IEEE PEDS 2017, Honolulu, USA
12 – 15 December 2017

Development of High-Efficiency
Permanent Magnet Synchronous Generator
for Motorcycle Application††

Toshihiko Noguchi, Yuki Kurebayashi, Tetsuya Osakabe , and Toshihisa Takagi†

Shizuoka University and Suzuki Motor Corporation
noguchi.toshihiko@shizuoka.ac.jp, kurebayashi.yuki.16@shizuoka.ac.jp

Abstract-High-efficiency design of a permanent magnet
synchronous generator is discussed in this paper.
Theapplication of the generator is a motorcycle, and improvementof the efficiency is focused on to reduce the fuel consumption of
the motorcycle. The generator has an outer rotor configuration,and has a single-phase winding for compact motorcycles andthree-phase windings for large motorcycles on the stator. There
are a lot of restrictions on designing this kind of auxiliarymachines because of the cost, mechanical and electricalcompatibility with conventional motorcycles, productivity, andso forth. The new design is conducted through the FEM based
three-dimensional electromagnetic analysis, and approximately
10-% power loss is reduced through the development, which cancontribute the mileage improvement or the reduction of the fuelconsumption of the motorcycle.

INTRODUCTION
Engine efficiency of motorcycles has been improved so far
by directly increasing the combustion efficiency. One of theepoch-making improvements was a drastic change of theengine operation from two-stroke cycle to four-stroke cyclein the past. Another improvement has been achieved with ahigh-performance catalyzer and a fuel injection system,
which contribute not only the combustion efficiency
improvement but also quality improvement of the exhaustgas.
Many efforts are still being made for further
improvement of the mileage or the fuel consumption [1]. In
recent years, great advancement of the mileage improvement
is expected from the view point of further environmental
conservation because of the rapid motorcycle market growthin Asia. As described above, it goes without saying that the
combustion efficiency improvement and the mechanicalpower loss reduction are indispensable to achieve low fuelconsumption of the motorcycles. These improvements,
however, are somehow approaching to a technical limit, and
it is rather difficult to overcome the limit. Therefore,
auxiliary machines have been focused on to improve the total
efficiency of the motorcycle and to reduce the fuelconsumption [2]. The typical auxiliary electric machine is apermanent magnet (PM) synchronous generator to charge a
12-V battery and to feed electric equipment such as lamps
and meters.

On the other hand, a commonly used rectifier to charge the
12-V battery is a short circuit type, which makes thegenerator windings short when the battery is fully charged;
thus, the generator has been designed to have a highsynchronous inductance to reduce the power losses when thewindings are shorted. However, an open type rectifier isrecently used for the motorcycle, which opens the generator
windings when the battery state of charge (SOC) is highenough. Assuming use of the open type rectifier, the paper
discusses development of the new generator that can improveoverall fuel efficiency of the motorcycle [3] [4].
CONVENTIONAL MOTORCYCLE GENERATOR
A. Overview of Conventional Generator
Fig. 1 and TABLE I show photographs and specificationsof a conventional generator for a motorcycle. As can be seenin the figure, the generator is a PM synchronous generator
with a 12-pole outer rotor and an 18-slot stator with threeTABLE I. SPECIFICATIONS OF CONVENTIONAL GENERATOR.
Normalized rotor diameter
1
Normalized stator diameter
0.824
Normalized axial length of stator core
0.144
Normalized air gap length
5.73 x 10-3
Coil diameter
1.15 mm
Number of coil-turns per pole
42
Winding resistance for one phase
0.30 
Normalized thickness of iron core plate
3.82 x 10-3 (EMSP)
Normalized thickness of iron core plate
9.16 x 10-3 (SPCC)

Fig. 1. Photographs of conventional generator for motorcycle.

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978-1-5090-2364-6/17/$31.00 ©2017 IEEE

Generator

80

RectifierMeasuedSimulated

Battery

LoadLine voltage (V)

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40
20
0
-20
-40
-60

Fig. 2. Electric circuit diagram of auxiliary equipment of motorcycle.

-800.05 (ms/div)(a) No-load line-to-line voltage waveform at 3000 r/min.

SPHC

EMSP

Fig. 3. Simulation model of conventional generator.
Generator

Iline

Line Voltage (Vrms)

SPCC

Load

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Measured
Simulated
0

2

4

6

8 10 12 14 16 18 20 22 24

Line current (Arms)

Vline

Copper loss (W)Measured
Simulated

0

2

4

6

8 10 12 14 16 18 20 22 24

Line current (Arms)(c) Copper loss characteristics.

Iron loss (W)phase concentrated windings [5]. The rotor is directly
connected to a crankshaft of the engine, and rotatessynchronously with the engine. The unique point of themachine is a stator iron core structure, which uses SPCC
plates sandwiching the laminated electromagnetic steel plates(EMSP) and the SPCC plates are folded by 90 deg toward theaxial direction at the teeth edge. This construction seems tobe effective to retrieve more magnetic flux from the PM on
the rotor.
An electric circuit diagram for the motorcycle auxiliary
system is shown in Fig. 2, including the generator and athyristor full-bridge rectifier. The rectifier is used in two
operating modes, i.e., an open mode and a conduction mode.
All the thyristors are turned off in the open mode, and areturned on in the conduction mode. These modes are altered
to regulate the battery voltage around 13 V on the basis of the
duty cycle control. The aim of the machine design is to
reduce the power losses of the generator with the thyristor
rectifier at a designated operating point. Therefore, someexperimental tests in the following sections have been
conducted under the condition of the 13-V DC-bus voltage
regulated by an electronically controlled load so as not to turnoff the thyristor rectifier.
Figs. 3 and 4 represent the 3-D model of the conventional
generator and the circuit diagram used for evaluation of only
the generator, respectively.
In the case of operation
characteristic measurement of only the generator, three-phasevariable resistive loads are simply connected to the generator.
B. Operation Characteristics of Conventional Generator
Figs. 5 (a) to (e) show various operation characteristics ofthe conventional generator at 3000 r/min, comparing the

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Measured
Simulated

0

2

4

6

8 10 12 14 16 18 20 22 24

Line current (Arms)(d) Iron loss characteristics.

Efficiency (%)

Fig. 4. Circuit diagram for measuring generator operation characteristics.

(b) V-I characteristics.
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20
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Measured
Simulated

0

2

4

6

8 10 12 14 16 18 20 22 24

Line current (Arms)(e) Efficiency characteristics.
Fig. 5. Operation characteristics of conventional generator.

experimental test results with FEM based electromagnetic
analysis results. A simulator JSOL’s JMAG-Designer 16 ® isused for the FEM based electromagnetic analysis [6]. The
rotating speed of 3000 r/min has been specified because thisoperating point is the most frequent condition in the
motorcycle driving modes. Temperature of the PM and the770Loss (W)

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Copper loss (Measured)
Iron loss (Measured)

Rectifier off

Rectifier on

Loss (W)(a) Power losses at 3000 r/min.
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50
40
30
20
10
0

Eddy current loss (SPHC)
Hysteresis loss (EMSP)
Eddy current loss (EMSP)
Eddy current loss (SPCC)
All iron loss

Simulated

Measured(b) Details of no-load iron losses at 3000 r/min.

Loss (W)winding resistance has been kept constant at every
measurement point through the computer simulations. It canbe found from the figures that there are some errors between
the analysis and the experimental results in a heavy load
range. The errors seem to be caused by the winding
resistance variation due to its temperature rise in theexperiments. As can be seen in the Fig. 5 (c), the dominant
power loss is copper loss when the rectifier is in the
conduction mode. The iron loss, however, is reduced asshown in Fig. 5 (d) because of the armature reaction magnetic
flux that cancels the PM flux, which results in the magnetic
flux density reduction in the stator iron core.
Fig. 6 (a) shows power losses in the two operation modesof the rectifier at 3000 r/min. The copper loss is dominant in
the conduction mode of the rectifier whereas only the ironloss is measured in the open mode. The detailed analysis
result of the iron losses in the open mode at 3000 r/min isindicated in Fig. 6 (b), and the iron loss analysis results withrespect to the rotation speeds are depicted in Fig. 6 (c). As
shown in Fig. 6 (b), the most part of the iron losses appears inthe stator iron core, especially as eddy current losses. It
should be noted that the eddy current loss of the SPCC plates
is significant for their small physical volume among all ironlosses. Fig. 7 shows a 3-D FEM analysis result of the SPCC
plate at 3000 r/min, where the eddy current loss distribution isindicated. Since the thickness of each SPCC plate is 2.4
times of the laminated EMSP and the SPCC plate is folded in
the axial direction at the end of the tooth as described
previously, the eddy current loss concentrates on the SPCC
plate. Fig. 8 shows output current characteristics of therectifier under the condition of a 13-V constant battery
voltage and a saturated machine temperature. The simulationand the measured results agree very well, and the output
current is limited under 28 A even in the higher speed range
because of increase of the synchronous reactance.

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120
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80
60
40
20
0

All iron loss (Measured)
Other iron loss (Simulated)
SPCC iron loss (Simulated)

1300

3000

4500

6000

Rotating speed (r/min)(c) Power loss characteristics with respect to rotating speed.
Fig. 6. Detailed iron loss characteristics of conventional generator.

IMPROVED DESIGN OF MOTORCYCLE GENERATOR

Fig. 7. Eddy current loss generated in SPCC plate at 3000 r/min.

Output current (A)The rotor back yoke has not been modified in the new
design because the mechanical compatibility with theconventional model is important. In addition, some different
materials for the PM are available, but it is difficult to change
the PM size due to the compatibility reason. The improved
generator has newly been designed according to the following
restrictions [7]:1) less than 400-V output voltage at 10500 r/min;2) 18.5-A or more battery input current at 1300 r/min; and3) less than 35-A battery input current at 10500 r/min.
The above design restrictions are imposed due to the thyristor
rectifier absolute ratings and the power requirement of theactual motorcycle. In the first step of the new design process,the SPCC plates are eliminated from the stator iron core, andhigher quality EMSP is employed instead of the SPCC toreduce the iron loss by 40 %. Since the copper loss takes
major part of the total power loss during the conduction modeof the thyristor rectifier, the diameter of the stator windings

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10

Simulated
Measured

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0.0 1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0

Rotationg speed ×103 (r/min)Fig. 8. Rectifier output current characteristics of conventional generator.

are designed larger to reduce the copper loss. In order tomake the thicker windings possible, the slot cross section isenlarged by 7.1 % by making the slot deeper than the771

TABLE II. EFFECT OF PM OVERHANG.
Axial length of PM
No-load voltage at 3000 r/min
37 mm
40.98 V (0 %)32 mm
40.18 V (-2.0 %)25 mm
37.29 V (-9.0 %)120
iron loss of stator (20p18s)
iron loss of stator (12p18s)

100

Loss (W)

80
60
40
20
0

1300

3000

4500

6000

Rotating speed (r/min)

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36
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33

3

4

5

6

7

8

9

10

Slot opening width (mm)No-load line-to-line voltage (V)Number of coil-turns per poleFig. 9. Comparison of no-load iron losses.
400
395
390
385
380

3

4

5

6

7

8

9

10

Slot opening width (mm)(a) Number of turns to limit voltage.
(b) Voltage at 10500 r/min.
Fig. 10. Number of winding turns to limit no-load output voltage less than
400 V at 10500 r/min.
20.5
20.0

Output current (A)Output current (A)

19.5
19.0
18.5
18.0
17.5
17.0
16.5

3

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5

6

7

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9

Slot opening width (mm)

10

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30

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8

9

10

Slot opening width (mm)(a) 1300 r/min.
(b) 10500 r/min.
Fig. 11. Output current characteristics with respect to slot opening width.
35

Eddy current loss (SUS)
Hysteresis loss (EMSP)
Eddy current loss (EMSP)

30

Loss (W)conventional model. It is necessary to increase the axial
length of the stator iron core to compensate for the reduced
magnetic flux due to the elimination of the SPCC plates. The
newly designed stator axial length is 5.8 % longer than that of
the conventional model, resulting in the same weight of thestator iron core owing to the enlarged slot areas. As for thePM, the class-5 or the class-6 ferrite PM has been employed
to mount on the outer rotor with a stainless steel (SUS) sleeve.
The SUS sleeve is attached on the PM surface for its
mechanical reinforcement whereas it has a drawback to
generate additional eddy current loss.
TABLE II shows a relationship between the axial length of
the PM and the electromotive force at 3000 r/min, where allof the PM length are longer than the stator axial length. As
indicated in the table, the electromotive force strongly
depends on the PM overhang, but saturates even though the
PM axial length becomes longer than 32 mm. Therefore, the
32-mm long PM in the axial length has been selected.
The number of the poles has been determined bycomparing the total power losses between the 12-pole and the
20-pole machines with the same 18-slot stator. Fig. 9 isestimated stator iron losses in no load condition of the 18pole and the 20-pole machines at several rotating speeds. It
can be seen that the 12-pole machine is superior to the 20pole one from the viewpoint of the stator iron loss reduction.
The peak-to-peak output voltage Vp-p at the 10500-r/min
maximum rotating speed has to be limited to 400 V becauseof the maximum voltage rating of the thyristor rectifier. On
the other hand, the output current at the 1300-r/min lowest
speed has to be over 18.5 A to charge the battery even in theidling situation. Therefore, it is necessary to reduce thesynchronous inductance to satisfy the above requirements.
One of the effective methods of reducing the inductance isadjustment of the slot opening, which has an impact on theleakage magnetic flux. In other words, the number of the flux
linkage to the stator windings is affected by the slot opening,
so the number of the winding turns must be adjusted to satisfy
the first requirement. Fig. 10 (a) shows the relationshipbetween the slot opening width and the number of thewinding turns, which makes Vp-p at no load less than 400V.
In addition, Fig. 10 (b) indicates Vp-p at 10500 r/min under theconditions shown in Fig. 10 (a), i.e., combinations of thenumber of the winding turns and the slot opening width. Figs
11 (a) and (b) are output current characteristics with respect
to the slot opening width at 1300 r/min and 10500 r/min. Fig.
11 (a) shows a relationship between the number of thewinding turns per slot and the slot opening width, and Fig. 11(b) shows a characteristic of the terminal voltage Vp-p at
10500 r/min with respect to the number of the turns. The
number of the turns can be determined at every slot opening
width from the figure, so it has been checked whether thesecond and the third design requirements are satisfied or not.
The output current characteristic with respect to the slot
opening width is shown in Fig. 11. It is confirmed from thefigure that the synchronous inductance increases and theleakage inductance deceases as the number of the winding

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4

5
6
7
8
Slot opening width (mm)9Fig. 12. Iron loss characteristics with respect to slot opening width.

turns increases and that the current monotonically goes up
with the slot opening width. The appropriate slot opening
widths that satisfy the design requirements are 6 mm for 36 T,
7 mm for 37 T, 8 mm for 38 T, and 9 mm for 39 T. Fig. 12shows comparison of the total iron losses and their contents
for every slot opening width from 4 to 9 mm. As can befound in the figure, 7-mm slot opening width is the best
choice to reduce the total iron loss. Since the generator musthave a marginal output characteristic, a model with 7-mm slot
opening width and 37-turn windings is employed. The

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80
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Line voltage (V)TABLE III. SPECIFICATIONS OF IMPROVED DESIGN GENERATOR.
Normalized rotor diameter
1
Normalized stator diameter
0.824
Normalized axial length of stator core
0.153
Normalized air gap length
7.63 x 10-3
Coil diameter
1.20 mm
Number of coil-turns per pole
37
Winding resistance for one phase0.22 
Normalized thickness of iron core plate
3.82 x 10-3 (EMSP)

MeasuedSimulated

40
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0
-20
-40
-60
-80

0.05 (ms/div)(a) No-load line-to-line voltage waveform at 3000 r/min.

Line Voltage (Vrms)

50
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30
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20
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10
5
0

Measured
Simulated
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Line current (Arms)

Fig. 13. Photographs of improved design generator for motorcycle.
SPHC

Copper loss (W)(b) V-I characteristics.
200
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140
120
100
80
60
40
20
0

Measured
Simulated

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Line current (Arms)(c) Copper loss characteristics.
EMSP

SUS

PROTOTYPE GENERATOR AND TEST RESULTS
Figs. 13 and 14 are photographs of the newly designed
prototype and its 3-D model for electromagnetic analysis,
respectively.
Figs. 15 (a) to (e) show operation
characteristics of the prototype generator at 3000 r/min,comparing the experimental results with the analysis results.
As can be seen in the figures, the five characteristics are aswhat are designed, and both of the copper loss and the ironloss are remarkably reduced, which improves significantly
the efficiency of the generator.
Fig. 16 shows no-load iron loss analysis results at 3000
r/min. The extra iron loss is generated in the SUS sleeve,

Measured
Simulated

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Line current (Arms)(d) Iron loss characteristics.

Efficiency (%)

diameter of the coils has been increased by 0.05 mm owing to
the slot area expansion and the reduction of the number of theturns, resulting in lowering the winding resistance per phasefrom 0.30 to 0.22 . TABLE III shows the final major
specifications of the newly designed generator. An SUS
sleeve is attached to the PM for mechanical reinforcement
and for preventing the PM from chipping in an assembly
process. An air gap is slightly wider than that of theconventional model due to the sleeve although the rotor innerdiameter has not been changed from the conventional one.

Iron loss (W)Fig. 14. Simulation model of improved design generator.

100
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10
0

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80
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60
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Measured
Simulated

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28

Line current (Arms)(e) Efficiency characteristics.
Fig. 15. Operation characteristics of improved deign generator.

which is also confirmed in the 3-D electromagnetic analysis
result shown in Fig. 17. The figure shows an eddy current
loss distribution on the sleeve, and it can be seen that theeddy current loss is mainly generated between the stator teeth.
Fig. 18 is comparison of the loss analysis results in the open
and the conduction modes of the rectifier between the

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40
35
30

Loss (W)conventional generator and the prototype. The total power
loss of the prototype is remarkably reduced regardless of therectifier modes. Fig. 19 is V-I characteristics of theconventional generator and the prototype at the idling speed,
where it is found that the short-circuit line current isincreased owing to the low synchronous inductance design ofthe prototype. Fig. 20 shows output current characteristics
when the battery voltage is kept at 13 V. It is confirmed fromthe figure that the output currents of the prototype are lessthan the predicted values obtained by the electromagnetic
analysis, and that the characteristic does not satisfy the design
requirements. The reason why the output current of the
prototype has been less than the target values is temperaturerise of the PM and voltage drops in the wire harness
connecting the rectifier to the battery. Therefore, it is desired
to redesign the improved generator eligible to the target
output characteristics, taking the above issues into account.

Eddy current loss (SPHC)
Hysteresis loss (EMSP)
Eddy current loss (EMSP)
Eddy current loss (SUS)
All iron loss

25
20
15
10
5
0

Simulated
MeasuredFig. 16. Details of no-load iron losses at 3000 r/min.

CONCLUSION

[1]

[2]

[3]

[4]

[5]

[6]

[7]

A. Triwiyatno, E. W. Sinuraya, J. D. Setiawan, and S. Munahar, “SmartController Design of Air to Fuel Ratio (AFR) and Brake Control
System on Gasoline Engine,” 2nd International Conference on
Information Technology, Computer, and Electrical Engineering, 2015.
M. Comanescu, A. Keyhani, and M. Dai, “Design and Analysis of 42-V
Permanent-Magnet Generator for Automotive Applications,” IEEE
Transactions on Energy Conversion, vol. 18, no. 1, 2003, pp. 107-112.
T. Osakabe, and T. Takagi “Study on Efficiency Improvement ofCompact Generator for Motorcycle,” SAE Conference Technical Paper,
2014-32-0138, 2014.
D. Takeuchi, T. Noguchi, T. Osakabe, S. Sako, and T. Takagi, “Studyon Efficiency Improvement of Compact Generator for Motorcycle,”
IEEJ Industry Applications Conference, vol. III, 2013, pp. 211-212.
A. Olano, V. Moreno, J. Molina, and I. Zubia, “Design andConstruction of an Outer-Rotor PM Synchronous Generator for SmallWind Turbines; Comparing Real Results with Those of FE Model,”18th International Conference on Electrical Machines, 2008.
M. Jenal, E. Sulaiman, M. Z. Ahmad, S. N. U. Zakaria, W. M. Utomo,
S. A. Zulkifli, and A. A. Bakar, “Design Study of Single and ThreePhase Synchronous Generator Using J-MAG Designer,” International
Conference on Clean Energy and Technology, 2013.
P. Eklund and S. Eriksson, “Air Gap Magnetic Flux Density Variations
Due to Manufacturing Tolerances in a Permanent Magnet Synchronous
Generator,” 22nd International Conference on Electrical Machines,
2016.

Iron loss (Measured)
Copper loss (Measured)

200

Loss (W)

150
100
50
0

Rectifier off Rectifier on

Rectifier off Rectifier on

Conventinal model

Improved modelFig. 18. Power loss comparison between conventional and improved models
in two operation modes of rectifier.

Line voltage (Vrms)

REFERENCES

Fig. 17. Eddy current distribution generated in stainless steel sleeve.

24
22
20
18
16
14
12
10
8
6
4
2
0

Conventional model (Measured)
Improved model (Measured)0 2 4 6 8 10 12 14 16 18 20 22 24 26

Line current (Arms)

Fig. 19. V-I characteristics of conventional and improved models at 1300
r/min (idling speed).
4035Output current (A)This paper has discussed a high-efficiency design of a
motorcycle generator. As a result of the investigation
described above, it is possible for the prototype to reduce thetotal loss by 54.8 % (35.2 W) under no load (the open modeof the rectifier) condition and by 10.1 % (17.3 W) underloaded (the conduction mode of the rectifier) condition at therotation speed of 3000 r/min, compared with the conventional
generator. Although the prototype surpasses the conventionalmodel from angles of the power losses and the efficiency, therequired specifications are not satisfied slightly yet; hence,
the further investigation will be made for a future work.

30
25
20
15
10

Simulated
Measured

50
0.0 1.5 3.0 4.5 6.0 7.5 9.0 10.5 12.0

Rotationg speed ×103 (r/min)Fig. 20 Rectifier output current characteristics of improved generator.

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