The 18th National Conference on Electrical Drives “CNAE 2016”
Novel Topologies of Low-Speed Axial-Flux
Permanent-Magnet Micro-Wind Generators
M.M. Radulescu, S. Breban and M. Chirca
Department of Electric Machines and Drives, Technical University of Cluj-Napoca, Cluj-Napoca, Romania
Abstract – New topologies of direct-drive micro-wind
generators originated from double-sided inner-stator axial-flux
permanent-magnet (AFPM) generators are proposed in this
paper. The analyzed novel designs of coreless stator with nonoverlapping concentrated winding, sandwiched between two
outer rotor disks with embedded spoke-type ferrite magnets,
possibly combined with Nd-Fe-B magnets in quasi-Halbach
arrangement, may represent good-performance and costeffective topologies for low-speed AFPM generators in microwind power applications.
Index Terms – Double-sided axial-flux generator, direct drive,
coreless stator, concentrated windings, spoke-type ferritemagnet rotor, quasi-Halbach different-grade magnet
arrangement, low-speed micro-wind energy conversion systems
I.
INTRODUCTION
Small-scale wind turbines can be classified based on their
physical size (rotor diameter or swept area) and rated electric
power. Micro-wind turbines are considered by IEC 61400-2
Standard as those having 1 to 7 kW rated power and rotor
swept area limited to 40 m2. The efficiency of small-scale
wind turbines is rather weak compared with that of their largescale counterparts due to aerodynamics, operation in lowwind-speed built environment and unsuited designs.
The double-sided axial-flux permanent-magnet (AFPM)
machine topology with inner coreless stator and two twin
outer PM-rotors is considered as the topology
of
choice for low-speed micro-wind generator applications [1,
2]. Compared with their radial-flux counterparts, AFPM
generators have the advantages of more compact structure due
to the flat shape with short axial-length, larger power-toweight ratio and torque density, more flexible PM-field and
armature-winding design, better cooling and modular
construction, which make them suitable for mechanical
integration with micro-wind turbines [3, 4].
The coreless inner-stator configuration (i) has no iron
losses; (ii) eliminates cogging torque, which makes it easier
for the wind turbine to start at very low wind speeds; (iii) is
easy to manufacture; (iv) has no attractive magnetic forces on
each side to the external rotor disks, provided that the stator
winding is located precisely on the centre plane between the
two rotor disks.
As the magnetic losses in rotor PMs and disks are very
small and can be neglected, the rotor disks can be
manufactured from solid iron. Since direct-driven micro-wind
AFPM generators have to operate at low speeds in order to
match the micro-wind-turbine speed (2–10 m/s), and to
produce electricity within a reasonable frequency range (25–
70 Hz), they have rather large diameters and multipolar rotor
design with the disadvantages of inherent risk of excessive
leakage flux between neighboring PMs, caused by the small
pole-pitch, and of the increase in the material cost of the
machine. However, by adopting new topologies for the
double-sided AFPM generators with suitable PM-rotor
design, the overall cost of micro-wind turbine systems can be
significantly lowered, whereas the manufacturing process
may be simplified, and the complexity of the required
equipment may be reduced.
This paper is organized as follows. In Section II, the first
new PM-rotor topology for double-sided inner-coreless-stator
AFPM machines to be used as direct-driven generators in
micro-wind turbine applications is proposed and discussed
through design analysis. Section III presents and analyzes two
further improved variants of the first novel proposed topology
of AFPM micro-wind generators. Conclusions are drawn in
Section IV.
II.
CONVENTIONAL VS. FIRST NOVEL TOPOLOGY
OF AFPM MICRO-WIND GENERATOR
The double-sided AFPM generator with double-outerPM rotor, inner-coreless-stator configuration is illustrated in
Fig.1. The rotor disks consist of surface-mounted axiallymagnetized Nd-Fe-B PMs. The micro-wind turbine can be
directly attached to the outer rotors, thus reducing the system
weight and cost. Due to the relatively large magnetic airgap,
saturation in rotor back-iron is not of major concern. In its
turn, the coreless (slotless) configuration of the stator could
eliminate iron losses and cogging torque, and also lead to high
efficiency, light weight and low starting torque, which are
beneficial for small-scale wind power applications. The
relatively large magnetic airgap requires more rotor-PMs for
sufficient excitation, however, with the advantages of
minimized high-harmonic components of airgap flux density
distribution, as well as of increased power density and factor.
The main active parts in the double-sided AFPM
generator are the rotor-PMs and the slotless stator winding, as
depicted in Fig.1. Alongside of rotor-pole/ stator-coil
combination, the size and shape of rotor-PMs and stator coils
are the key parameters with great impact on machine
performances. However, the values of these parameters are
Rotor
Nd-Fe-B PMs
Coreless
stator winding
Rotor back-iron
Fig. 1. Topology of conventional double-sided AFPM
micro-wind generator [5].
ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 2016, Special Issue, ISSN 2344-5637
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The 18th National Conference on Electrical Drives “CNAE 2016”
tightly coupled, making the design optimization of AFPM
generator very challenging [5].
The main reason of proposing new alternative topologies
vs. the conventional double-sided AFPM micro-wind
generator of Fig.1 is the replacement of rare-earth Nd-Fe-B
rotor-PMs with high cost and limited supply by ferrite PMs.
However, ferrite magnets have low residual flux density and
coercitive magnetic field strength, so that magnetic-fluxconcentration rotor design of the AFPM generator is needed.
The first novel topology of the double-sided inner
coreless-stator AFPM generator proposed by the authors [6]
is shown in Fig.2. It provides an outer PM-rotor disk structure
with embedded spoke-type ferrite PMs (in shape
of
paralelipipedic bars) with alternating circumferential
magnetization, and interspersed iron pieces, constituting the
rotor magnetic poles, and thus avoiding the rotor back-iron.
Such a magnetic-flux-concentration spoke-type arrangement
offers the following benefits: (i) the flux density in the rotor
magnetic poles is higher than in the ferrite PMs; (ii) the axial
length of the machine may be extended to increase the
magnetic flux linkage of the machine; (iii) the effective airgap
is considerably reduced due to the embedded (not surfacemounted) rotor-PMs.
The inner coreless stator contains a three-phase,
single-layer, concentrated, non-overlapping winding with
planar coils of trapezoidal shape and surface corresponding to
the rotor poles of both airgap sides. The coreless-winding
stator is cast with composite material of epoxy resin and
hardener to build a rigid disk structure.
The main data for the basic new design of double-sided
inner-coreless-stator AFPM micro-wind generator are given
in Table I.
Rotor-PM flux-density distribution from 3-D finiteelement field analysis of the basic innovative design for the
double-sided inner-coreless-stator AFPM micro-wind
generator is depicted in Fig.3, which proves that magnetic
saturation in the ferrite-magnet (FM) rotor disks is not of
concern, since maximum value of 1.5 T has been achieved.
Table I
Main design data for the first proposed new topology
of double-sided AFPM micro-wind generator
Fig. 3. Ferrite-magnet (FM) flux density distribution
in the two outer rotor-disks of the first proposed novel topology
of double-sided AFPM micro-wind generator [6].
III. IMPROVED VARIANTS OF THE FIRST NOVEL TOPOLOGY
OF AFPM MICRO-WIND GENERATOR
The second new topological variant proposed by the
authors [7] for the double-sided inner coreless-stator AFPM
generator is shown in Fig.4.
The only rotor structural change resides on partial
cutting-out of the ferromagnetic core of each rotor pole with
the twofold purpose of increasing the airgap flux density
(Fig.5) and decreasing the overall weight of the AFPM
generator.
Comparative 3-D finite-element (FE) analysis results for
the two novel proposed topologies of the double-sided innercoreless-stator AFPM micro-wind generator, running at the
rated speed of 480 [rpm], and supplying the same three-phase
resistive load of 50 Ω/ phase, are given in Table II.
ferrite
magnets
Table II
Comparative FE-analysis results for the first
and second novel topologies of double-sided
AFPM micro-wind generators
Fig. 2. First proposed novel topology of double-sided AFPM
micro-wind generator [6].
Comparative AFPM generator
design analysis results
Rotor weight [kg]
AFPM generator overall weight [kg]
Stator-phase voltage, rms value [V]
1st new
topology
32.4
52.24
145
2nd new
topology
18.6
40.22
150
ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 2016, Special Issue, ISSN 2344-5637
The 18th National Conference on Electrical Drives “CNAE 2016”
interspersed
iron pieces
Nd-Fe-B magnet
with axial
magnetization
Fig. 4. Second proposed novel topology of double-sided AFPM
micro-wind generator [7].
B=1,7 [T]
ferrite magnets
with circumferential
magnetization
for flux concentration
Fig. 6. Third proposed novel topology of double-sided AFPM
micro-wind generator.
The corresponding FE-based field-analysis results for the
third novel proposed topology of the double-sided innercoreless-stator AFPM micro-wind generator, running at the
rated speed of 480 [rpm], and supplying the same three-phase
resistive load, are given in Table III, and compared with those
of the first proposed novel topology.
Table III
Comparative FE-analysis results for the first
and third novel topologies of double-sided
AFPM micro-wind generators
Comparative AFPM generator
design analysis results
Average electromagnetic torque [Nm]
Stator-phase voltage, rms value [V]
IV.
Fig. 5. Ferrite-magnet flux density distribution
in the two outer rotor-disks of the second proposed novel
topology of double-sided AFPM micro-wind generator [7].
The third new rotor topology proposed by the authors as
improved variant of the first spoke-type ferrite-magnet rotor
topology for the double-sided inner coreless-stator AFPM
generator, is shown in Fig.6.
In this rotor structure, each magnetic pole consists of five
parts: two ferrite-magnet bars with paralelipipedic shape and
alternating circumferential magnetization for flux
concentration, two interspersed iron pieces, which are
separated by a central bar of Nd-Fe-B magnet having
paralelipipedic shape and axial magnetization. It is thus
created for each magnetic pole of the rotor a quasi-Halbach
arrangement of different-grade magnets. With proper design
of this PM-rotor structure, sinusoidal airgap flux density
waveform with increased amplitude and high electromagnetic torque can be obtained without requiring any back
irons for closing the magnetic flux path in rotor disks.
373
1st new
topology
61.8
145
3rd new
topology
104
200
CONCLUSIONS
The design analysis of novel proposed topologies for
low-speed AFPM micro-wind generators having one innercoreless stator with three-phase non-overlapping concentrated
winding, and two outer rotor disks with embedded spoke-type
ferrite magnets, has been reported in this paper.
Two improved variants of the novel PM-rotor structure,
one by partial cutting-out of the ferromagnetic core of each
rotor pole, the other by combining frrite magnets with
Nd-Fe-B magnets in quasi-Halbach array arrangement, have
also been proposed and comparatively analyzed.
The design analysis results prove that the novel proposed
topologies for low-speed double-sided AFPM generators are
well suited for micro-wind power applications.
ACKNOWLEDGMENT
This work was supported by Romanian Executive Unit for
Financing Higher Education, Research, Development and
Innovation (UEFISCDI) from the research project with the
code PN-II-PT-PCCA-2011-3.2-1696.
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ACTA ELECTROTECHNICA, Volume 57, Number 3-4, 2016, Special Issue, ISSN 2344-5637