International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 9, No. 1, March 2018, pp. 457~464
ISSN: 2088-8694, DOI: 10.11591/ijpeds.v9.i1.pp457-464
457
Design and Analysis of In-Wheel Double Stator Slotted Rotor
BLDC Motor for Electric Bicycle Application
S. Farina1, R.N. Firdaus2, F. Azhar3, M. Azri4, M. S. Ahmad5, R. Suhairi6,
A. Jidin7, T. Sutikno8
1,2,3,4,5,6,7Faculty
of Electrical Engineering, Universiti Teknikal Malaysia Melaka, Malaysia.
Machine Design, Power Electronics and Drives Research Group, CeRIA, UTeM.
6Electrical Technology Section, Universiti Kuala Lumpur-British Malaysian Institute, Malaysia.
8Universitas Ahmad Dahlan, Indonesia.
1,2,3,4,5,6,7Electrical
Article Info
ABSTRACT
Article history:
This paper discusses about design and analysis of double stator slotted rotor
(DSSR) BLDC motor for electric bicycle application. Usually single stator
(SS) BLDC motor is used in an electric bicycle. This type of motor has low
performance and need to be charged regularly. The objective of this research
is to design and analysis DSSR motor that have high torque. At starts, design
specification for the electric bicycle is calculated. Next, design process for
DSSR is carried out by using the desired parameter. Lastly, analysis for
double stator slotted rotor is simulated using FEM. Result for average back
emf, average inductance, inner stator flux density, outer stator flux density,
average torque and estimate torque constant is obtained. Result for average
torque from FEM archieve the requirement of motor torque for DSSR design
where the maximum average torque is 16.2 Nm. This research will give
benefit to mankind and society in term of environment protection and energy
consumption.
Received Sep 7, 2017
Revised Oct 31, 2017
Accepted Dec 21, 2017
Keyword:
Double stator
BLDC motor
Copyright © 2018 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
R.N. Firdaus,
Faculty of Electrical Engineering, Universiti Teknikal Malaysia, Melaka, Malaysia
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Email: norfirdaus@utem.edu.my
1.
INTRODUCTION
Recently, the accelerated development in the number of vehicle in use has seriously impacted on
worldwide energy consumption and environment. Compared to the internal combustion engine vehicles,
electric vehicles contribute significantly to the energy saving and environmental protection, and on account
of these benefits, they constitute today’s direction for the automotive industry [1]. With growing concerns on
environment protection and energy conservation, electric vehicles have gained increasing attention [1-3].
Different from the internal combustion vehicles, electric vehicle have an electric motor embedded in the
powertrain. Since the efficiency in the energy conversion of an electric motor together with the associated
power electronics supply is much higher than internal combustion vehicles, electric vehicle need less energy
to move [4]. Furthermore, exploiting the capabilities of the electric motors, additional abatement in energy
consumption can be achieved.
For instance, the start-stop and the regenerative braking features can further reduce the energy
consumption by approximately 20%–30% [4]. Therefore, electric vehicle are convenient, not only for
increasing the efficiency in the energy utilization, but also for cutting out environmental pollution in an equal
proportion [4]. There are many type of electric vehicle which include electric scooter, electric skateboard,
hoverboard, and electric moped scooter. Electric bicycle is one of the electric vehicle which uses electric
motor for propulsion. There are a great variety of electric bicycle available worldwide from electric bicycle
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458
ISSN: 2088-8694
that have a small motor to assist the rider pedal power to more powerful electic bicycle which tend closer to
style functionality and performance. Electric bicycle uses rechargeable batteries and can travel up to 25 km/h.
Single stator (SS) BLDC motor is often used for electric bicyle. The problem with SS motor is that it produce
low power and low torque. When using SS motor , the electric bicycle will have low performance and need
to be charge regularly. To overcome this problem, double stator slotted rotor motor (DSSR) is introduce.
DSSR have high torque torque and high power. Due to the special structure of double stator, where it has two
stator that will double all the parameter, DSSR higher performance compared to SS which could increase the
usage time.
Double stator motor is widely used in electric vehicle because of its high power and high efficiency
[4-11]. Some author uses double stator for hybrid electric vehicle (HEV) [12]. Compared with conventional
permanent magnet electric machines, double stator has the advantage that currents of both the inner and outer
stators produce electromagnetic torque and two air-gaps can deliver the output torque, thus improving the
torque density and providing a high starting torque for cold cranking. Because of the nature of double stator
windings, the machine can flexibly change their connections, hence providing a constant output voltage over
a wide speed range for battery charging [12-14]. Another author present paper about design of the double
stator permanent magnet synchronous starter and generator used in electric vehicles permanent magnet
double stator intergrated starter generator for hybrid electric vehicle (HEV).
The double stator permanent magnet synchronous starter generator was designed to have different
number slots in inner stator and in the outer stator and poles at double attached magnet rotor is also different.
Two machines consist of the outer stator, the inner stator and double attached magnet rotor two machines
with different combinations of poles and slots to can be designed separately to achieve optimum performance
and to meet the demand of different operation modes [15]. Another researcher studied performance analysis
of double-stator starter for the hybrid electric vehicle. When double stator works as a motor at low speeds,
the two armature windings are in series, so the output torque of the motor is large. When it works as a
machine, the composite voltage vector of the two stators windings can be altered through shifting the relative
position of the two stators. All of the above characteristics meet the demands of the motor used in the HEV
very well [16].
2.
DESIGN SPECIFICATION
The electric bicycle performance need to be estimated in term of total force needed by the bicycle
when moving towards the road. Equation (1) shows the total force where fg is the hill climbing force, while
fair and fr is the force when bicycle is moving through air and rolling force respectively. Figure 1 shows the
diagram of force characteristic in typical bicycle and all the force needed by the bicycle in order to move
forward.
ftotal f g f air f r
(1)
The hill climbing force , fg is given in equation (2) where m is the mass of the bicycle, g is the gravitational
force and θ is the road gradient;
f g (m)( g )(sin )
(2)
The second force is fair where this force appears when bicycle moves forwards and moving through air, where
cd is drag coefficient, ρ is air density, A is moving area, vr is relative speed in air. Equation (3) define about
this force.
f air (cd )( )( A)(
vr 2
)
2
vr va vb
(3)
(4)
Relative speed in air is equal to air speed and ground speed which is shown in equation (4) where va is air
speed while vb is ground speed.
Int J Power Electron & Dri Syst, Vol. 9, No. 1, March 2018 : 457 – 464
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f air
fg
fr
A
Figure 1. Diagram of force characteristic in typical bicycle
The last force is fr which is the rolling force. This rolling force is cause by the bicycle weight including the
rider and tire friction on the road. Equation (5) shows the rolling force while equation (6) shows the rolling
coefficient where it depends on tire pressure and tire type where, W is weight in pound while X and Y is the
tire pressure values;
f r (Crr )( mg )
Crr X
(5)
Y
W
(6)
By using equation (2), (3), (4), (5) and (6) the specific condition parameter is obtained. The condition is
based on basic collected data by the researcher. Parameter for typical bicycle is shown in Table 1. From the
table, mass of bicycle and weight of rider is 40 kg and 70 kg respectively.The road radient is 5 % while air
speed is assume 0 ms/s. The speed change form 0 km/h to 25 km/h. Tire radius of the bicyle is 0.365. The
drag coefficient, frontal area and air density is 1, 0.4 m2 and 1.197 kg/m3 respectively.
Table 1. Data of typical bicycle
Parameter
Mass of bicycle, mb
Weight of rider, wr
Road radient, θ
Air speed, Va
Speed range, n
Tire radius, Tr
Drag coefficient, Cd
Frontal area, A
Air density, p
Kg
Kg
%
m/s
km/h
m
m2
Kg/m3
Value
40
70
5
0
0-25
0.365
1
0.4
1.197
To obtained torque, equation (7) is used. For the calculation of motor torque and human torque, equation (8)
and (9) is applied, respectively. Where ftotal is obtained from equation (1), Tr is tire radius, and R is ratio of
motor torque over human torque.
T f total Tr
(7)
Tm otor T R
(8)
Design and Analysis of In-Wheel Double Stator Slotted Rotor BLDC Motor… (S. Farina)
460
ISSN: 2088-8694
Thum an T Tm otor
(9)
1.0
30
Ttotal
0.4
18
12
Thuman
0.2
Speed, Vr [km/h]
Ratio, R
Ratio
0.6
Torque, T [Nm]
24
0.8
30
20
24
16
18
12
Tmotor
12
6
6
0
0
8
Pout
Vr
4
Tmotor
0.0
0
5
10
15
20
Speed, Vr [km/h]
25
0
(a) Torque requirement
50
100 150 200
Speed, n [rpm]
0
250
(b) Output power estimation
Figure 2. Requirement for the bicycle in this research
3.
DESIGN OF DSSR
Outer
stator
Rotor
Outer
coil
Inner
coil
Inner
stator
Magnet
a) Basic structure
Int J Power Electron & Dri Syst, Vol. 9, No. 1, March 2018 : 457 – 464
b) Flux line
Torque, Tmotor [Nm]
Output power, Pout [W]
Figure 2 shows torque requirement for the bicycle in this research. Figure 2 (a) is the torque
requirement where total motor torque is the torque produce by load which is proportional to force and
distance of the bicyle. Maximum torque for motor at speed 20 km/h is 10 Nm while minimum torque produce
is 8 Nm. Total human torque is the torque contribute by human which is produce by rotating bicyle pedal
manually. Maximum torque produce by human force is 16 Nm while minimum torque is 14.5 Nm. Total
torque is the result after adding up both motor torque and human torque. Total maximum and minimum
torque produce by both force is 26 Nm and 22.5 Nm, respectively. The different between torque produce by
motor and human is 50%. As torque of the motor increase, human torque will be increased. Maximum ratio
for motor torque over human torque is 0.5. Figure 2 (b) shows output power estimation. The power is
estimated for torque produce by motor only. At speed 200 rpm, the power estimated is 19 W while torque
produce by motor is 10 Nm. Motor speed at this point is 23.9 km/h when being converted from revolutions
per minute [rpm]. From the figure, it can be seen that output power increase as speed of the motor increase
and speed in kilometres per hour (km/h) is perpendicular with speed in rpm.
Int J Power Electron & Dri Syst
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∅ri
∅iso
∅ro
∅osi
2T
0T
c) Magnetic flux density
d) Parameter configuration
Figure 3. Double stator configuration
Figure 3 shows overall double stator configuration which consist of basic structure, flux line,
magnetic flux density and parameter configuration. Figure 3 (a) is the basic structure of DSSR. In a DSSR,
there is two stator, which is inner stator and outer stator, coil winding at each stator, permanent magnet and
rotor. Figure 3 (b) is the flux of DSSR. The flux line moves from permanent magnet towards stator back to
permanent magnet, completing a circle from north to south. Figure 3 (c) is magnetic flux density of DSSR.
Further explanation of magnetic flux density is explained in Figure 4 (c) and 4 (d). Figure 4 (d) shows the
double rotor configuration.
The detail value for each configuration is explain in Table 2. Table 2 is design parameter of DSSR
BLDC. Based on the table, the Outer stator diameter, ∅osi is 116 mm while the inner stator outer diameter,
∅iso is 82 mm. The rotor outer diameter, ∅iro and Rotor inner diameter, ∅ri is 81 mm and 61 mm,
respectively. The mechanical air gap for both inner and outer is 0.5 mm. Both mechanical air gap have the
same value so that flux will flow equally in both air gap. This motor is design for three phase configuration
with number of turn for outer stator is 100 and 58 for inner stator. The number of slot and pole for this double
rotor is 18 and 20 respectively. Permanent magnet volume is 3.78 х 103 while permanent magnet size is 5.4
mm х 2 mm х 35 mm. The coil diameter for winding purpose is 1.0 mm. Stack length for the double stator
motor is 35 mm.
Table 2: Design parameter of DSSR BLDC
Item
Outer stator
[mm]
Number of turn
Inner stator outer diameter, ∅iso
Rotor outer diameter, ∅ro
Rotor inner diameter, ∅ri
Number of slots
Number of poles
Number of phase
Permanent magnet volume
Permanent magnet size
Coil size
Stack length
Outer air gap
Inner air gap
[mm]
[mm]
[mm]
[mm]
[mm]
[mm]
[mm]
[mm]
[mm]
Element
Outer stator diameter
Outer stator
Inner stator
82
81
61
18
20
3
3.78 х 103
5.4 х 2 х 35
1.0
35
0.5
0.5
Value
116
100
58
4. ANALYSIS OF DSSR FOR ELECTRIC BICYCLE APPLICATION
Figure 5 shows FEM analysis result of double stator slotted rotor BLDC motor for electric bicycle
application. Condition of each parameter is explained at each graph. Figure 5 (a) is average backemf.
Backemf during zero current where the changes of backemf, E is shown interm of changes magnetic flux, ϕ
towards time, t as shown in equation (9). The equation of backemf is according to Faraday,s law where ϕ is
the magnetic flux of the motor and t is the changes in time.
Design and Analysis of In-Wheel Double Stator Slotted Rotor BLDC Motor… (S. Farina)
462
ISSN: 2088-8694
E
d
dt
(9)
Based on Figure 5 (a), the minimum backemf is 20 V while maximum backemf is 100 V. Backemf
increase propotionally to the increase of speed. Speed of back emf is varied from 200 rpm to 1000 rpm. As
motor rotate at higher speed, more magnetic flux is created which influenced the increment of backemf.
Figure 5 (b) is result for average inductance which is from FEM analysis. Current at this point is
varied from 2 A to 10 A while speed is at the range of 200 rpm to 1000 rpm. Relationship between back emf,
inductance and current is shown in equation (10) where the induced voltage back emf, E is equal to motor
inductor’s inductance, L and the rate of change of current, i through the inductor. Maximum inductance is
0.035 H for current of 2 A while minimum inductance, is 0.025 H for current of 10 A. Inductance increase
linearly with the increase of current but there is no significant difference of inductance value when speed is
increase at all current level. From current 2 A to 4 A there is only some significant change of inductance.
This is the same for current 8 A to 10 A.
EL
di
[V]
dt
(10)
Figure 5 (c) and 5 (d) shows result for inner stator flux density and outer stator flux density,
respectively when current is varied from 2 A to 10 A and speed is in the range of 200 rpm to 1000 rpm.
Maximum inner stator flux density is 1.9 T while minimum inner stator flux density is 1.1 T. For outer stator
flux density, maximum flux density is 2.0 T at current 10 A while minimum flux density is 1.3 T at 2 A. The
similarity between inner stator flux density and outer stator flux density is that the flux density increase when
current is increase. Both flux density in occur in range of 1.0 T to 2.0 T. The difference between both flux
density in term of maximum and minimum value is 5 % and 12 %, respectively. The best flux density appear
at current 6 A. When current is change to 8 A and 10 A, motor start to saturate.
Figure 5 (e) shows average torque result. The result is obtained during current 2 A to 10 A while
speed is in the range of 200 rpm to 1000 rpm. The maximum average torque is 16.2 Nm while minimum
average torque is 4.2 Nm. This is in the range of motor torque which had beed discussed in Figure 2. Torque
increased when current is increased but maintain for different speed. There is only small increment of torque
from current 8 A to 10 A. This shows that the best operating current of the motor is at 6 A. The relationship
between torque with current is shown in equation (11) where P is number of pole, Z is number of conductor,
ϕ is flux per pole, I is armature current and A is number of parallel path.
T
PZI
[Nm]
2A
(11)
Figure 5 (f) shows torque vs speed result. This result is obtained by using equation (12) and (13).
Stall torque of the motor is 600 Nm where the output rotational speed is zero. Stall torque is the maximum
torque can be applied to the shaft and cause the motor to stop rotating. The maximum output speed of the
motor is 140 rpm. This is the motor condition when no torque is applied to the shaft. Operating requirement
for the motor is shown in the graph where motor operates at the range point. Torque and speed characteristic
is at 48 V.
The relationship between torque constant, current and speed is shown in equation (12) and (13)
Where V is the voltage source, kt is constant current, ke is contant voltage, Rc is coil resistance, and ω is speed
during no load condition. From the equation, it can be seen that torque increased linearly with the increased
of kt and ke but speed during no load decreased with the increased of ke.
Tc
Vkt2 k e
[Nm]
Rc
V
[rpm]
ke
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(12)
(13)
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100
Inductance, L [mH]
60
40
20
30
10 A
20
8A
6A
10
4A
2A
0
0
200
0
400 600 800 1000 1200
Speed, n [rpm]
0
200
a) Average backemf
2.5
2.5
8A
10 A
2.0
1.5
1.0
2A
4A
0.5
6A
0.0
0 200 400 600 800 1000 1200
Speed, n [rpm]
8A
10 A
2.0
4A
1.5
2A
1.0
6A
0.5
0.0
0
200 400 600 800 1000 1200
Speed, n [rpm]
c) Inner stator flux density
d) Outer stator flux density
1000
20
10 A
800
16
Torque, T [Nm]
Average torque, Tave [Nm]
400 600 800 1000 1200
Speed, n [rpm]
b) Average inductance
Outer stator flux density, Bos [T]
Inner stator flux density, BIs [T]
463
40
80
Average backemf, E [V]
ISSN: 2088-8694
8A
12
6A
8
4A
4
Operating
requirement
600
400
200
2A
0
0
0
200 400 600 800 1000 1200
Speed, n [rpm]
0
e) Average torque
40
80 120 160
Speed, n [rpm]
200
f) Torque vs speed
Figure 4. FEM analysis
4. CONCLUSION
In this paper, design and analysis of double stator slotted rotor BLDC for electric bicycle had been
discussed. Firstly, electric bicycle performance need to be estimated in term of total force needed by the
Design and Analysis of In-Wheel Double Stator Slotted Rotor BLDC Motor… (S. Farina)
464
ISSN: 2088-8694
bicycle when moving towards the road. As a result expected, torque and power is estimated and double stator
was designed in term of basic structure, flux line, magnetic flux density and parameter configuration. Lastly,
analysis for double stator slotted rotor using FEM analysis and result for average back emf, average
inductance, inner stator flux density, outer stator flux density, average torque and estimate torque vs speed is
obtained. Result for average torque from FEM result archieve the requirement of motor torque for DSSR
where the maximum average torque is 16.2 Nm..
ACKNOWLEDGEMENTS
The author would like to thank Skim Zamalah UTeM for supporting this research in order to ensured
the project be successful.
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maaspaliza azri