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CHAPTER 2

REVIEW OF LITERATURE

2.1 INTRODUCTION

The literature relevant to this thesis may be broadly classified into:

Literature on efficiency improvement in three phase induction motors, Fault

diagnosis and testing of three phase induction motors, and Standards relevant

to motor manufacture. Significant works have been reported in each of these

areas in the literature. The intention of this chapter is to provide a broad

outline on the various developments that have taken place in the field of

energy conservation / efficiency improvement in motor drive systems and to

show that the development of the approaches discussed in this thesis is

imperative.

2.2 LITERATURE ON EFFICIENCY IMPROVEMENT OF

THREE PHASE INDUCTION MOTORS

The literature on efficiency improvement can be classified under

the following heads:

2.2.1 Design Parameters Influencing Efficiency

Buschart (1979) presents the results of a motor-efficiency study

conducted on a project that required nine medium-voltage motors and

discusses motor-efficiency economics, motor-design parameters that affect

efficiency, motor application factors relevant to efficiency, and efficiency of

26

testing to be adopted. Hasuike (1983) describes 12 variable elements which

affect the improvement of efficiency. With the objective to discuss the factors

which determine the efficiency and rating of poly-phase AC induction motors,

Umans (1989) discusses the loss mechanisms and their relation to the

performance and design characteristics of the motor. Bonnett (1980) has

discussed in detail the various design considerations affecting motor

efficiency i.e. amount of copper wire in the slots, stator slot size, length of

coil extension, lamination steel length, rotor bar size, rotor bar conductivity,

bearing selection, increased air gap etc. Bonnett (1993) updates users on the

opportunities available to achieve higher performance levels. He summarizes

the actions that can be taken by those who specify and design electric motors

to improve the efficiency. Numerous actions taken by those who specify the

motor requirements that have far-reaching effects on the motor-efficiency like

System Voltage, Operating Voltage, Motor Size and Loading, Motor Speed,

Load Shedding, Adjustable-Frequency Drives applied to Centrifugal Pumps

and Fans etc. have also been discussed. Bonnett (1994) updates users on the

relationship and trade-offs between efficiency and power factor.

2.2.1.1 Core Design

New Core materials: Core losses of an 18.5 kW asynchronous

motor were measured and computed by Binesti and Ducreux (1996), and have

reported that a 1 to 3 % improvement in efficiency is possible by replacing the

conventional soft laminations by new materials. High-Efficiency Motors

require cores with higher magnetic flux density, lower iron loss, and lower

mechanical hardness. To address the same, Takashima et al (1999) report of a

new non-oriented electrical steel sheet (50RMA350) with Si, Al, and rare-

earth metals successfully developed to give a higher efficiency inverter drive

model motors than a conventional material. Walters (1999a) expounds on the

benefits of using steel with high permeability. If steel permeability is

27

increased the magnetising current will fall considerably in the 1.5kW motor

and will help reduce the dominant copper losses, even though the air gap

ampere turns will remain sensibly constant. He describes of a core material

Polycor 420, which has a mean loss of 4 W/Kg, occurs in the core better than

some low-loss silicon steel but with a much higher permeability. Diaz

et al (2007) contribute to the understanding of the rotational losses in stator

motor cores in two ways. For determination of the core condition in motors,

certain instrument developers including Phenix (online) claim to have

products that can determine the core loss density.

Core length, Core diameter increase, annealing of stator core:

Park et al (1995) present the shape design of stator slot of 3-phase cage

induction motors for iron loss reduction. For optimum shape design, the

sensitivity analysis by discrete approach is employed and the Gradient

Projection method for non-linear constraint problems is chosen for an

optimization algorithm. Boglietti et al (2005) have taken for analysis some

possibilities for increasing the induction motor efficiency i.e. rotor with

copper bar included in the slot before the aluminum die cast, increase of the

core axial length, and annealing of the stator core using production

technological process modifications. This approach is known as the “no

tooling cost” (NTC) strategy because it does not require a complete redesign

of new laminations with a consistent cost in terms of investments. The

problem of increasing the efficiency in case of electro-mechanical energy

conversion by induction motors is considered from the technical and

economical point of view by Cistelecan et al (2007) and they have shown that

in order to fulfill the technical performance of the induction motors, the

increase in outer diameter of the lamination is compulsory, when compared to

the actual MEC motors. Examples of design are given for general purpose

induction motors in the range of frames 90-132. Kaga et al (1982) report that

due to wedging, the decrease in starting torques for the induction motors was

28

found to be only 3% .The efficiency of the ferrite wedged induction motors

was improved by about 4 % at the rated output and 11 % at a quarter output,

compared to that without the soft ferrite.

Effect of Process in Core Manufacture: Electrical machine

laminations are shaped by cutting electrical steel plates. In the process of

making the laminations, usually called stamping, frequently used cutting

techniques are laser cutting, guillotine cutting, and punching. However, the

application of such techniques on electrical steel introduces significant local

strains near the cutting edges, which influence its magnetic properties

drastically. The mechanical stress effects have been extensively studied by

Ossart et al (2000) and Maurel et al (2005). Producers take this effect into

account by introducing an empirical factor in the machine design or by

annealing the shaped components. There has been a need to develop a

numerical procedure which is capable of optimizing electrical devices, taking

into account the local material degradation and featuring high accuracy and

acceptable calculation time. Crevecoeur et al (2007) have developed a space

mapping procedure to meet these requirements and have quantified the

influence of the material degradation on the design taking switched reluctance

motor as an example. Fujisaki et al (2007) describe a method to analyze the

effects of shrink fitting and stamping, which are important processes in

manufacture of motor cores from electrical steel, on the mechanical stress

distribution and iron loss in motor cores. The mechanical stress distribution is

evaluated by a structural finite-element method, and the iron loss is evaluated

by combined analysis of the electromagnetic field (by a finite-element

method) and mechanical stress.

2.2.1.2 Different Winding Methods

Chen and Chen (1998) have reported of a winding based on a novel

principle of star-delta mixed series connection, by means of which either high

29

efficiency can be obtained or raw materials can be saved without worsening

overall performance. Deshmukh et al (2006) show how the simple method of

short chording of winding can improve the efficiency of three phase induction

motors with variable PWM supply. He reports that the efficiency of the motor

with the shortest chorded winding was as much as 22% higher than that of the

full-pitched motor at 16-kHz switching frequency due to harmonic

cancellation. Toliyat and Lipo (1994) analyse concentrated winding for

adjustable speed drive applications and reports of a decrease in the

magnetizing inductance and increase in torque per ampere in the five-phase

motor.

Copper Loss: Hasuike (1983) computes that the efficiency and the

stator conductor's space factor increases in accordance with the increase in the

diameter of the stator conductors. The maximum limit of the stator

conductor's space factor can be increased by development of the

manufacturing technology.

2.2.1.3 Rotor Design

Williamson and Mc Clay (1996) describe the use of a formal

optimization procedure to determine the design of a rotor slot to obtain

maximum efficiency. The method involves the use of an equivalent-circuit

model coupled to a finite-element field-model to calculate machine

performance. Kirtley et al (2007) highlight the importance of rotor conductor

bar shape to accommodate the high electrical conductivity of copper to

achieve high starting torque and to further reduce stray load losses.

Cowie et al (2003) report on the performance of motors with die-cast copper

rotors. Rotor I2R losses were reduced by 29 to 40% and motor total

losses were reduced by 11 to 19% resulting in increased motor efficiencies of

no less than 1.5 percentage points.

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New research and production techniques allow construction of ac

motors with die-cast copper rotors, allowing even higher efficiency levels and

greater longevity. Using copper rotors can reduce rotor losses and improve

die-casting consistency compared to die casting with aluminum. Malinowski

and Cormick (2004) discuss the challenges of production of die-cast rotor that

include tooling stresses and thermal shock from the higher melting point of

copper versus aluminum. Substantial progress in understanding and managing

the porosity problem characteristic of high pressure die-casting has also been

made. The results are applicable to die-casting in general and apply to die

casting of the rotor in aluminum as well as copper. Together with

development of the heated nickel-base alloy die system to achieve

economically attractive die life, there is a significant improvement to the

ability to manufacture the copper rotor.

Brush et al (2004) report of a project to test the suitability of the

copper rotor technology upgrade for motors used for water pumping in

agriculture in India. It was carried out by a cluster of motor and pump

manufacturers at Coimbatore, South India. Copper rotors were cast by a small

Indian die casting firm for all the tests. Rotor laminations designed for

aluminum were used in this direct substitution evaluation. Motors were built

and tested by six motor manufacturers. Field test of motors fitted to pumps

pumping water for agricultural use and one test of a motor driving a doffing

machine in a textile plant were then conducted. Kirtley et al (2007) report of

renewed interest in the use of copper in the squirrel cage because of its

substantially higher electrical conductivity. Short die life resulting from high

temperature has, in the past, made cast copper rotors uneconomical. Because

of higher energy costs and improvements in metallurgy of casting apparatus,

the economics of the situation appears to have changed and a number of

manufacturers of induction machines are taking a look at die-cast copper

rotors.

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2.2.1.4 Finite Element Methods based Design

The calculation of the magnetic field in a squirrel cage induction

motor forms the basis of all design procedures. Belmans et al (1992) discuss a

method to analyse the field in the machine, starting from a classical design

scheme but using the finite element technique. Weerdt et al (1997) describe

the steady state analysis of squirrel cage induction motors using a two-

dimensional finite element solution. Bellini et al (2006) investigate the rotor

quantities measured on an actual device and computed by a model

reproducing the machine. The knowledge of induction motor rotor quantities

is essential to precisely define the machine energy efficiency and it is useful

for the implementation of diagnostic techniques.

2.2.2 Issues in Energy-Efficient Motors

2.2.2.1 Concept of Energy Efficiency and Range of application

Biller (1978) reports that the ac motors ranging from 1-125 hp

account for over 53 percent of the total motor energy consumption. And yet, it

is this same group of motors (NEMA sizes) that have been standardized and

designed for low initial cost. As a result, the NEMA size motors have room

for significant efficiency improvement. Haataja and Pyrhonen (1998)

illustrate though Energy-efficient motors are cost-effective in all power

ranges, the most remarkable saving potential by using Energy-efficient motors

in Europe is found within the power range, below 37 kW, as 80% of the total

saving potential is in this power range. Walters (1999 a) elaborates on the

concept of Energy-efficient motors, advantages of them, the pay back period

when normal efficiency design motors are replaced with Standard efficiency

designs, the barriers to Energy-efficient motor sales. Walters (1999b) also

describes of the measures to be taken to promote Energy-efficient motor sales

32

like framing of Standards, imposing regulations and incentives for using the

motor.

2.2.2.2 Issues

Based on the review of various industrial motors available, Bonnett

(1997) dispels the fear that the drive for increased efficiency would diminish

motor life and performance. Further, he reports of the experience of many

users that the Energy-efficient motor offers increasing reliability and longer

life for most industrial applications.

Heath and Bradfield (1997) have dealt with how the use of an

“electronic detection” inverse time circuit breaker that can be appropriately

applied when an instantaneous trip breaker would nuisance-trip as the first-

half cycle transient inrush and locked rotor current, as ratios to the full load

current are higher for Energy-efficient motors..

As the cost of energy increases and energy resources become

scarce, Brethauer et al (1994) considered it essential that the impact of

efficiency be factored into economic decisions concerning new motor

purchase, motor repair, and motor replacement.

Biller (1978) has discussed the application advantages of using high

efficiency motors in large processing plants. Pillay and Fendley (1995)

present an in-depth survey of motors in a refinery and a chemical plant and

determine the potential for energy and demand saving.

DOE fact sheet on buying an Energy-efficient Motor shows how to

obtain the most efficient motor at the lowest price and avoid common

problems. DOE (2006) provides the best practices for Motor Systems. Brook

Hansen conceived the idea of higher efficiency cage induction motors which

33

could be sold profitably at Standard efficiency design motor price. Williams

et al (1996) details the related development work that took the idea of the new

‘W’ series motors into reality. This could be achieved by increasing the

copper volume, by varying size and shape of the rotor slot, development of

new magnetic steel, improving thermal design, aerodynamic design to

maximize axial flow of air, reducing fan noise, careful lamination geometry

and motor manufacturing methods, designing and sourcing a low cost, low

loss oil seal, change in varnishing approach with the use of impregnating

varnish. By these means and many other means they finally arrived at the

largest result – a range of motors having an overall efficiency increase slightly

greater than 3 % and was able to be sold at Standard efficiency motor prices.

2.2.3 Efficiency under Part Loading

Huget (1983) discusses the importance of operating motors at peak

efficiency for maximum energy savings. He stresses that proper motor

application can result in significant energy cost savings over the life of the

motor. Most oversized three phase induction motors operate with low

efficiency and power factor, which is, by far, the most important cause for

poor power factor in industrial installations. Mohan (1980) has shown that in

a lightly loaded induction machine, a substantial amount of energy that is

wasted can be conserved by voltage control. Several voltage control

arrangements consisting of SCRs were compared in their ability to improve

the motor efficiency, and in terms of the amount of harmonic currents

generated. Rowan and Lipo (1983) have quantitatively examined part-load

efficiency improvement of induction motors by controlling stator voltage. The

analysis has included many practical considerations such as motor and SCR

non linearities. Ferreira and Almeida (2006) show how the performance of the

oversized three phase induction motors can be improved, both in terms of

efficiency and power factor, with the proper change of the stator winding

34

connection, which can be delta or star, as a function of their load.

Sundareswaran and Jos (2005) propose a low-cost microprocessor-based

hardware used for the control of thyristorised star/delta switch. Ferreira and

Almeida (2008) have suggested that in the low-load operating periods, motor

performance can be improved both in terms of efficiency and power factor, if

the magnetizing flux is properly regulated.

DOE (1997) Fact sheet assists in decisions regarding replacement

of oversized and under loaded motors. Durham and Lockered (1988) propose

a method of sizing motors by using motor efficiency curves and pumping-unit

torque characteristics which will provide the least energy consumption and

more starting torque. Salles et al (2000) deal with the electric tracking of the

load variation of an induction machine supplied by the mains, using the

machine itself as a torque sensor.

2.2.3.1 Different Load Conditions and Operating Temperature

Auinger (2001) reports that the electric motor used as a part of

electrical equipment on machines is operated under: different load conditions

and different operating temperatures and these differ from the controlled

conditions as specified by the Standards e.g. the American IEEE Standard 112

specifications. Both these parameters have an impact on the efficiency and

continuous rating. DOE (1998) fact sheet briefly discusses several load

estimation techniques. Determining if motors are properly loaded enables to

make informed decisions about when to replace motors and which

replacements to choose.

2.2.4 Efficient Control of Motors

Kirschen et al (1985) examine the problems associated with the

implementation of an optimal efficiency controller in variable frequency

35

induction motor drives. A simple method for minimizing on-line, the global

system losses based on the adaptive control of the rotor flux in a field-

oriented drive system is presented. Moreira et al (1991) propose a method of

efficiency maximization that utilizes sensing of the third-harmonic component

of air gap flux. When an induction motor is driven under light loads, the

efficiency of the order of 10% obtained by reducing the air-gap flux of the

motor by adjusting the stator current and slip frequency in the case of a

current source inverter induction motor drive system is reported by Kim et al

(1984). An optimal method to control the speed of a wound rotor induction

motor by variable external rotor impedance is presented by Baghzoug and

Tan (1989). A high efficiency, unity power factor VVVF drive scheme for an

induction motor is presented by Morimoto et al (1991). Chen and Yeh (1992)

deal with the determination of the input voltage and frequency for the optimal

efficiency operation of induction motors and report that VVVF is superior to

that of constant flux operation.

Fuchs and Hanna (2002) report that deployment of inexpensive

thyristor / triac controllers improves efficiency. In typical hybrid electric

vehicle (HEV) propulsion applications, the traction motor and the drive are

used over the entire torque/speed operational range. In view of this fact,

Williamson et al (2007) aim at modeling the inverter and motor losses /

efficiencies over typical city and highway driving schedules. de Almeida

Souza et al (2007) introduce a new technique for efficiency optimization of

adjustable-speed drives combining online search of the optimal operating

point and a model-based efficiency control, with an emphasis on vector-

controlled induction motor drives. Zenginobuz et al (2004) deal with the

performance optimization of medium/high-power induction motors during

soft starting by eliminating the supply frequency torque pulsations, and by

keeping the line current constant at the preset value. Dong and Ojo (2006)

present a new induction-machine model, which accounts for the varying core-

36

loss resistance and saturation dependent magnetizing inductance, uses natural

and reference frame independent quantities as state variables, yielding a

reference rotor flux that ensures a minimum loss and yields an improved

efficiency of the drive system especially when driving part load.

Yatim and Utomo (2007) present a new method of improving the

energy efficiency of a Variable Speed Drive (VSD) for induction (LMC)

motors by obtaining the flux level that minimizes the total motor losses. An

induction motor drive, including a loss-model based efficiency controller, is

presented by Aguilar et al (2008). To improve the robustness of the control

system, a self-tuning controller has been designed.

2.2.5 Influence of Harmonics on Efficiency

Due to the increasing requirement of precise control and equipment

performance of a modern facility, the appearance of voltage harmonics in the

power system has drawn great attention. Its impact on three phase induction

motors is one of the major concerns in industrial power systems. Users should

be aware that harmonics from ASD’s may negate motor efficiency savings.

Smith and Stratford (1984) review some of the effects of harmonics on power

systems and how ac and dc adjustable-speed drives affect these harmonics.

They suggest that factoring the effects of thyristor drives into the design of

plant electrical power systems can help ensure that such systems are not

subject to major failures without apparent cause.

Faiz et al (2001) compare Phase-controlled inverter and the

sinusoidal pulse-width-modulation (SPWM) control technique based on the

frequency spectrum, total harmonic distortion, distortion factor and power

factor. They show the advantage of the SPWM technique over the phase

control technique. Faiz et al (2004) suggest that the available definitions of

unbalanced voltages are not comprehensive and complete. Therefore, the

37

results of these analyses on motor performance are not very reliable. To prove

this claim, a three phase 25-hp squirrel-cage induction motor is analyzed

under different unbalanced conditions. It is shown that it is necessary to

define a more precise unbalanced factor for more accurate results. The

magnetic air gap radial force waves in a three phase induction motor excited

with a non-sinusoidal supply from a solid-state converter are investigated

analytically, taking into account the interaction between the flux density

waves due to the harmonic currents by Yang and Timar (1980) and a method

for the prediction of the frequencies of the magnetic force components caused

by the harmonic currents is presented. Non-sinusoidal voltage has detrimental

effects on induction motor performance, and derating of the machine is

required.

Sen (1990) discusses in detail the derating of induction motors

required when the harmonic content is above 5%. Cummings (1986) develops

an expression defined as the harmonic voltage factor (HVF) for estimating the

effect – additional losses and heating that will occur and that may require

derating - of harmonic voltages on poly phase squirrel-cage motors. Lee et al

(1998) use a real load test to investigate the performances of a three phase

induction motor under different voltage distortion factors (VDF). The

monitored qualities include motor efficiency, temperature rise, and torsional

vibration.

Auinger (2001) discusses the sensitivity of a squirrel cage induction

motor to voltage unbalance and/or harmonics. IEC 60034-16 includes data on

how the voltage unbalance and harmonics influence the thermal conditions,

i.e. increased winding temperature or the necessity to reduce the continuous

output. The efficiency is simultaneously, significantly influenced.

Papazacharopoulos et al (2001) present a new model enabling consideration

of high harmonic effects on PWM induction motor drive efficiency.

38

Harmonic iron losses in the motor due to switching frequency are considered

by convenient modifications of the equivalent circuit parameters. The need

for real time monitoring and analysis of harmonic variation has been dealt

with by Moreno-Eguilaz and Peracaula (1998) and a digital implementation of

a high performance 1.5 kW induction motor drive is presented. The system is

useful to optimize efficiency and to evaluate harmonic pollution at different

speed and load conditions.

Hsu et al (2007) present the case study of power quality assessment

of large synchronous motor starting and loading in the integrated steel-making

cogeneration facility and propose a power quality index (PQI) due to voltage

variation in the assessment period to find the impact of motor starting and

loading on the power quality of the cogeneration system. IEEE Standard 1159

(1995) covers the recommended practice encompassing the monitoring of

electric power quality of single-phase and poly phase ac power systems.

Till now the literature relevant to design of Energy-efficient motors

by changing various design parameters, Standards relevant to motor

manufacture, issues involved in Energy-efficient motor operation, the issue of

part loading of motors, efficient control of Induction motors and influence of

harmonics on efficiency have been discussed. The possibility of winding not

adhering to the designer’s specification while manufacturing custom designed

motor and its significance in the efficiency reduction has not been reported in

the literature. Hence, to find out relevant literature which discusses the

possible means of determination of improper winding, literature on Fault

diagnosis and testing is reviewed.

2.2.6 Simple Practices for Efficient Operation

Darby (1986) provides practical tips that effect efficiency including

appropriate lubrication, cleaning, and operating the motors in environments

39

with ambient temperature at or below the design level for the motor. Bortoni

et al (2007) present the results of research conducted to assess the effects of

preventive maintenance and repairs on three phase squirrel-cage induction

motors and conclude that simple practices such as cleaning and lubricating the

motor can contribute to better performance and increase in overall efficiency

of the system.

2.2.6.1 Mechanical Modifications and Efficiency

The mechanical modifications or customized features specified by

the motor user to meet a specific application requirement may affect motor

operating efficiency. Maru and Wennerstrom (1983) review five major motor

losses along with examples of the effect of modifications, such as special

bearings, shaft seals, and high altitude, on these individual losses and overall

motor efficiency.

2.2.7 Rewinding and Efficiency

While discussing other ways of efficiency reduction during a

rewind, Darby (1986) reports as to how losses increase after rewind. IEEE

Standard 1068 (1996) provides general recommendations for users of motors

that need repair as well as owners and operators of establishments that offer

motor repair services. The use of this recommended practice is expected to

result in higher quality, more cost-effective, and timely repairs. IEEE

Standard 1068 (1990) (Section four) on motor repairs outlines the method to

be used to ensure that no damage occurs during rewind. AEMT (2003) study

report describes how proper motor repair can prevent any loss of efficiency in

rewinding. This guide is aimed at service centers to help them maintain

efficiency levels. Cao et al (2006) describes of the most significant changes to

the loss in induction motors caused by the repair process, cautions to achieve

higher levels of efficiency during rewind, and good practices to be followed

40

during rewind. Department of Energy, USA (2005) offers tips to extend the

operating life of motors. EASA (2006) recommends a practice for repair of

electrical power apparatus, which has been accepted as a Standard by ANSI.

A service centre evaluation guide is provided by DOE (1999).

2.3 LITERATURE ON FAULT DIAGNOSIS AND TESTING OF

THREE PHASE INDUCTION MOTORS

2.3.1 Fault Diagnosis

Reliable electrical motor performance is vital to profitable plant

operation. This can be accomplished using several different testing methods,

ranging from online monitoring to offline testing. Motor Reliability Working

Group (1985) reports that 30%–40% of induction motor failures are due to

stator winding breakdown. Benbouzid et al (1999) address the application of

motor current spectral analysis for the detection and localization of abnormal

electrical and mechanical conditions that indicate, or may lead to, a failure of

induction motors. The subject of on-line detection and location of inter-turn

short circuits in the stator windings of three phase induction motors is

discussed by Marques et al (1999), and a non-invasive approach, based on the

computer-aided monitoring of the stator current Parks Vector is introduced.

Gleichman (2002) discusses some of common winding failure modes, and

some results of common tests and technologies.

Bangura et al (2003) develop the foundations of a technique for

detection and categorization of dynamic/static eccentricities and bar/end-ring

connector breakages in squirrel-cage induction motors. This approach can

distinguish between the “fault signatures” of each of the following faults:

eccentricities, broken bars, and broken end-ring connectors in such induction

motors. Kim et ak (2003) develop a speed-sensorless fault diagnosis system

for induction motors, using recurrent dynamic neural networks and multi

41

resolution or Fourier-based signal processing for transient or quasi-steady-

state operation respectively. Siddique et al (2005) present a comprehensive

review of various stator faults, their causes, detection parameters / techniques,

and latest trends in the condition monitoring technology. Cruz and Cardoso

(2005) report of the use of the multiple reference frames theory for the

diagnosis of stator faults in three phase induction motors. A fault indicator,

the so-called swing angle, for broken-bar and interturn faults is investigated

by Mirafzal and Demerdash (2006a). This fault indicator is based on the

rotating magnetic-field pendulous- oscillation concept in faulty squirrel-cage

induction motors.

Mohammed et al (2006) examine the behavior of three phase

induction motors with internal fault conditions under sinusoidal and non

sinusoidal supply voltages. This includes two types of faults, rotor broken bar

and stator faults. A discrete wavelet transform (DWT) was then used to

extract the different harmonic components of the stator currents. A robust

interturn fault diagnostic approach based on the concept of magnetic field

pendulous oscillation, which occurs in induction motors under faulty

conditions, is introduced by Mirafzal et al (2006b).

Khan et al (2007) present a real-time implementation of an online

protection technique for induction motor fault detection and diagnosis. The

protection system utilizes a wavelet packet transform (WPT)-based algorithm

for detecting and diagnosing various disturbances occurring in three phase

induction motors. Vereb et al (2007) investigate electrical breakdown of inter

conductor insulation, which presents an obstacle to the increase of the rated

voltage of electrical machines as a means to improve efficiency of electrical

energy exploitation. Chetwani et al (2005) present a technique based on the

monitoring of the current when the machine is normally operated and analyze

the same in frequency domain for detection of the faults. The technique can

detect online the presence of various faults such as broken bar in the rotor

42

cage of induction motor, bearing faults, eccentricity faults and stator turn to

turn short.

2.3.2 Testing Methods

2.3.2.1 Surge Test

The surge testing is an established method for diagnosing winding

faults. In the surge comparison test, two identical high voltages, high-

frequency pulses are simultaneously imposed on two phases of the motor

winding with third phase grounded. Moses and Harter (1957), Thorsen and

Dalva (1997) discuss how an oscilloscope is used to compare the reflected

pulses indicating the insulation faults between windings, coils, and group of

coils. The pulse-pulse surge testing is a predictive field method to show the

turn-turn insulation weakness before the turn-turn short occurs. IEEE

Standard 522 (1992), Schump (1989), Zotos (1994) discuss an electronic and

portable device “Surge Tester” used to locate insulation faults and winding

dissymmetry.

2.3.2.2 Insulation Tests

Armature or stator insulation can fail due to several reasons. Tavner

and Penman (1987) report that Primary among these are: high stator core or

winding , temperatures; slack core lamination, slot wedges, and joints; loose

bracing for end winding; contamination due to oil, moisture, and dirt; short

circuit or starting stresses; electrical discharges; leakage in cooling systems.

There are a number of techniques to detect these faults. Stone and Kapler

(1998) report that for large generator and motor stator windings rated 4 kV

and above, online partial-discharge (PD) test methods give very reliable

results. Nandi et al (2005) report that for low-voltage motors, stator fault

detection procedures are yet to be standardized. They give a comprehensive

list of techniques available for stator fault detection in low voltage motors.

43

2.3.2.3 Automated Standard Tests

Neft and Cancino (1990) describe an automated testing facility of

induction motors designed to perform seven standard tests. In a mass

production environment it is very costly to perform these tests on every

motor. Hubbi and Goldberg (1993) present a method for quality control of

induction motors. The method uses the results of the no-load and the blocked

rotor tests and defines three acceptance regions in the Ibr-Io, Po-Pbr and Pbr -Ibr

planes.

2.3.2.4 In-Service Tests

Lu et al (2006) present the results of an up-to-date literature survey

on efficiency estimation methods of in-service motors. They suggest four

efficiency estimation methods as candidates for non-intrusive in-service

motor efficiency estimation requirements.

2.4 LITERATURE ON STANDARDS RELEVANT TO

INDUCTION MOTOR MANUFACTURE

IEEE Standard 43 (2006) describes the recommended procedure for

measuring insulation resistance of armature and field windings in rotating

machines rated 1 hp or greater. It applies to synchronous machines, induction

machines, dc machines, and synchronous condensers. IEEE Standard 112

(2004) covers Instructions for conducting and reporting the more generally

applicable and acceptable tests of poly-phase induction motors and

generators. The purpose of IEEE Standard 118 (1978) is to present methods

of measuring electrical resistance which are commonly used to determine the

characteristics of electric machinery and equipment. IS 13730 (Part 0 /Set 1)

44

(2003) specifies the general requirements of enamelled round copper winding

wires with or without a bonding layer.

It could be found from the review of literature that the detection of

improper stator winding in a motor of a particular rating whose winding work

is complete has not been reported in the literature.

2.5 CONCLUSION

From the review of literature, it could be concluded that plenty of

literature have been reported which can be categorized under the following

heads:

Design of Energy-efficient motors by changing various design

parameters

Standards relevant to motor manufacture

Issues like quality, reliability and short circuit protection of

Energy-efficient motors

Efficiency reduction due to part loading of motors and the

ways to minimize the reduction in efficiency

Efficient control of Induction motors

Origin, Measurement, Effects and Mitigation of harmonics

From an elaborate literature review, it could be concluded that:

The possibility of winding not adhering to the designer’s

specification while manufacturing custom designed motors

and its significance in the efficiency reduction has not been

reported in the literature.

45

It could be found that the reduction of efficiency due to

improper winding present in the motor due to drop in number

of turns and conductor size reduction is reported, however,

theoretically.

Means for quality assurance of custom designed motor by

testing stator winding data present in the motor of a particular

rating whose winding work is complete is not reported.

Development of non-destructive techniques to determine the

winding data in finished stator windings is imperative.