Abstract—Thi s 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 p ower-split HE Vs, in which the po wer-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 of Toyota 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 currents into/from the rotating armature windings. With no doubt, this will degrade the reliability of the whole system. Fig. 1. Architecture of power-train of To yota 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 effect of 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 , Guoq ing Xu 1, 2 , Yuan yuan Wu 1, 2 , Zhou Cheng 1, 2, 3 , Jianjian Song 1, 2 1 Shenzhen Institutes of Advanced Technology, Chinese Academy of Science, Shenzhen, China 2 The Chinese University of Hong Kong, Hong Kong, China 3 Wuhan University of Technology, Wuhan, China E-mail: [email protected]c.cn
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7/21/2019 A Novel Power-train Using Coaxial Magnetic Gear
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
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.
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