Design, principle, constraints, ARIOUS TYPES OF

and optimal control

V electric machines can be
considered for an applica-
tion to integrated starter
generator (ISG). Permanent magnet (PM)
synchronous machines present interesting
performances; nevertheless, it remains
difficult to reach a significant constant
power zone under good efficiency condi-
tions and power factor. The loss of control
of electronics at high-speed operations
may lead to high-terminal voltages,
which can become dangerous for the
whole vehicle. In this study, we examine
the main advantages and drawbacks of the
induction machine, reluctant, and PM
machine. Then we present more precisely
the wound rotor synchronous machine
(WRSM) performances for ISG applica-
tions. All these machines must follow
strict rules and specifications (low length,
high torque, speed, and efficiency). Dif-
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ferent kinds of machines have already

been evaluated.
The switched reluctance generator has
received a great deal of attention for air-
craft engine applications, where robustness
and high-speed operations are imposed.
Such a realization was described in 1989
[1]. The rotation speed reaches up to
25,000 r/min. In automotive applications,
U.S. NAVY PHOTO BY JOURNALIST 2ND CLASS STEPHEN P. WEAVER the rotation speed is limited to 6,000 r/
min for integrated structures. Neverthe-
less, some recent studies have been carried
out [2] and evidenced low efficiencies and
high-voltage ripples in the generator
BY GUY FRIEDRICH mode. Induction machines have also been
& ANTHONY GIRARDIN
investigated in an integrated structure
[3]–[5] or in a belt-driven structure [6].
These studies showed a reduction in the
maximum available power for high-speed
operations. The power factor decreases
26
Digital Object Identifier 10.1109/MRA.2009.932592 with high speed, but efficiencies remained
1077-2618/09/$25.00©2009 IEEE
greater than 80% for a wide speed account this phenomenon for our
range. The main advantage of this optimal control. A simple analytical
kind of machine is its ability to sup- A DEFAULT ON method for considering the iron
port high rotor temperature (up to losses is proposed and used in the
250
C) and its intrinsic fault-tolerant THE POWER optimization process. Optimal cur-
function due to its passive rotor. A rents, simulations, and experimental
default on the power converter does CONVERTER results are discussed and evidence
not generate high current or voltage DOES NOT that WRSM is a challenger to the
on the vehicle network. Its main draw- induction and the synchronous PM
back is the oversizing of the power GENERATE HIGH machine for ISG application.
converter due to the magnetizing cur-
rent, which is difficult to reduce CURRENT OR Machine and
because the application requires, for Control Structure
the integrated structure, a relatively VOLTAGE ON THE
high air gap (0.5 mm) due to the Constraints Due to ISG Application
vibrational constraints. VEHICLE As other automotive applications, ISG
Synchronous PM machines, thanks NETWORK. is very constrained:
to the drop of the costs of PMs, may 1) low length
be a good choice for starter-generator 2) high torque at low speed with
(SG) applications. The application minimum power taken on the
requires a wide constant power zone, and so interior mag- battery (170 N  m at 400 Arms, 8 kW)
nets are required. The principle of flux weakening with 3) operating points at high speed (fi 6,000 r/min)
this kind of machine has been well known since 1980 4) power and voltage limited by the battery: 8 kW,
[7]–[9], and its application to ISG has been described in 21–36 V in the motor mode (starter or boost) and
[10] and [11]. High efficiencies are reached despite high 42–50 V in the generator mode (power depending
Id negative value for high-speed operation. Recently, this on the battery technology)
kind of machine has been studied in a multibarrier struc- 5) limited battery energy storage
ture, which is called PM-assisted reluctance motor gener- 6) current limited by inverter or thermic conditions
ators [12], [13]. The recent apparition of bistate (150 fi 400 Arms)
magnetic materials allows the use of this kind of structure 7) high-temperature variation (25
C fi 180
C).
for high peripheral-speed operation [14]. The main prob- These constraints create specific consequences (high-
lem of such machines is the apparition of high-voltage magnetic saturation) and limitations (current, voltage,

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terminals in the case of a lack of control of the power power, and energy).
electronics in a high-speed operation (no id control). This
possible high voltage imposes supplementary protection Design
systems to disconnect the electric machine from the net- Figure 1 shows a cross section of a WRSM dedicated to an
work to limit damages on the vehicle. ISG application. The procedure for designing is not
In this context, wound-rotor synchronous machine described in this article and is based on an optimal
have great advantages: low cost (no rare earth PM) and approach that has been previously presented for a PM
three control variables (id, iq, and if) allows new optimiza- structure [15]. The main characteristics of the optimal
tion possibilities compared with PM machine (id and iq). design are summarized in Table 1.
A special design of the rotor minimizes the required field
current and allows the use of a rotating transformer if a Control Scheme
total brushless configuration is required. Beyond these This machine is used as a starter and a generator. Its con-
advantages, it is interesting to note a significant improve- trol is unified by using a unique torque control: positive
ment of the safety, thanks to the possibility of canceling of torques for motor operations and negative for generator.
the field current, and therefore, limiting the risks of high Figure 2 shows this control scheme.
voltages for high-speed operations.
The flux-density level in the air gap is limited only by
the quality of sheets used thus authorizing significant spe-
cific torque.
Moreover, during the generating mode, the converter
can be operated in a synchronous rectifier mode, thus
authorizing an increase in the efficiency and a reduction in
the electromagnetic disturbances and converter losses by
avoiding an operation in the pulsewidth modulation
(PWM) mode. X
After a presentation of ISG constraints, the machine Y
design, and control structure, models of the WRSM and
its environment are described. The cross-saturation
phenomenon is detailed, and we propose an off-line 1
27
finite element (FE) method to calculate, taking into WRSM cross section.
 Models wd ¼ fd (id , iq , if ): (1)
THE MATLAB wq ¼ fq (id , iq , if ): (2)
Machine
The WRSM is modeled by classical OPTIMIZATION
Park’s equations (d-q reference frame). The functions fd and fq are calcu-
Nevertheless, taking into account the TOOLBOX lated by interpolation of tables,
magnetic saturation does not allow the which were realized with the FE soft-
use of inductances. PROVIDES A ware FLUX2D [16]. For different
operating points (iq , iq , and if ), the
Saturation
NONLINEAR three-phase fluxes (wabc ) are eval-
Because of magnetic saturation, flux CONSTRAINED uated (internal function), and then,
cannot be expressed as inductances the direct and quadrature fluxes are
functions. Each flux wd and wq is a OPTIMIZATION deduced [17].
nonlinear function of the currents Figure 3 shows the results of the FE
id ; iq , and if : ROUTINE. software FLUX2D calculation: wd for
different values of iq with If ¼ 0 A
and If ¼ 24 A, and then wq for differ-
TABLE 1. MAIN RESULTS. ent values of id with If ¼ 0 A and If ¼ 24 A.
Figure 3(a) (If ¼ 0 A) shows a classic evolution of wd
Optimal design
as a function of id for iq ¼ 0 (upper curve). The important
External diameter 257 mm influence of the cross saturation by comparing in the
Internal diameter 134 mm same figure the drop of wd for a given value of id and a
high value of iq is noted. For example, for id ¼ 346 A, wd
Main sizes is reduced by about a half when iq varies from 0 to its
Iron length 53 mm maximum value.
Figure 3(c) shows the influence of If on wd . The
Air gap 1 mm
coupling between wd and iq is lower but still present.
Air-gap diameter 214 mm This reduction of coupling is due to the high value of If
Electrical parameters saturating the d axis.
Figure 3(b) and (d) show a classical evolution of wq as a
Stator resistance 17 mX function of iq. The cross-saturation phenomenon is still
present, but the influence of If is relatively low.
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Rotor resistance 1.5 X

Figure 2 shows the importance of considering the cross
Maf (unsaturated) 4 mH saturation for the determination of the optimal currents.
Ld (unsaturated) 0.2 mH
Iron Losses
Lq (unsaturated) 0.17 mH
Iron losses remain one of the most difficult phenomena
If max 23 A to model. Many publications deal with this problem,
Other parameters but it is difficult to find a model that is sufficiently
simple and low time consuming to be evaluated for
Pole number 12 each iteration of the optimizer loop. On the other
Stator filling factor 0.4 hand, iron losses in WRSM are linked to four parame-
ters, Id , Iq , If , and xs , and so, even tabulated data cal-
Materials
culated by an off-line FEs method would require lots of
Iron UGINE FEV 700-P-65 simulations to reach a sufficient accuracy. Our choice
is to use the classic Bertotti approach and develop an
analytical model, which will be
called by the optimizer. For the
Battery evaluation of iron losses, we ne-
glect the saturation phenomenon
to determine the peak air-gap flux
Id *
Optimal
density, and so, we can use con-
T* Control Iq* PWM Inverter WRSM stant values of Ld , Lq , and Maf . The
Laws analytical model used is summar-
If* ized in Figure 4.
Ia
The iron losses model uses the
classic Park transform to determine
Ω the peak air-gap flux density (sinu-
Ib
θ soidal repartition). We consider a
d/dt ratio equal to two between iron and
2 air-gap flux density value (tooth/slot
28
Optimal torque control. ratio ¼ 1). Once the maximal flux
density is calculated, we apply the an energetic model is known. The
simplified Bertotti formula [18]: power-switches voltage drop is taken
WOUND ROTOR into account by an equivalent dc-bus
PIron ¼ C0 f Bam
þ Kcf ( f Bm ) , 2 voltage drop.
|fflfflffl{zfflfflffl} |fflfflfflfflfflffl{zfflfflfflfflfflffl}
(3) SYNCHRONOUS
Hysteresis eddy current Battery
MACHINE The battery is modeled by a voltage
with C0 and a linked to the magnetic ALLOWS HIGH source in a series with an internal
material, whose values are deduced resistance as shown in Figure 6.
from steel data sheets. Kcf ¼ ((pd)2 = POWER FACTOR Maximum power supply equals to
(6qmv )) (d, q et mv : with d, thickness
of the sheet; f, fundamental frequency; EVEN IN FLUX- Pbmax ¼
E2b
: (4)
q, steel resistivity, mv steel density; and 4Rb
Bm peak value of the flux density (sinu- WEAKENING
soidal hypothesis). An example of In the generator mode, the terminal
results for a constant speed and field
MODE. voltage is regulated at a constant value
current is given in Figure 5. Excess (around). The battery is thus modeled
losses are neglected. by a simple voltage source Vch .

Inverter Electromechanical Equations

Converter behavior is considered as ideal. Nevertheless, its For a given operating point (id , iq , if , and xs ), the follow-
efficiency can be considered in the optimization process if ing electrical and mechanical data are performed:

PSID for If = 0 A PSID for If = 24 A

0.065 0.065
0.06 0.06
0.05 0.05
0.04 0.04
0.03 0.03
0.02 0.02
PSID (Wb)

PSID (Wb)

0.01 0.01
0 0

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–0.01 –0.01
–0.02 Iq = 0 A –0.02 Iq = 0 A
–0.03 Iq = 200*sqrt(3)A –0.03 Iq = 200*sqrt(3)A
–0.04 Iq = 400*sqrt(3)A –0.04 Iq = 400*sqrt(3)A
–0.05 Iq = 600*sqrt(3)A –0.05 Iq = 600*sqrt(3)A
–0.06 –0.06
–0.065 –0.065
–1,040 –693 –346–173 0 173 346 693 1,040 –1,040 –693 –346–173 0 173 346 693 1,040
ld (A) ld (A)
(a) (c)
PSIQ for If = 0 A PSIQ for If = 24 A
0.065 0.065
0.06 0.06
0.05 0.05
0.04 0.04
0.03 0.03
0.02 0.02
PSIQ (Wb)

PSIQ (Wb)

0.01 0.01
0 0
–0.01 –0.01
–0.02 –0.02
–0.03 –0.03
–0.04 –0.04
–0.05 –0.05
–0.06 –0.06
–0.065 –0.065
–1,040 –693 –346–173 0 173 346 693 1,040 –1,040 –693 –346–173 0 173 346 693 1,040
lq (A) lq (A)
Id = –400*sqrt(3)A Id = 100*sqrt(3)A Id = –400*sqrt(3)A Id = 100*sqrt(3)A
Id = –200*sqrt(3)A Id = 200*sqrt(3)A Id = –200*sqrt(3)A Id = 200*sqrt(3)A
Id = –100*sqrt(3)A Id = 400*sqrt(3)A Id = –100*sqrt(3)A Id = 400*sqrt(3)A
Id = 0 A Id = 600*sqrt(3)A Id = 0 A Id = 600*sqrt(3)A
(b) (d)
3
29
Calculation results: (a) wd for If ¼ 0 A, (b) wq for If ¼ 0 A, (c) wd for If ¼ 24 A, and (d) wq for If ¼ 24 A:
 1) current (rms) 6) electrical and mechanical power
sffiffiffiffiffiffiffiffiffiffiffiffiffi Pe ¼ vd id þ vq iq þ Rf I2f , (14)
i2d þ i2q WRSM IS A
Irms ¼ (5) Pm ¼ Tm X (15)
3
CHALLENGER 7) efficiency
2) flux (table) FOR PERMANENT  sign(Tm )
Pm
wd ¼ wd (id , iq , if ), (6) g¼ (16)
MAGNET Pe
wq ¼ wq (id , iq , if ) (7)
MACHINE FOR 8) battery voltage
3) total losses
ISG pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ffi
Piron losses ¼ Piron (id ,iq ,Maf ,xs ), Eb þ E2b  4Rb Pb
Ub ¼ (17)
(8) APPLICATIONS. 2
Pcopper losses ¼ 3Rs I2rms þ Rf I2f
(9) where Pb is the power given by
the battery. It is equal to the
4) electromagnetic torque electrical power divided by the inverter efficiency.
h i If voltages are considered as sinusoidal, their maxi-
Tem ¼ p wd iq  wq id (10) mal rms values are considered equal to

Ub
5) voltage Vsup ¼ pffiffiffi : (18)
2 2
vd ¼ Rs id  xs wq , (11)
vq ¼ Rs iq þ xs wd , (12) 9) The maximum current is equal to the maximum
sffiffiffiffiffiffiffiffiffiffiffiffiffiffi inverter current in the starter mode and is limited
v2d þ v2q by the maximum current density in the WRSM
Vrms ¼ (13) in the generator mode.
3

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Iinverter max (starter)

Ilim ¼ (19)
/ Js max (generator):
Ld id + Maf If
with Js max being the maximal stator current density.
Lq iq Iron Piron (Losses)
Losses Optimal Control
Model
ωs
Principle
Controlling the WRSM is equivalent to injecting currents
4 id , iq , and if , which minimizes the total losses with
Iron losses determination. respect to different constraints (torque, current, voltage,
and power).
X
120 1,800 8(T  , X), If (id , iq ) n min
 
Plosses
id , iq , if
1.6e+003 1,600
100 1.2e+003
1,400
Quadrature Current

80 400 800 1,200

1,000
60
800 Rb
200 600
40 600
1.8e+003 400
20 1.4e+003
1e+003 200 Ub
Eb
0 0
0 2 4 6 8 10 12 14 16 18 20 Ib
Field Current
5 6
30
Example of iron losses evaluation for Id = 0 and 2,000 r/min. Battery electrical model.
with
id (A)
Tm ¼ T  , 153
Vrms  Vsup , 140
130 –50
120
Irms  Ilim , 110
100 –100
Pe  Pbmax: 90
80 –150
70
60

T (N · m)
The MATLAB optimization tool- 50 –200
box [19] provides a nonlinear con- 40
30
strained optimization routine. It 20 –250
minimizes an objective function f and 10
0
tries to maintain constrained func- –10 –300
tions g negative: –20
–30
–40 –350
x n min

f (x ) –50
–60
x –400
–72

1, 00
1, 50
1, 00
2, 0
2, 00
2, 50
2, 00
3, 50
3, 00
3, 50
3, 00
4, 50
4, 00
4, 50
4, 00
5, 50
5, 00
5, 50
5, 00
6, 50
0
with

500
750
1, 0

75

00
0
25

0
2
5

0
2
5
7
0
2
5
7
0
2
5
7
0
2
5
7
gi (x ) 5 0, 8i ¼ 1 . . . Nconstraints : Ω (r/min)
(a)
iq (A)
Objective Function
153
Here the objective function equals the 140 500
total losses: 130
120 400
110
f ¼ PTotal losses : (20) 100
90 400
80
Constraints Function 70 200
T (N · m)

60
The following are the constraints functions. 50
40 100
1) Mechanical torque is equal to the 30
command torque: 20
10 0
0

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  –10
gt ¼ jTm  T j  ejT j: (21) –20 –100
–30
–40
e is a percentage (05e51) that –50 –200
defines precision. –60
–72
2) Current [see (5)] is lower than the
1, 00
1, 50
1, 00
2, 50
2, 00
2, 50
2, 00
3, 50
3, 00
3, 50
3, 00
4, 50
4, 0
4, 50
4, 00
5, 50
5, 00
5, 50
5, 00
6, 50
0
500
750
1, 0

00

00
limit [see (19)]:
0
25

0
2
5
7
0
2
5
7
0
2
5
7

2
5
7
0
2
5
7

Ω (r/min)
gi ¼ Irms  Ilim : (22) (b)
if (A)
3) Applied voltage [see (13)] is 153
less than the available voltage 140
130 22
[see (18)]:
120 20
110
gv ¼ Vrms  Vsup : (23) 100 18
90
80 16
4) In the starter mode, electrical 70
60
T (N · m)

14
power [see (14)] is limited by the 50
40 12
battery maximum power [see (4)]: 30
20 10
gp ¼ Pe  Pbmax : (24) 10
0 8
–10
–20 6
Algorithm –30
–40 4
The algorithm is shared by both parts. –50
1) In the first step, the maximal tor- –60 2
que speed zone is established. –72
1, 00
1, 50
1, 00
2, 50
2, 00
2, 50
2, 00
3, 50
3, 00
3, 50
3, 00
4, 50
4, 00
4, 0
4, 00
5, 50
5, 00
5, 50
5, 00
6, 50
0
500
750
1, 0

Speed is imposed by the applica-

25

00
0
25

0
2
5
7
0
2
5
7
0
2
5
7
0

5
7
0
2
5
7

tion (0–6,000 r/min). Maximum Ω (r/min)

torques (positive and negative) are (c)
calculated at standstill by a first 7
31
constrained optimization. Optimal control currents: (a) id (A), (b) iq (A), and (c) if (A).
 2) For each couple (T  , X), id , id , Optimal Control Currents
and if are calculated by optimi- The three calculated currents id , iq ,
zation as seen earlier. THE MAIN and if are displayed in Figure 7.
The optimizer MATLAB func- DRAWBACK OF 1) In the starter mode (positive
tion ‘‘fmincon’’ [20] that allows torques), a reduction of the Id
tabulated data was used. In our case, THE WRSM IS THE component in the starter mode
the tabulated data were wd ; wq com- allows a maximum value for Iq.
puted by a commercial FE software SUPPLY OF ITS For high torque, If reaches its
(FLUX 2D). maximal value. Nevertheless,
ROTOR WITH the optimizer reduces this cur-
Simulation Results rent even in the constant tor-
In this section, we present the simula- A DC CURRENT. que zone to reduce total losses.
tion results. 2) In the generator mode (nega-
tive torques), Id and If are
simultaneously controlled to
improve the efficiency. Iq
Efficiencies = f (Ω,T) presents a classical evolution.
153
140 Performances
130 0.9
120
110 0.8 Efficiency
100
90 The isoefficiencies lines displayed in
0.7
80 Figure 8 show a good efficiency in the
70
60 0.6 generator mode at usual speeds for an
T (N · m)

50 internal combustion engine (ICE) and a

40 0.5
30 high torque at low speeds in the motor
20 0.4 mode to easily crank the ICE. We can
10 notice high-efficiency values (greater
0 0.3
–10 than 0.75) in the generator mode in a
–20 wide torque-speed zone even at idle
–30 0.2
–40 speed (750 r/min).
–50 0.1 In a previous publication [21], the
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–60
–72 authors have developed an internal PM
(IPM) machine based on the same
250
500
750
1, 0
1, 00
1, 50
1, 0
2, 50
2, 00
2, 50
2, 0
3, 50
3, 0
3,250
3, 00
4, 50
4, 0
4, 50
4, 00
5, 50
5, 00
5, 50
5, 0
6, 50
0
50

50

00

00

50

00
0
2

7
0
2

5
7

2
5
7
0
2

specifications. Even if the comparison

Ω (r/min) is always difficult, we can say that the
8 efficiency performances are equivalent
Isoefficiencies. in a wide operating zone. Nevertheless,
the WRSM has a higher efficiency for
high-speed and low-torque operations.
Power Factor = f (Ω,T ) This phenomenon is due to the high Id
153 1 negative value, which generates high
140 copper losses to reach this operating
130 0.98
120 point. For low-speed and high-torque
110 0.96 operations, the IPM presents higher
100
90 efficiencies. Some results are compared
0.94
80 in the Table 2.
70
60 0.92
T (N · m)

50 Power Factor
40 0.9
30
The values of the power factor are
20 0.88 shown in Figure 9. This figure shows
10 the main interest of the WRSM: the
0 0.86
–10 power factor remains very high
–20 (greater than 0.94) in the whole tor-
–30 0.84
–40 que-speed plane. This allows to
–50 0.82 reduce copper losses but especially
–60
–72 0.8 the dc bank of capacitors, which is
one of the larger and more fragile part
250
50 0
750
1, 0
1, 00
1, 50
1, 0
2, 50
2, 00
2, 50
2, 0
3, 50
3, 0
3,250
3, 00
4, 50
4, 0
4, 50
4, 00
5, 50
5, 00
5, 50
5, 0
6, 50
0
50

50

00

00

50

00
0
2

7
0
2

5
7

2
5
7
0
2

of the power converter, especially for

Ω (r/min) high-temperature conditions.
9 This high-power factor in the wide
32
Power factor. operating range is the main advantage
 TABLE 2. MAIN RESULTS.
10 N  m at 30 N  m at
Efficiencies 4,000 r/min 1,000 r/min
ISG 0.75 0.87
WRSM 0.82 0.78

of the WRSM compared with induction machine or IPM for

ISG applications.
10
Experimental Results
Rotor of the WRSM.
Electric Machine
To validate the preceding simulations and to consider the Conclusions
application of the WRSM to the ISG, a prototype was This study presented an application of the principle of a
designed and produced. Figure 10 shows the rotor of this WRSM for an ISG application. A principle of optimal
machine. A significant filling factor has to be noted. This control was also presented. The model used by the opti-
optimization result is linked to the minimization of the mizer considers the particularly significant phenomenon
rotor losses, not only to increase the efficiency of the sys-
tem but also to reduce the power to be injected in the
rotor, to minimize the power of the rotor converter and
its associated rotating transformer in the case of a brush-
less realization.
Figure 11 shows the small length of the machine and
the particular care taken to the realization of very small-
length end windings.

Experimental Results
The machine was assembled on a test bench to estab-
lish the efficiency cartography. Figure 12 shows the

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experimental results in the shape of dots and com-
pares them to simulations (colored zones). A good
correlation between measurements and simulations,
which enables the validation of the suggested
approach, was observed. 11
Stator of the WRSM.
Rotor Supply
The main drawback of the WRSM
is the supply of its rotor with a dc Efficiencies = f (Ω,T )
current. For the presented results, 153
0.03
the rotor has been supplied by slip 140
130 0.9
rings. This solution has been effi- 120
ciently used in our prototype even 110 0.8
100
for high-speed operations. Never- 90 0.7
theless, another study has been car- 80
ried out to evaluate the feasibility 70
60 0.6
T (N · m)

to supply the rotor with a rotary 50 0.65

40 0.5
transformer. Comprehensive re- 30
sults may be found in [22]. The 20 0.74 0.4
active part of the transformer is of 10
0 0.3
20 mm, with the structure used. –10 0.77 0.77 0.75
The maximal total power losses –20 0.77 0.72 0.72
–30 0.74 0.2
reach 240 W (for If ¼ 25 A). –40 0.73
Nevertheless, this maximal current –50 0.1
–60
is only required for the starter –72
mode. In the generator mode, espe-
250
50 0
750
1, 0
1, 00
1, 50
1, 00
2, 50
2, 0
2, 50
2, 0
3, 50
3, 0
3,250
3, 00
4, 50
4, 0
4, 50
4, 00
5, 50
5, 0
5, 50
5, 0
6, 50
0
00

50

00

00

00

50

00
0
2
5
7

5
7

2
5
7

cially for high-speed operations,

the field current is reduced to 4 A. Ω (r/min)
In that case, the transformer losses 12
33
are reduced to 50 W. Isoefficiencies calculated and some points of measures.
 of cross saturation for this kind of [9] S. Morimoto, Y. Takeda, T. Hirasa, and K.
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Associating the advantages of the induction machine Application to a permanent magnet machine,’’ Proc. IEEE IEMDC,
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IEEE INDUSTRY APPLICATIONS MAGAZINE
JULY j AUG 2009
WWW.IEEE.ORG/IAS

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