ABSTRACT
Temperature rise is of much concern in the short and long term operations of
induction machine, the most useful industrial work icon. This work examines
induction machines mean temperatures at the different core parts of the
machine. The system’s thermal network is developed, the algebraic and
differential equations for the proposed models are solved so as to ascertain the
thermal performances of the machine under steady and transient conditions.
The lumped parameter thermal method is used to estimate the temperature rise
in induction machine. This method is achieved using thermal resistances,
thermal capacitances and power losses. To analyze the thermal process, the
7.5kW machine is divided geometrically into a number of lumped components,
each component having a bulk thermal storage and heat generation and
interconnections to adjacent components through a linear mesh of thermal
impedances. The lumped parameters are derived entirely from dimensional
information, the thermal properties of the materials used in the design, and
constant heat transfer coefficients. The thermal circuit in steady-state condition
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consists of thermal resistances and heat sources connected between the
components nodes while for transient analysis, the thermal capacitances were
used additionally to take into account the change in internal energy of the body
with time. In the course of the simulation using MATLAB, the response curves
showing the predicted temperature rise for the induction machine core parts
were obtained. To find out the effect of the decretization level on the symmetry,
the two different thermal models, the SIM and the LIM models having eleven
and thirteen nodes respectively were considered and the results from the two
models were compared. The resulting predicted temperature values together
with other results obtained in this work provide useful information to designers
and industries on the thermal characteristics of the induction machine.
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TABLE OF CONTENTS
Title page ………………………………………………………….………………..….iii
Approval page ………………………………………………………………….….…..iv
Certification page………………………………………………………….…..….……v
Dedication page…………………………………………………………….…..….…..vi
Acknowledgement………………………………………………………….…..….….vii
Abstract……………………………………………………………………..…..….….viii
Table of contents…………………………………………………….….……..….……ix
List of figures…………………………………………………………….……..……..xii
List of tables………………………………………………………….…….…..….….xiv
List of symbols…………..……………………………………….……………..……..xv
Chapter One: INTRODUCTION ………………………………………………..….…..1
1.1 Background of study…………….……………….…………………………….…1
1.2 Statement of Problem …………………..……….…………….……….….….…3
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1.3 Purpose of Study ………………………………..…………………………………..3
1.4 Significance of Study ………………………………………….………………….4
1.5 Scope of Study..……………………………………….……………………………….5
1.6 Arrangement of Chapters ……..………………………………….……………….5
Chapter Two: LITERATURE REVIEW …………………………….…………………..6
Chapter Three: HEAT TRANSFER MECHANISMS IN ELECTRICAL MACHINES
3.1 Heat Transfer in Electrical Machines…………….……………….….………12
3.2 Modes of Heat Transfer …………………..……………..…….………….…13
3.2.1 Conduction ………………………………………………….……………………14
3.2.2 Convection ……………………………………………………………………..16
3.2.3 Radiation …………………..…………………………………….……………….18
3.3. Heat Flow in Electrical Machines ………………….…………..……..…..…20
3.3.1 Heat Transfer Flow Types …………………………………………………..20
3.3.2 Heat Transfer Flow System …………………………………..……….……..21
3.3.3 The Boundary Layers……………………………………………….…………22
3.4 Determination of Thermal Conductance………………………….….……….23
3.5 Thermal-Electrical Analogous Quantities ………………………….….……25
3.5.1 Thermal and Electrical Resistance Relationship …………….….…..…….26
Chapter Four: THERMAL MODEL DEVELOPMENT AND PARAMETER COMPUTATION
4.1 Cylindrical Component and Heat Transfer Analysis…………….……………28
4.2 Conductive Heat Transfer Analysis in Induction Motor ………….…….……28
4.3 Convective Heat Transfer Analysis in Induction Motor………….…….…….34
4.4 Description of Model Components and Assumptions …………….…….….35
4.5 Calculation of Thermal Resistances…………………………….…………….45
4.6 Calculation of Thermal Capacitances ………………………..…………….…56
Chapter Five: LOSSES IN INDUCTION MACHINE
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5.1 Determination of Losses in Induction Motors .…………………….………..69
5.1.1 Stator and Rotor Copper Losses ……………………..…………….…….. 69
5.1.2 Core Losses …………………………………………….…….……..….……70
5.1.3 Friction and Windage Losses ………………………….………..………….70
5.1.4 Differential Flux Densities and Eddy-Currents in the Rotor Bars ………..71
5.1.5 Stray-Load Losses …………………………………………………………….72
5.1.6 Rotor Copper Losses …………………………………………………….……72
5.1.7 No Load Losses …………………………………………….…………….…..73
5.1.8 Pulsation Losses ………………………………………………………………74
5.2 Calculation of Losses from IM Equivalent Circuit…………………………..74
5.3 Loss Estimation of the 7.5 kW Induction machine ….….………………….79
5.4 Segregation and Analysis of the IM Losses……… ………………………..82
5.5 Performance Characteristics of the 10 HP Induction machine…..………..83
5.6.1 Motor Efficiency /Losses ……………………….…………………………….86
5.6.2 Determination of Motor Efficiency ……………………..………….………….86 5.6.3
Improving Efficiency by Minimizing Watts Losses …………………………87
5.7 The Effects of Temperature ……………………………..….…….…………..88
Chapter Six: THERMAL MODELLING AND COMPUTER SIMULATION
6.1 The Heat Balance Equations ……………………………………………….……90
6.2 Thermal Models and Network Theory ……………………………….……..…90
6.3 The Transient State Analysis ……………………………….………….………98
6.4 The Steady State Analysis …………………………………………………..104
6.5 Transient State Analysis results.………………….….………………..……..108
6.6 Discussion of Results …………………………….……………………..…….116
Chapter Seven: CONCLUSION AND RECOMMENDATION
7.1 Conclusion…………………….……………….….……………………..…….118
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7.2 Recommendation …………….………………….……………………..…….119
REFERENCES …………………………………………………………………..…..….…..120
APPENDIX………………………………………………..……………….……..……….…..131
CHAPTER ONE
INTRODUCTION
1.1 Background of Study
This thesis is concerned with the thermal modelling of the induction
machine. With the increasing quest for miniaturization, energy
conservation and efficiency, cost reduction, as well as the imperative to
exploit easier and available topologies and materials, it becomes
necessary to analyze the induction machine thermal circuit to the same
tone as its electromagnetic design. This would help in achieving an early
diagnosis of thermo-electrical faults in induction machines, leading to an
extensively investigated task which pays back in cost and maintenance
savings. Since failures in induction machines occur as a result of aging of
the machine itself or from severe operating conditions then, monitoring
the machine’s thermal condition becomes crucial so as to detect any fault
at an early stage thereby eliminating catastrophic machine faults and
avoidance of expensive maintenance costs. Faults in induction machines
can be broadly classified into thermal faults, electrical faults and
mechanical faults. Currently, stator electrical faults are mitigated by recent
improvements in the design and manufacture of stator windings.
However, in case of machine driven by switching power converters the
machine is stressed by voltages including high harmonic contents. The
latter option is becoming the standard for electric drives. A solution is the
development of vastly improved thermal system cum insulation material.
On the other side, cage rotor design is receiving slight modifications,
apart from that, rotor bars breakage can be caused by thermal stress,
electromagnetic forces, electromagnetic noise and vibration, centrifugal
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forces, environmental stress, for example abrasion of rotor, mechanical
stress due to loose laminations, fatigue parts, bearing failure, e.t.c.
In the design of the induction machine, the manufacturers take many
factors into consideration to ensure that it works efficiently. One of the
most important factors in the design of an induction motor is its thermal
limits for different operating conditions because if a machine works
beyond its thermal limit for a prolonged time, the life span of the machine
is reduced.
The lumped-parameter thermal method is the most popular method used
to estimate the temperature rise in electrical systems. The thermal model
is based on thermal resistances, thermal capacitances and power losses.
To analyze the thermal process, the electrical system is divided
geometrically into a number of lumped components, each component
having a bulk thermal storage and heat generation and interconnections
to flanking components through a linear mesh of thermal impedances. It
may be a simple network as demonstrated in [1] or may have many tens
of nodes. For any given configuration, the designer looks for a matching
design tool for the analysis. Motor-Cad is a design tool used by some
authors in [2-3] for thermal analysis of electrical motors. This design tool
gives a detailed model, based on the geometry and the type of the motor.
It was predominantly used to analyze the parameter sensitivity of the
thermal models. In [4], D. A. Staton et al also used Motor-Cad to
determine the optical thermal parameters for electrical motors. Here in,
the thermal circuit is solved in matlab as is the case in [5] through a
system of linear equations.
The lumped parameters are derived from entirely dimensional
information, the thermal properties of the materials used in the design,
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and constant heat transfer coefficients. The thermal circuit in steady-state
condition consists of thermal resistances and heat sources connected
between the components nodes while for transient analysis, the heat
thermal capacitances are used additionally to take into account the
change in internal energy of the body with time. The associated
equivalent thermal network, would have the heat generation in the
component concentrated in its midpoint. This point represents the mean
temperature of the component.
1.2 Statement of Problem
The main limiting factor for how much an electric machine can
continuously be loaded is usually the temperature. When a machine
exceeds its thermal limit there are various outcomes: The oxidation
process in insulation materials is accelerated, which eventually leads to
loss of dielectric property. Bearing lubricants may deteriorate or the
viscosity may become too high, resulting in reduced oil film thickness.
Other problems are mechanical stress and changes in geometry caused
by thermal expansion of the machine elements. Statistics show that
despite the reliability of the induction machine, there is a little annual
failure rate in the industries and from research it has been shown that
most of the failures are caused by extensive heating of different motor
parts involved in the machine operation.
1.3 Purpose of Study
The objectives of this research work include:
To study the various parts or components of the induction machine;
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To study the thermal behaviour or temperature limits of the induction
machine and its components under various operating conditions;
To review the losses and methods of heat transfer in the induction
machine;
To develop an accurate thermal model for an induction machine;
To predict the temperature in different parts of the induction machine
using the thermal model and software program and lastly,
To investigate how the machine symmetry is affected by the nodal
configuration.
1.4 Significance of Study
The essence of this research work is to develop a thermal model for
an induction machine that will enable the prediction of temperature in
different parts of the machine. This is very important first to the
manufacturer or designer of an induction machine because with these
predictions one can decide on the insulation class limits the machine
belongs to. Also modern trends in the construction of machines is moving
in the direction of making machines with reduced weights, costs and with
increased efficiency. In order to achieve this, the thermal analysis
becomes very crucial in deciding on what types of insulators and other
materials that would be used to make these machines.
In industries, the knowledge of the thermal limits of machines increases
the life span of their machines and reduces downtime; thereby increasing
production and profit. Finally, it is hoped that this work would be an
important tool for other researchers who may desire to carry out further
work in this topic or similar topics.
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1.5 Scope of Study
This research work reviews the thermal characteristics of the
induction machine in general and focuses on the thermal modelling of
totally enclosed natural ventilated induction machine.
1.6 Chapter Arrangement
Chapter one introduced the work by presenting the background of the
study and the statement of problem. The purpose, significance and scope
of the work were also presented in this chapter. Chapter two exclusively
took care of the literature review while in Chapter three, the heat transfer
mechanisms in electrical machines were discussed. The thermal model
development and parameter computation were treated in Chapter four. It
involved the conductive and convective heat transfer analyses and details
of the calculation of the thermal resistances and capacitances.
In Chapter five, the losses in induction machine were discussed while in
Chapter six, the thermal modelling and computer simulation were carried
out, the simulation results were also presented in this chapter. Lastly,
Chapter seven was presented in the form of conclusion and
recommendations.
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