a novel power-train using coaxial magnetic gear
DESCRIPTION
paperTRANSCRIPT
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 1/6
Abstract—This paper proposes a novel power-train for power-
split hybrid electric vehicles (HEVs). The key is to integrate two
permanent magnet motor/generators (M/Gs) together with a
coaxial magnetic gear (CMG). By designing the modulating ring
of the CMG to be rotatable, this integrated machine can achieve
both power splitting and mixing, and therefore, can seamlessly
match the vehicle road load to the engine optimal operating
region. With the one-side-in and one-side-out structure and the
non-contact transmission of the CMG, all the drawbacks aroused
by the mechanical gears and chain existing in the traditional
power-train system can be overcome. Moreover, the proposed
power-train possesses the merits of small size and light weight,
which are vitally important for extending the full-electric drive
range of HEVs. The working principle and the design details are
elaborated. By using the finite element method, the
electromagnetic characteristics are analyzed. Finally, system
modeling and simulation are conducted to evaluate the proposed
system.
Index Terms—hybrid electric vehicle, power-train, coaxial
magnetic gear, power-split, Halbach array.
I. I NTRODUCTION
According to the types of the power-train, hybrid electric
vehicles (HEVs) can be generally classified as series HEVs,
parallel HEVs and power-split HEVs, in which the power-split
HEVs combine the benefits of both the parallel types and the
series types, hence offering the merits of ultralow emissions
and high fuel economy [1]-[2]. The power-train of power-split
HEVs is also termed as electronic-continuously variable
transmission (E-CVT) system, which was firstly adopted by
Toyota Prius in 1997 [3]. Then, several derivatives such as the
GM-Allison compound E-CVT, the Timken compound split
E-CVT and the Renault compound split E-CVT were
introduced [4]-[5].
Fig. 1 depicts the basic architecture of the power-train ofToyota Prius. It mainly consists of a planetary gear set, two
electric motor/generators (M/Gs) and two power electronic
inverters. The engine shaft is connected to the planet carrier,
and the rotor shafts of the two M/Gs are attached to the sun
gear and the ring gear, respectively. The ring gear is connected
to the final driveline through a silent chain and a counter gear
to drive the wheels. By controlling the switching modes of the
inverters, multiple power flows among the engine, the M/Gs
and the battery can be achieved. Thus, this power-train can
seamlessly match the vehicle road load to the engine optimal
operating region. However, the reliance on mechanical gears
(including the planetary gear set and the counter gear) and the
silent chain inevitably causes the drawbacks of transmission
loss, gear noise and regular lubrication. In order to overcome
these shortcomings, the combination of two concentrically
arranged machines was proposed to realize power splitting and
mixing for HEVs [6]-[8]. Unfortunately, slip rings and carbon
brushes have to be equipped to inject/withdraw currentsinto/from the rotating armature windings. With no doubt, this
will degrade the reliability of the whole system.
Fig. 1. Architecture of power-train of Toyota Prius
Recently, a high performance coaxial magnetic gear (CMG)
has been proposed [9]-[11]. It can provide non-contact torque
transmission and speed variation using the modulation effectof permanent magnet (PM) fields. Since all the PMs are
involved to transmit torque, the CMG can offer as the torque
density as high as its mechanical counterpart. Thus, it has
promising industrial applications, such as EV drives [12] and
wind power generation [13]. The purpose of this paper is to
develop a novel power-train for power-split HEVs, in which
the CMG is adopted to supersede the mechanical planetary
gear set. By integrating the two PM M/Gs together with the
CMG, the one-side-in and one-side-out mechanical structure
can be achieved so as to eliminate the silent chain and the
A Novel Power-train Using Coaxial Magnetic
Gear for Power-split Hybrid Electric Vehicles
Linni Jian
1, 2
, Guoqing Xu
1, 2
, Yuanyuan Wu
1, 2
, Zhou Cheng
1, 2, 3
, Jianjian Song
1, 2
1 Shenzhen Institutes of Advanced Technology, Chinese Academy of Science, Shenzhen, China2 The Chinese University of Hong Kong, Hong Kong, China
3 Wuhan University of Technology, Wuhan, China
E-mail: [email protected]
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 2/6
counter gear employed in the traditional power-train. Thus, the
aforementioned drawbacks aroused by the mechanical
planetary gear set can be overcome. Moreover, due to the high
torque density of PM machines and the high extent of
integration, the proposed power-train possesses the merits ofsmall size and light weight, which are vitally important for
extending the full-electric drive range of HEVs.
II. SYSTEM CONFIGURATION AND OPERATING PRINCIPLE
Fig. 2 shows the architecture of the proposed power-train.
The key is to integrate the two M/Gs into a CMG. PMs are
mounted on both the inside and outside surfaces of the rotor of
M/G1, which also serves as the inner rotor of the CMG.
Similarly, PMs are mounted on both the inside and outside
surfaces of the rotor of M/G2, which also serves as the outer
rotor of the CMG. The stator of M/G1 is deployed in the inner
bore of the CMG and the stator of M/G2 is located at the periphery of the CMG.
The engine shaft is connected to the modulating ring of the
CMG, and the rotor shaft of the M/G2 is attached to the final
driveline. The working principle of the CMG lies on the
magnetic field modulation of the modulating ring [10]. This
modulating ring consists of several ferromagnetic segments
which are symmetrically deployed in the airspace between its
inner rotor and outer rotor. By defining p1 and p2 as the pole-
pair numbers of the inner and outer rotors of the CMG,
respectively, and n s as the number of ferromagnetic segments
on the modulation ring, the CMG can achieve stable torque
transmission when they satisfy the following relationship:
21 p pn s += (1)
When the modulating ring is fabricated as a stationarycomponent, the CMG can offer fixed-ratio variable speed
transmission, and the corresponding speed relationship is
given by:
22
1
21 ω ω ω r G
p
p−=−= (2)
where ω1, ω2 are the rotational speeds of the outer rotor and
inner rotor respectively, Gr is the so-called gear ratio, and the
minus sign indicates that the two rotors rotate in opposite
directions.
In order to achieve multi-port power flow distribution,
herein, the modulating ring is purposely designed as another
rotational component. The corresponding speed relationship is
governed by:
( ) 01 321 =+−+ ω ω ω r r GG (3)
where ω3 is the rotational speed of the modulating ring.
Without considering the power losses occurred in the CMG, it
yields:
0332211 =++ ω ω ω T T T (4)
(a) (b)
(c)
Fig. 2. Proposed power-train. (a) Architecture. (b) Cross-section of integrated machine. (c) Constitution of integrated machine.
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 3/6
0321 =++ T T T (5)
where T 1, T 2 and T 3 are the developed magnetic torques on the
inner rotor, the outer rotor and the modulating ring,
respectively.
Equations (3)-(5) demonstrate that when the modulating
ring is designed to be rotatable, the CMG possesses similar
functions as the planetary gear set [3], thus offering the
capability of both power splitting and mixing.
TABLE I. POWER SPECIFICATIONS OF M/GS
Rated power of M/G1 15 kW
Rated speed of M/G1 2500 rpm
Rated phase voltage of M/G1 100 V
Rated power of M/G2 30 kW
Rated speed of M/G2 950 rpm
Rated phase voltage of M/G2 100 V
III. MACHINE DESIGN
The gear ratio Gr of the CMG and the power specificationsof the two M/Gs are governed by the expected performance of
the vehicle to be designed. Herein, Gr =2.6 which is the same
with that in Toyota Prius is adopted and the power
specifications of the two M/Gs are listed in Table I.
Fig. 3. Construction of PM poles and rotors.
The pole-pair numbers of the CMG are governed by (2).
Hence, p1 and p2 equal to 5 and 13, respectively, are chosen to
result in the gear ratio of 2.6. Consequently, n s equals 18 can
be deduced from (1). For this integrated machine, thedecoupling of electromagnetic fields in the CMG and the two
M/Gs is very important since it can directly affect the
controllability of the system. Thus, the Halbach arrays are
employed to constitute the PM poles on the two rotors. Asshown in Fig. 3, each pole of PM is divided into 4 and 2
segments for the inner rotor and the outer rotor, respectively.
The corresponding magnetization directions are indicated by
the arrow lines. It is well known that the Halbach arrays canoffer the merit of self-shielding. Therefore, the back iron of
the two rotors can be designed to have small thickness, leading
to save iron material and reduce the system size. Moreover,
non-magnetic shielding cases are sandwiched within the tworotors so as to offer further decoupling. The adoption of
Halbach arrays can also help reduce the cogging torque and
increase the torque transmission capability of the CMG [14].
(a)
(b)
Fig. 4. Slots and winding connections of two M/Gs. (a) M/G1. (b) M/G2.
TABLE II. SPECIFICATIONS OF PROPOSED MACHINE
No. of stator slots of M/G1 12
No. of pole-pairs of M/G1 5
No. of phases of M/G1 3
No. of stator slots of M/G2 24
No. of pole-pairs of M/G2 13
No. of phases of M/G2 3
No. of ferromagnetic segments 18
Length of airgaps 1 mm
Thickness of PMs 6 mm
Thickness of modulating ring 15 mm
Thickness of shielding cases 4 mm
Thickness of back iron of rotors 4 mm
Inside radius of stator of M/G1 30 mm
Outside radius of stator of M/G1 86.5 mm
Inside radius of stator of M/G2 153.5 mm
Outside radius of stator of M/G2 195 mm
Effective axial length 200 mm
Fractional-slots and concentrated windings are adopted in
the stators of the two M/Gs. The concentrated winding refers
to the armature winding that encircles a single stator tooth. It
can offer several significant advantages over the distributed
winding [15]: 1) Reduction in the coil volume and hence the
copper loss in the end region; 2) Shortened machine axiallength; 3) Reduction in machine manufacturing cost; 4)Compatibility with segmented stator structures that makes it
possible to achieve higher slot fill factor values. Moreover, it
has been proven that by adopting fractional-slots and
concentrated windings, the d -axis inductance of the armature
windings can be dramatically increased, which helps achieve
optimal flux weakening of surface-mounted PM machines [16].This feature is advantageous for widening the speed
adjustment range of the M/Gs. Fig. 4 illustrates the slots and
winding connections on the stators of the two M/Gs. Thenumbers of slots are 12 and 24 on M/G1 and M/G2,
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 4/6
respectively. Thus, the slot-per-phase-per-pole (SPP) of 2/5
and 8/13 are achieved for the M/G1 and M/G2, respectively.
IV. FINITE ELEMENT A NALYSIS
The electromagnetic characteristics of the proposedintegrated machine can be analyzed by using the finite element
method (FEM). The specifications are listed in Table II.
Firstly, the electromagnetic field distributions of the
proposed machine under no-load and full-load are illustratedin Fig. 5. It can be observed that almost no flux line goes
through the shielding cases, which demonstrates that due to
the adoptions of Halbach arrays and shielding cases, excellent
field decoupling among the CMG and the two M/Gs can beachieved. With no doubt, this can help improve the
controllability of the whole system. Therefore, the
electromagnetic behavior of each component (namely theCMG and the two M/Gs) can be analyzed and modeled
individually in spite of its integration.
(a) (b) Fig. 5. Electromagnetic field distributions in integrated machine. (a) No load.
(b) Full load.
(a)
(b)
Fig. 6. Back EMF waveforms at rated speeds. (a) M/G1. (b) M/G2.
Secondly, as shown in Fig. 6, the back electromotive force
(EMF) waveforms can be obtained by rotating the two rotors
of M/Gs at their rated speeds. Thirdly, the torque transmission
capability of the CMG can be assessed by calculating the
Maxwell’s stress tensors in the airgaps while keeping the outerrotor and the modulating ring standstill, and rotating the inner
rotor step by step. Fig. 7 depicts the resulting torque-angle
curves. It can be found that the torque-angle curves vary
sinusoidally, in which the maximum torque values denote the pull-out torques. Thus, the pull-out torques of the inner rotor,
outer rotor and modulating ring are 226.7 Nm, 589.7 Nm and
815.9 Nm, respectively. The ratio of the pull-out torques on
the two rotors is 2.601, which has a very good agreement withthe designed gear ratio of 2.6. In addition, the above three
pull-out torques agree well with (5) since there is a phase
difference of π between the curve of the modulating ring andthe curves of the two rotors.
Fig.7. Torque-angle curves of CMG.
Fig. 8. Vehicle road load.
V. SYSTEM MODELING AND SIMULATION
As shown in Fig. 8, the vehicle road load F l consists ofthree main components: aerodynamic drag force F d , rolling
resistance force F r and climbing force F c [2]:
cr d l F F F F ++= (6)
20386.0 AvC F d d ρ = (7)
α cos Mg C F r r = (8)
α sin Mg F c = (9)
where C d is the aerodynamic drag coefficient, ρ is the air
density, A is the vehicle frontal area, v is the vehicle velocity,
M is the vehicle mass, g is the gravitational acceleration, C r is
the rolling resistance coefficient, and α is the angle of incline.
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 5/6
The motive force F available at the wheels is required to
overcome the above road load F l and to drive the vehicle with
an acceleration a. If F l is greater than F , it becomes a
deceleration and a is a negative value. It is expressed as:
( )l
r
F F M k
a −=1 (10)
where k r denotes a correction factor that there is an apparent
increase in vehicle mass due to the inertia of rotational masses.
Fig. 9. Rotational parts of integrated machine (from left to right: inner rotor,
modulating ring and outer rotor).
Fig. 9 indicates the rotational parts of the integratedmachine. Their motions are governed by:
dt
d J T T m
1111
ω =+ (11)
dt
d J T T e
333
ω =+ (12)
dt
d J T T T fd m
2
222
ω =−+ (13)
where J 1, J 2, J 3 are the moments of inertia of inner rotor of
CMG (rotor of M/G1), outer rotor of CMG (rotor of M/G2) andmodulating ring of CMG, respectively; T fd , T m1 and T m2 are the
load torque of final driveline, output torque of M/G1, and
output torque of M/G2, respectively. In addition, assuming thatthe initial torques are zero at the time instant t =0, the
developed magnetic torques in the CMG can be given by [17]:
( ) ⎥⎦⎤
⎢⎣⎡ −+−= ∫
t
s P dt N p pT T 0
3221111 sin ω ω ω (14)
( ) ⎥⎦⎤
⎢⎣⎡ −+−= ∫
t
s P dt N p pT T 0
3221122 sin ω ω ω (15)
( ) ⎥⎦⎤
⎢⎣⎡ −+= ∫
t
s P dt N p pT T 0
3221133 sin ω ω ω (16)
where T p1, T p2 and T p3 are the pull-out torques developed on
the inner rotor, outer rotor and modulating ring, respectively.
On the other hand, the M/Gs can be modeled by following
equations:
k
d
k
s
k
qk
q
k k
qdt
d ir v λ ω
λ ++= (17)
k
q
k
s
k
d k
d
k k
d dt
d ir v λ ω
λ −+= (18)
k
q
k
q
k
q i L=λ (19)
k
pm
k
d
k
d
k
d i L λ λ += (20)
( )k
d
k
q
k
q
k
d k mk ii pT λ λ −=2
3 (21)
where superscript k =1 is for the case of M/G1, and k =2 is for
the case of M/G2, subscript q represents the q-axis component,
and d represents the d -axis component; v, i, L and λ are thestator voltage, stator current, stator inductance and stator flux
linkage; ω s is the rotational speed of the stator flux linkageand λ pm is the flux linkage excited by the PMs.
Fig. 10. Power distributions under FUD 48 driving mode.
For simplicity, the engine is assumed to be shut down and
the vehicle is driven under the full-electric mode. As shown inFig. 10, the FUD 48 driving mode is engaged. The maximum
speed is 48 km/h and the time span is 55 s. The power
distribution between the two M/Gs is defined as: 1) during the
time period of 0-18 s, the M/G1 and M/G2 supply 1/3 and 2/3
of the propulsion power; 2) during the time period of 18-48 s,
the M/G2 supply all the power needed; 3) during the time period of 48-55 s, the M/G1 and M/G2 collect 1/3 and 2/3 of
the dynamic power of the vehicle.
(a)
7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
http://slidepdf.com/reader/full/a-novel-power-train-using-coaxial-magnetic-gear 6/6
(b)
Fig. 11. Simulated responses of rotational speed, electromagnetic torque andstator current. (a) M/G1. (b) M/G2.
Fig. 11 shows the simulated responses of the M/G1 and
M/G2. It can be observed that the rotational speeds of the two
M/Gs are with opposite directions, and the ratio of theirquantities is consistent with the gear ratio. This is in
accordance with the speeds relationship constrained by (3)
since the engine is shut down during the whole process.
Moreover, during the acceleration stage (0-18 s), the torque
and speed of each M/G are with the same direction, whichmeans that the two M/Gs deliver power to the driveline
simultaneously. Then, when the vehicle runs at the high speed
range (18-48 s), the torque of M/G1 is zero while the M/G2 solely supplies the power needed. Finally, during thedeceleration stage (48-55 s), the torque and speed of each M/G
are with opposite directions, which means that the two M/Gsrecycle the braking power of the vehicle, namely regenerative
braking.
VIII. CONCLUSIONS
In this paper, a new power-train for power-split HEVs has
been designed, analyzed and modeled. The key is to integratetwo PM M/Gs together with a CMG. First, by purposely
designing the modulating ring of the CMG to be rotatable, this
integrated machine can achieve both power splitting and
mixing, and therefore can seamlessly match the vehicle roadload to the engine optimal operating region. Then, with the
one-side-in and one-side-out structure and the non-contact
transmission of the CMG, all the drawbacks aroused by the
mechanical gears and chain existing in the traditional power-
train can be overcome. Finally, the proposed power-trainsystem possesses the merits of small size and light weight,
which are vitally important for extending the full-electric drive
range of HEVs.
R EFERENCES
[1] M. Ehsani, Y. Gao, S.E. Gay and A. Emadi, Modern Electric, Hybrid
Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design.Boca Raton: CRC Press, 2005.
[2] C. C. Chan and K. T. Chau, Modern Electric Vehicle Technology.
Oxford: Oxford University Press, 2001.
[3] S. Sasaki, “Toyota’s newly developed hybrid powertrain,” IEEE International Symposium on Power Semiconductor Devices & ICs, June
1998, pp. 17-22.
[4] K. T. Chau and C. C. Chan, “Emerging energy-efficient technologies for
hybrid electric vehicles,” Proceedings of the IEEE , vol. 95, no. 4, pp.821−835, Apri. 2007.
[5] J. M. Miller, “Hybrid electric vehicle propulsion system architectures of thee-CVT type,” IEEE Trans. on Power Electron., vol. 21, no. 3, May 2006,
pp. 756-767.[6] M. J. Hoeijmakers and J. A. Ferreira, “The electric variable
transmission,” IEEE Trans. Ind. Appl., vol. 42, no. 4, July/Aug. 2006, pp.1092-1100.
[7] L. Xu, Y. Zhang and X. Wen, “Multioperational modes and controlstrategies of dual-mechanical-port machine for hybrid electrical
vehicles,” IEEE Trans. Ind. Appl., vol. 45, no. 2, pp.747−755, Mar./Apr.,
2009.
[8] S. Eriksson and C. Sadarangani, “A four-quadrant HEV drive system,”
IEEE Vehicular Technology Conference, Sep. 2002, pp. 1510-1514.
[9] K. Atallah and D. Howe, “A novel high performance magnetic gear,”
IEEE Trans. Magn., vol. 37, no. 4, pp. 2844−2846, Jul. 2001.
[10]
K. Atallah, S. D. Calverley and D. Howe, “Design, analysis andrealisation of a high-performance magnetic gear,” IEE Proc. - Electric Power Appl., vol. 151, no. 2, pp. 135-143, Mar. 2004.
[11] L. Jian, and K. T. Chau, “Analytical calculation of magnetic field
distribution in coaxial magnetic gears,” Progress in Electromagnetics
Research, vol. 92, pp. 1-16. 2009.[12] K. T. Chau, D. Zhang, J. Z. Jiang, C. Liu and Y. J. Zhang, “Design of a
magnetic-geared outer-rotor permanent-magnet brushless motor for
electric vehicles,” IEEE Trans. Mag., vol. 43, no. 6, pp. 2504-2506. Jun.
2007.
[13] L. Jian, K. T. Chau and J. Z. Jiang, “A magnetic-geared outer-rotor permanent-magnet brushless machine for wind power generation,” IEEE
Trans. Ind. Appli., vol. 45, no. 3, May/June 2009.
[14] L. Jian, K. T. Chau, Y. Gong, J. Jiang, C. Yu and W. Li, “Comparison of
coaxial magnetic gears with different topologies,” IEEE Trans. Magn.,
vol. 45, no. 10, pp. 4526−4529, Oct. 2009.
[15] Ayman M. EL-Refaie, “Fractional-slot concentrated-windings
synchronous permanent magnet machines: opportunities andchallenges,” IEEE Trans. Ind. Electron., vol. 57, no. 1, pp. 107-121, Jan.
2010.[16] Ayman M. EL-Refaie and T. M. Jahns, “Optimal flux weakening in
surface PM machines using fractional-slot concentrated windings,” IEEE Trans. Ind. Appli., vol. 41, no. 3, pp. 709-800, May/June 2005.
[17] L. Jian and K. T. Chau, “A coaxial magnetic gear with Halbach permanent magnet arrays,” IEEE Trans. Energy Conv., vol. 25, no. 2,
June 2010.