The 2nd Power and Energy Conversion Symposium (PECS 2014)
Melaka, Malaysia
12 May 2014
Performance Analysis of 12S-10P Hybrid-Excitation
Flux Switching Machines for HEV
Nurul ‘Ain Jafar, Siti Khalidah Rahimi, Siti Nur Umira Zakaria and Erwan Sulaiman,
Department of Electrical Power Engineering, Faculty of Electrical and Electronic Engineering,
Universiti Tun Hussein Onn Malaysia, Locked Bag 101,
Batu Pahat, Johor, 86400 Malaysia
aienjafar@yahoo.com and erwan@uthm.edu.my
Abstract- Research and development on flux switching
machines (FSMs) have been emerged as an attractive machine
recently due to their unique characteristics of all flux sources
located on the stator allowing a simple and robust rotor structure
whilst providing high torque and power density suitable for
various applications. They can be categorized into three groups
that are permanent magnet (PM) FSM, field excitation (FE)
FSM, and hybrid excitation (HE) FSM. Both PMFSM and
FEFSM has only PM and field excitation coil (FEC), respectively
as their main flux sources, while HEFSM combines both PM and
FECs. In this research, design and performance of a three phase
12S-10P HEFSM in which the field excitation coil (FEC) located
external to the PM are investigated. Initially, the design
procedures of the proposed machine including parts drawing,
materials, condition and properties setting are explained. Then,
coil arrangement test is analyzed to all armature coil slots to
confirm the polarity of each phase. Moreover, the flux interaction
analysis is performed to investigate the flux capabilities at
various current densities. Finally, torque and power
performances are investigated at various armature and FEC
current densities.
I.
INTRODUCTION
A demand for vehicles using electrical propulsion drives is
getting higher and higher from the standpoints of preventing
global warming and saving fossil fuel recently. Hybrid
Electric Vehicles (HEVs) which combine battery-based
electric machines with an internal combustion engine (ICE)
have been manufactured to reduce the pollutant emissions that
can cause global warming due to demand of personal
transportation. As viable candidates of electric machines for
HEV drives, there are four major types of electric machines,
namely, DC machines, induction machines (IMs), switch
reluctance machines (SRMs), and permanent magnet
synchronous machines (PMSMs) as illustrated in Fig. 1. DC
machines should be widely accepted for electric vehicles (EV)
and HEV drives because they can use a battery as a DC-supply
as it is and have an advantage of simple control principle
based on the orthogonal disposition of field and armature
mmf. However, the essential problem of dc drives, which is
caused by commutator and brush, makes them less reliable
and unsuitable for a maintenance-free drives [1-5].
IMs are widely accepted as the most potential candidate of
non-PM machine for the electric propulsion motor in HEVs
due to their reliability, ruggedness, low maintenance, low cost
and ability to operate in hostile environments. Moreover, low
efficiency at low speed-light load region of HEV drives due to
the secondary loss may essentially degrade fuel consumption
or system efficiency. Furthermore, IM drives have some
drawbacks such as necessity of high electric loading to realize
high torque density and low power factor, which are more
serious for the large power motor and that push them out from
the candidate race for HEV electric propulsion system [6-7].
(a) DC
(c) IM
(b) IPMSM
(d) SRM
Fig.1. Four major candidates of electric machines for HEV drives
SRMs are gaining much interest as non-PM machine and
are recognized to have a potential for HEV drive applications.
They have several advantages such as simple and rugged
construction, low manufacturing cost, fault tolerant capability,
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The 2nd Power and Energy Conversion Symposium (PECS 2014)
Melaka, Malaysia
12 May 2014
simple control and outstanding torque speed characteristic.
Thus, SRM can inherently operate with an extremely long
constant-power range. Nevertheless, several disadvantages
such as vibration and acoustic noise, larger torque pulsation,
necessity of special converter topology, excessive bus current
ripple and electromagnetic interference noise generation make
them unattractive [8-10].
As one of the vehicles, many automotive companies have
commercialized Hybrid Electric Vehicles (HEVs) in which
Interior Permanent Magnet Synchronous Motors (IPMSM)
have been employed as a main traction motor in terms of high
torque and or power density, and high efficiency over most of
operating torque-speed range. This is due to the restriction of
motor size to ensure enough passenger space and the
limitation of motor weight to reduce fuel consumption. As an
example, the historical progress in the power density of main
traction motor installed on Toyota HEV show that the power
density of each motor employed in Lexus RX400h ‟05 and
GS450h “06 has been improved approximately five times and
more, respectively, compared to that installed on Prius ‟97
[11-12]. Even though the torque density of each motor has
been hardly changed, a reduction gear has enabled to elevate
the axle torque necessary for propelling the large vehicles such
as RX400h and GS450h. As one of effective strategies for
increasing the motor power density, the technological
tendency to employ the combination of a high-speed machine
and a reduction gear would be accelerated.
With the significant achievements and improvements of
permanent magnet materials and power electronics devices,
the brushless machines excited by PM associated with FEC
are developing dramatically. This type of machine is called
hybrid excitation machine (HEM) that can be classified into
four categories. The first type consists of both PM and FEC at
rotor side such as combination rotor hybrid excitation (CRHE)
machine and synchronous/PM hybrid AC machine [13]. The
second type consists of PM in the rotor while FEC is in the
stator [14]. The third type consists of PM in the rotor while the
FEC is in the machine end [15-16]. Finally, the fourth type of
HEM is the machine, which has both PM and FEC in the
stator [17-18]. It should be emphasized that all HEMs
mentioned in the first three have a PM in the rotor and can be
categorized as “hybrid rotor-PM with field excitation
machines” while the fourth machine can be referred as “hybrid
stator-PM with field excitation machines”. Based on its
principles of operation, the fourth machine is also known as
“hybrid excitation flux switching synchronous machine
(HEFSSM)” which is getting more popular recently.
HEFSSM with all active parts, namely permanent magnet,
DC field excitation coil and armature coil located on the stator
has an advantage of robust rotor structure being similar to the
switch reluctance machine (SRM), which makes it more
suitable for high-speed drive applications compared to “hybrid
rotor-PM with field excitation machines” and conventional
IPMSM [19]. Moreover, since all active parts are located in
the stator body, it is expected that a simple cooling system can
be used for this machine compared with a water jacket system
used in IPMSM. In addition, the additional DC FEC can also
be used to control flux with variable flux capabilities. Various
combinations of stator slot and rotor pole for HEFSSM have
been developed for high-speed application. For example,
12Slot-10Pole HEFSSM has been proposed such as in [20-21].
However, the proposed machine has a separated PM and Ctype stator core that makes it difficult to manufacture, and the
design is not yet optimized for HEV applications while the
machine in [22] has a limitation of torque and power
production in high current density condition.
In this paper, based on the topology of HEFSSM discussed
above, a combination of 12Slot-10Pole HEFSSM that
proposed for HEV applications is demonstrate in Fig. 2. The
proposed machine is composed of 12 PMs and 12 FECs
distributed uniformly in the midst of each armature coil. In
this 12Slot-10Pole machine, the PMs and FECs produce six
north poles interspersed between six south poles. The threephase armature coils are accommodated in the 12 slots for
each 1/4 stator body periodically. As the rotor rotates, the
fluxes generated by PMs and mmf of FECs link with the
armature coil alternately. For the rotor rotation through 1/10 of
a revolution, the flux linkage of the armature has one periodic
cycle and thus, the frequency of back-emf induced in the
armature coil becomes ten times of the mechanical rotational
frequency. Generally, the relation between the mechanical
rotation frequency and the electrical frequency for this
machine can be expressed as
fe  Nr . fm
where fe is the electrical frequency, fm is the mechanical
rotation frequency and Nr is the number of rotor poles
respectively.
222
Armature coil
Stator
Rotor
Shaft
Permanent
Magnet
Fig. 2. Proposed design of 12S-10P HEFSM
FEC
The 2nd Power and Energy Conversion Symposium (PECS 2014)
Melaka, Malaysia
12 May 2014
II.
Fig.3. principle operation of HEFSM (a) θe=0° (b) θe=180° flux moves
from stator to rotor (c) θe=0° and (d) θe=180° flux moves from stator to rotor
OPERATING PRINCIPLE OF HEFSM
The term flux switching is created to describe machines in
which the stator tooth flux switches polarity following the
motion of a salient pole rotor. All excitation sources are
located on the stator with the armature coils and FECs
allocated to alternate stator teeth. The advantages of this
machine are easy cooling of all active parts in the stator and
robust rotor structure that makes it better suitability for highspeed application compared to conventional IPMSM. On the
other hand, the field excitation can be used to control flux with
variable flux capabilities.
Generally, the FSM can be categorized into three groups
that are permanent magnet flux switching motor (PMFSM),
field excitation flux switching motor (FEFSM), and hybrid
excitation flux switching motor (HEFSM). The operating
principle of the proposed HEFSM is illustrated in Fig 3, where
the red and blue line indicate the flux from PM and FEC,
respectively. In Fig 3 (a) and (b), since the direction of both
PM and FEC fluxes are in the same polarity, both fluxes are
combined and move together into the rotor, hence producing
more fluxes with a so called hybrid excitation flux.
Furthermore in Figure 3 (c) and (d), where the FEC is in
reverse polarity, only flux of PM flows into the rotor while the
flux of FEC moves around the stator outer yoke which results
in less flux excitation. As one advantage of the DC FEC, the
flux of PM can easily be controlled with variable flux control
capabilities as well as under field weakening and or field
strengthening excitation.
III.
DESIGN RESTRICTIONS AND SPECIFICATIONS
The design restrictions and specifications of the proposed
HEFSM for HEV applications are listed in TABLE 1. The
electrical restrictions related with the inverter such as
maximum 650V DC bus voltage and maximum 360A inverter
current (Arms) are set. By assuming only a water cooling
system is employed as the cooling system for the machine, the
limit of the current density is set to the maximum of
30Arms/mm2 for armature winding and 30A/mm2 for FEC. The
outer diameter, the motor stack length, the shaft radius and the
air gap of the main part of the machine design being 264mm,
84mm, 30mm and 0.7mm, respectively. In the other hand, the
target maximum torque and power is 303Nm and 123kW,
respectively. The weight of PM is limited to 1.3kg.
Commercial FEA package, JMAG-Studio ver.13.0, released
by JRI is used as 2D-FEA solver for this design.
TABLE 1. Design Restrictions and Specifications
HEFSM
650
360
30
Items
Max. DC-bus voltage inverter (V)
Max. inverter current (Arms)
Max. current density in armature
winding, Ja (Arms/mm2)
Max. current density in excitation
winding, Je (A/mm2)
Stator outer diameter (mm)
Motor stack length (mm)
Shaft radius (mm)
Air gap length (mm)
PM weight (kg)
Max. torque (Nm)
Max. power (kW)
30
264
84
30
0.7
1.3
303
123
IV. DESIGN RESULTS AND PERFORMANCES BASED
ON FINITE ELEMENT ANALYSIS
(a)
(b)
(c)
(d)
A. Coil Arrangement Test
In order to validate the operating principle of the HEFSM
and to set the position of each armature coil phase, coil
arrangement tests are examined in each armature coil
separately. Initially, all armature coils and FEC are wounded
in counter clockwise direction. The flux linkages of each coil
are evaluated and the armature coil phases are classified
according to conventional 120° phased shifted between all
phases or called as U, V, and W. Fig. 4 displays the flux
linkage at condition of PM with high FEC current density.
Moreover, the magnetic flux linkage in all conditions is in the
same phase with essential sinusoidal waveform. The
maximum amplitude of flux linkage generated under this
condition is approximately 0.11619Wb.
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The 2nd Power and Energy Conversion Symposium (PECS 2014)
Melaka, Malaysia
12 May 2014
Flux[Wb]
U
V
amplitude of induced voltage generated is also increased up to
340.90V and becoming more distorted.
W
0.15
Je=0 A/mm2
Je=30 A/mm2
Emf[V]
0.1
Je=15 A/mm2
400
0.05
300
200
0
0
30
60
90 120 150 180 210 240 270 300 330 360
100
-0.05
Electric Cycle[°]
0
0
-0.1
30
60
90 120 150 180 210 240 270 300 330 360
-100
-0.15
Electric
cycle[°]
-200
Fig. 4. UVW phase of magnetic flux
-300
B. Flux Linkages
The flux path of the proposed HEFSM produced by both
PM and FEC with maximum current density Je of 30 A/mm2
is demonstrated in Fig. 5. From the figure, it is obvious that
the flux flow from stator to the rotor and return through
nearest rotor pole to complete one flux cycle. Moreover, most
of the fluxes are distributed and saturated at rotor air gap.
Nevertheless, huge number of fluxes flow into rotor in high
gap flux density can lead the unnecessary flux leakage and
flux cancellation. Besides, more flux flow into the rotor can
cause higher back-emf which is unnecessary for the system.
-400
Fig. 6. Back emf at 1200r/min rated speed
D. Cogging Torque
The cogging torque profile of the proposed HEFSM under
the conditions of PM only, FEC only and combination of PM
and FEC is depicted in Fig. 7. Obviously, 6 cycles of cogging
torque are generated to complete one electric cycle. The peakpeak cogging torque of PM only and PM with maximum FEC
current density Je of 30 A/mm2 are approximately 32.244Nm
and 28.187Nm, respectively. It is clear that the cogging torque
of PM only is slightly higher than PM with maximum FEC
current density due to weakening effect of the flux linkage.
Since the cogging torque values produced must not exceed
10% of the average torque, the generated cogging torque of
the proposed HEFSM is unnecessary for the performance of
the machine due to production of high vibration and noise.
Therefore, further design refinement and optimization should
be conducted to get the target torque and reduce the cogging
torque.
Torque
[Nm]
20
PM Only
Je=30
PM + Je=30
15
Fig. 5. Flux line of PM with maximum current density, Je of
30A/mm2
10
C. Induced Voltage
The induced voltage of the proposed machine is generated
at the speed of 1200 r/min. Fig. 6 illustrates the open circuit
waveforms of the proposed design for different current density
conditions. The voltage with Je of 0A means that the induced
voltage is generated from flux of PM only. The amplitude of
fundamental component is 258.23V and is slightly sinusoidal.
When the Je is varied from 0 A/mm2 to 15 A/mm2, the
amplitude of induced voltage is increased to 301.10V. This is
due to the strengthening effect by the additional generated flux
of FEC. With increasing Je to the maximum of 30 A/mm2, the
224
5
0
0
30
60
90 120 150 180 210 240 270 300 330 360
-5
-10
Electric Cycle[°]
-15
-20
Fig. 7. Cogging torque
The 2nd Power and Energy Conversion Symposium (PECS 2014)
Melaka, Malaysia
12 May 2014
E. Torque vs FEC current density, Je at various armature
current densities, Ja characteristics
The drive performances of the initial design machine in
terms of torque versus FEC current density characteristics of
the proposed motor is plotted in Fig. 8. The graph shows that
with increasing in Je, the torque also increase linearly with the
same pattern at 0Arms/mm2 until 15Arms/mm2. Meanwhile,
similar torque characteristic is obtained at 20Arms/mm2,
25Arms/mm2 and 30Arms/mm2 in which the torque pattern is
constant. The maximum torque is approximately 183.32Nm
when Ja of 20Arms/mm2 and Je of 30A/mm2 are set. This is due
to low armature coil flux that limits the force to move the
rotor. To overcome this problem, design improvement and
optimization will be conducted in future.
Torque[Nm]
250
200
150
184.70Nm
100
50
Electric
cycle[°]
0
0
30
60
90 120 150 180 210 240 270 300 330 360
Fig. 9. Instantaneous torque characteristics
Ja=0
Ja=15
T[Nm]
Ja=5
Ja=20
Ja=10
Ja=25
V.
CONCLUSIONS
In this paper, performances analysis of the proposed
HEFSM for HEV application has been presented. The coil
arrangement test have been carried out to identify each phase
of armature coil and to locate the initial position of the rotor.
The performances of the proposed motor such as flux
capability, cogging torque and back-emf characteristic have
also been investigated and demonstrated. The initial design
performances demonstrate that the design refinement is
required to achieve the target performances. Thus, by further
design modification and optimization, the target performances
HEFSM are expected can be achieved.
200
180
160
140
120
100
80
60
40
Je A/mm2
20
0
0
5
10
15
20
25
30
Fig. 8. Torque vs Je at maximum Ja
F. Instantaneous torque characteristic at maximum current
density condition
Furthermore, the instantaneous torque characteristics at
maximum torque condition in which the FEC current density
and armature current density are set to 30A/mm2 and
30Arms/mm2, respectively as demonstrated in Fig. 9. The
average torque reaches 184.70Nm with the peak-peak torque
of approximately 43.26Nm. Since the peak-peak torque is
higher than required minimum value, estimation of flux
linkage which contributes to high cogging torque will be
investigated and optimized.
VI.
ACKNOWLEDGEMENT
This work was supported by UTHM Scholarship from
Centre of Graduate Studies, Universiti Tun Hussein Malaysia
(UTHM), Batu Pahat, Johor.
VII.
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