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Design of the axial-flux permanent magnet coreless generator

for the multi-megawatts wind turbine

Hideki Kobayashi, Yuhito Doi, Koji Miyata, Takehisa Minowa

Magnetic Materials R&D Center, Shin-Etsu Chemical Co., Ltd.

2-1-5 Kitago, Echizen-shi, Fukui, Japan
Design of the axial-flux permanent magnet coreless generator
for the multi-megawatts wind turbine

Hideki Kobayashi, Yuhito Doi, Koji Miyata, Takehisa Minowa



In general, an axial magnetic field type permanent magnet generator is able to increase output density per volume
more than radial magnetic field type permanent magnet generator. Moreover, in the case of a coreless generator,
which does not have any core in the stator, it excels in starting performance as cogging torque does not occur. In
addition, generating efficiency is high as there is no hysteresis loss. Therefore, we have designed supersize
generating equipment, so that a coreless axial magnetic field type permanent magnet generator can be utilized in
megawatt scale wind turbines. The designed generator is structured with multiple layers, consisting of 2 stators and
3 magnet rotors, and large rectangle NdFeB permanent magnets are incorporated in rotors. The unit size of NdFeB
magnet block was decided from the maximum limit of industrial availability through the powder sintering process. The
larger rectangle magnets can be produced by assembling of them.
The designed one, which has a diameter of 10 meters and an axial length of 1 meter showed the 6.5MW electrical
output in direct drive operation without speed increasing gear. As a result of research on the influence of different
numbers of poles, the highest power-to-weight ratio was obtained with 48 poles. The further output can be obtained
by further stacking of rotors and stators.
Moreover, we have reviewed the assembly method of a rotor when utilizing a large permanent magnet. We have
also produced a 1:20 scale miniature machine, and verified the analysis values.

NdFeB magnet designed and produced the prototype;

I. INTRODUCTION and evaluated the generator that adopted the method to
The popularity of wind power generations has rapidly increase the generated voltage by piling the rotor in axial
expanded in recent years in order to address global direction to configure the magnetic circuit in a coreless
environmental problems, as well as energy depletion. generator of the axial gap type.
The wind power generation system consists of
technologies from various fields, including the power
generator. The power generator is categorized into the II. NdFeB Magnet
induction generator and the synchronous generator. The Two types of the rare earth magnet exist, namely the
synchronous generator uses NdFeB magnets not only SmCo magnet and NdFeB magnet, and presently
for small wind power generators, but also for large wind approximately 98% of the total production quantity stems
power generators as it has the advantage of having no from the Nd family; with the NdFeB magnet acting as the
excitation loss. However, in the case of the permanent rare earth magnet.
magnet synchronous generator with the core, it has The NdFeB magnets consist of the sintered type and
limitations in the starting performance due to the brake the bond type, and are made from a mixture of resin and
by the iron loss of the core, and the cogging torque plastics (etc.) with magnetic powder. The sintered
caused by the magnetic attraction between the magnet makes up the bulk of the magnet production,
permanent magnet and the stator core. and the manufacturing process of the NdFeB magnet
On the other hand, the wind turbine using a coreless follows powder metallurgy as outlined in Fig. 1.
generator has fine starting performance without iron loss, The difference in normal powder metallurgy has to do
though it produces lower generated voltage compared to with compression the molding under the magnetic field.
one with the core. To increase the generated voltage, By arranging the crystal orientation of the fine powder to
the diameter of the generator is increased, however, the one direction using an external magnetic field, the
nacelle becomes large and it may affect the exposed mechanical pressure is applied to the mold, and the
area of wind for a small generator. resulting product provides the magnetic anisotropy. The
The authors committed themselves to investigating compactness of the pressured powder is then sintered at
the wind power generator, and the manufacturer of the high temperature, and through the liquid phase sintering,
the compacted matter shrinks, and the density increases III. PERMANENT MAGNET GENERATOR
to the near true density. The volume shrinks in the half, The permanent magnet generator is suitable for the
however obtaining true dimensional accuracy is difficult. wind power generator as it requires high-efficiency
After sintering, a diamond wheel does grinding, and the because there is less excitation loss compared to the
dimension and surface treatment is applied to achieve induction generator.
the final product. The permanent magnet generator is categorized into
the radial type and the axial type in terms of the direction
Reduction of
rare earth oxides
to metals
Melting Pulverization
Pressing in a
magnetic field
Sintering Aging
of the magnetic flux from the magnet and the generator.
As shown in Fig. 2, in the radial type, the magnet is
positioned on the surface of the rotor that is coupled to
the shaft, and the magnetic flux is generated in the radial
Nd, Dy, Fe, B rough fine
direction perpendicular to the shaft toward the stator
arranged on the outer side of the rotor. In the axial type,
Shipping (Magnetization) Inspection
Machining property
the disk shape rotor is coupled to the shaft, and the
magnet is positioned on the surface of the disk to
generate the magnetic flux parallel to the shaft.

Fig.1 Manufacturing process of NdFeB magnet.

There are several products using the NdFeB magnet
that most people are unaware of. Furthermore, in the
field of wind power generation, the utilization of the
NdFeB magnet for the large and small types of wind
power generators is studied. Table 1 shows product
examples using NdFeB magnet.
Table 1 Application product examples of the NdFeB magnet radial axial

Field Products Fig. 2 The direction of the magnetic flux from the rotor of
radial type and axial type
Computer VCM for HDD
Air conditioners, Refrigerators With the radial type, only the magnetic field is used
Home Washing machines that is generated in the cylindrical gap between the outer
appliances Vacuum cleaners stator and the inner rotor so that the output density
Digital cameras, Electric shavers cannot be increased.
Portable telephones, Speakers, Also, with the radial type, in order to increase the
AV Equipment magnetic field in gap, magnetic materials such as
DVD, CD, Portable music players
laminated silicon steel plate is used in the stator side.
Elevators, Industrial robots Accordingly, the starting performance decreases by the
Injection molding machines magnetic attraction generated between the magnet and
NC machines, Linear motors the stator, while the efficiency decreases because of the
HV, Car air-conditioners, iron loss caused by the generation of the alternate
Driving motor and Generators for magnetic field in the stator side. A possible
Automobile countermeasure to these predicaments, involves rotor
electric cars etc.,
arrangement at the inner and outer periphery, as well as
Power steering and various sensors
the configuration of the double ring rotor that sandwich
MRI, Motor for railways, the stator without using the magnetic material in the
Others Wind power generators, stator side. However, as they are still of radial type, the
Electric bicycles inner space of the rotor does not contribute to power
generation [1], [2].
On the other hand, the axial type can improve the
output density as it can adopt the large magnetic surface
by making the rotor dimension thin in the direction of the
rotation shaft [3], [4]. In Reference [5], the volumes of
the radial type and the axial type are compared. As
shown in Fig. 3, the axial type can be downsized [5].
between the gaps of the rotors. The magnetic field is not
2.5 Shaft and others
only added, but is also increased as the permeance
inside the magnet is improved. Also, the rotor arranged
2 Rotor at the edge in the axial direction is constructed with
Volume [L]

1.5 Magnet ferromagnetic material attached with the magnet in order

Coil to suppress the external leakage of the magnetic flux of
1 the magnet. At the same time, recycling the flux
Stator yoke increases the magnetic field in the gap, as it is
Stator teeth structured to increase the magnetic field in the gap
0 between the rotors by forming the closed magnetic
RFM AFM circuit among the magnet of the stacked rotor and the
edge back yoke.
Fig. 3 Volume comparison for 50 kW motors The coreless coil is arranged in a ring shape in the
stator, and the stator itself is positioned in the gap of the
By positioning the stator on both sides of the rotor, the rotor to generate the electromotive force by receiving the
magnetic flux on both sides of the magnet can be alternating magnetic field.
utilized. In addition, by piling the rotor and the stator in As each stator receives the alternating magnetic field
the direction of the shaft, a plurality of the air gap can be of the same phase, the generated voltage can be
applied. As described above, configuring the pile increased proportionally to the stator number by
plurality of the rotor and stator creates large air gap in connecting a coil in series at the same position of each
the volume given to the generator, and thereby stator.
increases output density. By using this method, instead The increase of the generated voltage is directly
of increasing the diameter of generator in order to raise related to the increase of the power output, and the
the output, the stacking number of rotors and stators can generated voltage is proportional to the strength of the
be increased in the axial direction. This also results in an magnetic field. It is essential to make the gap as small
increase in the degrees of freedom of the generator as possible in order to increase the magnetic field.
shape. Additionally, it is preferable to avoid making the
According to the remarkable performance thickness of the rotor arranged inside more than that of
improvement of the NdFeB magnet in recent years, even the magnet, and preferable to make a structure that
the coreless structure that has no magnet material in the allows both sides of the stator coil to approach the
stator becomes possible to generate high magnetic field. magnet as often as possible. For that purpose, the rotor
In addition, by stacking the rotor and making the closed has the structure wherein the magnet is embedded in a
flux magnetic circuit, the magnetic field in the gap can be disk hole made of non-magnetic material, while the
intensified. Moreover, the coreless permanent magnet stator has the structure wherein the coil is embedded in
generator of axial type has neither the cogging torque the disk made of insulating material to avoid generating
nor the iron loss of the stator, so one can expect a high eddy current. One can conclude that such a generator
efficiency and good starting performance. with the stacked magnetic circuit is smaller, lighter and
We studied the generator with the stacked rotor that costs less compared to a plurality of coreless generators
forms the magnetic circuit in the coreless permanent being placed side by side simply.
magnet synchronous generator of axial type.
Coil (stator) Magnet (rotor)


First, we manufactured a small test generator to
confirm the performance of the multi-layered axial
generator [6]. The conceptual diagram of this model is
shown in Fig. 4.
The basic structure involves the stacking of the axial
gap coreless generator in the axial direction. The rotor
coupled to the rotation shaft and the stator fixed to the S N
case is arranged alternatively.
A plurality of the permanent magnet is fixed to the
rotor with NS alternatively in the ring shape, and the Nonmagnetic frame（stator）
Back yoke (rotor)
magnetization direction is aligned with that of the
Fig. 4 The structure of the stacked magnetic circuit
neighboring rotor and generates the magnetic field
The main specifications are shown in Table 2. The
appearance and the inside are shown in Fig. 5.
Test generator
The three-dimensional magnetic field analyses was 1000
done during the design stage, and the electromotive

Output power [W]

force was derived at each revolution speed from the
magnetic flux quantity that interlinks the coil. 600
Fig. 6 is the results of the wind tunnel experiment. For
comparison purposes, the output curves are plotted with 400
that of another generator, and it is understood that the 200
present generator generates higher output at low and
middle wind speeds. As this generator has no cogging 0
0 2 4 6 8 10 12 14
torque and no iron loss, it was also confirmed by this
Wind speed [m/s]
experiment that it has an advantage in low and middle
wind speeds. Fig. 6 Wind tunnel experiment results

Table 2 Prototype generator specification

Rated revolution speed [rpm] 840 (1) Design
No load EMF [V] 206 (840rpm) Then, we designed 5-7 MW generators with a
diameter of 10 m and a rated speed of 13 rpm. It is more
Number of rotors 7
difficult to hold the magnet in a larger generator than in a
Number of poles 12 smaller one. As shown in Fig. 7, we fixed the magnet to
Rotor size [mm] φ140×6 the disk-shaped yoke in the inner rotor as in the outer
Gap between rotors [mm] 6 rotor.
Number of stators 6 As a first step in designing this generator, we
researched whether its output differs depending on the
Number of coils 9
number of poles. Fig. 8 shows three different rotors with
Stator size [mm] φ170×4 the different numbers of poles. Fig. 9 shows the average
Winding number of a coil 53 magnetic field distribution in the air gap between the
Outside size [mm] φ182×142 rotors in three cases. Here, the term average magnetic
Weight [kg] 8.5 field refers to the averaged fluctuations in the
circumferential direction of the magnetic field. The
Cooling Natural
horizontal axis in Fig. 9 shows the distance from the
center of the gap toward the surface of the magnet. As
can be seen from Fig. 9, the strongest magnetic field is
achieved with 48 poles. From this result, we can expect
that the strongest output will be achieved with this
number of poles.
Table 3 shows the results of calculated output and
other specifications. The allowable current was
calculated based on the heat transfer coefficient
obtained from the test of first small generator. While the
output with 12 poles is high due to the large number of
coil windings, the highest output per unit weight of the
coil and magnet is achieved with 48 poles. The loss of
a) Appearance b) Sectional view (3-D CAD) this generator is only copper loss, and the generation
Fig. 5 Appearance and Sectional view of test model efficiency is approx. 0.97. Fig. 10 shows an image of a
generator with 48 poles.
Rotor yoke

Fig.7 The structure of multi-megawatts axial generator

Fig. 10 Image of a 6.5 MW generator (48 poles)

(2) Rotor assembly

As described above, while an axial magnetic field type
permanent generator offers the advantage of high torque
12 poles 48 poles 72 poles density, it has a complicated structure that makes it
Fig. 8 Magnet and coil shapes with three different pole difficult to manufacture. The generator we designed has
numbers a structure that can achieve high power generation with
a large magnet even in a coreless generator and
0.4 increase generating efficiency without iron loss or
0.39 cogging torque. This structure makes the magnet unit
oversized, which leads to questions regarding the
Averaged Magnetic field　B[T]

0.37 feasibility of manufacturing and assembling such a large
0.36 permanent magnet. We therefore considered the
0.35 manufacture of large permanent magnets and rotor
0.34 assembly based on current technology.
0.33 12 poles The generator designed with 48 poles above has the
48 poles magnetic pole measuring W 376 mm x H 1740 mm x T
72 poles
0.31 50 mm. A process to incorporate magnets of this size
0.3 into rotor disks and to stack rotors with a specified gap
-40 -30 -20 -10 0 maintained is required. NdFeB magnets are
manufactured by arranging the crystal orientation of
Fig. 9 Magnetic field distribution in the air gap NdFeB magnet powder in a particular direction using an
external magnetic field, applying mechanical pressure to
the mold and sintering it through powder metallurgy. In
Table 3 Comparison between variant pole numbers line with this process, the electromagnet used to apply
Pole number 12 48 72 the magnetic field is incorporated into the press. Due to
Rated power [MW] 7.8 6.5 5.4 the limitations of the electromagnet’s performance, the
maximum size of a magnet manufactured in one press is
Magnet weight [ton] 52 47 46
approx. 100 mm square. As the pole of the generator we
Coil weight [ton] 42 22 18 designed is huge, it is made by assembling magnet
Efficiency 0.96 0.97 0.97 blocks measuring 100 mm x 100 mm x 50 mm. In
Drive type Direct drive addition, as NdFeB magnets are fragile, threaded holes
cannot be made in them. They are therefore bonded to
Rated rotation speed [rpm] 13
an iron plate (called the back plate), which is then bolted
Diameter of rotor [m] 9.6 to the rotor disk.
Cooling Natural Since assembling the magnetized magnet blocks to
the rotor disk one by one is intensive in terms of time
and labor, we separated the pole into four units as
shown in Fig. 11, incorporated each of the separated
units onto the rotor and bolted them to the rotor disk.
The magnetizing process for the magnets requires a
magnetic field exceeding 2 T. As we have magnetizing
equipment that uses superconductive coils, we can
magnetize a magnet of 500 mm cubic. This means we
can magnetize a unit of W 376 mm x H 436 mm.
Magnetic blocks before magnetizing are bedded and
bonded to a back plate. Then, the unit is completed by
one magnetizing process using the superconductive Rotor yoke
magnetizing equipment. This method is simple and easy
compared to magnetizing and fixing each magnet blocks.

Fig. 14 Sliding the magnets into the rotors is relatively

Fig. 11 Pole comprising four units Let us consider a method to embed these magnetized
magnet units into the rotor. As shown in Fig. 13, it is
rather difficult to control the unit due to the strong
magnetic attractive force toward the rotor in the method
where the unit is lowered to the rotor. In addition, the
magnetic attractive force generated between the rotors
when they are facing finally reaches approx. 2700 kN for
a gap of 140 mm, which also makes control rather
Accordingly, we decided to embed the units by sliding
them onto the rotor. Despite an attractive force of 10 kN
or more between rotor and unit, the unit can be moved
with a relatively light force by greasing the area between
the rotor and the back plate to reduce friction. We can
control the units easily against the magnetic force in
such a way that they are pushed from the outer diameter
side to a specified position along the guide. In addition,
this method enables us to embed the units while
maintaining the gap between the rotors aligned
beforehand so that they face each other, as shown in
Fig. 14. When the units are completely embedded, the
Fig. 12 Superconductive magnetizing equipment rotor assembly is complete. Then, coils are inserted into
the rotor gap from the outer diameter side, and the coils
are assembled and connected each other to complete
the manufacture of the stator.
A single axial-type generator comprises these two
rotors and one stator. The generated voltage can be
increased by stacking these in the axial direction.


To confirm the design, we manufactured and reviewed
a 1:20-scale model. Table 4 shows the specifications of
this model and the production machine.

Fig. 13 Embedding the magnets from the top is difficult.

Table 4 Specifications of the 1:20-scale model and the
production machine
scale model
Number of poles 48 48
6.5 MW 2.1kW
Rated power
( 13 rpm ) ( 180 rpm )
Number of stators 2 2
Number of rotors 3 3
Magnet length [mm] 1740 87
Magnet width [mm] 376 18.8
Magnet thickness [mm] 50 2.5
Coil thickness [mm] 60 3
(Between the surfaces 140 7
of the magnets）
Rotor diameter [mm] 9600 480
Fig.16 The 1:20-scale model manufactured
Magnet weight 47 ton 5.9 kg
Coil weight 22 ton 2.8 kg
Torque / rotation
Efficiency 0.97 0.81 measuring equipment Generator
Cooling Natural Natural

To assemble this 1:20-scale model, we employed the

method of arranging the rotor disks so that they face
each other and sliding the units onto the rotors. However,
we used the poles as one magnet unit instead of
separating them. We calculated the attraction between
the magnet units and the rotor as approx. 80 N by
Drive motor
analyzing the magnetic field. Fig. 15 shows how the
magnet units are embedded on the rotor. This process

can be easily performed using a simple tool. We Variable load


completed two pairs of rotor assemblies and stacked Generator
them into the whole assembly. As the leakage of the
magnetic field from the back of the rotor is minimal, the
attraction between the rotor assemblies is also very little,
which facilitates their manufacture. We then embedded
Revolution counter
and connected the coils. Fig. 16 shows the completed
1:20-scale model. Fig.17 Evaluation system
Output (Meas.)
Output power, Copper loss [kW]

1.2 Output (Cal.）

Copper loss (Meas.)
Copper loss （Cal.）




0 1 2 3
Magnet unit Load current [A]

Fig.18 Comparison of measured/calculated output and

copper loss for 180 rpm
Fig.15 Magnet unit embedding
The evaluation system shown in Fig.17 was used to VIII. CONCLUSIONS
measure the output of the manufactured test generator.
The generator was set to a revolution speed range of 0- 1. We designed a large coreless generator of the axial
180 rpm using the driving motor, and the output power type.
was measured by varying the load resistance value for 2. Analysis showed that a magnetic field in gap of rotors
each revolution speed. Fig. 18 shows the results for varies with number of poles. At large rotors which
output and copper loss. As can be seen from Fig. 18, the have a gap of 140 mm, when the number of poles
measured values were almost the same as the equals 48, the magnetic field is maximum and the
calculated ones, and an output of approx. 1 kW per power-to-weight ratio is highest. The rated power of
stator was obtained at 180 rpm. In addition, we observed 6.5 MW was obtained with the rotor diameter at 9.6 m,
almost only copper loss and obtained a generation 48 poles, 36 coils, 2 stators and the magnet weight at
efficiency of approx. 80%. These results confirmed that 47 tons. The magnetic pole dimension was
our magnetic field analysis and design were valid. 376x1740x50 mm and it was possible to magnetize it
using the magnetizer of a superconductive coil by
dividing it into four.
VII. DESIGN OF 10 MW GENERATOR 3. The magnet can be assembled relatively easily by the
method of first setting the two rotor yokes with the
In the WindPACT project of the NREL, it is said that specified gap and then assembling the magnet.
the small type drive train can be achieved through the 4. The results of measuring the output from a 1:20-scale
combination of a low speed generator and single stage test generator confirmed that our analysis was
gearbox [7]. correct.
We therefore designed 10 MW generators that 5. The study of a 10 MW generator showed that when
support three driving systems (direct drive, speed x 10 the operation speed is increased 100 times, the
and speed x 100 with gearbox) to calculate and compare magnet weight could be reduced to 1/20 of that of the
the weights of magnets to be used. We assumed the direct drive.
rated revolution speed of the wind turbine as 10 rpm. In
the direct drive system, because the revolution speed
drop from 13 rpm to 10 rpm and rated power rise from REFERENCES
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