High Efficiency Permanent Magnet Drive Systems for Electric Vehicles

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High Efficiency Permanent Magnet rive Systems for Electric Vehicles

D. J. Patterson

Centre or Energy Studies, Northern Territory University, Darwin NT 0909 Australia

Phone +61 8 89 466389, Fax +61 8 89 466993, e-mail:-

[email protected]

Abstract his paper discusses the recent history of the

development of Electric Vehicles EVs), and the range of

traction systems in use. A case is made for permanent magnet

machines

PMs),

particularly those using axial flux geometry.

The speed limitation of PMs is addressed, and two simple

methods of surmounting this limitation are presented. Issues in

axial flux machine design relevant to the pursuit of very high

efficiency are presented. The development of a very high

efficiency hard switched controller is discussed, where the

specific requirements of the drive system allow substantial

reduction of switching rates, thus allowing stringent control of

switching losses

I

INTRODUCTION

A. E

V

Background

The first phase of recent research into electric vehicles

(EVs) began in the

70s

prompted by sudden increases in oil

prices. This phase waned gradually as the most pessimistic

projections of ultimate price and depletion of known

resources did not eventuate.

The second phase, driven by urban air pollution concerns,

began with the California Air Resources Board 1990 mandate

which has now also been adopted by three north eastem

states in the USA. This phase has already resulted in the

recent availability of the General Motors EV1 for lease in the

states of California and Arizona. Several other large

manufacturers are about to enter the market. Whilst the

mandate has been modified

in

some areas, the 2003 targets

remain.

Urban air pollution concerns are now moving out of the

United States. The Northern Territory Universitys own small

Indonesian Vehicle project, in association with the

Indonesian government Agency for the Assessment

of

Technology (BPPT), aimed at use in central Jakarta, is but

one example of small project work going on in many of the

huge population centres

of

the world such as Cairo, New

Delhi, a nd Bangkok.

The significant problem with these vehicles is that in

attempting to mimic the range performance of competing

internal combustion engine technology in conventional

vehicles (CVs), very large, expensive, heavy and short lived

battery packs have resulted, whilst still not achieving genuine

comparability with CVs. This has resulted in considerable

research in to hybrid vehicles, which have both a heat engine

and an electric motor, and a lesser amount

of

storage.

Such hybrids, in their many configurations promise the

most likely solutions for completely acceptable general

transportation in the near future. [l ], [2], [3].

Third and fourth phases are now gathering momentum.

The third is driven by global warming concerns, since road

transport, mainly cars,

is

the fastest growing and otherwis

most

intractable source of carbon dioxide releases

[4].

Wher

electricity is generated from natural gas, as it is in th

Northern Territory, Australia, significant greenhouse ga

reductions are possible by using EVs in place of CVs [ 5 ] ,[ 6

The fourth phase is driven by power supply authoritie

word wide, who are seeing EVs as an opportunity to se

more product, and to do so when the major loads on suppl

are not present, ie in the evening.

B.

Traction System Backg round

There are now three types of machine under consideratio

for

EVs;

induction machines

(IMs),

permanent magne

machines (PMs), and to a lesser extent switched reluctanc

machines (SRs) [7], [SI. IMs have been chosen for thei

reliability, long history

of

industrial operation, and low co

by, for instance, General Motors or the GM EVl and b

Ford [9],

[ l o ] .

However the lower weight and highe

efficiency of PMs, together with substantial reductions in th

cost of rare earth magnets, is attracting a large proportion o

current research activity [Ill, [12], [13], [14], [l5], [16]

This paper, while covering EV issues in general wil

discuss as an example an in the wheel permanent magne

brushless axial flux drive system or a solar powe red vehicl

for an international race [20]. Whilst solar powered vehicle

will never come into general use because of the very low

power available (approximately 1 kW), the design pressure

on weight, efficiency, and reliability are producin

theoretical analyses, power electronic processing system

motors and drives with wide applicability in the general are

of electric and hybrid vehicles.

Fig. 1 shows the Northern Territory University entry o

the road during the 1996 World Solar Challen ge in Australia

SRs are under consideration or a range of auxiliar

machines, generators, starters, power steering drives, etc an

are discussed as possibly producing similar efficiencie

compared with PMs at less cost. Whilst many researchers ar

~ 1 ,9 1 .

Fig 1 The

Fuji

- Xerox Desert Rose on the road during the 1996 World

Solar Challenge

0-7803-3932-0

39 1

Authorized licensed use limited to: KING SAUD UNIVERSITY. Downloaded on October 27, 2009 at 17:40 from IEEE Xplore. Restrictions apply.


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aware of the benefits

of SRs

[4], as yet little has been

published on their use for traction.

The single significant problem with PMs is their speed

limitation, determined by battery voltage. For a given supply

voltage the machine has a rectangular torque speed

characteristic a s sketched in Fig. 2, which is considerably

different from that generally considered as desirable

ix

traction and

EV

work.

Most vehicle drive systems (eg automobiles, diesel electric

trains) have an instantaneous power limitation not present in

electric vehicles. This implies a reduction in the achievable

torque as speed increases, known as operation in the constant

horsepower region. This is extended over the very large

speed range required of vehicles by gearboxes or automatic

transmissions, allowing considerably higher speeds at lowier

torques. That this characteristic is desirable is in part

simply due to experience, since the concept of a vehicle

which slows up for gradients, but can negotiate very steep

inclines at low speeds is very much imprinted on human

experience. However much research in this area has

highlighted the reduction in torque expected at higher speed,

in terms of required profiles, and the high acceleration

required at low speeds in urban travel [21].

Much research has been aimed at extending the speed

range of PMs into the so called flux weakening regime.

Two novel approaches addressing this specific concern

[141,[151.

will be presented in this paper.

C. Signi jkance

of

System Eficiency

There is a substantial difference between the peak or rated

system efficiency of a traction system as recorded in the

laboratory, and that achieved under normal driving

conditions.

Recent work has reported on an EV with a

motor/controller syste c. ith an efficiency at rated speed and

power of 92%. In simulations, this vehicle was put cyclically

through a four mode operation, (i) 12.5 km/h for 20 sec, (li)

acceleration to 52 k m k over 11 sec, (iii) hold for 30 sec., (iv)

then braking, using regeneration),

to

the low speed over 19

sec. The simulation showed that over 60% of the energy used

was consumed by motor and controller losses [181.

Attempts to build totally electric vehicles quite rapidly

focus the mind on the efficiency of the all parts of the

vehicle, including the rapidly proliferating subsidiary

Continuous capability,

l /

rushless DC

motor

Desirable

Torque characteristic for

-~

Speed

Fig. 2. Torque-speed characteristics for various drives.

electrical systems, from entertainment through

airconditioning to active suspension [22]. The pressures on

such systems to achieve very high efficiencies are

dramatically increased when the only source of energy is on

board electrical storage.

D. Race Background

The World Solar Challenge is a race for solar powered

vehicles across the continent of Australia from the northern

shore to the southern shore, a distance

of

some 3000 km.

This race was first held in Novem ber 1987, and was repeated

in

1990, 1993 and again in 199 6. Universities provide a

suostantial number of the entrants, however participation by

the research arms of the automotive industry has increased

significantly since the first race was won by the General

Motors Sunraycer [23]. The race is proving to be an

important test bed for developing

EV

technology, and is

attracting increasing attention for this aspect [24],

[ 2 5 ] .

The

1996 race had 48 starters including the vehicle shown in Fig.

1

11

PERMANENT MAGNET

DRIVE

YSTEMS

In

traditional brushed dc machines the required speed

extension, or operation in constant horsepower region, has

been achieved by direct control of the field. Flux weakening

is also applied in typical induction motor variable speed

drives operating above rated speed.

Control of the flux in a permanent magnet machine is

possible in machines designed with large armature reaction

effects. Much research has gone into the interior permanent

magnet machine in an attempt to address this issue, however

the large circulating currents involved significantly degrade

the efficiency [141, [151, [161. In the most efficient format for

BDCMs, using surface permanent magnets, and given the

magnetic properties of the Neodymium Iron Boron magnet

material commonly used, armature reaction effects are

minimal, and controlling the flux by electrical means is very

difficult.

A

Benefits

o

Axial Flux Geometry

Axial flux geometry, where in its simplest form the

machine is made from

two

opposing disks, one fixed and one

rotating, has two substantial advantages over the more

common radial flux geometry, where a drum rotates inside a

cylinder. [ l l] , [12], [17], [19] [26], [27].

Firstly there are significant volume savings over the more

usual radial flux geometry, for which much of the internal

volume of the rotor does not contribute to power output. This

volumetric efficiency, as well as details of design trade-offs

between the two geometries is addressed by Millner [l 11.

Secondly and more importantly a very simple technique

for flux weakening relying on mechanical adjustment of the

air-gap, which does not impinge significantly

on

the

efficiency, becomes possible. Within a surprisingly broad

band, increasing the airgap increases the copper loss as the

torque constant decreases, but decreases the iron

loss

as the

392



3/6

flux density reducLs, keeping the overall efficiency relatively

constant.

Axial flux ma chines can have a stator disk with rotor disks

either side, [26], [171, a single rotor sandwiched between two

stator disks, [111 or simply a single sided arrangement with

one stationary and one rotating disk, as discussed here.

Whilst it is tempting to use the intemal stator version,

particularly in an ironless configuration, the thermal

management adva ntage of an iron stator in broadlheavy duty

traction applications is not insignificant.

Similarly the single rotor double sided machine appears

very attractive, however its construction is rather more

difficult and the ability to adjust the airgap is made

considerably more complicated. There is

a

strong attractive

force between the disks in the single sided version, but it is

not unmanageable with a dedicated thrust bearing or deep

groove ball bearings. The work reported in this paper is on

single sided axial flux machines, although some of the results

would have more general applicability. It should be noted

that not all axial flux machine applications are for in

-

wheel

drives, where the added unsprung weight can provide a

substantial mechanical design challenge.

B.

The Specific Machine Developed

This motor used in the race

in

1993 is shown

in

Fig 3and

has been reported earlier.

The motor for the 1996 race involved several

improvements,

so

that system efficiencies have risen from

about 91% at rated conditions to over 94%.[28]. These

machines can be readily dismantled and reassembled with a

range of sp acers on the shaft, providing different air gaps, for

expected race condition s, Production versions of this

machine were used by 6 other competitors in the 1996 race in

Australia, and 19 out of 36 competitors used the traction

system in the USA Sunra yce in June of 1997.

Construction wc.:;

is

currently progressing at Northem

Territory University on a prototype system providing such

adjustment without the need for dismantling,

so

that

adjustment can be ma de during operation.

Genesis the Sunrayce 97 entry from Messiah College,

Grantham, Pennsylvania featured a production motor with an

adde d fully automated servo driven gap variation system, put

to good effect during the race to fine tune the efficiency of

the motor, under operating conditions.

C. Managing the Loss Mechanisms

Whilst much of the control of the loss mechanisms in axial

flux machines follow relatively standard procedures, there

are two aspects of the machines discussed here that are

worthy of mention, flux distribution within an axial flux

machine of this particular type, and cogging torque in

machines with iron stators.

I

Flux distribution: The accurate solution of flux

distribution in axial flux machines is complicated by

two

issues, firstly lines of flux do not remain in a single plane as

they do in radial flux machines, typically travelling on

~

393

Fig

3 .

An

early version

of

the

motor,

disassembled

cylindrical surfaces across the airgap, and radially in t

circuit before returning across the air gap. Two dimension

finite element analysis gives reasonable approximation

when sections of such cylinders are flattened out.

Secondly the sides of slots are parallel, and therefore n

radial, so that less iron area is available for flux at the inn

radius of the toroid. Accurate analysis, and the imperatives

section I.

C.

above highlighting the importance of lo

control, indicate that three dimensional finite eleme

analysis

is

essential for complete understanding of fl

distribution., and machine optimisation.

2

Cogging

torque: Cogging torque, indicated by t

tendency of the rotor to index, or align with the slot pattern

invariably present in permanent magnet machines, and

very strong in concentrated winding situations as in th

machine, where the slot number of

36

is a simple multiple

the pole number of

12.

The first versions of this machine, with

a

normal runni

torque at

1 kW,

72 kph of

15

Nm , exhibited a cogging torq

of 10 Nm. Whilst this is in itself not a loss mechanism, sin

energy stored in the magnetic spring is returned, and giv

the inertia of the vehicle such cogging is never noticed

operation, it is nevertheless a very interesting diagnostic.

The existence of preferred rotor positions is an indicati

that at those positions, the magnetic circuit reluctance

lowest, and therefore the flux density will be at its highest.

there is a cogging torque then movement away from t

preferred position must imply increasing reluctance and th

decreasing flux density, implying a flux ripple at the s

frequency. Whilst in normal operation, the main flux patte

has a frequency of close to 60

z

the slot induced ripple w

be at 180

Hz,

and thus can contribute significantly to ir

loss,

not only in the stator, but also in the rotor.

Cogging torque is a well known phenomenon, however t

usual techniques to control it such as skewing the magne

simply reduce the cogging torque at any angle by averag

it across the skew angle [29]. Whilst this can be effective

reducing torque ripple, it does not remove the flux ripple

the slot frequency, and the associated

loss

The torque simp

occurs at different times in different places to give a n

average cogging torque o f zero.



4/6

For a typical six step controller, the minimum magnet pole

width is 120 electrical degrees, leaving

60

electrical degrees

between poles. However machines constructed with this

minimum magnet width will usually exhibit very hig,h

cogging torque.

Widening the m agnet w ill significantly impac t on this. Thle

principle has been reported for radial

f l u x

machines, wifh

general solutions given for some formats [30]. Individual

researchers have reported adjustment

of

magnet cross section

as well as width to minimise cogging torque [181.

The results of the optimisation process for an axial flux

machine designed for application in direct driving the rear

wheel of a bicycle are shown below in fig.

4.

This machine stator has an outer diameter of 160 mm and

an inside diameter of

100

mm. The machine has been

designed with 16 poles, one slot per pole per phase, thus at

the inner radius the slot pitch is 6.5 mm, and the minimum

magnet width is thus 13 mm. The plots show the force on a

magnet (and hence

on

a rotor) as the magnet is displaced

from its central position (centred over one slot) to a position

of one half a slot pitch. The parameter is the magnet width

as

it is gradually increased from the minimum. The reduction in

cogging torque is clear, as is the return of cogging torque

as

the width is increased past the minimum. Thus the flux ripple

can be reduced, and hence iron loss reduced although the

resultant at the minimum cogging torque, optimising only

magnet width, still results is some higher order

flux

ripple.

It should also be noted that the back emf waveform for the

windings is not unaffected by the magnet width adjustment,

and good design will include analysis of the impact on back

emf waveform, since this will impact on controller

efficiency, and the average torque constant of the motor

[3 11.

I11

THE POWER LECTRONICONTROLLER

The requirements for vehicular drives are very different

from those often considered under the topic of advanced

drives. Whilst accurate control

of

average torque is essential,

accurate control of the instantaneous torque

is

not, neither is

torque control response in very short times, because of the

inertia of the vehicle, and power limitations in the prime

mover.

The dominant requirement is that of maximising

efficiency. This is true not only for the racing vehicle but for

200

Min O5mm

2

. m f Min l Omm

A Min l 5mm

* Min 2Omm

2

.

100

.

X

x m

:

-200

x

Min 2 5m m

Displacement

mm

Fig. 4 Cogging force vs tangential displacement of magnet from central

position, with magnet width as the parameter.

~

394

EVs

in general as discussed in section

I.

C., not because of

the cost of energy, but because of the cost of carrying the

energy in battery systems. Further, managing the heat load

from an inefficient controller can add significantly to the

required infrastructure.

Earlier work on such controllers has highlighted many of

the issues, including the difficulty of measuring very high

efficiencies. This has resulted

in

the development of a very

simple, accurate calorimetric technique. This entails

enclosing the complete controller in a thermally well

insulated container with all connections made, injecting

controlled amounts of heat via

a

load resistor mounted on the

controller heat sink, thus calibrating the enclosure

temperature above ambient against power injected. A small

fan inside the enclosure ensures evenness of temperature at

all points inside the enclosure, so that the internal volum e can

be treated as a point source of heat. The m otor is then driven

on load by the controller, and the internal temperature of the

enclosure above ambient is measured. This work is reported

elsewhere [32]. For the first generation controller, hard

switched at 15

ItHz,

the controller losses at

1

kW, 72 kph

were

34.2 W.

This lead to the development of a second generation

controller discussed below.

A Switching

Loss

Control

At the present time, since it is possible by the techniques

described below to dramatically limit switching losses

without compromising operation

so

that conduction

loss is

dominant, soft switched inverters are not able to compete, at

least up to power levels of 10 0 kW.

Since reversing an EV

is

required only rarely, and then

under conditions of stopping, and conscious selection by the

driver, the motor control need only be two quadrant. This

implies that high levels of regenerative braking are not

available at low speeds. However since the energy of the

vehicle is proportional to the square of the velocity, little is

lost by using mechanical braking for determination of final

stopping position. Thus since only two phases are energised

at any one time by the controller, involving four switches,

one can be fully on one fully off, and only two switches act

under PWM control to regulate the current. The flyback

current is conducted through the opposite FET, driven to

conduct in reverse after a dead time, since the drop of an ON

FET

i n

the reverse direction is substantially lower than either

a body diode or a Schottky diode. A hysteresis band current

control scheme is used, with consequent very

low

switching

frequencies at both low and high speeds, the highest

switching frequency being at half speed. Very large current

ripples in such traction applications are not significant, and

the controller has a current ripple band of

10A

superimposed

on an average current which is 30A under typical operating

conditions. This results in a switching frequency which is

below 6 kHz at all times, and for the solar car operation,

when operating in a band

of

the top

15%

of speed, is never



5/6

Hall Po

i

s

Power

Fig. 5 Block diagram

of

the high performance controller.

above a few hundred

Hz.

This controller is shown in Fig

5,

and whilst the calorimetric procedures h ave yet to be applied,

all indications are that the losses at

1

kW, at a speed range

from 75 km/h to

90

k m h the losses are less than

10

W, of

which

8.5

W are conduction

loss.

B. Body Diod e Reverse Recovery

A major source of

loss

in well designed,

low

inductance

layout, high current high switching speed hard switched

controllers results from the reverse recovery of the body

diode in the

MOSFET,

carrying current during flyback dead

times

[33].

Careful use of parallel Schottky diodes, enables carrying

of the majority of the current during the dead time by the

Schottky diode. Figs

6

and

7

show the reverse recovery

phenomenon in a constructed controller, with and without the

Schottky diodes.

C.

Layout Issues

Low inductance layout of high speed high current

switching is important not only to prevent overvoltage

conditions resulting from high diidt

s

in the circuit, but also

to allow the curr ent to comm utate as rapidly as possible from

one switch to another, minimising switching loss [34].

D. General E

V

work

For general

EV

work,

at

power ranges from

10 k W to 100

kW,

and where the vehicle is likely to spend a large amount

of

time at a speed well below maximum speed, the benefits

of

soft

switching, in terms of device stress, machine stress,

and RFIIEMI co ntrol are very attractive. Much work is being

directed to this important area

[ 3 5 ] .

IV FURTHER

WORK

Laboratory design work is currently being carried out on a

version of the controller with

a

boost converter on the DC

bus to provide occasional performance above a rated

(Battery voltage limited) speed. This is a second approach,

Fig

6

Measured reverse current in the

lower

FET body diode, carryin

forward current of 10 A as the upper FET is switched on.

Fig 7 Repeat of Fig 6with paralleled Schottky diodes

in

place

being studied in parallel with the mechanical adjustment

the air gap in the axial flux machine. Two versions are und

analysis, the first using a hard switched boost converter fo

Southeast Asian city street application, and a resonant li

version for high performance vehicles [36].

V CONCLUSIONS

The pressures

/

requirements

for

maximising efficiency

EVs

are substantial, and it is believed that the benefits of t

permanent m agnet machine, particularly in its volumetrica

efficient axial flux form are significant. It is proposed that t

perceived limitation, that

of

not easily yielding to consta

horsepower operation, can be readily surmounted. In fact tw

solutions, that of using the mechanical gap variation in t

motor, and that of the use of bus boosting, are simply a

effectively achievable. These could well be used in conce

The use of gap variation is also important in optimising t

machines efficiency under operational conditions.

395



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VI ACKNOWLEDGMENTS

[IS] M Terashima, T Ashikaga, T Mizuno, K Natori, N. Fujiwara, and M.

Yada, Novel Motors and Controllers for High-Performance Electric

Vehicle with Four In-Wheel M otors, fEE E tran s Indu stria l

The author would like to acknowledge the value of the

substantial interaction with New Generation Motors

Corporation covering many discussions, ideas, arid

Clectronics, Vol44 no I , Feb

1997,

pp 28 - 38

improvements The author

is

indebted

o

Chris oKeefe,

who

[191 D

J

Patterson, Recent Advances I n the Design and Construction of

Axial Flux Permanent Magnet Machines. Proceedings, IEAust

roduced the data shown in fig. 4.

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Generator in the Northern Territory - Results Proccedings IECON

97, New Orleans, November, 1997

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Machine. and loss measurement using a simple calorimetric

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Tuckey and D J Patterson, Integrating a Resonant dc Link

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High Efficiency Permanent Magnet Drive Systems for Electric Vehicles