Energy Efficiency for
Fractional Power Loads
12
© PHOTO DISC
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Single-phase switched reluctance motors
offer several advantages
T
is checked by simulation. Promis-
HE SINGLE-
ing improvement in efficiency that is
phase switched reluctance motor is presented as a
favorable to the SSRM drive is observed in the
robust and rather economic solution for
simulations. The designed SSRM prototype is then built,
fractional power-load applications where a low-starting
and the electronic converter is implemented. General
torque is required. This work reports the results obtained
achievements are: that the SSRM presented 13% less mag-
with the replacement of a single-phase capacitor-run
netic material and 13% less copper when compared to the
induction motor (SPIM) by a single-phase switched reluc-
SPIM. Under steady-state operation, the measured input
tance motor (SSRM). The load is a 500-mm industrial fan
power required by the SSRM is 37.5% smaller than that
originally driven by the SPIM. A 4/4 SSRM prototype and
drawn by the SPIM to drive the load in similar conditions.
associated electronic converter are designed and operation
Simulation and experimental results are presented and
BY DARIZON A. ANDRADE, ROGÉRIO S. COSTA,
RODRIGO S. TEIXEIRA, & AUGUSTO V. FLEURY
1077-2618/06/$20.00©2006 IEEE
compared and found to be in good agreement. Overall, the
work indicates a potential alternative in the pursuit of
energy conservation technology.
The present work is mainly concerned with drive efficiency for fractional power loads. The SSRM is presented
as a potential alternative for driving small loads that are
mainly inertial. Centrifugal pumps, indoor fans, industrial
blowers, emeries, and vacuum cleaners are examples of
suitable loads. As a target application, an off-the-shelf
industrial blower that was originally driven by a singlephase, capacitor-run induction motor was chosen. The
research comprises the design and construction of a 4/4
SSRM with auxiliary windings as proposed in [3] and the
development of a microcontroller-based driving strategy to
start and run the blower at its original rated speed. The
result was a considerable savings in magnetic material and
copper and a rather smaller input power at full-speed operation as observed in the measurements that will be shown
in later sections. Additionally, the requirement of a static
converter to feed the motor inherently resulted in a continuous controllable speed drive that is an unparalleled
advantage for the SRM drive.
The work that is presented in the following sections
discusses the principle of operation of the SSRM, the prototype design, driving strategy for starting and steady
state operation, modeling and simulation results, steadystate measurements, and performance comparisons with
the single-phase induction motor.
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Search for Better Drivers
The driving of small loads invariably uses single-phase
induction or universal motors. Despite having low manufacturing costs, these motors present two serious inconveniences: universal motors have limitations such as short
lifetime and maintenance requirements due to the presence
of brushes, while fractional power single-phase induction
motors operate with very low efficiency, rarely higher than
50%. Although the losses of individual units are relatively
low, most of these machines in use in all areas of activity
result in an accountable waste of energy. This is not in line
with the concept of energy efficient and reliable systems.
In the search for a low maintenance and high efficiency
driver for these loads, the single-phase switched reluctance
motor (SRM) appears. Switched-reluctance machines are
known to be rugged and reliable and highly efficient.
They present high torque/volume ratio and, due to constructive characteristics, present very low manufacturing
costs [1]. Single-phase SRMs are doubly salient machines
with an equal number of rotor and stator poles; configurations of 6/6, 4/4, and 2/2 are common. The phase winding
is concentrated and wound around the stator poles. The
rotor is composed of only laminated magnetic circuit, Principle of Operation
which makes it rather robust, free of copper losses, and The SRM torque production is based on the tendency of
the alignment of the stator and rotor poles. When the
suitable for operation at high speeds.
Recent literature has documented design and driving phase winding is energized, the rotor rotates until a
strategies for SSRMs. For example, [2] presents a work position of alignment is reached, as shown in Figure 1(a).
where an SSRM is used to replace a universal motor for a This position corresponds to a condition of maximum
vacuum cleaner application and reports that this motor is flux linkage and also maximum winding inductance.
much more efficient, with greater suction power and a Due to magnetic circuit geometry with stator and rotor
lifetime that is four times longer. A number of researchers salient poles, the inductance of the winding is a function
are dedicated to driving strategies [3]–[6] where the main of rotor position. This dependency in the case of a 4/4
goal is to reduce the number of semiconductors while motor is is shown in Figure 2.
The torque produced is generally expressed by the folattaining desired driving features.
Like single-phase induction motors, SSRMs are not lowing equation:
fully self-starting devices. There are some specific rotor
1
∂L
positions for which auxiliary means are required to get the
Tm = · i 2 ·
,
(1)
2
∂θ
machine started. This has caught the attention of the scientific community as indicated by the great number of existing patents [7]–[9]. For instance, [7],
[10], and [2] make use of appropriate parking
magnets so that the rotor is always parked in a
position that allows for starting torque when the
winding is excited. Reference [8] uses the
property of core magnetic saturation to correctly
position the rotor for starting. Reference [9] uses
a mechanical structure to set the rotor in motion.
A design for a 4/4 machine with interpoles carrying auxiliary windings that allow for self-starting
in any rotor position is presented in [3]. It also
allows for starting in any direction of rotation, a
feature that qualifies the SSRM for four-quadrant
operation. This is a very sound solution as the
auxiliary windings, which are more than simply
(a)
(b)
auxiliary starting aids, can be used for monitoring
1
the rotor position during running thereby openCross view of the 4/4 SSRM FEM analysis.
ing grounds for rotor-position estimation.
13
where Tm , i are the motor torque and phase current
respectively and ∂L/∂θ is the rate of change of inductance with rotor position.
In (1), it is important to observe that the winding current and the rate of change of inductance with position are
the only elements that determine the torque produced by
the machine. This equation also shows that positive and
negative torque is possible, independent of current direction. If the rate of change of inductance is positive, positive torque is produced, while negative torque occurs if
the winding is energized during the inductance negative
slope. As a result, to effectively produce positive torque,
current should exist in the winding only when the slope is
positive. Therefore, the knowledge of the instantaneous
rotor position is a requirement.
Additionally, the torque profile is highly uneven as
torque is only being produced during 50% of the inductance cycle. This imposes limitations in the applications of
SSRMs particularly during start up, inertial loads being
the most appropriate.
60
14
Inductance (mH)
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50
40
30
20
10
0
−90−80 −60 −40 −20 0
20 40
Rotor Position (°)
60
80 90
2
Inductance profile (FEM analysis).
TABLE 1. COMPARISON OF MOTOR DIMENSIONS.
Main Dimensions of the Motor
Stator diameter
SSRM
126.6 mm
Bore diameter
63.9 mm
Stack length
41.0 mm
Stator pole arc
Rotor pole arc
Airgap
45◦
46.6◦
0.25 mm
Stator pole width
24.45 mm
Rotor pole width
25.05 mm
Stator yoke thickness
15.9 mm
Rotor yoke thickness
15.0 mm
Global volume
518.1 cc
Bore volume
131.5 cc
Copper weight
0.471 kg
Prototype Selection
As discussed above, the SSRM operates with a maximum duty cycle of 0.5, therefore, it has a torque discontinuity that results in high torque ripple. Those
applications that present low sensitivity to this characteristic should be addressed [1], and in fact there are
many suitable low-starting torque mechanical loads that
filter the torque oscillations quite well, with virtually
no speed oscillations. Indoor fans, industrial fans, and
blowers among many others are good examples. In the
present work, an off-the-shelf, five-blade 500-mm diameter industrial fan was chosen. It is a drive that comprises only the load directly attached to the motor shaft.
Therefore, the differences in input power when driving
the load at a given speed with distinct driver systems are
due only to the motors and their controls, which allows
for performance comparison.
Prototype Design
To get the design specifications for the SSRM, the fan
was driven with the original induction motor and rated
quantities of operation were recorded. It was verified
that the four poles, 60-Hz, 127-V SPIM drives the load
at 1,600 r/min, absorbing 144 W from mains supply.
Evaluation of losses in the SPIM based in its equivalent
circuit [15] indicated an efficiency of around 50% at
rated speed. This accounts for a shaft output power of
72 W. This information set the design specifications for
the SSRM: a 4/4 topology with output power of 100 W
at 1,600 r/min with an external stator diameter of 126.6
mm, the same as that of the capacitor-run induction
machine, which makes the retrofit in the blower structure easy to do. The design was developed according to
the usual criteria [1], [6], [11]–[14]. The cross view of
the topology is shown in Figure 1 including the structure for auxiliary windings [3]. Main dimensions of the
designed SRM are given in Table 1 together with some
dimensions of the SPIM for comparison purposes.
The measured stack length of the induction machine
magnetic circuit is 47 mm, while the
stack length of the designed SRM is
41 mm, which is 13% smaller. The
∈ SPIM
two machines have the same stator
126.6 mm
diameter, and so the volume of the
SRM is smaller in the same amount.
72.0 mm
Additionally, the bore diameter of the
47.0 mm
induction motor is 72.0 mm against a
bore diameter of 63.9 mm for the
—
SSRM giving a bore volume of 191.3
—
cm 3 cc for the induction motor and a
—
bore volume of 131.5 cm 3 cc for the
SSRM motor, which is 31% less. The
—
copper weight for the induction motor
—
is 0.541 kg, while for the SSRM is
—
0.471 kg, again 13% smaller. The copper weight for the SRM main winding
—
is only 0.302 kg. It should be noted
591.6 cc
that in case of a motor being designed
191.3 cc
for custom application, with just one
direction of rotation, only one pair of
0.541 kg
auxiliary windings is needed thereby
reducing the magnetic and copper material even more.
These facts indicate how the SRM is able to make better
use of magnetic and electric material. Pictures of the
motors are shown in Figure 3.
Control Logic and Control Circuit
Unlike driving the induction motor, which is connected
to the ac power supply, the switched reluctance motor
requires a static converter to synchronize the flow of current in the windings with the rotor position for useful
average torque production. The feedback signal that
defines the periods of main winding conduction comes
from the rotor position sensor. This comprises a slotted
disk fixed in the shaft and an optical switch as shown in
Figure 4. The signal coming from the optical switch
indicates the conduction periods. When this signal is
high, corresponding to positive inductance slope, the
main winding switches are enabled (conduction window), otherwise their gates are grounded.
Figure 5 shows the structure implemented to drive
the SSRM. It comprises the well-known asymmetrical
bridge to drive the main winding driven by fully digital control hardware. There are alternatives to this
power converter [3]–[6]. Certain applications can benefit from simpler and less expensive structures, but at
the present stage it was decided to have this in view of
the flexibility desired in a research development. As a
particularity of implementation, a very low-conduction
loss (RD(ON) = 0.04 MOSFETS) was chosen.
starts by briefly turning on the two main winding
switches with controlled currents to bring the rotor to
a position of 0◦ . In the sequence, these switches are
turned off and the auxiliary winding is energized to
bring the rotor in alignment with the interpoles. This
guarantees a position for positive torque production
from the main winding. From this point on, only the
(a)
The control logic was implemented exclusively into a
single microcontroller. It receives the signals coming
from the rotor position sensor (the only feedback signal
in the system), makes the required processing, and outputs gate signal commands to drive the converter switches. Interfacing between microcontroller hardware and
power switches is made with optical couplers. The control logic implementation is 100% software based. It is
highly flexible, which allows for quick and easy changes
or adjustments at no extra cost.
(b)
3
SPIM and SSRM comparison: (a) SSRM and SPIM stators and
(b) SPIM and SSRM rotors.
Starting Up Strategy
There are some specific rotor positions in the SSRM
that make torque production impossible. For the present case, these correspond to the rotor positioned at
0◦ , ±45◦ , ±90◦ , and so on. For these positions, there
will be no torque production when the derivative of
the inductance with a zero position, as depicted in Figure 2, is even with the current in the windings (1). For
these positions, there will be no torque production as
the derivative of the inductance is zero, as depicted in
Figure 2. In such cases, auxiliary means are called for
to set the rotor in motion. Based on the chosen topology, the strategy to achieve starting for any rotor position is shown in the flowchart of Figure 6. The control
logic first verifies if the conduction window is enabled.
If it is so, indicating that the rotor is parked in a
region where positive torque is possible, the routine
energizes the main winding for the starting sequence.
If the conduction window is closed, the control logic
45°
Optical
Sensor
45°
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Control Logic and Implementation
Motor Shaft
4
Rotor position sensor.
15
D2
S2
S1
Circuit of the static converter.
iaux.
C
127 V ~
ia
S1
main winding is energized following the enabling times of the
conduction window. Because the
Main
Auxiliary
front-end converter is a diode
bridge rectifier supplied directly
IRFP 260
from mains, the dc link voltage,
smoothed by the capacitor, is
D1
constant. This voltage applied
Daux
to the motor winding leads to
high current levels when the
Au x
Ph A
starting is at zero and low
speeds as there is no counter emf
in the winding. These high current levels are not adequate in
IRFP 260
the presence of the static switches and need to be limited. Using
Saux
the flexibility brought by the
Rotor
digital control environment and
Position
S2
Saux
to keep costs down, an indirect
current control was implementMicrocontroller-Based
ed, which reduced the voltage
Control Strategy
implemented. The voltage
(PIC 16F84A)
applied in the main winding is
5
chopped with a preprogrammed
frequency that is a function of
the rotor speed. The microcontroller promptly calculates the rotor speed from the
rotor-position-sensor feedback signal.
Steady-State Operation
When the rotor reaches about 85% of the rated speed,
the switching strategy is changed to single pulse. From
this point on, the switching is as follows: in the beginning of the conduction cycle the switches (S1 and S2)
are turned on. After a predetermined period of time,
the upper switch (S1) is turned off and the current freewheels in the path is formed by the closed switch (S2)
and corresponding diode with zero voltage across the
winding. In the sequence, S2 is also turned off and
inverse voltage is applied to the winding, leading to a
faster current decrease and recovering of energy to the
dc link capacitor. Simulated and experimental results of
this operation will be shown in the following sections.
Position
Detection
Y
Conduction
window
enabled?
N
Briefly energize main
winding for alignment.
Energize auxiliary
winding.
Energize main
winding for starting.
0.85 rated
speed?
Single-Phase Switched Reluctance Motor
40
Ia
W
35
N
Ia (A) - W (rad/s)
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Start
Current Control
Strategy
30
25
20
15
10
5
Y
0
Steady-State Strategy
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Time (s)
16
6
Flowchart of starting and steady operation strategy.
7
Simulation: SSRM phase current and speed at starting.
Vm
Tl
0
where
Vm , rm , im
Tl , Lm
J, D
ω, θ
0
0
im
= 12 im ∂L∂θm θ −D 0 · ω
0
1
0
θ
im
L m 0 im ∂L∂θm θ
· ω̇ , (2)
+ 0 −J
0
0
0
−1
θ̇
rm
are main phase voltage, resistance and
current
are load torque and phase inductance,
respectively
are inertia and viscous friction, respectively
are speed and rotor position, respectively.
Single-Phase Switched Reluctance Motor
14
12
10
8
6
4
2
0
5.61
5.615
5.62
5.625
5.63
5.635
5.64
Time (s)
200
Capacitor-Run Single-Phase Induction Motor
Ia × 10
Va
Pm
150
100
50
0
−50
−100
−150
−200
0
8
0.01
0.02
0.03
Time (s)
0.04
0.05
Simulation: Changing from controlled current to single pulse
AC input voltage, current and active power to the single-
supply.
phase capacitor-run induction motor drive.
10
IA (A) - VA (Scaled)
Single-Phase Switched Reluctance Motor
8
6
4
2
0
−2
−4
−6
−8
1>
IA
VA (Scaled)
2>1) Conduction Window: 10 V 5 ms
2) Phase Current: 1 A 5 ms
11.56 11.565 11.57 11.575 11.58 11.585 11.59
11
Time (s)
9
Simulated ac supply voltage and current.
Conduction window and phase current in the SSRM at starting. Scales—phase current: 1 A/div time: 5 ms/div.
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Ia (A) - Gate S2 (Scaled)
16
The units are given in International System of Units
(SI), and the load torque varies with the square of
rotor speed. The simulation comprises the whole system, that is, the mains supply, the converter, the
motor, and load. This integrated simulation environment is possible from some simulation platforms, and
in the present case the Simpower Systems toolbox
from MATLAB was used.
Figures 7–9 show the simulated results. In Figure
7, the controlled starting current and corresponding
speed profile are shown. According to (1) and Figure
2, the motor torque profile is similar to the current
profile, existing only during current conduction. This
sets the requirement of low-starting torque inertial
load that can continue in movement after a pulse of
torque until the next current conduction period is
reached. In Figure 8, the transition from current-controlled to single-pulse supply is shown when the rotor
has reached about 85% of the final speed. As seen in
the single-pulse operation, the current rises quickly
under dc link voltage. Soon after, the upper switch is
turned off and the current decreases in free-wheel
mode until finally the conduction-window signal is
closed. When this occurs, the remaining power switch
Current × 10 (A) Voltage(V) Active Power(W)
Mathematical Modeling and Simulation
The mathematical modeling is important to evaluate the
motor dynamic behavior and through the results obtain
quantitative and qualitative evaluation performance
parameters, which can be analyzed and provide good
feedback to adjust design characteristics. In the present
case, a linear model was used for these purposes. In state
variables the model is given by
17
is opened and reverse voltage is applied in the winding, which forces the current quickly to zero. Figure 9
shows the simulated ac voltage and current supply.
The advantage of having the diode bridge and dc-link
smoothing capacitor is that current from the supply to
the dc link only flows when the ac voltage becomes
higher than the capacitor voltage, which leads to the
ac-side currents always having the same signal as the
supplying voltage. The outcome is that the voltampere product is always positive and therefore the drive
operates with a unity-power factor. These and several
other simulations indicated promising results particularly in the input active power to drive the specified
load. Consequently, the design was approved and the
prototype was built.
Current × 10 (A) Voltage(V) Active Power(W)
Experimental Results
In this section, experimental results of the blower being
driven by the single-phase induction motor and by the
SSRM motor are presented and compared.
2>1) Conduction Window: 10 V 2 ms
2) Phase Current: 2 A 2 ms
Single-Phase Switched Reluctance Motor
200
Ia × 10
Va
Pm
150
100
50
0
−50
−100
−150
−200
0
0.01
0.02
0.03
Time (s)
0.04
0.05
14
12
AC input voltage, current and active power to the SSRM drive.
Main winding current at single pulse operation—rated
speed. Scales—phase current: 2 A/div time: 2 ms/div.
1,000
Single-Phase Switched Reluctance Machine
la × Va
800
Product (VA)
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1>
18
Figure 10 shows the applied voltage, 127 V rms,
input current, 1.27 A rms, and input power, Pm = 144
W, for the induction machine drive with a power factor
0.89 lagging.
The remaining of experimental results presented concern the SSRM operation. Figure 11 shows a detail of
main winding current during the motor acceleration. As
explained before, to keep the starting current at acceptable
levels the voltage across the winding is chopped with a
frequency that is speed dependant. Figure 12 shows the
current profile when the rotor is running at steady-state
speed and the single pulse switching strategy is as
explained before.
Figure 13 shows the speed evolution during starting. The transition period from controlled switching
to single pulse operation can be observed in the plot
for the speed around 1,400 r/min. With the programmed driving strategy, the SSRM can be turned
off and on at any time (the case of a voltage sag). If
mains supply is temporarily lost, when it is back on,
600
400
200
0
0
1>
1) Rotor Speed: 200 RPM 5 s
0.01
0.02
0.03
Time (s)
0.04
0.05
15
13
Starting up: Rotor speed profile (scales—200 r/min/div—5 s/div).
AC mains supply volt-ampere product or instantaneous
input power.
Input Power (W)
Converter
18
17
Blower driven by SSRM and corresponding static converter.
The SSRM.
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the converter gets the rotor speed
TABLE 2. STEADY-STATE PERFORMANCE COMPARISON
from the sensor pulses and determines
OF THE DRIVE SYSTEMS.
the adequate switching condition so
that no over current occurs.
Drive Characteristic
SSRM
SPIM
Figure 14 shows the ac side voltage
Mechanical load
Five-blades fan
Five-blades fan
and input power to the SSRM drive
Rated speed
1,600 r/min
1,600 r/min
that are to be compared with those of
Figure 10. Input ac voltage is also 127
100
100
Rate of flow (m3 /min)
V rms and measured input power is 90
Rate of flow (CFM)
3,531.6
3,531.6
W to drive the fan at the same speed
Electrical input active power
90 W
144 W
of 1,600 r/min. Figure 15 shows the
instantaneous input power. It is to be
at full load
observed that the voltampere product
Power
factor
1.0
0.89 lagging
is always positive, indicating power
CFM/watt
39.24
24.53
flow from supply to the load only.
Unlike the induction motor, there is
Supply voltage
127 V rms
127 V rms
no return of power from load to mains
supply with the SSRM drive. Therefore, the operation occurs at unity power factor, which
is another important operational advantage for the
SSRM drive. Table 2 shows the performance compariInput Power × Speed with SSRM Drive
son of the two drive systems at full load, an economy
100
of 37.5%, that is, 54 W in active power and operation
90
at unity power factor are observed in favor of the
SSRM drive. This means that this system operating 16
80
h a day, for example, will provide an economy of ener70
gy of 311 kWh at the end of one year as compared to
60
the original SPIM drive.
An additional feature of this drive is to be evidenced
50
when it drives loads with a torque that varies approxi40
mately with the square of speed. The load torque at low
30
speeds is very small, easing the starting procedure.
20
Most important is that the power required to drive the
load is proportional to the cubic of speed. Therefore, a
10
small reduction in speed means an appreciable reduc0
tion in input power. For instance, a 10% reduction in
0
200 400 600 800 1,000 1,200 1,400 1,600
speed leads to a 27% reduction in input power. For the
Blower Speed (r/min)
SSRM driven blower, the characteristic of active power
16
absorbed from mains was taken by measurements and is
shown in Figure 16, where the fast reducing input Input power x speed for the SSRM driven fan.
19
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20
power requirement can be confirmed. Although the
present work does not present a controllable speed
drive, one can appreciate the potential for economy of
energy using controlled speeds for these loads. For the
SSRM drive that already has the electronic converter
and rotor position feedback signal, this means just one
more step while for induction motor a whole strategy
for speed control would be required.
Figure 17 shows the experimental set up with the
SSRM and its converter. Notice that the converter is
small, and industrial manufacturing expertise can easily
combine it in the motor body structure. The set shown
is ready to be connected to the ac mains and run. Figure
18 shows the back end of the SSRM and the rotor position sensor. Regarding this sensor, it is instructive to
say that in the last decade several strategies for sensorless rotor position detection were proposed, and those
can also be considered for these cases. Cost sensitivity
will be the issue.
As a final remark, it is important to make clear that
the main focus of this article is in the economy of
energy provided in the studied situation. No closed
loop for speed or torque control was presented, only a
starting and current limitation condition possible from
the digital control was offered. The control strategy
used very little of the micro-controller capacity. More
elaborated solutions are possible just by software
sophistication with no additional hardware requirement. This again shows the potential offered by
switched reluctance motors and the associated electronic converter.
Conclusions
With the evolution of the micro and power electronics,
SRM drives are becoming rather attractive, once the
cost of motor plus converter becomes competitive. Fractional power loads are normally driven by poorly efficient motors and consequently make inefficient drives.
The power of isolated units is not relevant but as these
loads are used in great quantities the overall waste of
energy becomes accountable. This is not in line with
modern efficient sytems thinking. The main contribution of this article is to present a potential alternative
for driving these fractional power loads with a highly
efficient motor. For the target application chosen, an
SSRM motor was designed, built, and tested. The
immediate advantages are threefold: The designed
motor has a smaller overall volume, 13% less magnetic
material and copper weight, and, in rated operation,
saves 37.5% power to drive the same load at the same
speed compared to the original single-phase capacitorrun induction motor while operating with unity power
factor. The smaller volume indicates relevant reduction
in material cost and the reduction in input power during operation indicates that an eventual increase in cost
due to the converter can be recovered in a short time
span. An additional advantage of the SSRM is its inherent ability to work with continuous controllable speed
owing to the static converter. The microcontrollerbased control strategy makes the drive rather flexible
and attractive at a very affordable cost. Overall, the
work suggests an alternative for replacement of smallload drives that, in many cases, will help in the development of energy-conservation technology.
Acknowledgment
The authors gratefully acknowledge the support of
FAPEMIG—“Fundação de Amparo à Pesquisa do Estado de Minas Gerais—Brazil”—through research projects TEC 1056/02 and TEC 146/02, and CAPES for
scholarships.
References
[1] R. Krishnan, Switched Reluctance Motor Drives: Modeling, Simulation,
Analysis, Design, and Applications. Boca Raton, FL: CRC Press, 2001.
[2] J.Y. Lim, Y.C. Jung, S.Y. Kim, and J.C. Kim, “Single phase switched
reluctance motor vacuum cleaner,” in Proc. ISIE 2001, vol. 2, pp.
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Darizon A. Andrade (darizon@ufu.br) is with Electric Drives Lab.–Universidade Federal de Uberlândia Brazil.
Rogério S. Costa is with Centrais Elétricas de Goiás. Rodrigo
S. Teixeira is with Empresa Brasileira de Compressores.
Augusto V. Fleury is with the Universidade Católica de
Goiás. Andrade is a Member of the IEEE. This article first
appered in its original form at the 2004 INDUSCON.