the loss of up to two phases. This is of importance to some specific applications where fault tolerant ability and high reliability are required. For three-phase induction motor, if one phase of the motor or inverter is lost, a divided DC bus and neutral connections would be in need so as to control current independently in the remaining two phases. In order words, a zero-sequence component is necessary in a three-phase induction motor to provide an undisturbed rotating MMF after one phase is opened. From reliability standpoint, the zero sequence currents have detrimental effects on motor bearing failure, reducing the reliability and increasing the cost of maintenance of the motor. Six-phase induction machine adopts concentrated winding structure and makes use of the third harmonic currents to generate the nearly rectangular flux distribution, resulting in the improvement of flux density and increased output torque. Under asymmetrical faults conditions, the proper third harmonic currents are still superimposed on the fundamental currents of six-phase induction motor with the remaining phases so as to maintain a nearly rectangular air-gap flux just as under symmetrical conditions (Xu et al., 2002a).
The background of growth of multi-phase machines started in the early 1920s. However, in recent times, an enormous amount of job has been performed to investigate the particular benefits of multi-phase motors and drives. Multi-phase induction devices for industrial applications include electrical and hybrid vehicles, ship propulsion, more electrical aircraft and wind systems. One of the most frequently considered multiphase machines for these applications is asymmetrical six-phase or asymmetrical dual three-phase machine (Prieto et al, 2010). Induction motors have well known advantages of simple construction, reliability, ruggedness, low maintenance and low cost, which led to their wide spread use in many industrial applications. More research works have indicated that drives with six-phase have various merits over traditional three-phase ones, such as lower pulsation, reduction in harmonic currents, increase in current per phase without need to increase the phase voltage, greater reliability and fault tolerant (Renukadevi and Rajambal, 2011). With all these merits of multiphase induction motor, this work look at its six-phase modelling and analysis under asymmetrical faults.
1.2 STATEMENT OF PROBLEM
Due to the dominance of multiphase induction motor in terms of its usage and its wide spread industrial applications, it has become expedient to model and analyze a six-phase asynchronous motor under asymmetrical faults. Several works have been done on multi-phase machine with respect to various conditions, which the machine is subjected to.
In (Akpama et al., 2015), Transient analysis and modelling of six phase asynchronous machine, was carried out. Multi-phase induction machine was seen to be possible and advantageous. Three phase induction machines have almost replaced DC machine in the industry due to the simplicity of design, ruggedness, low-cost, low maintenance cost and direct connection to AC power source compared to DC motors. In fact, 85% of the total power consumption in industrial sector is from induction motors. As a consequence, fresh approaches are being attempted, fresh techniques of assessment and layout are being attempted that will enhance these systems performance as well as boost their effectiveness. Among the multi-phase machines category, owing to the ease of turning a three-phase machine to a six-phase system, six-phase machine has gained more publicity. In submarines, the idea is to use a six–phase motor to replace two three phase motors, and money is saved, redesigning the machine with higher reliability. In an event of loss of a phase, it does not stop the motor from running, as reliability is a criterion in designing a submarine. Moreover, benefits over the traditional three-phase motors are reducing the amplitude and increasing the frequency of torque pulsations, lowering the rotor harmonic current losses as well as DC link current harmonics. Multi-phase induction motor also has application in electric ship propulsion, traction including hybrid and electric vehicles and the idea of electric air-craft.
Tomer and Dubey, (2013), worked on a “complete modelling and performance analysis of six phase permanent magnet synchronous motor system”, which was fed by two voltage source inverter. The work was found to be reliable, with reduced stator current per phase and possess a high degree of freedom when compared with its three-phase counterpart. The effect of fault conditions were not considered. (Venter et al., 2012), worked on the realization of “3 & 6 phase” induction machine. They concentrated only on three and six phase conversion without examining its behavior under faulty conditions of the machine. Two series-connected five phase six phase two-motor system with single inverter supply has been examined. “The first was made of two-five machine supplied from a single five-phase inverter, whereas the second one comprised symmetrical six-phase machine, a three phase machine and single six-phase inverter”. The different configurations of the machines were not taken into consideration (Levi et al., 2008). In Aspsley and William, (2005) steady state behaviour of a multi-phase squirrel cage induction motor with any form of open circuit or short-circuit fault in the stator winding, was keenly examined without due consideration to losses. Dynamic modelling of an induction motor connected to an adjustable speed drive has been worked on without actually considering the multi-phase cases (Salon et al., 1989). Dynamic multi-phase induction machines models for smart drive applications under different operating conditions has been worked on, in which it was found that multi-phase supply system has potential benefits than three phase system and some interest have grown in the area of multi-phase a.c. motors. It however did not mention the effect of losses, which is a vital factor (Dhillon et al., 2013). Also in (Prieto et al., 2010), a modified continuous PWM technique for asymmetrical six-phase induction motor was analysed which numerous space vector PWM technique were developed for non-symmetrical six-phase induction motor drives. But, the core loss effect was not considered. Six-phase series-connected two-motor drive with decoupled dynamic control was also analysed, in which a six-phase two-motor drive, having a six-phase voltage source inverter (VSI), a six-phase induction machine as well as a three phase induction machine. However, different winding configurations of six-phase motor was not discussed (Jones et al., 2006). In (Che et al., 2014), current control methods for an asymmetrical six-phase induction motor drive, was carried out in which vector space decomposition approach was used, but this work was still lacking in asymmetrical faults effect findings. Thus, it has become expedient to model and analyze a six-phase induction motor under asymmetrical faults, which is believed will solve the problems, that were not considered by the previous authors.
1.3 AIM AND OBJECTIVES
The aim of this research work is modelling and analysis of six-phase asynchronous motor under asymmetrical faults.
The objectives of study include the following:
- To study the existing three-phase squirrel cage induction motor both in laboratory experiments and software simulation in single and double layer winding configurations.
- To reconfigure and remodel three-phase squirrel cage induction motor into six-phase squirrel cage induction motor in both single and double layer winding based on the three-phase motor parameters.
- To perform tests on the six-phase induction motor under no-load and load conditions.
- To subject the six-phase induction motor in both configuration to asymmetrical fault conditions.
- To validate the performances of the six-phase and three-phase motors in both configurations under asymmetrical faults using ANSYS Maxwell 2D.
1.4 SCOPE OF THE THESIS
The scope of this work is to model and analyse a six-phase asynchronous motor under asymmetrical faults condition. Particular attention will be given to speed, torque and core loss performances during asymmetrical fault so as to make comparism between faulty and normal conditions. The simulation program will be performed in ANSYS Maxwell 2D as it has become widely used and acceptable simulation software for machines.
1.5 SIGNIFICANCE OF STUDY
- The research work will make the application of six-phase induction motor more efficient, having known its behavior under different fault conditions.
- Better understanding of multi-phase induction motor will be enhanced by this research work.
- The research work, on completion, will serve as reference material for future research work on related topics.
- The research work will find significant application in submarine where asymmetrical faults are common features.
1.6 RESEARCH OUTLINE
Chapter one presents the introductory part, with brief history and fundamental background of the research topic. It includes aim, objectives, significance of work and the expected outcome of the research.
Literature review of relevant materials related to the research topic was presented in Chapter two. This includes previous and relevant works done on the research topic.
Chapter three presents the methods in which the research work was carried out. ANSYS Maxwell is the major software adopted and used to model and simulate the induction motors under normal conditions and asymmetrical faults condition.
Chapter four presents the result of the experiment and computer simulation as well as discussions on the various results while chapter five presents conclusion and necessary recommendations for future research on six-phase induction motor.
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