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NREL/CP-500-24996 ? UC Category: 1213

Axial Flux, Modular, PermanentMagnet Generator with a Toroidal
Winding for Wind Turbine

E. Muljadi
C.P. Butterfield
Yih-Huei Wan
National Wind Technology Center
National Renewable Energy Laboratory
Presented at
IEEE Industry Applications Conference
St. Louis, MO
November 5-8, 1998

National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, Colorado 80401-3393
A national laboratory of the U.S. Department of Energy
Managed by Midwest Research Institute
for the U.S. Department of Energy
under contract No. DE-AC36-83CH10093
Work performed under task number WE803020
July 1998

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Axial Flux, Modular, Permanent-Magnet Generator with
a Toroidal Winding for Wind Turbine Applications
E. Muljadi, C. P. Butterfield, Yih-Huei Wan
National Renewable Energy Laboratory
1617 Cole Boulevard
Golden, CO 80401
Tel. (303)384-6900, Fax (303)384-6999,

Abstract - Permanent-magnet generators have been used for
wind turbines for many years. Many small wind turbine
manufacturers use direct-drive permanent-magnet generators.
For wind turbine generators, the design philosophy must
cover the following characteristics: low cost, light weight,
low speed, high torque, and variable speed generation. The
generator is easy to manufacture and the design can be scaled
up for a larger size without major retooling.
A modular permanent-magnet generator with axial flux
direction was chosen. The permanent magnet used is NdFeB
or ferrite magnet with flux guide to focus flux density in the
air gap. Each unit module of the generator may consist of
one, two, or more phases. Each generator can be expanded to
two or more unit modules. Each unit module is built from
simple modular poles. The stator winding is formed like a
torus. Thus, the assembly process is simplified and the
winding insertion in the slot is less tedious.
We built a prototype of one unit module and performed
preliminary tests in our laboratory. Follow up tests will be
conducted in our lab to improve the design.
Using permanent-magnet (PM) generators for small wind
turbines is very common. Usually an AC generator with
many poles operates between 10-100 Hz. Because the
generator is directly driven by the wind turbine [1,3,5], it is
commonly known as a direct drive generator. Many
configurations use surface mounted three phase PM
synchronous generators with a rectifier connected to the
generator terminal.
Many types of generator concepts have been used and
proposed to convert wind power into electricity. An axial
flux generator with a different type of winding and a different
magnet arrangement was developed [1,2]. A modular
concept was proposed to reduce manufacturing costs [3]. The
transverse flux generator has a higher power density than a
traditional induction generator [4].
In this paper, a
combination of a modular, axial flux, and torroidal stator
winding are applied to a permanent-magnet generator.

Although the design is intended for wind turbine applications,
this PM machine can be used for many other applications.
A wind turbine generator must be light to minimize the
requirements for the tower structure. Since the wind turbine
operates at low rotational speed, the generator is built with
many poles. We designed, built, and tested a permanentmagnet generator for wind turbines.
Several unique
properties are included in this design. It uses a modular
concept. Each pole is constructed individually, thus the
number of poles is based on the requirements. The winding
is concentric, like a torus, making it easy to assemble. The
rotor core has a focusing capability with a variable magnet
area, so the air gap flux density can be adjusted independent
of the rotor radius. A single unit module of this generator can
have single or multiple phases. Additional unit modules can
be stacked in the axial direction to get more power. With this
modular concept, any failure in one unit can be replaced
immediately or can be bypassed, thus minimizing turbine
The dimension of the generator and the size of each
component should be based on the actual wind turbine for
which it is to be used. Because the purpose of the prototype
unit is to prove the concept, we designed and built it with
readily available components. A steady state analysis was
done to determine the initial electric loading and magnetic
loading. The initial loss calculation was derived. The next
step of the calculation was done using finite element analysis.
The flux density in the critical components, and the map of
the core losses were found. No-load, rated, and short-circuit
conditions can be predicted from this analysis. Any changes
made were reiterated by using steady state analysis. Thus the
process was repeated until the final design is ready.
A test was conducted in the lab to find the parameters of
the generator and any unpredicted anomalies. Data were
collected for no-load and full-load conditions.
The first section of this paper is devoted to introducing the
background of the PM generator in wind turbine applications.
The second section introduces the generator components. In
the third section we present our analysis of the PM generator.

In the fourth section we describe testing, and lastly, in the
fifth section the conclusions are summarized.
In this paper we discuss only one unit module of the
generator. The generator consists of an eighteen-pole
permanent magnet. The stator and the rotor cores are made
of pre-cut transformer lamination silicon steel (gauge 26,
M19). The stator and rotor cores can be made on a per pole
basis, reducing the cost of complete dies required to stamp a
conventional lamination configuration. The geometry of the
stator and the rotor core could have been optimized, however,
this project focuses on the proof of concept.
A. Rotor
The cross section of the stator and rotor pole is illustrated
in Figure 1. Each pole is constructed from two identical corestacks and the permanent magnet is sandwiched in between.
The rotor is constructed to allow an expansion in the axial
direction, for example, to increase the magnet surface. The
flux directions at the top (outer radius) and the bottom (inner
radius) of the rotor pole are the opposite. Around the
perimeter of the rotor, the flux direction of one pole is
opposite of the flux direction in next pole, as shown by the
white arrows in Figure 2. The ratio of the magnetic surface
area to the pole surface area determines the focusing factor.
The chosen geometry enables the designer to increase the
length of the rotor core without affecting the stator geometry
and vice versa.
The rotor poles are attached to a non-magnetic disk that
holds the rotor cores. The shaft is attached to the disk to
rotate the rotor core. A non-magnetic stainless steel belt is
strapped around the rotor core to keep the rotor poles in
place. Since the rotor speed is low, centrifugal force created
when the rotor rotates is not very high. There are nine pole
pairs on the rotor. Between two rotor poles, there is a small
gap to minimize interpolar magnetic leakage.
B. Stator
The stator consists of two stator sides. There are nine poles
attached to each stator side. The poles on each side are
attached to a plate (not shown in Figure 2) which holds the
stator core

rotor core


Non magnet disk

Figure 2. PM Generator with Toroidal Winding
stator poles in place. In the prototype, one side of the stator
core can be rotated (within a limited angle range) with respect
to the other stator side. Thus the position of the stator cores
in one side can be shifted with respect to the other sides. The
shift can be adjusted to control the phase shift between the
first stator side and the second stator side.
C. Stator winding
The stator winding is wound like a torus or a washer. With
a toroidal form, the stator winding can be easily assembled
and automated for production. The stator winding between
the stator poles is exposed to open air, which improves
One advantage of wind power systems is the location of the
generator. It is mounted on a tower above the ground. The
cooling mechanism is better up on the nacelle than inside a
ground level building because the generator is always
exposed to air flow that is proportional to the generator load.
During low wind speeds, the heat transfer from the winding is
lower, however, the heat generated in the winding is lower,
too. The opposite is true at high wind; more heat is generated
in the winding, but more air flow is available to transfer the
heat away.
In this paper, one module unit is built for a single phase
generator. The stator windings at the two sides are connected
in parallel to generate a single phase output. The rotor shaft
is attached to the stator sides through the bearings, which are
attached to the stator plate. The rotor core has a width of
6.35 cm (2.5 in.) and a diameter of 29.2 cm (11.5 in.). The
overall width of the generator is 16.5 cm (6.5 in.), excluding
the two stator plates.
D. Expansion for multimodule generation system

rotor core

Figure 1. One pole of the stator and rotor core

The power from the stator can be actively controlled using
power switches (IGBTs) or passively controlled using a diode


Number of phases per unit module = 1 (two windings in
The electric loading:
Stator current = 11.0 Amp RMS (at per phase voltage 58 Volt
The wire chosen is AWG 12
The current density in the slot J = 3.4x106 Amp/m2
Predicted copper losses at rated current = 42 watts

To 60 Hz

B. Finite element analysis
To analyze the magnetic circuit, the finite element method
was used to compute the flux density in the generator
components. The main purpose of this analysis is to get the
overall picture of the saturation levels in different parts of the
generator, the iron losses in the components of the generators,
and the worst case of demagnetization on the permanent
magnet. In the finite element analysis presented here, the
generator uses a ferrite permanent magnet.
No-load condition. In the no-load condition, the magnetic

Figure 3. Expansion for multimodule generation.
rectifier. Figure 3 shows a possible configuration of the
power converter to process the power generated by the
generator. The generator may consist of one or more
modules. In this configuration, only three unit modules are
shown. Each unit module of the generator is paired with one
leg module of power switches on the power converter side.
Thus the power converter and the generator can be expanded
in a similar fashion. The power generated is converted back
to the utility via a three-phase inverter, which can be
controlled to produce good power quality.
The analysis of the generator is based on the wind turbine
requirements. The steady state analysis was performed as the
first step to get the first cut of design criteria. The finite
element analysis was performed to refine the magnetic
analysis. Finally, a dynamic analysis was performed in the
lab to validate generator performance under dynamic
A. Steady state analysis
The prime mover for this generator is a wind turbine. One
characteristic of wind turbines is that the rotational speed is
lower than most prime movers. To avoid using a gearbox,
the generator is direct driven. Multiple poles must be used to
allow slow speed operation.
From steady state analysis, the following criteria are
Number of poles = 18
Max operating frequency = 100 Hz (at 667 rpm)

Figure 4. Flux density at no-load condition
path is analyzed to see the magnetic flux density in different
parts of the magnetic paths. With the stator core in each side
shifted by 180o the maximum flux in the core happens when
the stator core and the rotor core are aligned. Figure 4 shows
the flux lines at the no-load condition. Only one side of the
stator core is shown. Some flux leakage is shown such as at
both ends of the rotor poles. The rotor core has low flux
density with the highest flux density at the parts closest to the
air gap. As shown in Figure 4, the maximum flux density

element analysis, the permanent magnet used is ferrite,
however, in this experiment the permanent magnet chosen is
rare earth permanent magnet (NdFeB).

Figure 5. Flux density at no-load condition
occurs at the corner of the U-shaped stator core. Figure 5
shows the magnitude of the flux density along the horizontal
line in the middle of the air gap. The maximum flux density
at no load is 1.55 Tesla. The flux density at the air gap is
0.9 Tesla and the flux density at the permanent magnet is
0.24 Tesla. The stator core and the rotor core have a flux
density below the saturation point.
Inductive load at rated current. In this condition, the
magnetic path is analyzed to see flux reduction at the air gap
at the least favorable power factor. The generator is loaded to
have rated current.
Short-circuit condition. In this condition, the magnetic
path is analyzed to see the demagnetization effects on the
permanent magnet. In order to analyze the worst case
scenario, the stator core and the rotor core are perfectly
aligned and the short circuit current is applied to the stator
core. In this case the short circuit current is about ten times
the rated current. The result is tabulated in Table 1.

Figure 6. Open circuit woltage
B. Voltage and current waveforms
The open circuit voltage is measured at the terminal output
of winding 2 (open circuit). The stator cores are shifted
toward each other by 180 electrical degrees. The voltage
waveform is captured from the scope, digitized, and plotted
in Figure 6 and Figure 7.
In Figure 7, the generator is loaded with resistive load up to
rated load at 100 Hz. The voltage across the terminal output
of the generator is a unity power factor load. Thus the
current waveform is reflected by this terminal voltage

Table 1. Flux Density Comparison at Different Magnetic
Paths for Different Conditions
Inductive Load (rated)
Short Circuit

B airgap
0.91 T
0.89 T
0.70 T

B max
1.55 T
1.50 T
1.05 T

B at PM
0.244 T
0.239 T
0.193 T

A. Experimental set up
The experiment was conducted to observe the performance
of the generator. The generator is driven by a motor via a
belt. The motor is a four pole motor, with rated speed of
1800 rpm. The motor is fed by a PWM variable frequency
drive. The generator speed is driven to 667 RPM. The
output frequency at this rpm is 100 Hz. The experiment is
conducted only on a single unit generator. In the finite

Figure 7. Terminal voltage across resistive load
C. Parameter Determination Test
A simple modified test is used to get the parameters of the
permanent magnet [6]. The experiment is shown in Figure 8.


- The axial flux design makes it easier to increase the flux
density in the air gap.

/ V,E = ?

Winding 2 (open)

Winding 1

is minimized. The design can be readily changed, such as
the number of poles in one unit or the number of unit
modules in a generator system.

- The toroidal form of the stator winding makes it easy to
fabricate. The geometry of the stator winding and stator
core make the heat dissipation more effective.
- To scale up the output power of the generator, more units
can be stacked in the axial directions. The power converter
required to process the power is readily compatible with
the generator. Each unit module of the generator is
matched with each leg of the power switches.




The authors wish to thank Jerry Bianchi for his assistance
during the test set up and Jim Adams for his help during the
fabrication of this generator.

Figure 8. Experimental set-up
One side of the generator (winding 1) is connected to a rated
load at unity power factor. The generator is driven to
generate a rated frequency. The other side of the winding
(winding 2) is an open circuit. The voltage output of winding
1 is called terminal voltage V and the open circuit voltage of
winding 2 is called open circuit voltage E. The angle
difference between V and E is called ?, which is the torque
angle of the generator at this load. The power, current, and
voltage output of winding 1 is recorded.
The parameters can be computed from the test data, and the
results are listed in Table 2 below.
Table 2. Results from Test Data
Vopen circuit
Rotor Speed

8.41 mH
75 volts
11 Amp
667 rpm

4.38 mH
Vrated load
100 Hz


0.22 ohm
58 volts
650 watt

The proposed generator is investigated for application in
wind power generation. In the first stage of implementation,
a proof of concept of the generator is investigated. The
magnetic and electric loading are shown to be within the
limits of common practice of machine design. The generator
has the following advantages for wind turbine generation:
- The modular concept is suitable for the commercial
production of machines of limited quantities and with
different sizes and output requirements. The components
are manufactured on a per pole basis. The tooling required

We wish to acknowledge our management at NREL and
the U.S. Department of Energy (DOE) for encouraging us
and approving the time and tools we needed for this project.
DOE supported this work under contract number DE-AC3683CH10093.
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July/August 1996, pp. 882-887.
[3] E. Spooner, A. Williamson, " Modular, Permanent-magnet
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IEEE Industry Applications Society, Oct. 6-10, 1996, San
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[4] S. Huang, J. Luo, T.A. Lipo, " Analysis and Evaluation of
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pp. 378-384
[5] E.F. Fuchs, A.A. Fardoun, P.Carlin, R.W. Erikson,
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Technology, Design and Applications, " Marcel Dekker, Inc.
New York, 1997.

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