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688 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 15, NO. 4, JULY 2000

Energy, Environment, and Advances in PowerElectronics

Bimal K. Bose, Life Fellow, IEEE

AbstractThe technology of power electronics has gone throughrapid technological advancement during the last four decades, andrecently, its applications are fast expanding in industrial, commer-cial, residential, military and utility environments. In the global in-dustrial automation, energy conservation and environmental pol-lution control trends of the 21st century, the widespread impact ofpower electronics is inevitable. The paper begins with a discussionon global energy generation scenario and the corresponding en-vironmental pollution problem. The mitigation of this problem isthen discussed with particular emphasis on energy saving with thehelp of power electronics. A brief but comprehensive review of therecent advances of power electronics that includes power semicon-ductor devices, converters, machines, drives and control is incor-porated in the paper. Finally, a prognosis for the 21st century has

been outlined.Index TermsConverter, drive, energy, environment, machine,

power electronics, power semiconductor device.

I. INTRODUCTION

POWER electronics has now firmly established its impor-

tance as indispensable tool in industrial process applica-

tions after decades of technological evolution. Fortunately, we

are now living in an era of industrial renaissance when not onlypower electronics but also computers, communication, informa-

tion, and transportation technologies are advancing rapidly. The

advancement of these technologies has brought the geographi-

cally remote areas in the world closer day by day. We now livein a truly global society, particularly with the advancement of

internet communication . The nations of the world have now

become increasingly dependent on each other as a result of this

closeness. In spite of great diversity among nations, one thing

we can safely predict with certainty that in the 21st century,

wars in the world will be fought on economic rather than mil-

itary fronts. In the new global market, free from trade barriers,

the nations around the world will face fierce industrial compet-

itiveness for survival and improvement of living standards. In

the highly automated industrial environment struggling for high

quality products with low cost, it appears that two technologies

will be most dominating: computers and power electronics with

motion control.Let me first give a broad historical perspective. As you

know, before the industrial civilization in the world that started

around 200 years ago, people were essentially dependent on

manual and animal labor. Life was simple and unsophisticated,

Manuscript received January 29, 1999; revised January 20, 2000. Thiswork was presented at the IEEE-PEDES Conference, Australia, 1998, and theIEE-IAS Conference, Japan, 1997. Recommended by Associate Editor, L. Xu.

The author is with the Department of Electrical Engineering, University ofTennessee, Knoxville, TN 37996-2100 USA.

Publisher Item Identifier S 0885-8993(00)05559-9.

and the environment was relatively clean. Then, in 1785, the

invention of steam engine by James Watt of Scottland brought

industrial revolution. It was the beginning of mechanical

age, or age of machines. The advent of internal combustion

engine in the late nineteenth century gave further momentum

to the trend. Gradually, industrial revolution spread from

Europe to America, and then to the rest of the world. In

1888, Nickola Tesla invented commercial induction motor.

The commercial dc machine came somewhat earlier, and

synchronous machine came somewhat later. The introduction

of electrical machines along with commercial availability of

electrical power started the new electrical age. It is interesting

to note that the Croatia-born engineer Tesla came to USA towork with Thomas Edison, the inventing wizard of nineteenth

century. Interestingly, with 1093 U.S. patents, Edison is often

considered as the greatest inventor in history. He had only three

months of formal schooling, and believed that genius is 99%

perspiration and only 1% inspiration. Edison was a fervent

advocate of dc machines and firmly believed that ac machines

had no future. He even refused to attend any discussion related

to ac machines. Gradually, Edison and Tesla became fierce

enemies because of their diagonally opposite viewpoints. The

invention of the transistor in 1948 by Bardeen, Brattain, and

Schokley brought us to the threshold of modern solid-state elec-

tronics age. Then, in 1956, Bell Telephone Laboratory invented

thyristor (PNPN transistor) which was later commercialized

by General Electric Co. Then, gradually came the modern eras

of integrated circuits, computers, communication and robot

technologies. It is often said that solid-state electronics brought

in the first electronics revolution, whereas solid-state power

electronics brought in the second electronics revolution. It is

interesting to note that power electronics essentially blends the

technologies brought in by the mechanical age, electrical age,

and electronics age. It is truly an interdisciplinary technology.

II. ENERGY AND ENVIRONMENTAL ISSUES

A. Energy ScenarioEnergy has been the life-blood for continual progress of

human civilization. Since the beginning of industrial revolution

around two centuries ago, the global energy consumption has

increased by leaps and bounds to accelerate the human living

standard, particularly in the industrialized nations of the world.

In fact, per-capita energy consumption has been a barometer

of a nations economic prosperity. The USA has the highest

living standard in the world. With only 5% of world population,

it consumes 25% of total energy. Japan, on the other hand,

consumes 5% of total energy with 2% of world population.

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BOSE: ENERGY, ENVIRONMENT, AND ADVANCES IN POWER ELECTRONICS 689

India and China together, with 38% of world population,

consumes only one-tenth of that of USA.

Globally, as indicated in Fig. 1(a), 87% of total energy is gen-

erated from fossil fuel (coal, oil and natural gas), 6% is gener-

ated in nuclear plants, and the remaining 7% comes from renew-

able sources (mainly hydro and wind power) [1]. The U.S. en-

ergy generation [Fig. 1(b)] approximately follows the same pat-

tern [2]. As indicated, 42% of U.S. energy comes from oil mostof which is consumed in automobile transportation. Currently,

USA imports more than 50% oil from outside. Fig. 2 shows the

electricity generation of USA, Japan, China and India by dif-

ferent types of fuel [3]. In USA, 37% of total energy is gener-

ated in electrical form of which 55% comes from coal and 20%

comes from nuclear plants. It is interesting to note that China

and India generate most of the electricity from coal (74% and

71%, respectively).

Unfortunately, the world has limited fossil fuel and nuclear

power resources. Fig. 3 shows the idealized potential energy de-

pletion curves of the world [4]. The world has large reserve of

coal. It is then followed by oil, natural gas and uranium fuel,

respectively. The natural uranium fuel is expected to last hardlyfor 50 years. Of course, breeder reactor can generate more nu-

clear fuel. Oil is expected to last hardly for more than 100 years,

and gas for 150 years. However, coal is expected to last for more

than 200 years. How can we operate our automobiles and aero-

planes when oil gets totally exhausted? Of course, fossil fuels

can often be converted to another form. Will the wheels of civi-

lization come to screeching halt beyond the 22nd century when

fossil and nuclear fuels become totally exhausted? By energy

conservation, the fuel depletion curves can be extended in time.

The wind and solar power, not shown in Fig. 3, can be explored

extensively. Can we expect a break-through in fusion power to

solve our future energy needs? Unfortunately, in spite of pro-

longed and expensive research, fusion power has not yet shown

any practical sign of future promise. The U.S. Department of

Energy has recently down-scaled fusion research in the labora-

tories of USA. It appears that cheap and abundant energy supply

which we are now enjoying will be overin future and our society

will be forced to move in an altered direction.

B. Environmental Issues

Unfortunately, environmental pollution and safety problems

contributed by increased energy consumption are recently

becoming domination issues in our society. Nuclear power

plants have safety problems. Besides, nuclear plant waste

remains radioactive for thousands of years. Even with the latesttechnology, we do not know how to satisfactorily dispose of the

nuclear waste. The U.S. society vehemently opposes expansion

of nuclear power in spite of having stringent safety standards by

Nuclear Regulatory Commission (NRC). Anti-nuclear slogan

is now spreading in many countries of the world.

Burning of fossil fuels emits gases, such as CO , SO , NO ,

HC, O and CO, besides generation of fly ash by coal. These

gases create environmental pollution problems, such as global

warming (green house effect), acid rain and urban pollution.

Global warming (a few degrees in hundred years) may cause

melting of polar ice cap and corresponding inundation of

low-lying areas of the world. In addition, the resulting world

Fig. 1. Energy generation scenario.(a) Globaltotalenergy. (b)US total energy.

climate change may adversely affect our agriculture andvegetation. Of course, preserving the worlds rain forests and

widespread forestation can alleviate this problem. The acid

rain, mainly caused by SO and NO due to coal burning,

damages vegetation. Then, of course, there is urban pollution

problem mainly by IC engine vehicles.

Fig. 4 compares the present and projected emissions of four

different countries, i.e., USA, China, India, and Japan for elec-

tricity generation [3]. USA consumes largest amount of elec-

tricity, and therefore, emission generated by USA is the largest.

Japan is a smaller country and it has much stricter air pollu-

tion standard. However, fast developing countries like China and

India, mainly dependent on coal, are increasing pollution at a

much faster rate. Japan is already facing a problem by coal-gen-erated pollution in near-by China. Environmental pollution is

truly a global problem, and the Kyoto conference in 1997 tended

to address this problem.

How can we solve or mitigate the environmental pollution

problems? As a first step, all our energy consumption can be

promoted in electrical form, and then advanced emission control

standards can be applied in central fossil fuel plants. The prob-

lems then becomeeasier to handlewhen compared to distributed

consumption of coal, oil and natural gas. As emission control

technologies advance, more and more stringent controls can be

enforced in central power stations. Wind energy is the cheapest

(5 to 6 cents/kWH) environmentally clean and safe renewable

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Fig. 2. Electricity generation by fuel types for selected countries.

Fig. 3. Idealized energy depletion curves.

energy source, and the world has truly vast potential of it. It

has been estimated that if 10% of raw wind potential could be

utilized (ignoring the economic constraints), the whole worlds

electricity needs could be met [6]. The recent advances in vari-

able speed wind turbines, power electronics and variable speed

drive technologies with advanced control are promoting large

expansion of wind energy programs in USA, Europe, China and

India [6]. Wind and solar power (also environmentally clean)

are particularly attractive for people of the emerging countries

who are not tied to electric power grids. It has been estimated

that currently around two billion people (33% of world pop-

ulation) are isolated from power grids. Of course, wind and

solar power, being statistical in nature, require back-up energy

sources. The solar photovoltaics are currently expensive ($3.5 to

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Fig. 4. Present and projected emissions of selected countries for electricitygeneration.

$4.5/W), and have low efficiency (13% to 16%). However, with

the present research trend, the cost and efficiency are expected

to improve significantly in future [8].

Urban pollution can be prevented by widespread use of elec-

tric vehicles and subway transportation. Note that wind power,

photovoltaics, electric car and subway systems heavily depend

on power electronics which will be discussed later. Conservation

of energy by more efficient use of electricity, and thus reduction

of fuel consumption, is a definite way to reduce pollution be-

sides having economic payoff. It also helps saving precious fuels

for the future. The role of power electronics in energy saving

will be discussed later. Unfortunately, availability of cheap en-

ergy promotes its wastage. It has been estimated that 33% of

total energy is simply wasted in the USA because of consumer

negligence [1]. In Japan, energy is typically four times more ex-

pensive, and therefore, the urge for energy saving is great.

III. ELECTRIC/HYBRID VEHICLES

Since electric and hybrid vehicles can play dominant role in

environmental pollution control and they constitute a major ap-

plication area of power electronics, the paper will remain in-

complete without some discussion of them. The history of EV

goes back to nearly a century, but serious R & D in this area

started during Arab oil embargo in the 1970s. The prime focus

at that time was oil saving in order to reduce dependence on

imported oil. However, in the 1980s, the environmental pol-

lution (particularly urban pollution) problem became a serious

concern in our society. Currently, oil conservation as well as en-

vironmental pollution control are the motivating factors that are

urging worldwide R & D activities in EV/HV. In 1990, Cali-

fornia Air Resource Board (CARB) established rules (with sev-

eral revisions thereafter) that mandated 10% of all vehicles sold

in California by 2004 must be zero emission vehicles (ZEV).

The California mandate made wide reverberations not only in

the other states of USA but in Europe and Japan also.Fig. 5 shows the fuel chains for gasoline-fuelled IC engine

vehicles (ICEV) and EV charged with coal-based power where

typical efficiency figures are incorporated. It is assumed that

electricity for EV is mainly generated in coal-based power sta-

tion. ICEV has very poor efficiency, and calculation indicates

that nearly 10% of gross energy of oil well is delivered to the

wheels. On the other hand, EV has much higher efficiency, and

nearly 20% of gross energy of coal mine is delivered to the

wheels. However, note that the urban pollution due to ICEV is

replaced by equivalent central power station pollution due to EV.

As mentioned before, the emission control in a central power

station is much easier to handle than that of individual vehicles.

Considering the potential importance of electric and hybridvehicles, in September 1993, the U.S. President along with

the big three automakers (GM, Ford and Chrysler) declared

a PNGV (Partnership for Next Generation Vehicles) program

[9] with the goal of producing state-of-the art hybrid vehicle

by the year 2004. The vehicle should have fuel efficiency (80

miles/gallon) three times the present value, range of 380 miles,

useful life of 100000 miles and very low emission (0.125

HC/1.7 CO/0.2 NO gms/mile). Of course, the vehicle should

be cost-effective and acceptable by customers.

Although electric and hybrid vehicles have been commer-

cially introduced recently by a few companies around the world,

they are currently far below the consumers acceptance level in

cost and performance. Fortunately, EV/HV is very intensive in

power electronics that has reached an acceptable level of ma-

turity in the recent years. It is essentially the limitation of bat-

tery technology that is inhibiting the acceptance of EV in the

market place. Todays propulsion battery is too heavy and ex-

pensive. Besides, it has low cycle life, gives low vehicle range,

and has the limitation of slow charging. Even with the capa-

bility of fast charging, the present utility distribution network

can not handle the charging power if large number of EV bat-

teries are fast-charged simultaneously. With the limited energy

storage capability of battery, EV is essentially suitable for lim-

ited range city driving. HV can truly replace ICEV, but the cost

is much higher. Among the possible energy storage devices [11]for EV/HV, such as battery, flywheel and ultra-capacitor, the

battery is the most viable candidate, and will remain so in near

future. Battery has been the prime focus for research for a long

time, but the technological evolution has been very slow. In

1990, the U.S. Advanced Battery Consortium (USABC) was es-

tablished to sponsor R & D activities in this area. It established

near-term, mid-term and long-term research goals, as indicated

in Table I [12]. At present, lead-acid battery is possibly the best

compromise in energy density, power density, life-cycle, cost

and other criteria. It has been widely used in developmental

EVs that include GM commercial EV1. Nickel-Cadmium bat-

tery hashigher energy density and longer life, but it is expensive.

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Fig. 5. Fuel chains for gasoline-fuelled ice vehicle and electric vehicle with coal-based power.

TABLE I

BATTERY TECHNOLOGY STATUS

Sodium-Sulfur battery has reasonably high energy density, high

cycle life, but itspowerdensity is low and operating temperature

is high. Nickel-Metal-Hydride and Lithium batteries appear to

be more promising EV battery, but currently they are very ex-

pensive.

The discussion on EV/HV will remain incomplete without

a few words about power devices. Power device helps in-

creasing the vehicle range, acceleration capability at high

speed, gradeability and maintaining a minimum state-of-charge

for the storage device. Although gasoline engine has been

the traditional power device, other candidates such as diesel

engine, gas turbine and fuel cell can also be considered. Diesel

engine has improved efficiency, but it is noisy and gives fumes.

For these reasons, it has not found customer acceptance in

USA. Fuel cell is particularly a very interesting device because

it is static, has high efficiency, and gives no emission when

used with hydrogen fuel. Of course, there will be emission if

natural gas is used as a fuel through a reformer. Table II shows



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TABLE IICHARACTERISTICS OF VARIOUS FUEL CELL TYPES

the different fuel cell types and their characteristics [13]. An

expanded review of power and energy storage devices for next

generation electric and hybrid vehicles is given in [11].

IV. IMPORTANCE OF POWER ELECTRONICS

The importance of power electronics in industrial automa-

tion, energy conservation and environmental pollution control

has already been touched in the previous sections. As the costof power electronics is falling and the system performance is

improving, its applications are proliferating in industrial, com-

mercial, residential, utility, aerospace and military systems. It

is expected that this trend will continue with high momentum

in this century. Modern computers, communication and elec-

tronic systems get life blood from power electronics. Modern

industrial processes, transportation and energy systems benefit

tremendously in productivity and quality enhancement with the

help of power electronics. As mentioned before, the environ-

mentally clean sources of power, such as wind, photovoltaics

and fuel cells which will be highly emphasized in future, heavily

depend on power electronics.

The growing importance of power electronics which isbeing increasingly visible now-a-days is the energy saving

of electrical apparatus by more efficient use of electricity.

It has been estimated that roughly 15% 20% of electricity

consumption can be saved by extensive application of power

electronics. According to EPRI (Electric Power Research

Institute) estimate, 60%65% of generated electricity in the

USA is consumed in motor drives, and majority of these drives

are for pumps and fans. Again, the processes by most of these

pump and fan drives can get benefit by variable flow control.

The example applications are ID/FD (induced draft/forced

draft) fans and boiler feed pumps in fossil power station. Tradi-

tionally, variable flow is obtained by throttle control while the

driving induction motor runs at constant speed. It can be shown

that power electronics controlled variable speed drive with

fully open throttle can save as much as 30% energy at light load

condition. The extra cost of power electronics can be recovered

in a period depending on the cost of electricity. Light load

machine operation at reduced flux (flux programming control)

can further improve drive efficiency, typically by 20%. In fact,

as the cost of power electronics goes down drastically, the use

of variable frequency starter in the front-end of the machine willpermit flux-programming control even for constant speed drive

applications. Another example application is variable speed

load-proportional air-conditioner/heat pump drive that can

save as high as 30% energy in comparison with conventional

thermostatically-controlled system. Interestingly, because of

high energy cost, typically 70% of room air-conditioners in

Japanese homes use variable speed drives to save energy. On

the other hand, variable speed air-conditioners are practically

unknown in the USA because of cheap energy. It has been

estimated that typically 20% of generated energy is consumed

in lighting. It is well-known that fluorescent lamps are two to

three times more efficient than incandescent lamps. Using high

frequency power electronics ballast can boost fluorescent lamp

efficiency typically by additional 20%. Note that energy saving

not only gives economic benefit but the cooling burden also

becomes less, and there is the benefit of reduced environmental

pollution because of reduced generation.

V. ADVANCES IN POWER ELECTRONICS AND DRIVES

A. Power Semiconductor Devices

Todays progress in power electronics has been possible pri-

marily due to advances in power semiconductor devices. Apart

from device evolution, the inventions in converter topologies,



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PWM techniques, analytical and simulation methods, control

and estimation techniques, computers, digital signal processors,

ASIC chips, control hardware and software, etc. have also con-

tributed to this progress. It is interesting to note that histor-

ically the power electronics age began with the invention of

glass-bulb mercury-arc rectifier at the beginning of this cen-

tury (1901). However, the modern era of solid state power elec-

tronics started with the invention of thyristor (or silicon-con-trolled rectifier) in the late 1950s. Gradually, other devices,

such as triac, gate turn-off thyristor (GTO), bipolar power tran-

sistor (BPT or BJT), power MOSFET, insulated gate bipolar

transistor (IGBT), static induction transistor (SIT), static induc-

tion thyristor (SITH), MOS-controlled thyristor (MCT), and in-

tegrated gate-commutated thyristor (IGCT) were introduced. Of

course, power semiconductor diode which is not included in the

list, is equally important. As the evolution of new and advanced

devices continued, the voltage and current ratings and electrical

characteristics of the existing devices began improving dramati-

cally. Power electronics evolution generally followed the device

evolution. Again, device evolution has been heavily influenced

by the advances in microelectronics. Suffice it to say that if de-vice evolution stopped after invention of thyristor, power elec-

tronics evolution could have also stopped in a primitive age.

In fact, the device evolution along with converter, control and

system evolution has been so spectacular in the last decade of

20th century, we can define it as the decade of power elec-

tronics."

Thyristor, the general workhorse of power electronics, dom-

inated the first generation of power electronics, typically in the

period of 19581975. Even today, thyristors are indispensable

for handling very high power at low frequency for applications,

such as HVDC converters, phase-control type static VAR

compensators, cycloconverters and load-commutated inverters.

It appears that the dominance of thyristor in high power

handling will not be challenged at least in the near future. Triac

is basically an integration of anti-parallel thyristors, and is

suitable for low frequency resistive type load, such as heating

and lighting control. The advent of high power GTOs pushed

the force-commutated thyristor inverters, once so popular, into

obsolescence. The device continues to grow in power rating

(most recently 6000 V, 6000 A) for multi-megawatt voltage-fed

and current-fed (with reverse blocking device) converter

applications. Slow switching of the device that causes large

switching loss restricts its switching frequency to be low (a

few hundred Hz) in high power applications. Large dissipative

snubbers are essential for GTO converters. Of course, regen-erative snubber can be used to improve converter efficiency.

Power MOSFET, unlike most other devices, is a majority

carrier device. Therefore, its conduction drop is high for higher

voltage devices, but the switching loss is low. It is very popularin low voltage high frequency applications, such as switching

mode power supplies (SMPS) and other battery-operated power

electronic apparatus. Recently, trench-gate devices with lower

conduction drop have been introduced. The device does not

have any competitor in future. The advent of large Darlington

BJTs in the 1970s brought great expectations in power

electronics. Recently, and almost suddenly, the device has

been totally ousted in competition with power MOSFET (low

voltage range) and IGBT (higher voltage). The introduction of

IGBT in the 1980s was an important milestone in the history

of power semiconductor devices. Its switching frequency is

much higher than that of BJT, and square SOA (safe operating

area) permits easy snubberless operation. The power rating

(currently 3500 V, 1200 A) and electrical characteristics of the

device are continuously improving. IGBT intelligent power

modules (IPM) have been available with built-in gate driver,control and protection for up to several hundred kW power

rating. The present (fourth-generation) IGBTs with trench gate

technology have conduction drop which is slightly higher than

a diode, and much higher switching speed. MCT is another

MOS-gated device which was commercially introduced in

1992. The present MCT (P-type with 1200 V, 500 A), however,

has limited RBSOA (reverse-biased SOA), and switching

speed is much inferior than IGBT. MCTs are being promoted

for soft-switched converter applications (will be discussed

later) where these inferiorities are not barriers. Currently,

the U.S. Government, in collaboration with industries and

universities, has started a large R & D program in PEBB (power

electronic building block) development [23]. IGCT, basically ahard-switched GTO, has been commercially introduced most

recently [24] (currently 4500 V, 3000 A). The claimed advan-

tages over GTO are lower conduction drop, faster switching,

monolithic bypass diode, snubberless operation, and ease of

series operation.

Although silicon has been the basic raw material for power

semiconductor devices for a long time, several other raw mate-

rials, such as silicon carbide and diamond, are showing signifi-

cant future promise. These materials have large band gap, high

carrier mobility, high electrical and thermal conductivities and

strong radiation hardness. Therefore, devices can be built for

higher voltage , higher temperature, higher frequency and lower

conduction drop. Unfortunately, processing of these materials is

very complex. SiC power devices (diode, power MOSFET and

thyristor) have already been demonstrated in laboratory, and are

likely to be commercially available in early part of this century.

It is expected that eventually most of silicon power devices will

disappear from the market.

B. Converters

A converter uses a matrix of power semiconductor switches

to convert electrical power at high efficiency. It is interesting

to note that converter technology has generally followed the

device evolution, although the most common type of con-verter operating on utility system is the Graetz bridge which

was introduced long time ago. Diode or phase-controlled

converters using thyristor type devices distort line current

wave, and thus create utility power quality problems. The

latter, particularly, deteriorates line power factor. Rigorous

IEEE-519 and IEC-1000 standards have been formulated to

restrict harmonic loading on the utility system. The IEEE-519

standard limits harmonic loading at the point of common

coupling by the consumer (but not the individual equipment),

whereas IEC-1000 restricts harmonic generation by individual

pieces of equipment. Since diode and thyristor type converters

are very common and constantly growing on utility system,



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various types of passive, active and hybrid filters have been

proposed to combat these problems. Of course, high power

converters can solve this problem by using multi-pulsing

technique. Active filters and static VAR compensators (SVC)

can be based on voltage-fed or current-fed converter topology,

but the former type is generally preferred because of lower cost

and improved performance. The active power line conditioner

(APLC) has the dual functions, i.e., active filtering as well asVAR compensation. These types of apparatus are attractive for

retrofit applications to phase-controlled apparatus, but have not

found much popularity because of high cost. It is interesting to

note that recently very high power (48 MVA) SVC (static VAR

compensator) using GTO-based multi-stepped voltage-fed

converter has been installed in Japanese railway system [26].

In the 1960s, when inverter-grade thyristors appeared, var-

ious types of voltage-fed force-commutated thyristor inverters,

such as McMurray inverter, McMurray-Bedford inverter,

Verhoef inverter, ac-switched inverter and dc-side commutated

inverter were introduced. Many other commutation techniques

were highlighted in the literature. However, this class of

inverters gradually faced obsolescence because of the advent ofself-commutated GTOs and bipolar transistors. As mentioned

before, BJTs became obsolete with the advent of IGBTs.

Among the PWM techniques, sinusoidal voltage control PWM

and hysteresis-band instantaneous current control PWM be-

came very popular. Digital computation intensive space vector

PWM (SVM) with isolated neutral loads was introduced in

the 1980s. SVM performance is superior to sinusoidal PWM

but computation time restricts the upper limit of switching

frequency. Because of superior performance, the recent trend is

to replace current control PWM by voltage control PWM.

Since PWM inverter can operate both in inversion and

rectification modes, the unit can replace the phase-controlled

converter on line side solving the harmonics and power factor

problems. Fig. 6 shows the progression of modern voltage-fed

converter topology. The diode rectifier with boost chopper

shapes the line current to be sinusoidal at unity power factor,

but it is nonregenerative. A double-sided PWM converter

system, in addition, gives regeneration capability which is

important for a drive. The topology in this system can be either

two-level or three-level (neutral point clamped or NPC). In

fact, higher number of levels is also possible. Again, each

unit can have half-bridge, H-bridge, or three-phase bridge

configuration. Three-level topology, used in high voltage

high power applications, gives better harmonic performance

without increasing PWM switching frequency. Recently,attempts have been made to replace multi-megawatt thyristor

phase-controlled cycloconverters for machine drives with

the GTO-based three-level converter system. An example

is 10-MVA three-level convertersynchronous motor drive

for rolling mill drive introduced by Mitshubishi [29] a few

years ago. The system is more economical than cyclocon-

verter drive with hybrid APLC. Since IGBTs have higher

switching frequency, and their voltage and current ratings

are increasing recently, there is a growing trend to replace

two-level GTO converters in the lower end by three-level IGBT

converters. Again, with higher IGBT voltage rating, there is

also a trend to replace three-level IGBT converters by the

Fig. 6. Progression of voltage-fed converters.

corresponding two-level converters giving large size and cost

benefits. Japanese railway drive with 1500 V dc supply is an

example [30]. Fig. 7 shows the progression of current-fed con-

verters. Thyristor phase-controlled rectifier/inverter is the most

common example in this class. High power (multi-megawatt)

load-commutated thyristor inverter (LCI) synchronous motor

drives where machines operate at leading power factor are

very popular in industry. With induction motor, the same type

of converter has been used but with a capacitor bank at the

machine terminal for load commutation. Converters can again

be multi-stepped (12 or 24 pulse), as mentioned before, to

reduce harmonics in line and load currents, but the problem

of poor displacement factor remains. Force-commutated cur-

rent-fed auto-sequential commutated inverters (ASCI) have

been used for induction motor drives, but recently, they have

become obsolete. Double-sided PWM current-fed converters

using self-controlled reverse-blocking devices have essentially

the same features as those of voltage-fed topology [Fig. 6(b)].

However, the voltage-fed topology is superior and far morepopular in industrial drive applications.

In the recent literature, we have seen an enormous growth

of soft switching technology [31]. The traditional converters

with self-controlled devices use hard switching principle. Soft

switching of devices at zero voltage or zero current (or both)

tends to minimize or eliminate device switching loss, thus

giving improvement of converter efficiency. Besides, the other

advantages are: elimination of snubber loss, improvement

of device reliability, less dv/dt stress on machine insulation,

reduced EMI problem, and elimination of machine bearing

current problem and voltage boost effect at machine terminal

with long cable. Of course, some of these problems can be

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Fig. 7. Progression of current-fed converters.

solved by using low-pass LC filter at the machine terminal.

Soft-switched converters can be generally classified as resonant

link dc, resonant pole dc, and high frequency ac link systems.

The resonant link dc can be voltage-fed or current-fed type. The

former again can be classified as free-resonance or quasires-

onance types, or active or passive clamp types. Soft-switched

high frequency ac link system can be resonant (parallel or

series) or nonresonant type. This class has the advantages of

transformer coupling although the number of components is

larger. Unfortunately, in spite of technology evolution for more

than a decade, soft-switched converters have hardly shown in

the market place. In spite of the claimed potential advantages,

the need of extra components and control complexity are

possibly the reasons of their disfavor.

C. Machines and Drives

Electrical machine is the workhorse in a drive system. Its evo-

lution over the past century has been slow and much less dra-

matic than that of power semiconductor devices and converter

circuits. Electrically, mechanically and thermally, a machine is a

very complex system. The advent of modern digital computers,

improved modeling, simulation and CAD programs, and avail-ability of new materials have contributed to higher power den-

sity, higher efficiency, improved reliability, reduced cost, and

improved mechanical and thermal design of the machine in the

recent years. Advanced studies related to machine modeling,

control, state and parameter estimation, etc. are the exciting

R&D topics today for drive system engineers. Although dc ma-

chines have been traditionally used for variable speed appli-

cations, and yet count as majority today, the advent of solid

state variable frequency inverters, improved power semicon-

ductor devices, microprocessors, DSPs, powerful ASIC chips,

and advanced control and estimation techniques gradually en-

hanced the application trend of variable frequency ac drives.

Both induction and synchronous machines have been

widely used in variable speed drives. Although the cage type

induction motor is most commonly used in wide power range,

wound-rotor machines with slip power recovery control (known

as static Kramer and Scherbius drives) have been generally

used in limited speed range multi-megawatt drive applications.

Because of expensive machine and poor system performance,

particularly with phase-controlled converters , these drives arerecently being disfavored except for specialized applications.

Synchronous machines can be classified as reluctance machine,

permanent magnet (PM) machine, and wound-field machine.

The PM machines can again be classified as surface and inte-

rior magnet types, radial and axial field types, and sinusoidal

and trapezoidal flux types. Although PM machine is more

expensive (particularly with high energy NdFeB magnet) and

require absolute position sensor, it has higher power density

and improved efficiency that reduces the life cycle cost. While

the trapezoidal and sinusoidal surface magnet machines are

essentially restricted to constant torque region speed control,

the interior permanent magnet (IPM) machines can extend

operation into field-weakening region. Synchronous reluctancemachines often provide low cost and robust solution, and are

suitable for high speed applications. Design of this class of

machines has gone through a lot of improvement in recent

years. It was mentioned before that wound-field synchronous

machines are very popular in the highest power range. Although

somewhat expensive compared to induction motor, it has the

advantages of programmable power factor control (leading,

lagging or unity), improved efficiency and reduced moment of

inertia. Programmability of power factor makes the converter

economical and further boosts the drive efficiency.

The discussion on machines and drives will remain incom-

plete without some discussion on switched reluctance machine

(SRM) drives which have received so much attention in the re-

cent literature. This electronic motor is claimed to be simple,

rugged, low-cost, fault-tolerant, and can operate at high speed.

However, there are pulsating torque and acoustic noise prob-

lems, and absolute position sensor is required unlike induction

motor drive. Of course, position estimation and pulsating torque

compensation techniques have been proposed in the literature.

Unfortunately, machines can not be operated in parallel on a

single converter, bypass operation of converter(in case of con-

verter fault) is not possible unlike induction motor drive with

60 Hz supply, and estimation of feedback signals is difficult. In

spite of large commercialization effort, it is expected that SRM

drives will be restricted to very specialized applications.

D. Control and Estimation

Control and estimation of high performance ac drives have

been a very fascinating and challenging area of R & D in the

recent years, and literature in this area is extensive [32]. The

advent of powerful microcomputers, digital signal processors,

application specific ICs, personal computers, CAD and sim-

ulation tools, AI techniques, and advancement of control and

estimation theories has continuously extended the frontier of

control and estimation techniques. In this section, control and

estimation related to induction motor drive only will be briefly

reviewed. Although simple volts/Hz control is known for a long

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time, and is yet extensively used in low performance industrial

drives, the advent of vector or field-oriented control since the

1970s brought renaissance in modern high performance control

of ac drives. It has been widely accepted forapplications, such as

paper and textile mills, metal rolling mills, servos and machine

tools, elevators, electric vehicles, etc. where dc machine-like

performances are demanded. In spite of complexity, this type of

control is expected to be universal for ac drives in future. Vectorcontrol can be classified into indirect and direct methods, and

can be based on rotor or stator flux orientation. Currently, the

indirect method with rotor flux orientation is most popular in

industrial applications. A problem in indirect vector control is

detuning of slip gain (Ks) due to machine parameter variation

that causes the coupling effect. For a machine with unknown

parameters, the initial or off-line tuning can be easily made by

automated parameter measurement by inverter-injected signals.

This also helps self-commissioning of a drive where tuning of

feedback control parameters is also required. On-line tuning of

Ks is more difficult and has been attempted by methods, such

as injection of PRB (pseudo-random binary) signal in machine

-axis and observation on -axis, extended Kalman filter pa-rameter estimation, parameter estimation by solving machine

- model, MRAC (model referencing adaptive control) and

fuzzy MRAC methods. The direct vector control with rotor flux

orientation is much less parameter sensitive if the flux vector

is estimated from the machine terminal voltages and currents

(voltage model). However, zero or near-zero speed operation

requires flux vector estimation from speed and current signals

(current model or Blaschke equation) where parameter variation

problem becomes serious. In a wide range speed control with

this method, the voltage and current models can be hybrided for

flux estimation. The stator flux oriented direct vector control has

the advantage that the flux vector estimated by voltage model

is sensitive to stator resistance only which can be compensated

somewhat easily. However, in this method, there is inherent cou-

pling effect, and therefore, decoupling current compensation is

required in the flux control loop.

Speed sensorlessvector control isa popular R & D topic in the

recent literature. Speed estimation by processing the machine

voltage and current signals has been attempted by methods,

such as slip and stator frequency estimation, solving the dy-

namic machine model, slot harmonic voltages, MRAC method,

speed adaptive flux observer (Luenberger observer), and ex-

tended Kalman filter. Very recently, it has been attempted by

injecting auxiliary signals through the inverter. Although speed

sensorless vector- controlled drives are available commercially,speed estimation at zero speed or near zero speed yet remains a

challenge.

A type of performance-enhanced scalar control, called direct

torque and flux control (known as DTC) which was proposed

in the 1980s, is receiving a lot of attention in the recent liter-

ature. The DTC control gives adequately fast response, has the

simplicity of implementation due to absence of close loop cur-

rent control, traditional PWM algorithm and vector transforma-

tion. However, the inherent demerits of limit cycle operation,

such as pulsating torque, pulsating flux and additional machine

harmonic loss exist. Since the torque and stator flux vector are

estimated from machine voltages and currents, the low speed

limitation and parameter variation problem are similar to stator

flux oriented vector control. Recently, fuzzy and neuro-fuzzy

control techniques have been proposed to alleviate the perfor-

mance of DTC control.

In a high performance control where the machine parameter

variation and load torque disturbance can be problems, the con-

troller parameters (and sometimes the control structure) can be

varied adaptively to give the desired stability, robustness anddead-beat response. Various adaptive control techniques, such

as self-tuning regulator (STR), H-infinity control, MRAC and

sliding mode control (SMC) or variable structure system (VSS)

have been proposed in the literature. With vector control in the

inner loop, the transient model becomes dc machine-like, and

then it becomes much easier to apply adaptive control in the

outer loop.

In the recent years, intelligent control based on AI (artificial

intelligence) techniques is showing high promise for advanced

ac drive applications [33]. AI can be classified into expert

system (ES), fuzzy logic (FL), artificial neural network (ANN)

and genetic algorithm (GA). The ES, based on Boolean algebra,

uses hard or precise computation, whereas FL, ANN and GAuse soft or approximate computation. With a control based

on AI, a system is often said to be intelligent, autonomous,

adaptive, self-organizing or learning. A plant model is

often unknown, or ill-defined. Or, the system may be nonlinear,

complex, multivariable with parameter variation problem.

An intelligent control can identify the model, if necessary,

and give predicted performance even with wide range of

parameter variation. In ES, the expertise of a human being in

a certain domain is embedded into a computer program. The

ES has been applied in auto-tuning of P-I control, off-line and

on-line diagnostics in equipment, automated design, simulation

and test of power electronic systems. Fuzzy logic, unlike

Boolean logic, deals with problems that have vagueness or

imprecision, and uses membership functions and fuzzy rule

table to solve a problem. FL has been applied in adaptive

control for parameter variation, robust control with load

disturbance, nonlinear compensation, estimation of distorted

waves, state and parameter estimation, modeling of machine,

and tuning off-line P-I control in self commissioning of drive.

Fig. 6 gives an example of FL application in variable speed

wind generation system [37]. The wind turbine is coupled to

an induction generator which generates power for the grid

through a double-sided PWM IGBT converter system. The

machine speed is controlled in regenerative mode to track

the wind velocity with indirect vector control, whereas theline-side converter uses direct vector control to pump power

to the utility grid at unity power factor. The system uses three

fuzzy controllers. The controller FLC-3 gives robustness to

speed control loop against the pulsating torque of vertical

turbine and shock by wind vortex. The controllers FLC-1 and

FLC-2 maximize the system power output by on-line search,

as explained in Fig. 8. FLC-1 continuously tracks the wind

velocity with the machine speed to maximize the turbine

aerodynamic efficiency, whereas FLC-2 optimizes the machine

efficiency by on-line programming of the flux. Fuzzy control

in FLC-1 and FLC-2 is justified because it improves speed of

convergence and permits use of noisy signals. ANN is a more



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(a)

(b)

Fig. 8. (a) Fuzzy logic based control of variable speed wind generation system. (b) System operation with fuzzy controllers FLC-1 and FLC-2.

generic form of AI and tries to emulate the biological neural

network of human brain. It can be generally classified intofeedforward and feedback or recurrent type. Although many

different topologies of ANN are proposed in the literature, back

propagation or multiple perceptron type feedforward topology

is most commonly used. This type of network basically maps

input-output static patterns using associative memory property.

Dynamical systems can be emulated by feedforward ANN

with delayed inputs, outputs or feedback, or by recurrent ANN.

Various off-line and on-line training methods of ANN have

been proposed in literature. Recurrent ANNs are somewhat

difficult to train, but Kalman filter based off-line and on-line

training have shown a lot of promise. The ANNs have been

used in one or multi-dimensional nonlinear function genera-

tion, PWM techniques, zero-phase delay harmonic filtering

of waves, waveform FFT signature analysis, on-line diag-nostics, dynamic model emulation of machines, and inverse

dynamics-based drive control, feedback signal estimation, etc.

ANN-based intelligent control seems to have extremely bright

future. Fig. 8 gives an example of neural network application

in an inverse dynamics based adaptive control of a multiple

degree of freedom robotic manipulator [41] where it is intended

to control a multi-dimensional trajectory with the respective

command signals. The N-degree of freedom plant is nonlinear

and have mutual coupling. In the beginning, a feedforward

ANN is trained off-line to emulate the plant model. The ANN is

then used in feedforward manner to cancel the plant dynamics.

The feedback loop detects the tracking error due to training

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Fig. 9. Neural network based inverse dynamics adaptive control.

Fig. 10. Speed sensorless stator flux oriented vector control with neuro-fuzzy based performance enhancement.

imperfection and parameter variation, and generates the sup-

plementary corrective signals, as shown, which can be used

for on-line training of the ANN. A simpler version of Fig. 9

can be used in industrial drive. Fig. 10 shows an example of

a speed sensorless stator flux-oriented vector-controlled drive

[42] for electric vehicle that uses fuzzy and neural techniques

to enhance the drive performance. The drive is started with the

current model (or Blaschke equation) but without speed sensor,

and then transferred to direct vector control at zero speed but

finite torque (i.e., slip frequency). The voltage model based

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stator flux vector estimation uses programmable cascaded

low-pass filters (PCLPF) which permit integration of voltage

down to zero speed. A quasifuzzy stator resiatance estimator

provides correction to the flux estimator. The neuro-fuzzy

controller permits flux programming efficiency optimization

control [similar to FLC-2 of Fig. 8(a)] of the machine where an

ANN is used to replace the fuzzy controller.

VI. CONCLUSION AND PROGNOSIS FOR THE 21ST CENTURY

The paper gives a brief but comprehensive review of global

energy scenario, future depletion of fossil and nuclear fuels,

environmental pollution problem, and mitigation of environ-

mental problems with particular emphasis of energy conser-

vation with the help of power electronics. Electric/hybrid ve-

hicle technology that plays an important role in fuel saving and

pollution control has been reviewed in this context. The ad-

vances in power electronics that includes power semiconductor

devices, converters, machines, drives, control and estimation

have been reviewed with some emphasis on modern intelligent

control methods.

Finally, it may be possible to give some prognosis for the

21st century trend in power electronics. Of course, our present

knowledge and trends can only be extrapolated for the future.

Any breakthrough invention, like invention of transistor or

artificial neural network, can alter the entire scenario. Again, of

course, these are the authors own opinion based on his knowl-

edge and experience. First, it can be predicted with confidence

that power electronics applications will spread everywherein

every phase of industrial, commercial, residential, utility,

transportation, aerospace and military environments. The main

reason is drastic cost and size reduction with improvement of

performance and reliability of future power electronics systems.Besides general process applications, energy saving will be

the important motivation, as mentioned before. As our energy

demand tends to grow further to improve our living standard,

more rigid environmental regulations will tend to drive up

the energy cost, and thus will promote more conservation of

energy. With the present research trend, the cost of photovoltaic

energy will come down substantially. Environmental regula-

tions will promote photovoltaic and wind energy extensively.

The trend in wind energy is already evident. Our future hope

of environmentally clean and safe fusion energy appears very

uncertain at this point. A new and break-through approach is

needed in this direction. The battery technology is expected

to have a break-through promoting widespread applications ofelectric and hybrid vehicles in future. The scarcity of IC engine

fuels (and correspondingly high cost), and the urge to minimize

urban pollution problem, are bound to promote EV/HV in the

market eventually even with the expensive and bulky batteries.

Of course, extensive R & D will significantly improve other en-

ergy storage and power devices. Silicon carbide, and ultimately

the diamond power semiconductor devices, will usher another

renaissance in power electronicspushing the silicon material

essentially into control microelectronics only. High frequency,

high temperature, low conduction drop and improved radiation

hardness of the new power devices will boost efficiency of the

increasingly popular voltage-fed converters. It can be easily

predicted that phase-controlled converters which are currently

so popular on utility line will eventually phase extinction, thus

solving the power quality and power factor problems perma-

nently. High power multi-terminal HVDC systems, rolling mill

drives, pump and compressor drives, static VAR compressors

in flexible ac transmission (FACT) systems will use voltage-fed

inverters with these devices. Soft-switching converters may

appear briefly, but will then disappear, except in specializedapplications. Traditional matrix converters operating on utility

line do not show future promise because of larger count of

power semiconductors and expensive ac capacitor bank on the

line. We expect that the converter technology will have the

similar trend as VLSI technology, and the present generation of

converter circuit designers will tend to disappear. Future con-

verters will have automated computer-aided design and built as

integrated and intelligent building blocks, similar to the trend

of micro chips. Future power electronics R & D activity will be

essentially confined to devices and systems. With drastic cost

reduction, almost every ac machine (both variable and constant

speed) will be eventually coupled with a front-end converter.

Variable frequency starting (solid state starter) of a constantspeed machine will solve power quality problem as well as im-

prove operational efficiency at light-load by flux programming

control (the idea is similar to Nola controller). At the lower end

of power, intelligent machines will be built with integrated

converter and control in the same frame. Thomas Edisons

favorite dc machine will eventually face total demise in this

century. It appears that switched reluctance drives will not be

able to stand in competition with induction and PM machine

drives for general industrial applications. Its application will

be restricted to a few specialized applications, as mentioned

before. Reduced cost of high energy magnet and improved

efficiency will promote increased volume of PM machine

drives, particularly when energy cost increases. Considering

the present trend, it appears that the popular volts/Hz control

of ac drive will disappear and sensorless vector control will

be universally accepted, except for high precision speed and

position control systems that require near zero speed operation.

The simplicity of volts/Hz control and complexity of sensorless

vector control with powerful DSP are essentially transparent

to the user. Advanced intelligent control and estimation based

on fuzzy logic and neural network will find dramatically in-

creasing acceptanace in power electronic systems. ANN-based

advanced control and estimation, embedded in ASIC chips,

particularly shows very high promise.

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[10] R. K. Bennet, Electric cars? Fahgedaboudit!, Readers Dig., pp.125129, Jan. 1998.

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Intersoc. Energy Conv. Eng. Conf., Aug. 1996, pp. 143149.[12] D. Coates, Advanced battery systems for electric vehicle applications,

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low cost inverters, in Proc. IEEE IAS Ann. Meet. Conf. Rec., 1997, pp.15921599.

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Oxford Univ. Press, 1998.

Bimal K. Bose (S59M60SM78F89LF96)received the B.E. degree from the Bengal Engi-neering College, Calcutta, India, the M.S. degreefrom the University of Wisconsin, Madison, and thePh.D. degree from Calcutta University, Calcutta, in1956, 1960, and 1966, respectively.

He currently holds the Condra Chair of Excellencein Power Electronics at the University of Tennessee,Knoxville, where he is responsible for organizing thepower electronics teaching and research program forthe last thirteen years. He is also the Distinguished

Scientist of EPRI-Power Electronics Applications Center, Knoxville; Honorary

Professor of Shanghai University, China University of Mining and Technologyand Xian Mining Institute (also Honorary Director of its Electrical EngineeringInstitute),China; and Senior Adviser of Beijing Power ElectronicsResearch andDevelopment Center, China. Early in his career for eleven years, he served as afaculty member at Calcutta University (Bengal Engineering College). In 1971,he joined Rensselaer Polytechnic Institute, Troy, NY, as Associate Professorof Electrical Engineering and conducted its teaching and research program. In1976, he joined General Electric Corporate Research and Development, Sch-enectady, NY, as Research Engineer and served there for eleven years. He hasserved as a consultant in more than ten industries. His research interests extendacross the whole spectrum of power electronics, and specifically include powerconverters; AC drives; microcomputer control; EV drives; and expert system,fuzzy logic and neural network applications in power electronics and drives. Hehas published more than 150 papers, and holds 21 U.S. patents. He is author andeditor of a number of books that include Power Electronics and AC Drives (En-glewood Cliffs, NJ: Prentice-Hall, 1986), Adjustable Speed AC Drive Systems(New York: IEEE Press, 1981), Microcomputer Control of Power Electronics

and Drives (NewYork: IEEE Press, 1987) and Modern Power Electronics (NewYork: IEEE Press, 1992), and Power Electronics and Variable Frequency Drives(New York: IEEE Press, 1997). The bookPower Electronics and AC Drives hasbeen translated into Japanese, Chinese, and Korean, and is widely used as agraduate level text book

Dr. Bose received the IEEE Industry Applications Societys OutstandingAchievement Award for outstanding contributions in the application ofelectricity to industry in 1993; IEEE Industrial Electronics Societys EugeneMittelmann Award in recognition of outstanding contributions to research anddevelopment in the field of power electronics and a lifetime achievement inthe area of motor drive in 1994; IEEE Region 3 Outstanding Engineer Awardfor outstanding achievements in power electronics and drives technologyin 1994; IEEE Lamme Gold Medal for contributions in power electronicsand drives in 1996; and IEEE Continuing Education Award for exemplaryand sustained contributions to continuing education in 1997, the IEEEMillennium Medal in 2000, and the Premchand Roychand Scholarship andMouat Gold Medal from Calcutta University, in 1968 and 1970, respectively,

for his research contributions, and the GE Publication Award, Silver PatentMedal, and a number of IEEE prize paper awards. He was the Guest Editorof the Proceedings of the IEEE Special Issue on Power Electronics and

Motion Control, August 1994. He is listed in Marquis Whos Who in America.He has served the IEEE in various capacities that include Chairman of IESociety Power Electronics Council, Associate Editor of IE Transactions,IEEE-IECON Power Electronics Chairman, Chairman of IAS Industrial PowerConverter Committee, IAS member in Neural Network Council, and variousother professional committees. He is a member of the Editorial Board of thePROCEEDINGSOF THE IEEE (1995present), and has served in a large number ofnational and international professional organizations. He was a DistinguishedLecturer in IEEE-IA and IE Societies. In 1995, he initiated Power Electronicsfor Universal Brotherhood (PEUB), an international organization to promotehumanitarian activities of the power electronics community.

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