A Basic Study of Electrical Variable
Transmission and Its Application in Hybrid
Electric Vehicle
Shumei Cui, Yuan Cheng, and C.C. Chan, Fellow, IEEE
Abstract-A novel electromechanical converter-electrical
variable transmission (EVT) is introduced. EVT comprises two
concentric induction motors and two inverters and is suitable for
hybrid electric vehicles. Firstly, an elementary EVT model will
be given. Next, the operation modes in an EVT-equipped vehicle
are analyzed, and the prototype machine and the test bench with
results analysis are presented. It is concluded that EVT can
achieve the function of continuously variable transmission and
starter and generator which will optimize the operation of
internal combustion engine and enhance vehicle performance.
Index Terms-Continuously variable transmission, electrical
variable transmission, induction machine, modeling, hybrid
electric vehicle, operation modes.
I. INTRODUCTION
TITH
THETHEdevelopment
WITH
development of hybrid
hybr1d electric
electr1c vehicles,
veh1cles, many
novel electric machines appear, such as concentrically
arranged double-rotor machine [1]-[3]. Using induction
machines or permanent magnet machines, these converters
have a stator and two rotors and could transfer two mechanical
energies and exchange mechanical and electrical energies.
Electrical variable transmission (EVT) is one kind of these
machines. In principle, EVT comprises an induction generator
with two rotors, a normal induction motor, two power
converters and energy storage equipment. This structure
enables the function of continuously variable transmission
(CVT), starter and generator, and is especially suitable for
hybrid electric vehicles. Choosing EVT as the research object,
this paper builds the field-oriented control (FOC) model of the
double-rotor induction motor, followed by the analysis of the
EVT-equipped vehicle control strategy and motor control
methods for EVT in different operation modes. Finally, the
EVT prototype machine and test bench are introduced.
11. STRUCTURE OF EVT
EVT iS composed of stator, inner rotor and outer rotor (as
the stator, rotor and interrotor in [1], respectively) shown in
Fig.l1. It could be seen as a machine with two concentrically
Shumei Gui, Yuan Cheng, and C. C. Chan are with Harbin Institute of
Technology,
Harbin,
150001,
China
(cuism a hit.edu.cn,
chengyuanl(hit edu nr, ccchan@eee.hku.hk ).ancotl rtees
arranged induction machines. The outer machine is a normal
squirrel-cage induction motor. The inner machine has its
squirrel-cage winding on its outer part and its 3-phase winding
on its inner part with sliprings. Different from similar designs,
p
p
g
g
aitional to e exists b..etwe el dstationd inne ror,
which makes the electromagnetc field distribution much more
complicated than the simple combination of two induction
motors. In an EVT-equipped vehicle, the internal combustion
engine (ICE) shaft is connected to the inner rotor and the final
gear is connected to the outer rotor.
To simplify EVT structure
and control methods, the
foloin
alys is based EVT
illustratedi g2
a
on
split EVT illustratedin Fig.2.
followhng analysds
When the field coupling of two airgaps shown in Fig.1 iS
absent, EVT could be divided into the left double-rotor
machine (the primary machine) and the right machine (the
secondary
machine), so that only mechanical
'
. connection
Sr
Outerrotor
~ ~Stator
Secondary shaft
Primary shaft
X
X
_
e rotor
Ig ~K1nner rotor
P.
cm
P2
)2Z
,1
l
+ 1-,
Enrg
Fig.2. The split EVT
exists. This treatment helps to understand its operation modes
ancotlsrtei.
1-4244-01 59-3/06/$20.00 ©2006 IEEE
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III. MODELING OF THE SPLIT EVT
The secondary machine can use the mature model of
induction motor as reference, whereas the model of the
primary machine should take the effect of the rotating inner
"stator" into account. For the whole model of the split EVT, it
can be deduced from the torque equations of the two machines.
A. FOC modelfor the double-rotor machine
The primary machine could be considered as a normal
induction motor with the exception that the "stator" is rotating
and placed inside. If d-axis is aligned with the flux of the
outer rotor and the two rotors have the same rotating
directions, we can get the voltage equations for the primary
machine:
Udl Rlidl + P /dl - (ws - n p * m1 )!Vql
Rliql + P Ymql + (os - np *Wml )Y/dl
uql
O = Rizd2 + P Y'd2
=
V1d2
=
IV. THE ANALYSIS OF EVT OPERATION MODES
A. CVTMode
The output characteristics of EVT make it very suitable for
use in a vehicle as a continuously variable transmission. The
basic working principle of EVT working as a CVT could be
seen in [1] and [4]. Here we will look at how to apply EVT in
a vehicle. To make the problems simple, EVT losses are
neglected.
Fig.3 shows the ICE optimal operation line (OOL) where
ICE has specific throttle level, speed and torque. Once the
throttle level a is specified, the optimal ICE speed and torque
will be specified:
0mi f(a),
Tm1 g(a).
R2lq2 + oslYVd2
the equations of flux linkage:
Vdl = Lsidl + Lm'd2
Vql = Lsiql +Lmiq2
O
torque, the EVT output speed and torque, and the
electromagnetic torque generated by the primary and
secondary machine, respectively. TL is the load torque. J1 and
J2 are the moments of inertia.
(3)
Point P2 shows the torque and speed required at the final
gear when vehicle is running at a constant speed, and point
PI is the equivalent ICE operation point at OOL. Hence,
Lm1dl + Lr1d2
O = Lmiql + Lriq2
P1
and the synchronous speed and slip speed:
s
(2)
=flp
co., + c= WS + fp '0m2
Cos/ = scos = (miqlT(rCVd2)
Here, the subscripts 1 and 2 represent for the inner rotor
and outer rotor, respectively. From these equations we can see
that in the equations of conventional vector control, s is
replaced by s -np CWmI . Due to the rotation of the inner
rotor, its mechanical speed )ml should be considered into the
synchronous speed s . t1 is the inner rotor angular frequency,
and W)sl is the slip angular frequency. np is the number of pole
pairs.
=
P2 .
The EVT control strategy is that the primary machine
adopts speed control to change the speed by An from the
speed required at the final gear to the optimal speed of the ICE.
In the same manner, the torque is changed by AT with the
secondary machine to the optimal torque for the required
power. The control strategy keeps the operation of the ICE at
maximum efficiency during all driving conditions [5] and [6].
Speed and torque demand sent to EVT could derive from
the relationship of EVT ratio i, vehicle speed V (km/h), and
ICE speed
c0ml,'
i = Awm1 IV
A = 18r / 5io,
where
io is the ratio of final gear, r is the radius of wheels.
When the throttle level a is designated, the desired ICE
B. SplitEVTModeling
operation
speed and torque will be specified from (2) and (3).
To integrate two separate machines as one system are the
the
Finally,
speed demand sent to the primary machine equals
torque equations.
to
For the primary machine,
A\n* = mlt2 m (1-I) f ((X)- VIA,
_Jdo.,m
and from (1) the torque demand sent to the secondary machine
Tmi Tel=i dt
equals
to
for the secondary machine,
T +T -T -J 61(0m2
~~~~~AT'*TL+J T cm2 Tm -Tr J2 dV
2
dt\*= L+J dt -m=TL+A d gx
Tel + e2-L =
dt
an EVT-equipped vehicle, the throttle level has no direct
~~~~~~In
T
T + ]7e2~~~TL+Jldw
dw
m2
tml Jl
,j
~connection to the position of the acceleration pedal. Hence, a
(1)
Where W)ml,S Tmi, W m2 , Tm2, Tel, Te2 are the ICE speed and
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T(Nm?
\,P22 (wm,
T\\
V. THE EVT PROTOTYPE MACHINE AND TEST BENCH
T2)
A. The E VTPrototype Machine
-An
Because there is no mature method for designing double(/, m1 Tm17rotor machine, we combine two specially designed induction
machines into EVT by repeating choosing and calculating the
/ ">Power
yoke of the outer rotor. The structure of EVT prototype
machine is shown in Fig. 5, the photos of prototype machine,
/
,show
n in Fig.6.
n(rpm)
Some parameters and geometric dimensions of EVT
Fig. 3. EVT control strategy in CVT mode
Acceleration~~~~~~~
Brk
PeaPedal
ea
rototype are shown in Table I and Table II.
Acceleration
PedalC c
Brake
hl I
O N
.......................B. Test Bench
_ehe Cntrl Uni
3
Brake
--,lA
split EVT test bench, shown in Fig.7, has been built [8].
The primary machine is a 20kW conversion induction
machine with two rotors, and the secondary machine is a
ranisaxl
normal 20kW induction machine. Two 40kW induction
machines are used as the power source and the load machine
AItuator |nergy s
ure
1respectively. A modular system containing a CPU board and
IC
some I/O boards from dSPACE monitor and store test data.
ke
Some experiments have been done to verify the EVT
concept. From the results, EVT can be controlled according to
Fig. 4. Structure of an EVT-equipped vehicle
the CVT control strategy. But efficiency measurements are not
vehicle control unit is needed as shown in Fig. 4 to receive the carried out due to mechanical imperfections.
driver demands and calculate the demands of ICE and EVT. If
the required power exceeds the ICE output ability, external
electrical energy is needed. The power flow is shown in Fig.2
innr rotor.
when EVT working as a CVT.
Optimal Operation Line
ltl
XB&
B. Starter Mode
In starter mode, only the double-rotor motor works. The
primary machine works in torque control in order to make full
use of the fast response of the current loop [7].
Because of action and reaction, the direction of rotating
field is opposite to the rotating direction of the inner rotor.
Hence, special consideration should be taken into account,
because ICE has a specific rotating direction. When the ICE
shaft speed reaches the critical starting speed judged by ICE
control unit, ignition signal can be sent out.
i
slip rin
Fig. 5. The structure of EVT prototype
C. Generator Mode
In CVT mode, all the electricity generated by the doublerotor motor supplies the secondary machine. By increasing the
ICE throttle level, via a power electronic converter, the onboard network can be supplied and hence the on-board battery
can be charged.
The EVT could be in generator mode when vehicle stops.
At that time, the output power from ICE supplies the energy
storage equipment. The secondary machine can also work as a
generator to store the brake energy.
D.) Purea ElectricModeSpeed(rpm)
When an EVT-equipped vehicle iS operating In pure
electric mode, the ICE does not work. To avoid the rotation of
ICE, the primary machine does not work either. Hence, only
the secondary machine iS working and operating in torque
GEOMETRIC DIMENSIONS OF EVT PROTOTYPE MACHINE
Sao
ue oo
ne oo
Inedimtrm)27175
Outer diameter(mm)
310
236
166
mode.
Number of turns
Fig.6. Photos of EVT inner rotor, and inner/outer rotor set
TABLE I
PARAMETERS OF EVT PROTOTYPE MACHINE
Rated Power(kW)
Rated Voltage(V)
Slots
Primary Machine
20
200
48/44
Secondary Machine
20
200
36/24
1500
1500
TABLE II
60
1/2
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48
dSPACE
~~~REFERENCES
System [1] Hoeijmakers Martin J., Jan A. Ferreira, "The electrical variable
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Machine
Prmaryin Secindar
machinEVT
stMaechin
Fig.e7.EVT
test bench
Load Machine
VI. CONCLUSION
noel elctroechaical onveter,
This This
aper aperintroucesa
ntrodces anovelelectomechnicalconveter,
as well as the modeling of a double-rotor machine and
elementary analysis of different operation modes. EVT has the
potential of making vehicles more efficient. Experimental
results show that the split EVT could work well according to
control objective. The future work is in progress, including the
test of prototype machine, the electromagnetic coupling and
outer rotor saturation, the investigation of EVT operation
modes and control strategies, and the exploration of other
application possibilities.
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