A Novel Double-Stator Double-Rotor Brushless Electrical Continuously Variable Transmission System

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013 3909

A Novel Double-Stator Double-Rotor Brushless Electrical Continuously

Variable Transmission System

Shuangxia Niu, S. L. Ho, and W. N. Fu

The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

A novel double-stator, double-rotor brushless electrical continuously variable transmission (E-CVT) system is proposed. This machineincorporates an inner stator which can fully utilize the space inside the inner rotor so as to reduce the volume of the machine. Thisdesign is based on magnetic flux modulating and magnetic gear effect to make the E-CVT system work in different operating modes.Different control modes of this proposed machine are also discussed and analyzed, including pure electric mode, hybrid mode, andbattery charging mode. Time-stepping finite element method (TS-FEM) is used to analyze the dynamic performance of the machine.TS-FEM simulation results are reported to verify the validity of the proposed design.

Index Terms— Electric machine, electrical continuously variable transmission, finite element method, vernier machine.

I. I NTRODUCTION

BECAUSE of concerns of energy crisis and environmental

sustainability, hybrid electric vehicles (HEVs), which is acompromise of gasoline vehicles and pure electric vehicles, are

attracting much interests among researchers around the world.

The most prominent merit of HEVs is their fuel economy and

energy ef ficiency, most of which is by virtue of their highly ef fi-

cient power-split systems. HEV system uses electrical continu-

ously variable transmission (E-CVT) system for power splitting

in order to realize energy allocation, reduce the fuel consump-

tion and improve the energy ef ficiency of the whole system [1].

A wealth of E-CVT systems have been designed, developed

and applied in HEVs. For the Toyota Prius, its E-CVT propul-

sion system comprises of a planetary gear set and two elec-

tric motors/generators. Through inverters, the electromechan-

ical power flows ef ficiently among the engine, the motor/gener-ators (M/Gs) and the battery. The power in the internal combus-

tion engine (ICE) and electric machines can be effectively split

and combined together [2]. However, in that E-CVT system,

the planetary gear, which is a key element of the system, is a

mechanical device which has inevitable mechanical problems

such as frictional loss, high maintenance and audible noise. To

solve this problem, a series of gearless E-CVT propulsion sys-

tems are developed. The key is to use a double-rotor machine

to replace the mechanical gear to realize the power split and

combination [3]. At present, induction machine and permanent

magnet (PM) machine based double-rotor integrated E-CVT

propulsion systems have been developed. Although these de-

signs can effectively alleviate the mechanical problems and im-

prove the system ef ficiency, they require carbon brushes and slip

rings which reduce the reliability of the system. In order to elim-

inate the brushes, a new gearless and brushless E-CVT propul-

sion system using a double-stator PM brushless machine has

been developed [4]. Even though that design requires no carbon

brush and slip ring, an additional induction machine has to be

Manuscript received October 28, 2012; revised January 04, 2013, January 18,2013, andJanuary31, 2013; accepted February 14,2013. Date of current versionJuly 15, 2013. Corresponding author: S. Niu (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2013.2248347

cascaded with the PM unit to realize the power split. The main

drawback of that machine is bulkiness.

In recent years, magnetic gear system is emerging as a re-

placement of planetary gears to realize contactless transmissionof torque and speed [5]. In [6], a coaxial magnetic gear inte-

grated E-CVT system with a compact structure and improved

torque density has been studied. However, the complicated

structure of the machine which involves four airgaps and

three rotating parts are basically impractical. In [7], a compact

E-CVT system with two rotors and one stator is presented.

Since two sets of stator windings are housed inside one set of

slots in the stator, the outside diameter of machine needs to be

enlarged. In that design the secondary winding of the machine

and one rotor constitute a mutlipole PM machine. However,

the main demerit of that structure is the additional loss because

the rotor is mechanically connected to the ICE, not directly to

wheels of the vehicle.In this paper, based on magnetic gear effects, a novel E-CVT

system with two mechanical ports and two electrical ports is

proposed. The core component of this system is a brushless

double-stator double-rotor electric machine and the electrical

power transmission is totally contactless. There is no mechan-

ical problem arising from brushes in this design when compared

with the conventional ones. In essence, this is a fully magneti-

cally and electrically integrated design and the structure is com-

pact and its torque density is high. The shortcomings of the

machine in [7] are overcomed. The working principle of this

machine is described and the different control modes of this

system are also introduced. The performance including the static

and dynamic operations of the system is analyzed using a cir-cuit-field-motion coupled time-stepping finite element method

(CFM-TS-FEM).

II. SYSTEM CONFIGURATION AND WORKING PRINCIPLE

A. E-CVT System

Fig. 1 shows the configuration of the proposed E-CVT

system. The core of the system consists of a brushless

double-stator, double-rotor PM machine, two inverters and

an energy storage system (battery or ultracapacitor). These

inverters are connected to the inner and outer stator windings

with the energy storage system. As for the rotors, the inner

0018-9464/$31.00 © 2013 IEEE



3910 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Fig. 1. Proposed E-CVT system in HEVs.

one is connected to the shaft of the internal combustion engine

(ICE) and the outer one is connected to the final driveline.

During downhill or braking, when the engine power exceeds

the power required by the HEV, the surplus power from the en-

gine is stored in a battery. The outer stator and rotor serve as

a generator to convert the surplus mechanical power from the

wheels into electrical power which is stored in the battery or ul-

tracapacitor. During uphill or acceleration, energy from the bat-

tery is drawn out to provide additional power to assist the driv-

eline. This system can also realize full electric launch to save

fuel for journeys with many traf fic stops.

B. Machine Structure and Operating Principle

The salient feature of this E-CVT system is its double-rotor

double-stator PM machine. It is an integration of a multi-pole

fractional-slot PM (MFPM) machine and a double-rotor Vernier

PM (DVPM) machine, as shown in Fig. 2. The outer rotor is

commonly shared by the two machines. In contrast to traditional

PM machines, the PMs in the rotors have the same direction of

magnetization (radially outward), as indicated in Fig. 2(b). Non-

magnetized ferrite poles are equally inset between each pair of

PMs. Considering these two machines individually, the oper-

ating principle and design guideline are given below.

The inner stator, inner rotor and outer rotor constitute a

DVPM machine. In Vernier machines, the gear effects areobtained through magnetic field modulation in the two air-

gaps. The operating principle of the proposed design is almost

the same as that of magnetic gear, except that the inner ro-

tating magnetic field is provided by the stationary 3-phase

winding. Based on magnetic gear operating principle, for the

double-rotor Vernier PM machine, the fundamental rule is

(1)

where; is the PM number in the inner rotor and is

that in the outer rotor; is the inner stator winding pole-pair

number. For the proposed Vernier PM machine, the parame-

ters are , , and . By virtue of the

“magnetic gear effect,” the flux-modulation poles can modulate

Fig.2. Configuration of theproposedmachine.(a) Machine structure. (b)Frontview.

the low harmonic component of the inner airgap magnetic field

upon the specific high harmonic components in the outer airgap.

To reduce the end winding length, the inner stator is designed

with 10 poles in 9 slots, which is a traditional fractional-slot

concentrated winding combination.

In the DVPM machine, the speed of the outer rotor is denotedas , the speed of the inner rotor is , and the magnetic flux

rotating speed of the inner stator winding is . Since there are

three rotating magnetic fields, the relationship among its rota-

tional speeds and pole-pair number is similar to that of the plan-

etary gear, which is governed by

(2)

From (1) and (2), the rotating speed of working flux of the stator

winding can be expressed as

(3)

The frequency of stator winding supply can be expressed as

(4)

The outer stator and outer rotor constitute a MFPM machine

and the phase winding is designed with a fractional slot con-

centrated winding to reduce the slot number and end winding

length. In contrast to the machine in [7], the outer rotor is di-

rectly connected to the wheels of the vehicle. The advantage is

that mechanical power can be transmitted to the wheels directly.

The relationship between the stator winding pole-pair number

and slot number is expressed as

(5)



NIU et al.: A NOVEL DOUBLE-STATOR DOUBLE-ROTOR BRUSHLESS ELECTRICAL CONTINUOUSLY VARIABLE TRANSMISSION SYSTEM 3911

where, is the stator slot number and is the winding

pole-pair number. In this proposed machine, the outer stator pa-

rameters are , and is the largest common

divisor of and .

The advantages of the proposed structure of this double-rotor

double-stator brushless PM machine are summarized as the fol-

lowing:

1) Compact structure The compact structure can fully utilizethe limited space within the outer rotor. The volume of the

unit is reduced and the torque density is improved.

2) Flexible controllability The inner DVPM machine with

two rotating parts can be controlled in a manner similar to

that for planetary gear. The outer PM machine may serve

as either a generator to return energy to the battery or work

as a motor to assist the ICE to drive the driveline.

3) Short end winding Compared with conventional stator

windings, the “concentrated winding” structure can ef-

fectively reduce the end winding length to reduce copper

losses.

4) Magnetic gear effect Flux modulation caused by magneticgear effect in the machine can reduce the rated speed of the

machine while producing a relatively high torque.

III. A NALYSIS AND POTENTIAL APPLICATIONS

A. CFM-TS-FEM

A two-dimensional (2-D) CFM-TS-FEM coupling with elec-

tric circuit equations is used to simulate the operation of the

motor. The basic equations of transient magnetic field-circuit

coupled problem can be summarized as [8]

(6)

(7)

where; is the depth of the model in the -direction (axial di-

rection); is the symmetry multiplier; is the reluctivity of ma-

terial; is the conductivity; is the -component of the mag-

netic vector potential; is the polarity ( 1 or 1) to repre-

sent, respectively, the forward paths or return paths; is the

total cross-sectional area of the region occupied by the winding

in the solution domain; is the total conductor number of this

winding; is the number of parallel branches in the winding;, and are, respectively, the d.c. resistance, branch cur-

rent and branch voltage of the winding.

B. FEM Analysis Results

Using CFM-TS-FEM, the steady-state and transient perfor-

mance of the machine is analyzed. Fig. 3 depicts the back-emf

waveforms induced in the inner and outer stator windings with

an outer rotor speed of 545.5 rpm and an inner rotor speed of

444.4 rpm. The electrical degree of inner stator is referred to

5 pole pairs and that of outer stator is referred to 22 pole pairs.

The frequencies of outer and inner windings are 50 and 100 Hz,

respectively. Simulation results agree well with the description

given in Section II. The two sets of stator windings are con-

nected independently and each can be flexibly controlled by in-

Fig. 3. Back EMF waveforms. (a) Outer stator. (b) Inner stator.

Fig. 4. Radial flux densities and their harmonic spectra. (a) In outer airgap. (b)In middle airgap. (c) In inner airgap.

verters. The back EMF waveform in each phase is affected by

two sets of PMs and one set of stator winding, and the inter-

actions of these magnetic fields will result in slight imbalances

in the back EMF waveform of each phase. Fig. 4 shows the ra-

dial flux densities and their harmonic spectra in the airgaps with

only PM excitation. In the outer airgap, the highest 22 pole-pair

field harmonic interacts with the 22-pole PM outer rotor. The

22 and 27 pole-pair harmonic components are dominant in the

middle airgap. After modulation, the 22 pole-pair harmonic ishighly attenuated and only the 5 pole-pair and 27 pole-pair har-

monic components are prominent. Operationally the 5 pole-pair

harmonic component interacts with the stator windings.

C. Control Mode and Analysis

E-CVT technology assists HEVs to provide good fuel

economy and low emission. For different driver demands

it has different control strategies for different power combina-

tion and split. In this paper, the performance of E-CVT machine

at different control mode is analyzed with CFM-TS-FEM.

1) Pure Electric Drive Mode: HEV is best driven at pure

electric drive mode to reduce pollutant emission in urban area.

The rated power for this E-CVT is 2 kW, its rated torque is 35

Nm and rated speed is 545.5 rpm. The outer MFPM machine



3912 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013

Fig. 5. Torque waveforms during pure electric mode.

Fig. 6. Torque output versus phase current in pure electric mode.

serves as a motor to convert the electric power from battery

to mechanical output at the wheels. When the outer rotor runs

at 545.5 rpm, and there is no mechanical power from ICE, the

torque waveform on the outer rotor is around 35 Nm, as shown

in Fig. 5. The relationship between stator current and torque

output on the outer rotor is given in Fig. 6.2) Hybrid Drive Mode: During starting up or climbing, the

power demand for the traction driver is high, hence power from

the battery should augment the mechanical energy from ICE to

drive the wheels. For this machine, the inner DVPM machine

should be assisting, and coordinated with, the outer MFPM ma-

chine to drive the outer rotor. To drive the outer rotor at 545.5

rpm, the outer winding needs to be fed with a current of 200

Hz and, assuming the inner rotor speed is 222.2 rpm, the inner

winding should be fed with a current at 100 Hz. The torque

output on outer rotor and the torque input to inner rotor are

given in Fig. 7. Torque input from ICE is around 22 Nm and

torque output from the outer rotor is 53 Nm when the vehicle

is climbing. Cogging torque is approximately related to the in-verse of the smallest common multiple of the PM pole number

and stator slot number. Here, the outer PM pole number is 22,

slot number is 48, so the torque ripple is small. With 27 PM

poles and 9 slots in the inner stator, the torque ripple is however

larger. Skewing of the slots is necessary to reduce the torque

ripples.

3) Battery Charging Mode: During regenerative braking,

downhill or idling time, the mechanical energy at the wheels

or ICE is converted to electric energy and stored in the bat-

tery. With inner and outer rotor rotating at different speeds, the

Fig. 7. Torque waveforms during hybrid operating mode.

induced voltages in the inner and outer stator windings are as

shown in Fig. 3. Through the inverter, this energy can be used

to charge the battery or ultracapacitors for further uses.

IV. CONCLUSION

A novel double-stator double-rotor E-CVT system utilizing

magnetic gear effect to modulate the magnetic flux has been pro-

posed to realize different E-CVT operating modes. The salient

advantage of the design is that two rotors and two stators are

incorporated within one machine frame and one inner stator is

housed within the space inside the inner rotor. The structure is

compact. TS-FEM is used to analyze the performance of the

machine, verify its working principle and its validity in prac-

tical applications.

ACKNOWLEDGMENT

This work was supported by the Research Grant Councilof the Hong Kong SAR Government under projects PolyU

5176/09E and 4–ZZBM

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A Novel Double-Stator Double-Rotor Brushless Electrical Continuously Variable Transmission System