Preparation
A Design ofofAxial-gap
a Formatted
Switched
Technical
Reluctance
Work for
Motor for In-Wheel
the ICEM
Direct-Drive EV
Tohru Shibamoto, Kenji
J. W.
Nakamura,
Haggle, L. L.
Hiroki
Grigsby
Goto and Osamu Ichinokura
Elec. and Comm. Eng. Dept., Tohoku University
Abstract -- Switched reluctance motors (SR Motors) attract
attention as motor that use no rare earth materials. And it is a
candidate technology for electric vehicle application. In
addition, axial-gap structure has possibility of effective
utilization of in-wheel flat motor space. This paper mainly
discusses the design and the characteristics of axial-gap SR
motors. This study focuses on the volumetric constrains of inwheel drive system. First, the results of comparing the axialgap SR motors specification to the radial-gap one at same
volume are shown that utilize the available active volume. By
the results, a new flat volume axial-gap SR motor for in-wheel
direct-drive EV is designed. Finally, a new support link
structure is proposed for the large axial direction
electromagnetic force of axial-gap SR motor.
Index Terms—Switched reluctance motor, electric vehicle,
axial gap motor
I.
Nr
Ns
Np
tr
ts
l
Sst
Srt
g
Rin
m
VDC
IRMS
Bmove
L
Faxial
Rcase
lcase
NOMENCLATURE
Number of rotor poles
Number of stator poles
Number of turns per pole
Rotor pole thickness (mm)
Stator pole thickness (mm)
Motor axial length (mm)
Stator pole sectional area (mm2)
Rotor pole sectional area (mm2)
Mechanical air gap (mm)
Inner bore radius (mm)
Number of phases
Source Voltage (V)
Winding RMS value of current (A)
Moving flux density (in Tesla)
SRM Inductance (H)
Axial electromagnetic force (N)
Housing outer diameter (mm)
Housing axial length (mm)
materials, such as dysprosium, neodymium and so on, keep
rising in recent years. It seems that this aspect doesn’t
change.
Because of that, this paper focuses attention on Switched
Reluctance (SR) Motor for in-wheel direct-drive EV. SR
motors are the motor that utilizes the reluctance torque
originated in magnetic saliency between stator and rotor
poles. SR motors are robust and simple structures because
these are made from only steel and winding wire. SR
motors have advantages such as high temperature operation
because of absence of demagnetization, low-cost
manufacturing. And it has high torque density because the
motor can increase its torque even though stator poles are
magnetically saturated. In addition, by employing in-wheel
drive system, it can make car interior space larger, and
reduce the mechanical loss such as gear. But SR motors
also have disadvantages. One of them is that SR motors
torque density is smaller than PMSMs' one. On the other
hand, its magnetic saliency causes higher torque ripple,
noise, and vibration compared to other motors. Torque
ripple and so on can measurably get smaller by control [1].
However the torque density becomes assignment.
This paper focuses on to increase the torque density of
SR motors for in-wheel direct-drive system. In-wheel
direct-drive system EV that use Radial gap SR motor is
already investigated [2]. The specification of the SR motor
is shown in Fig. 1 (Nr=20, Ns=16). In-wheel drive system
EV is shown in Fig. 2. The axial length l is 66.4mm and the
outer diameter is 222mm. The rotor and stator pole
thickness are tr = 11mm and ts = 10mm. The volume is very
flat. This is because that the wheel space is very thin and to
fit in the motor in wheel. So, coil end space and inner
diameter become dead space as the volume becomes more
flat. If can use their spaces, it seems that effective
utilization of the available active volume is possible. So this
paper also focuses on the axial-gap structure which can use
inner bore space and coil end space effectively.
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II. INTRODUCTION
978-1-4673-0142-8/12/$26.00 ©2012 IEEE
66.4
51
N recent years, environmental problem, such as
exhaustion of fossil fuels, global warming and air
pollution, become big issue all over the world. For such
occasions, the enlargement of Kyoto Protocol is decided.
Vehicles, such as car, motorcycle, truck, bus and so on,
consume a lot of fuel and put out a lot of emission gas.
Improvement in vehicle fuel economy and exhaust
emissions must be needed under such circumstances. And
so Electric Vehicle (EV) and Hybrid Vehicle attract lots of
attention and have been developed in recent years. A few
hybrid cars and a few electric vehicles are commercialized.
But EV has some problems. Almost EVs use Permanent
Magnet Synchronous Motors (PMSMs). PMSMs have high
torque and excellent efficiency. However, the magnet
222
I
Fig. 1. Structure of the Radial-gap SR motor for EV.
(Number of phases m=4)
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Fig. 2. In-wheel drive system EV that mounts SR motor.
(The mounted motor is shown in Fig.1.)
III. AXIAL-GAP SWITCHED RELUCTANCE MOTOR
A phase torque of SR motors can be expressed as (1).
1 2 dL(θ )
dθ
2
τ k = ik
(1)
Where k represent index of phases, Wk`(θ, ik) is the
coenergy stored in the kth winding, ik is the phase current, θ
is the rotor position, L(θ) is the phase inductance. Fig. 3
shows the phase inductance L profile versus position θ of
the SR motor. Total motor torque can be expressed as
τ =∑τ k k
(2)
Fig. 4 shows a driving system of the SR motor using
asymmetry half bridge converter. A consecutive rotation
can be obtained through switching the transistors
sequentially to always produce positive torque based on the
rotor position from a position sensor such as the rotary
encoder.
From (1) and (2), total torque can be improved by
increasing inductance changes. The inductance can be
increased by increasing gap surface area.
The structures of axial-gap SR motor and radial-gap SR
motor are shown in Fig. 5. Axial-gap motors have the gap
to axial direction. By this change, dead space of coil end
become smaller and inner bore can become smaller as far as
the volume allow. Axial-gap structure could reduce the
leakage flux from stator to rotor because the flux flow axial
direction. And the saturated magnetic flux can improve.
Then the axial-gap SR motors inductance curve become
better than the radial-gap SR motors at flat volume. The
advantage makes axial-gap SR motors performance better.
IV.
SIMULATION RESULTS COMPARISON
A.
Design comparison axial-gap SR motor models
First, axial-gap SR motor with single stator and rotor
(single rotor model) is designed. Second, axial-gap SR
motor with single stator and double rotor (double rotor
model) is designed to get more torque by increasing gap
area. The single rotor model is shown in Fig. 6 and the
double rotor model is shown in Fig. 7. Their motors are
designed based on the radial-gap SR motor of Fig. 1. The
active volume of the 16/20 radial-gap SR motor is flat. The
outer diameter, phases, winding space factor, number of
stator poles Ns, winding diameter and so on are same. When
design the axial-gap SR motors, increase the cross-sectional
area of stator pole Sst. Then, the number of rotor poles Nr
are decreased to 12 from 20 and the bore radius Rin are also
decreased to 42.5mm form
Rotor
Stator
Fig. 3. Inductance profile versus rotor position of the 16/20 SR motor and
principle.
Coil
Fig. 6. 4-phases Single stator and single rotor type Axial-gap SR motor
(Ns=16, Nr=12).
Stator
Fig. 4. Drive system of the 16/20 SR motor using asymmetry half-bridge
converter. (4 phase model)[1]
Coil
Rotor
Fig. 7. 4-phases Single stator and double rotor type Axial-gap SR motor
(Ns=16, Nr=12).
Fig. 5. Structure comparison of Radial with Axial-gap SR motor.
50mm to increase Sst and Srt. In addition, the number of
winding turns per pole Np is increased to 82 from 57 to
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adjust the flux density Bmove as same as that of the base
radial gap machine. And Source Voltage Vdc is changed to
60V from 48V to adjust the rotation speed area (01000rpm). Their changes are based on expression expressed
in (5).
VDC = m⋅ n ⋅ Nr ⋅ p ⋅ N p ⋅ Bm ⋅ Sst (5)
n is rotational speed value (in rps) of the motor. p is number
of series connection. Their two models are simulated using
3D FEM (JMAG Designer).
B.
Comparison the simulation results
Fig. 8 shows a simulation result that is characteristic of
current density versus torque. At this graph the current
density is limited less 22.6A/mm2, the current IRMS is 40A.
Fig. 9 shows the comparison of efficiencies. Fig. 10 shows
comparison of torque waveforms. And Fig. 11 shows the
operational area of the SR motors that is unaligned and
aligned stator pole magnetization curve. The torque of SR
motor is determined by the size of operation area from Fig.
11, at low current, the torque of double rotor model axialgap SR motor is smaller than that of other models. It arises
with increasing of magnetic reluctance. At low current, to
get same torque, double rotor model axial-gap SR motor
need more current compared with other models. So the
characteristics become such result. From Fig. 9 the doublet
rotor axial-gap SR motor is the most efficient of the three
models. It is because axial-gap structure effectively uses the
flat volume. Axial-gap motors have larger stator pole
sectional area and more windings. So axial-gap motor needs
smaller current value to get same torque. From Fig. 8, the
torque of double rotor axial-gap model becomes higher at
same current density (22.6 A/mm2). But the torque ripple of
the double rotor axial gap model is larger than that of
radial-gap SR motor. It is because that axial-gap SR motor
could get more torque, but the motor core becomes more
saturated by enough magnet motive force. So the torque
ripple becomes higher than radial-gap model.
But, from Fig.8, the maximum torque of single rotor
model is about 1.7 times larger than that of radial-gap 16/20
SR motor. And the maximum torque of the double rotor
axial-gap model is the largest of all. The torque value of
double rotor axial-gap model is 92 N•m. It is because
double rotor design makes magnetic reluctance large. So
the magnetization gradient become gradual at linear zone
when the poles aligned and unaligned. The area bounded by
aligned and unaligned magnetization curve of the double
rotor axial-gap model becomes largest of all. The
operational area of double rotor model axial-gap model
becomes wider than that of radial-gap model and single
rotor axial-gap model. The torque of SR motors becomes
larger with increasing of magnetic operational area (such as
Fig. 11). And so the maximum torque of the double rotor
axial-gap model becomes largest of all. From their results
and considerations, the double rotor axial-gap model has
the largest torque of all. But axial-gap models have large
axial direction forces. To consider about axial
electromagnetic force must be needed to develop axial-gap
SR motor machines.
Fig. 8. Current density versus torque characteristics comparison with 16/12
axial-gap SR motors to 16/20 radial-gap SR motor.
Fig. 9. Efficiency versus Torque characteristics comparison with 16/12
axial-gap SR motors to 16/20 radial-gap SR motor.
Fig. 10. Torque waveforms comparison with 16/12 double rotor axial-gap
SR motor to 16/20 radial-gap SR motor.
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shown in Fig. 14 and Fig. 15. The current density limitation
is less 22.6A/mm2. The maximum torque is 302N•m. At
simulation this axial-gap SR motor achieve the target
torque 300N•m. And this motor can rotate 0-300rpm and
output enough torque to move. So EV can move at speed
range 0-30km/h by using this SR motor.
Axial-gap single rotor
Unaligned
Poles aligned
Radial-gap SR motor
(a)
Axial-gap single rotor
Unaligned
Fig. 12. 3 phases Single stator and double rotor type Axial-gap SR motor
for EV test machine (Ns=18, Nr=12 ).
Poles aligned
Axial-gap double rotor
(b)
Fig. 11. Magnetization characteristics comparisons: (a) with single rotor
16/12 axial-gap SR motors to 16/20 radial-gap SR motor (b) with double
rotor 16/12 axial-gap SR motor to single rotor 16/12 axial-gap SR motor.
V. DESIGN OF AXIAL-GAP SR MOTOR
FOR TEST MACHINE
A.
Target and simulation result
As described in above chapter, torque characteristic of
axial-gap SR motor is better than that of radial-gap SR
motor at same volume. And to make more gaps is effective
to increase torque at high current area and increase volume
density at same volume. Axial-gap SR in-wheel motor EV
is begun to make. This EV requires 3 phase motor and 12
inch wheel size motor. So another size axial-gap SR motor
that based on previous chapter simulation results is
designed. To move the 2 ton EV at 10% incline, the
maximum torque target is 300N∙m per motor. And the
speed range is 0-300rpm (car speed 0-30km/h). The volume
constraints are that outer diameter is 266mm and axial
length l is 130mm. Number of phase m is 3. The space
factor of winding is 70%. The air gap g is 0.3mm. This
axial-gap SR motor model is shown in Fig.12. This SR
motor has single stator and double rotors. This motor
winding turns Np is 310 turns per pole. The rotor pole
length is 12mm that is sufficient length between stator to
rotor back yoke that is gotten from simulation that change
rotor pole length. But this motor has stator back yoke in
consideration of support link. Total stator axial length is
67.1mm. This length is decided by the simulation result that
is shown in Fig. 13. The maximum torque is 301.8N∙m at
this stator axial length. This motor simulation results are
Fig. 13 Magnetomotive force versus Torque characteristics about 18/12 3
phase axial-gap SR motor.
Fig. 14. Current density versus Torque characteristics about 18/12 3 phase
axial-gap SR motor.
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Fig. 15. Speed versus Torque characteristics about 18/12 3 phase axial-gap
SR motor.
B.
Support link against axial force
This axial-gap SR motor torque is large. But this motor
also has the large electromagnetic axial force. Fig. 16
shows electromagnetic axial force of the SR motor model.
The maximum axial force is 9.65kN. This force decrease
the distance between stator and rotor. This SR motor gap g
is just 0.3mm. So support link that sufficiently receives the
axial force is needed. The support link is designed by
structure analysis. This axial-gap In-wheel SR motor is
motor is produced experimentally. This test machine is
shown in Fig. 17. This case is made up of motor, fixed axial
length, position sensor and rotational case. The experiment
environment is shown in Fig. 18 and Fig. 19. After this, the
characteristics of this axial-gap In-wheel SR motor test
machine is measured.
(b)
Fig. 17. 18/12 Axial-gap In-wheel SR motor test machine: (a) front side
of the test machine (b) Back side of the test machine.
Fig. 18. The torque meter and 18/12 Axial-gap In-wheel SR motor test
machine.
Fig. 16. Axial electromagnetic force of 18/12 double rotor axial-gap SR
motor that one rotor receives (at maximum torque).
Fig. 19. The experiment environment of 18/12 Axial-gap In-wheel SR
motor test machine
C. Experimental results
Basic drive experiment of the proto-type machine has
been done. The result that is characteristics of current
density versus torque is shown in Fig. 20. And the speed
versus torque characteristics is shown in Fig. 21. From Fig.
20, the measured characteristics of proto-type machine
nearly have the agreement with simulation result. In the
experiment, the current limit is 300A/phase. The current
limitation control works at over 210 N∙m. So the current
density become low at over 210 N∙m. That is the reason
why the speed versus torque characteristics disagreed at
(a)
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VII. REFERENCES
[1]
Ayumu Nishimiya, Hiroki Goto, Hai-Jiao Guo and Osamu
Ichinokura, "A Novel Torque Control for a SR motor EV," Elec. and
Comm. Eng. Dept., Tohoku University, Elec. and Inform. Eng. Dept.,
Tohoku Gakuin University, Proceedings of the2008 International
Conference on Electrical Machines, Paper ID 1176, 2008.
[2]
H. Goto, Y. Suzuki, K. Nakamura, T. Watanabe, H. J. Guo, and
O.Ichinokura, "A multipolar SR motor and its application in EV,"
Journalof Magnetism and Magnetic Materials, Vol.290-291,
pp.1338-1342, 2005.
[3]
R. Madhavan, “A Novel Axial Flux Segmented SRM for Electric
Vehicle Application,” IEEE XIX International Conferrence on
Electrical Machines-ICEM 2010, Rome.
[4]
Hirom Arihara, Kan Akatsu, “Characteristics of Axial type Switched
Reluctance Motor,” Energy Conversion Congress and Exposition
(ECCE), 2011 IEEE, 3582-3589.
Fig. 20. Experiment result that is currrent density versus torque
characteristics.
VIII. BIOGRAPHIES
Tohru Shibamoto was born in Aichi, Japan, on January 18,1989. He
received his B.S. degree in electrical engineering from Tohoku University
in 2011 respectively. He is currently a master student at Tohoku
University, Sendai, Japan. His research interests include design of the inwheel direct drive axial-gap SR Motors.
Hiroki Goto (member) was born in Shizuoka, Japan, on June 27,
1979. He received his B.S., M.S. and Ph.D. degree in electrical
engineering from Tohoku University in 2002, and 2004, respectively. He
is presently a research associate of the Graduate School of Engineering,
Tohoku University. He has worked on control and analysis of motors. Mr.
Goto is a member of the Magnetic Society of Japan (MSJ) and IEEE.
Kenji Nakamura received the B.E. and M.E. degrees from Tohoku
University in 1998 and 2000, respectively. He was with Tohoku University
as a Research Associate in the Graduate School of Engineering from 2000
to 2007. In 2006, he received the Ph.D. degree from Tohoku University,
where he is currently an Associate Professor. His current research interests
include design and analysis of reluctance machines and permanent magnet
machines. Dr. Nakamura is a member of the Magnetic Society of Japan
(MSJ), the Institute of Electrical Engineers of Japan (IEEJ), and IEEE.
Fig. 21. Experiment result that is speed versus torque characteristics.
high torque. From Fig. 21, the speed versus torque
characteristics nearly agree with simulation result below
210 N∙m. At over 210 N∙m, the speed become lower than
simulation result. That is also because of the current
limitation.
Osamu Ichinokura (Member) was born in Morioka, on August 28,
1951. He received his B.S., M.S. and Ph.D. degrees in electrical
engineering from Tohoku University in 1975, 1977 and 1980, respectively.
Since 1980, he has been with the Electrical Engineering, Tohoku
University. He is now a professor of the Graduate School of Engineering,
Tohoku University. His current research interests are in the areas of power
electronics and power magnetics. Prof. Ichinokura is a member of the
Magnetic Society of Japan (MSJ), the Society of Instrument and Control
Engineers (SICE), the Institute of Electrical Installation Engineers of
Japan, and IEEE.
VI. CONCLUSION
In this paper, it confirmed by the simulation that axialgap SR motor has torque advantage at flat volume against
conventional radial-gap SR motor. In addition, by making
two rotors, it makes more gap area, the torque at high
current increases. But low current torque decrease and shift
the characteristic to high current area. It seems that if make
more rotors, the characteristic shift to more high current
area. And now we design the 3-phase 18/12 axial-gap SR
motor that has two rotors and one stator. This motor
achieves the target torque. So this axial-gap motor and case
that receives axial electromagnetic force was made. And the
experiment results nearly agree with simulation result. For
future work, we will mount the in-wheel axial-gap SR
motors to the vehicle, and test the driving performance. .
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