Wu-2012-Designofaminiatur

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Design of a miniature axial flux flywheel motor with PCB winding for
nanosatellites
Conference Paper · August 2012
DOI: 10.1109/ICoOM.2012.6316334
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2012 International Conference on Optoelectronics and Microelectronics (ICOM)
Design of a Miniature Axial Flux Flywheel Motor
with PCB Winding for Nanosatellites
Junfeng Wu*
Changchun Institute of Optics, Fine Mechanics and Physics
Chinese Academy of Sciences
Changchun, China
e-mail: awublack@126.com
motor development, many authors have also explored the axial
flux motor application in the field of electric vehicle. As in [2],
A novel low-speed axial-flux-modulated motor is proposed for
in-wheel applications in hybrid electric vehicles.
Electromagnetic field analysis method of axial motor also
moves forward. Generally, there are several following
categories. The first one includes the technique for static and
Abstract—This paper presents the design and analysis of an axial
flux permanent-magnet (AFPM) machine for nanosatellites
application. It is a miniature axial flux surface mounted
permanent magnet flywheel motor, using a printed circuit board
(PCB) stator winding. The PCB stator has simplified the design
and construction and avoids unnecessary space, since size
reduction has become one of the most important aspects of
flywheel motor design for nanosatellites. The performances of the
machine are estimated by combination method of threedimensional (3D) electromagnetic finite element analysis (FEA)
and approximate theoretical analysis. The back EMF and
electromagnetic torque are derived. A comparison between the
analysis and experimental measurement results of the prototype
machine is also presented. Both simulated and experimental
results show that the proposed motor would be an acceptable
solution for the nanosatellite applications with advantages such
as simple structure and low-cost, as multi-layer circuit board
production techniques have made the production of printed
circuit coils cheaper and easier.
Keywords-Axial
flux;
electric
motor;
EMF;electromagnetic torque; flywheel; nanosatellite
I.
flywheel rotor
motor rotor
motor stator
circuit board
Figure 1. Structure of the proposed flywheel motor.
transient analysis of AFPM machines using 3-D analysis with
the availability of powerful software tools [3], and the time
required is markedly considerable. The second way is that the
disk machine is simplified to a 2-D FEM model to save solving
time on the basis of not influencing the research result. The
model length in the moving direction is equal to the arc length
at the average radius [4]. The third way is that flux density
distribution is described analytically and geometrically without
FE analysis, as in [5][6].
back
INTRODUCTION
There has been an increased interest in using nanosatellites
in space science missions
due to hundreds of small
nanosatellites can form an intelligent constellation of a
distributed network of instruments to obtain measurements that
are not possible with traditional single spacecraft architectures.
Miniature flywheel is the key actuator in the nanosatellites,
which when rotated increasingly fast causes the spacecraft to
spin the other way in a proportional amount by conservation of
angular momentum. As pancake shape is more useful for the
angular momentum demanding, it has been difficult to design a
radial flux motor that can achieve the required decrease in
thickness. For this reason, the axial flux spindle motor has
become a more suitable choice for this application due to its
low profile. They also allow design flexibility and are
relatively easy to manufacture. For example, a change in
dimensions of a printed circuit stator can be accommodated
without any major alterations to production equipment and
processes.
Figure 2. Rotor structure of the proposed flywheel motor.
In this paper the back EMF and electromagnetic torque of
the AFPM machines are predicted analytically based on
magnetic field static 3-D analysis. Therefore, small amount of
additional computational time is needed to determine the main
parameters influencing machine performance. This method is
faster than the transient 3-D FE method. (For this application
It is well known that axial flux motor design software has
been developed in more mature, as Ansoft Maxwell was used
to analyze the electric car engine. It is easy to modify, easy to
operate, and achieved good results [1]. With the axial flux
978-1-4673-2639-1/12/$31.00 ©2012 IEEE
mechanical
bearing
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2012 International Conference on Optoelectronics and Microelectronics (ICOM)
study. Mesh using triangular finite elements, during the static
analysis, the system will automatically split the model and
finish optimization.
the motor current is very small, almost zero, then the magnetic
field distortion is small. Armature reaction is negligible).
II.
AXIAL FLUX PERMANENT MAGNET MOTOR DESIGN
A. The Structure of Disk Type Motor
The axial flux flywheel motor is shown in Fig. 1, where the
stator is composed of the shaft, bearing, and printed circuit
board (PCB) winding. The PCB winding is designed to achieve
an ultra thin and low weight stator. There are eighteen coils in
the stator, and the rotor consists of the yoke and the twelvepole PM, as in Fig. 2.
The AFPM stators are three-phase. Each stator has six coils
per phase with 6 turns per coil printed on a two-layer circuit
board, as in Fig. 3.
Figure 5. Rotor structure of the proposed flywheel motor.
A 3D model of FEA is shown in Fig. 5, and the motor air
gap flux density distribution at the different radius of the
circumferential is shown in Figure 6. Seen from the figure, the
radial component of air gap flux density is a function of
electrical angle for different radius. Distribution of the air gap
flux density is almost sinusoidal along the air gap.
R16
R18
R20
R22
R23
Flux density in the gap(T)
0 .4
Figure 3. Winding connections with concentrating winding
The simplest coil is made up of a spiral pair located on
neighboring layers. The spirals are joined by a via located at
their common centre. As shown in Fig. 4, current enters the
coil from a terminal on the outer radius side of the substrate. It
flows inwards, towards the coil centre, through the tracks of
one of the spirals, continues through the via at the centre of the
spiral and then flows outwards, away from the coil centre,
through the tracks of the second spiral.
0 .3
0 .2
0 .1
0 .0
-0 .1
-0 .2
-0 .3
-0 .4
-1
0
1
2
3
4
5
6
7
R o t a t in g A n g le ( R a d )
Figure 6. Air gap flux density, with radius 16 18 20 22 23 mm, when the gap
high is 0.6mm.
Figure 4. Printed circuit coil, (a) Top layer spiral, (b) Bottom layer spiral.
B. Air-gap flux density distribution
In order to analyze the performance of the motor, it is
essential to estimate the air-gap flux density distribution.
Analytical method may be a good choice, and effective means
in theoretical analysis of AFPM motors. But if we want to see a
more intuitive design result of air-gap effect and to analysis the
reasonableness of the model structure, three-dimensional finite
element method is a more appropriate choice. Time
consumption is relatively small for the air-gap magnetic field
distribution of the static analysis.
Figure 7. Description of the concentrating winding
III.
A. Torque
In order to analyze the performance of the motor, it is
essential to estimate the required motor torque to drive the
flywheel, and calculate the motor torque coefficient.
Concentrating winding is used in this Disk motor, the valid
In this paper, Ansoft Maxwell 3 D version finite element
analysis software was used to calculate the air-gap magnetic
field distribution, and the results provide a reference for further
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PERFORMANCE ANALYSIS OF TORQUE AND BACK EMF
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conductive winding is placed in the front of the permanent
magnet. The position on this planer can be described by angle
and radial theta, as Fig. 7.Distribution of the air gap flux
density is sinusoidal along the air gap, and the gap flux density
Bg (θ) describes the flux density under mean radial position, as
Fig. 8.
K
i =1
1
I ∑ Bg (θi + θ0 )(r1i 2 − r2 i 2 )
2 i =1
1 K
− I ∑ Bg (θ j + θ 0 )(r1 j 2 − r2 j 2 )
2 j =1
ν
N
TAC (θ 0 ) = TA (θ 0 ) − TC (θ 0 )
= TA (θ0 ) − TA (θ 0 +
C
⊕
S
dT = Bg (θ )rIdr
1
Bg (θ )rdr = IBg (θ )(r2 2 − r12 )
2
(1)
r2
1
e = ω ∫ Bg (θ )rdr = ω Bg (θ )(r2 2 − r12 )
r1
2
(2)
K
π
3( K + 1)
1
IBg (θi )(r1i 2 − r2i 2 )
2
1
IBg (θ j )(r1 j 2 − r2 j 2 )
2
1
= ω ∑ Bg (θi + θ 0 )(r1i 2 − r2i 2 )
2 i =1
1 K
− ω ∑ Bg (θ j + θ 0 )(r1 j 2 − r2 j 2 )
2 j =1
eAC (θ 0 ) = eA (θ 0 ) − eC (θ0 )
(3)
= eA (θ 0 ) − eA (θ0 +
2π
)
3
(10)
(11)
Back EMF of winding A-C is
(4)
E AC (θ0 ) = aeAC (θ 0 ) P
(12)
C. Back EMF Prediction
Here, back EMF is obtained for the case where the rotor
speed is 100 rpm for the axial position at 0.6mm with
parameters listed in table 1. The result is shown in Fig. 9. The
theoretical results showed good agreement with those obtained
by 3D-FE analysis by Ansoft Maxwell 3D, as in Fig. 12.
(5)
Torque induced by winding A is
978-1-4673-2639-1/12/$31.00 ©2012 IEEE
i =1
K
Within π/3 of second half winding, the magnetic torque
induced by j-th conductor is
Tj =
K
i =1
In a three-phase balanced winding, the number of turns in
three windings is equal with the electrical angle between the
adjacent phases such as A & B is π/3. Same electrical angle is
also between the phases, B & C. So the electrical angle of the
half of winding A is π/3, within π/3 of first half winding, the
torque induced by the k-th coil wire located in the radial
direction is
Ti =
(9)
e(θ 0 ) = ∑ ei (θ 0 ) − ∑ e j (θ0 )
Using left lead as a reference point, the phase difference
between two conductors is
Δθ =
(8)
B. Back EMF
Because eA , eC have differ angle of 2π/3, the induced
EMF can also be defined in terms of the rate at which magnetic
lines of flux are cut by the wire, so the back EMF of the disk
motor have the same form
The whole torque induced by one conductor under theta is
r1
(7)
Where a is the layer number, and P is pole pairs number.
Figure 8. Description of the air gap flux density
T = I∫
2π
)
3
TW _ AC = aTAC (θ 0 ) P
⊙
The motor mechanical velocity is ω. The torque conducted
by conductor with longth dr under (r, θ) position is
r2
(6)
When θ0 vary from 0 to 2π, the Back EMF of winding A is
derived.
θ
B
i =1
K
=
Bg
A
K
TA (θ 0 ) = ∑ Ti (θ 0 ) − ∑ T j (θ0 )
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0.15
Back EMF(Volt)
0.1
X: 4.608
Y: 0.1104
0.05
0
-0.05
-0.1
-0.15
0
1
2
3
4
5
Electrical Angal (Rad)
6
Figure 12. EMF calculated by 3D FEM
Figure 9. EMF calculated by this method
TABLE I. DESIGN PARAMETER
Wire number
1
2
3
4
5
6
Inner radius(mm)
Outer radius(mm)
15
24.5
16
23.5
17
22.5
17
21.2
16
22.5
15
23.5
IV.
EXPERIMENT AND RESULTS
A.
Maxwell Result of back EMF
From the design outlined in the previous section, a
prototype is constructed as shown in Fig. 10. This Rotor is 7
mm high with an exterior diameter of 55 mm. The motor
utilizes the two-layer PCB winding. Flywheel system uses
three-phase disk-type brushless DC motor as a driving device.
The planar structure of the motor greatly reduces the axial
dimensions of system, thus making the structure of the motor
more compact. The stator of DC motor adopts the slotless
armature structure with no core to eliminate the problems of
pulsation, hysteretic losses and eddy current losses caused by
the cogging. This will greatly improve the efficiency of the
motor.
The manufacture of the stator of the motor chooses the
convenient PCB processing. The positive and negative layers
of the stator have 36 coils to form three-phase Y-connected by
the nodes. Current shift of the three-phase DC motor is
achieved by adding three Hall elements on the side of the
stator coil. The Hall elements can also be used to measure the
speed and precise location of the rotor. The experiment result
is shown in Fig. 11 with condition of rotor speed 100 rpm
and the axial position at 0.6mm. Back EMF comparison
between proposed method and experiment measurement is
shown in Fig. 13.
Figure 10. Prototyped results. (a) PCB winding. (b) Rotor.
Figure 11. EMF measured under speed of 33Hz
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Measurement
Calculate
4.0
3.5
Back EMF(Volt)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
200
400
600
800
1000
1200
1400
speed (rad/s)
Figure 13. Back EMF comparison of measurement and calculation
V.
CONCLUSION
This paper has presented a design procedure for an axialflux flywheel motor which has promising applications in
nanosatellite. The design aims to eliminate much of the
structure of a conventional radial-flux machine so that its
thickness is only 25 mm and exterior diameter is 60 mm with
vacuum box. A PCB winding is applied to reduce the structure
of stator. A prototype motor is fabricated and the performance
tested. Measured results from the prototype motor confirm the
validation of the proposed design method. The predictions are
shown to be in good agreement with 3-D FE results.
ACKNOWLEDGMENT
Thanks to Professor Yihui Wu for her helpful discussions
on motor design. This work was supported in part by National
Natural Science Foundation (No.6131190103).
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978-1-4673-2639-1/12/$31.00 ©2012 IEEE
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