A Survey On Low Speed Low Power Axial Flux Generator Design and

Neurocomputing 509 (2022) 272–289
Contents lists available at ScienceDirect
Neurocomputing
journal homepage: www.elsevier.com/locate/neucom
A survey on low speed low power axial flux generator design and
optimization using simulation
Prashan Premaratne a,⇑, Inas Jawad Kadhim b, Muhammad Qadim Abdullah a, Brendan Halloran a,
Peter James Vial a
a
School of Electrical Computer & Telecommunications Engineering, University of Wollongong, North Wollongong, NSW 2522, Australia
b
Electrical Engineering Technical College, Middle Technical University, Baghdad, Iraq
a r t i c l e i n f o a b s t r a c t
Article history: Axial Flux Generator is a permanent magnet generator commonly used for low-speed power generation
Received 9 April 2022 using wind power. This generator can generate useful amount of power even under very low revolutions
Revised 11 June 2022 per minute (rpm). Over the last three decades, many researchers and engineers have developed plethora
Accepted 12 July 2022
of designs with varying success. Due to the commercial availability of rare-earth magnets such as NdFeB
Available online 28 July 2022
with very high magnetic flux density at very low prices, the propensity to develop new designs have sky
rocketed. Yet, many designs even with the availability of many advanced analytical software tools have
Keywords:
failed to produce useful amount of power under very low wind speeds. When the world is heading
Dual axial flux generator
Cogging torque
towards a more conscionable climate policy, wind power remains in the forefront of green energy.
Radial flux generator However, one major challenge in utilizing wind power is the inability of the small-scale generators to
Optimization of parameters generate useful amount of power at very low wind speeds. This poses a formidable challenge to the sci-
entific community to come up with the simplest, affordable and the most efficient design in harnessing
wind power at very low wind speeds; that is the most common situation around the world.
This research is the result of comprehensive and an exhaustive effort in bringing the best designs into
the forefront of research by critiquing innovative designs for their strengths and weaknesses. Almost all
the work in the literature has failed to realize the impact of the crucial parameters such as coil shape, coil
distance and rare-earth magnet shape and their distances from each other on the eventual generator effi-
ciency. This has resulted in most of the designs producing a very low output at very low revolutions. This
article will present a comprehensive survey of best designs in axial flux generators followed by a math-
ematical analysis of coil shapes, magnet shapes and gaps between these coils and magnets. Then the arti-
cle will present a very comprehensive electromagnetic simulations with multiple iterations in an effort to
optimize the many parameters which have not been analysed in previous works. This new insight will
open the research to a new height in achieving high efficiency in future designs.
Ó 2022 Elsevier B.V. All rights reserved.
1. Introduction research has been done on axial flux generators as it provides ease
of fabrication and higher power density compared to the radial flux
Most prominent generators are based on two designs; radial design. Another compelling reason for this has been the availability
flux and axial flux generators according to the flux direction in of low cost yet highly strong rare-earth magnets. In the context of
the airgap inside the generator. The defining difference between power generation using renewable sources such as wind power
radial flux permanent magnet (RFPM) and axial flux permanent and tidal waves, different types of machines have unique advan-
magnet (AFPM) machines can be described as a ratio of power/di- tages over the other designs. For instance, in hydroelectricity gen-
ameter of the machine. Axial flux machine output power is propor- eration, the flow of water turning the turbines have a
tional to the third power of the outer diameter, whereas in radial predetermined speed similar to a generator driven by gasoline or
flux machines, the output power is directly proportional to the steam. However, in harnessing wind energy, generator has to be
square of the diameter of the stator [1,2]. In recent decades, much effective in utilizing any wind that is prevailing. This may not be
an ideal approach to large wind turbines that generates over hun-
dreds of Kilowatts. Small scale wind energy harnessing expects to
⇑ Corresponding author.
maximize power generation even at very low wind speeds as it
E-mail address: prashan@uow.edu.au (P. Premaratne).
https://doi.org/10.1016/j.neucom.2022.07.004
0925-2312/Ó 2022 Elsevier B.V. All rights reserved.
P. Premaratne, I.J. Kadhim, M.Q. Abdullah et al. Neurocomputing 509 (2022) 272–289
could be the typical winds available under certain climate condi-

tions in certain geographical location. Hence, wind energy harness-
ing has provided the greatest challenge to the scientific community
in developing the best practice. This challenge has been taken up
by hundreds of designers over the years specially since 2000 as evi-
denced by the volume of literature published over the twenty
years. However, at very low wind speeds, cogging torque associ-
ated with generators with stator core have fared worse as the data
shows. Many works have been devoted to designs with coreless
stator designs not only to avoid wasteful cogging torque but also
iron losses due to eddy currents and copper losses in the designs.
Apart from impeding generators starting at low wind speeds, cog-
ging torque would be the source of acoustic noise and vibration
due to torque ripple [3,5,6,14,13,12,11,10,9,8,7,4]. This approach
also has provided an added benefit of light-weight generators as
well as lower material cost and production costs. When trying to
Fig. 2. Single Stator Single Rotor Configurations.
harness wind energy even at very low speeds, designers have real-
ized the benefits of axial flux generators due to their ability to pro-
duce power at very low speeds using large number of poles emphasis on developments that provide low output power in the
without gear boxes. range of 100 W at very low wind speeds up to 70 rpm. This rpm
Dual rotor axial flux generator (DRAFG) is a type of permanent has been chosen as it could result in more useful energy generation
magnet generators which consists of rare-earth magnets such as at very low scales and the ability to increase design efficiency will
neodymium iron boron (NdFeB) placed in rotors on either side of a undoubtedly result in major technological breakthroughs. The sec-
fixed stator winding, similar to a sandwich with a very small airgap tion on Related work will discuss significant accounts of research or
creating a very strong magnetic flux between them. Thus, this con- tutorials that have been published which shed light on the axial
figuration induces a powerful Electro Motive Force (EMF) in the sta- flux generators specially at low power levels. Some publications
tor winding. The major advantage of this disk-type generator is that will span from very low scale to very large scale and these designs
when the diameter is large consisting of many individual coils in the whether innovative or not will undoubtedly advance the knowl-
stator (9, 12, 16 or more coils), a voltage of around 20 V can be gen- edge of the reader in understanding the theoretical developments
erated with very low rpm. Hence, many designers try to develop as well as simulations or hardware designs. The section after the
their own DRAFG for power generation for wind turbines and tidal Related work will be devoted to the authors’ own development of
current-based power generation. The design can generate much an optimized axial flux generator which has ironed out many
more power at higher rpms. However, there are many design consid- design bottlenecks that many designers have not focused on. These
erations for an effective stator and rotor design that would ensure include parameters in axial flux generators such as coil shape, con-
the energy is not wasted in the process as many designers have straints on coils shapes with respect to chosen magnets, the mini-
ended up with poor designs with very low efficiencies. Axial flux mum gap needed between coils, the needed gap between magnets
generators and axial flux permanent magnet motors share many and the reasons behind many inefficiencies associated with many
common aspects. Design considerations in axial flux generators typ- axial flux generators.
ically enhance the axial flux permanent magnet machines as evi-
denced by a plethora of literature [15–46,55,48–54,47,56–58].
There are very common two different types of axial flux generators; 2. Related work
a single rotor single stator axial flux generator and dual rotor single
stator axial flux generator. The main difference between these two Most of the generators generate AC power and the coil configu-
types of axial flux generators is the performance, weight and the cost rations are setup to generate three-phase power. Murphy et al. [65]
to build them because the DRAFG requires twice the number of per- describes that three-phase power is more reliable than single-
manent magnets than a single axial flux generator design [59–64]. A phase power for generators. Three-phase power has higher starting
typical design of a DRAFG is shown in Fig. 1 and Single Stator Single torque and each phase is able to boost the power factor. It allows
Rotor axial flux configuration is shown in Fig. 2. significant electrical loads control as the current distributed in
As outlined above, this article is very much geared towards ana- each phase is lower. Besides that, the three-phase system is also
lysing the development of axial flux generators with a great capable of producing three different waves of power in sequence
which ensures the power flow is constantly delivered through
loads. Comparing this to single-phase system that is only able to
produce single wave of power that may drop to zero during one
complete cycle. Even though the zero drop is undetectable by
human perception, power electrical equipment with high power
demand can easily encounter problems in the long-term operation.
The efficiency of three-phase comes with several drawbacks in
terms of cost of installation and maintenance, but it all depends
on the power needs and prioritization. Fig. 3 shows the two types
of common AC flux generators.
As shown in Fig. 3(a) an axial flux generator can comprise of
either single or dual rotors with one stator. In this configuration,
the magnetic rotor is sandwiched between two stators. Fig. 3(b)
shows a radial flux generator where the design is much more com-
plicated as the coil-magnet separation with minimum air gap is
Fig. 1. Dual rotor single stator generator with exploded view. difficult to achieve. Pop et al. [66] evaluated the capabilities of axial
273
Table 1
U-core, C–core and E-core shape stators [68].
U-core C-core E-core
Fig. 3. An Axial Flux Generator with dual stators (left) Radial Flux Generator (right).
Table 2
flux generator and the radial flux generator. The comparison made
Analytical results of back-EMF, Cogging Torque and THD for C–core and E-core shapes
was highly focused on the electromagnetic field created during the [68].
current induction between the magnet and the conductor. The
Stator Shape Back-EMF (V) Cogging Torque (Nm) THD
radial flux generator is a three-phase system with a configuration
consisting of 18 stator teeth that are glued to the rotor skeleton C–core 1.65 0.96 19.40%
and 6 coils. Similarly, the axial flux generator is also a three- E-Core 1.13 1.22 10.40%
phase system that has one rotor in between of the two stators.
The rotor is a 4 pole-pairs while permanent magnets used are
NdFeB type which are glued to the disc rotor. The authors state that
if both axial flux and radial flux have the same air gaps, then the They used a coreless stator which was stated to be highly efficient
machines should produce the same amount of electromagnetic tor- as it could eliminate the direct flow of magnetic field in between
que because the area of air gap is directly proportional to electro- the stator and rotors. The proposed design had two outer rotors;
magnetic torque. Axial flux generator is preferred over radial flux one stator in between the rotors and non-ferromagnetic holders
generator because of the area of the air gap in axial flux generator to counteract the forces on magnets during the process. Materials
can produce higher torque-to-weight ratio compared to conven- used in the design were 12 rectangular shaped Neodymium Iron
tional radial flux generator. Boron (NdFeB) magnets and 6 trapezoidal shaped coils. The results
Nasiri-Zarandi et al. investigated the effects of dimension of air– they obtained show that the increase of rpm greatly increases the
gap and the number of stator slots available on the performance of output voltage thus proportionally increasing the efficiency. How-
axial flux generator [67]. The experiments they carried out were ever, when it comes to comparing the simulation and practical
based on the Finite Element Analysis method and the performance results, the efficiency was found to be reduced by 6.2 percent in
was evaluated between simulation and the constructed prototype practice. The difference between simulation results and practical
in order to have a reliable comparison. Three different configura- results are shown in Table 3. The authors concluded that the shape
tions of axial flux generator were proposed in which the first of the magnets led to poor test results.
design had 24 slots of stator followed by the second design that Different shapes of permanent magnets potentially affects the
had 30 slots of stator and final design had 36 slots for stators. Each harmonic components that is also related to the back-EMF which
stator also had coils with the same amount of turns. The experi- can result in losses. Shokri et al. have conducted a research on
ment was performed using same rpm speeds, same dimensions how the different magnet shapes can affect the performance of
and the analytical results of the Total Harmonics Distortion axial flux generator using 3-D finite element analysis method
(THD) of the flux density and the output torque were evaluated. [70]. Three different shapes of permanent magnets were used
Their results showed that by increasing number of slots results which were ‘sector’ like magnets, sinusoidal shaped magnets and
in improving flux distribution in the machine as well as reducing cylindrical shaped magnets. The results analyzed in the experi-
the THD. ments were based on the induced phase voltage that each shape
Commonly, there are two types of stators in Axial Flux Genera- of magnets could generate and the cogging torque occurrence, as
tor; coreless and cored. Coreless Axial Flux Generator mainly con- shown in Fig. 4 and Fig. 5. Based on the results, sinusoidal shaped
sists of vacuum or air such that the flux dissipation is not magnets had the lowest cogging torque and the optimized induced
controlled while the cored stator is able to control the flux switch- phase voltage at an angle of 180 degrees. Thus, the best perfor-
ing. Zhang et al. conducted an experiment on different shape of mance can be obtained is by using sinusoidal-shaped permanent
cored stator effect to the back electromotive force and the cogging magnet arrangement.
torque [68]. Three different shapes of cored stator are shown in When the shaft of the permanent magnet generator is rotated
Table 1. The results shown in Table 2 suggests that the Back-EMF without any load, the shaft resists it and until a certain amount
(voltage) is the highest when using C–core shape in order to obtain
greater output torque and using E-core shape leads to better fault
tolerance [68]. U-shape stator requires twice the number of perma- Table 3
nent magnets than the E-core shape or the C–core shape. There- Comparison of simulation versus measured results [69].
fore, the experiments conducted were focused more on C–core Data Speed Phase Output Efficiency
and E-core shape stators. However, in terms of cost effectiveness, (rpm) Voltage Power
coreless-stator is preferred compared to cored-stator in order to (V) (W)
reduce volumetric usage of metal in the machine. Simulation 3000 56.68 460 84.30%
Hosseini et al. have conducted an experiment on a small and Hardware Measurement 3000 40 390 78.10%
low-cost axial flux coreless permanent magnet generator [69].
274
Table 4
Cogging torque analysis on different slot opening and two different teeth profiles [5].
Slot opening Trapezoidal tooth Parallel tooth

(mm) (Nm) (%) (Nm) (%)
2 0.71 9.9 0.54 7.5
4.25 1.33 18.5 0.26 3.6
6.5 1.67 23.2 1.01 14
8.75 1.1 15.3 1.19 16.5
Table 5
Fig. 4. Comparison between cogging torque of different shaped of magnets [70]. Cogging torque analysis on different case studies. [6]
Solutions Case 1 Case 2 Case 3

Magnet pole-arc 0.85 0.79 0.77
Skewing angle 0.55 0.75 0.52
Back-EMF 144.2 131.1 128.4
Cogging torque 15.2 10.8 13.3
Kurt et al. and Gor et al. studied the loses and efficiency of elec-
tromagnetic design of permanent magnet generator [71,72]. Their
proposed design consisted of two rotors at both sides of the stator
containing 32 rare magnet discs and a stator in between with 24
coils. The machine with its 3-phase design was capable of generat-
Fig. 5. Comparison between induced phase voltage of different shaped of magnets
ing 340 W at 1000 rpm with an air gap of 5 mm. The authors found
[70].
that the core losses increased as the speed of rotor increased.
Besides that, large amount of losses was accounted for copper
losses and mechanical losses. Figs. 7, 8 results show that the core
of torque is exerted on it. This torque is called Cogging torque and
losses were increasing at higher resistance while the copper losses
is a hindrance to the self-starting capability of a wind generator at
were decreasing at higher resistance due to current reduction at
low wind speeds. It not only resists turning but also produces noise
high resistance. Hence, speed also play an important role in deliv-
and mechanical vibrations in wind turbines. Designers take variety
ering efficient performance of an axial flux generator.
of approaches to minimize this cogging torque and can be totally
Ani et al. have evaluated the performance of a simple core struc-
eliminated with a coreless stator design. Wanjiku, et al. have con-
ture of a small axial flux generator for small wind turbines [73].
ducted an analytical quasi-3D analysis on the influence of tooth
They explained in detail the design criteria and manufacturing pro-
profiles and slot openings to the cogging torque of an axial flux
generator [5]. In order to mitigate the cogging torque effectively,
the stator and rotor need to be modified. The authors proposed
analytical quasi-3D algorithm in the study of these cases. The
author used two profiles for the experiments which were trape-
zoidal teeth (a & c) and parallel teeth (b & d) as shown in Fig. 6. Slot
opening is the opening in between each magnet. The authors intro-
duced 4 different slot openings for the experiment. The results
from the experiment showed that the smaller the slot opening
the lower the cogging torque and the parallel tooth profile offered
lower cogging torque compared to trapezoidal tooth profile. The
results are shown in Table 4. Woo, et al. [6] also conducted an anal-
ysis on cogging-torque-effect by experimenting on three different
cases involving magnet pole-arc, skewing angle, back-EMF and
cogging torque. The results from Table 5 suggest that the magnet
pole-arc and skewing angle also provides significant effect to the
cogging torque and case 2 has the best performance to offer among
Fig. 7. Core loss against ohmic load for different speeds [72].
the three cases.
Fig. 6. (a & c) trapezoidal teeth (b & d) parallel teeth [5].
275
a detailed justification for comparison basis and the design data are
optimized and verified by finite-element analysis and commercial
generator test results. They attempt to justify their comparison
reasonable by keeping several parameters constant or in a limited
range. The rated phase voltage for each machine is designed to be
220 V and the power factor is maintained at 0.9 which is typical
value for wind generators. The outputs of the machines are chosen
to be 1KW, 10KW, 20KW, 50KW, 100KW, 150KW and 250KW with
a rpm value of 100 for machines below 50KW capacity and 50 rpm
for generators above 50KW which are typical rpm values found in
commercial direct-drive wind generators. They conclude from
their findings that axial-flux slotted machines have a smaller vol-
ume for a given power rating leading to power density being very
high for all the machines they investigated. They also point out the
importance of mechanical balance when designs with larger outer
Fig. 8. Copper loss against ohmic load for different speeds [72].
radii axial flux generators are designed. Two stator axial flux con-
figuration was found to be superior over one-stator configuration
despite having higher copper loses due to more coils in two stators.
cess in order to get low cost axial flux generator. The cost of small Wind energy generators could benefit from outer-rotor-radial flux
wind turbines may reach $1,8000/kW, which is quite high for res- generators over inner-rotor-radial flux generators due to better
idential area especially in rural areas. Coreless axial flux generator passive cooling ability and ease of installation. The Torus construc-
was chosen because of the coreless criteria that can eliminate the tion was found to be preferable despite construction requiring
cogging torque activity in the magnetic field leading to a simple more magnets for low-speed low power requirements without
design of low speed generator. requiring geared systems.
Since the permanent motors and permanent generators have Soderlund et al. [77] developed a two axial flux generators with
many common aspects when analysing their construction, it is power capacities of 5KW and 10KW and compared the measured
quite common to compare motors instead of generators. However, values against the calculations. The stator outer diameter wad kept
there is an important aspect when referring to a motor, which is at 40 cm and an inner diameter of 23 cm with 40 coils with 32
the torque that a motor generates where a generator is not dis- turns in each. The total number of magnetic poles of NdFeB was
cussed to have. One of the first comprehensive comparison of 14 and a coil resistance of 0.64/ohm per phase. The rotation speed
radial flux vs axial flux generator topologies are discussed by of the generator was assumed to be 50 rpm so that 50 Hz AC power
[74]. This comparison attempts to analyse the efficiency, physical output could be obtained. In their two configurations (one for 5KW
size and material cost for traditional radial field permanent magnet and the outher 10KW), some parameters were slightly different.
brushless machine and four unique configurations of axial field They concluded that small tolerances were to be achieved in the
permanent-magnet brushless machines. These consist of a single- construction for a reliable generator.
gap slotted axial field machine, a dual-gap slotted axial flux gener- Toroidal slotless axial flux generator was presented by Chal-
ator, a single-gap slot-less axial flux generator, and a dual-gap slot- mers et al. [78]. Their design consisted of many magnets due to
less axial flux generator. The generators are compared for five dif- slotless generators requiring more magnets to maintain adequate
ferent power generation levels from 250 W to 10KW at different magnetic flux in the circuit. They constructed both 1.5KW and
rotational speeds from 1000 revolutions per min (rpm) to 5KW experimental generators with 24 magnetic poles.
2000 rpm. The comparisons also provide estimates for associated Chen et al. [79] devised a generator based on insulated iron dust
development costs including material requirements such as cop- pressed slotted soft magnetic composite material-based stator to
per, steel and rare-earth magnets such as NdFeB weights. Further- reduce eddy currents in realising a high performance yet low cost
more, the authors estimate associated copper and iron losses, generator. As shown in Table 6, their design achieved 1KW at
moment of inertia, power per unit active weights and volumes 600 rpm which is not typically achieved by direct driven wind
for five different power generation levels. One of the conclusions power applications. This was mainly due to the small size of their
that the paper made was that axial flux generators were small in design of only 8 cm outer diameter.
volume for a given power rating making the material requirement Ferreira et al. [80] constructed a design which consists of an
small compared to the radial flux generators. As it is the case in internal rotor and two slotted stators on either side. The design
many such evaluations, the publication does not provide much used N30, much lower strength NdFeB magnets that are commer-
insight to construction of optimisation of axial flux generators cially available resulting in a power generation of only 340 W at
which is much more vital for enhancements.
Even though not specifically targeted at generators, [75] com-
pared axial flux against the radial flux machines. Their comparison Table 6
Main parameters of the axial flux permanent machine prototype [79].
based on the delivered electromagnetic torque and torque density
which is not directly impacting generators. One of the more posi- Output Power (kW) 1 Stator reactance (X) 2.2
tive outcomes for a generators was that their study concluded that Phase voltage (V) 24 Torque/volume (Nm/m3) 11430
larger dimensions facilitated higher pole numbers resulting in Speed (rpm) 600 Torque/weight (Nm/kg) 1.5043
higher voltages for a generator. They concluded that there is signif- Number of slots 18 Magnet weight (kg) 0.183
Inner radius (mm) 20 Copper weight (kg) 0.45
icant advantage in constructing axial flux generators when pole Outer radius (mm) 40 Core weight (kg) 0.909
number increases over 10 and the ratio of (axial length)/(outer sta- Turns per slot 110 Total loss (W) 100.44
tor diameter) is less than 0.3. Air gap flux density (T) 0.66 Copper loss (W) 48.05
Silaghi et al. [76] published an article that was very versatile for Air gap length (mm) 0.8 Iron loss (W) 11.98
Magnet length (mm) 4 Efficiency (%) 48.88
industry applications where one can determine the best configura-
Stator length (mm) 50 Poles 6
tion for axial or radial flux designs with power levels varying Stator resistance (X) 8.3 Rated current (A) 1.4
between 1 to 200KW for seven power levels. This account contains
276
600 rpm. However, their claim for low-speed direct drive generator Kaliyev et al. [84] aimed at generating around 50 W at low wind
ambition is not fulfilled in this design as a typical direct drive wind speeds to assist African nations with low power demand for daily
speed is no more than 70 rpm. Another disadvantage of this design uses. The rotor was an inner rotor with embedded cylindrical mag-
is the cogging torque due to slotted design as well as the usage of nets embedded in epoxy sandwiched between two stators with of
circular magnets as will be discussed in the optimization section of 14AWG wound trapezoidal coils. A total of 12 coils were used in
this article. both stators and they managed to retain an air gap of 1 mm, which
An interesting approach to maximize the power generation was was impressive. Their testing was done at 750 rpm which is 10
attempted by Nazlibilek et al. [81]. Experiments were conducted in times faster than typical wind speeds. Their design had probably
two ways: first they were carried out by manual switching and generated less than 50 W at 750 rpm however, the efficiency or
then by neuro-fuzzy controller. At low speed, all stages can be con- the losses associated with this design is pronounced. When mag-
nected in series to get more voltage, and can be connected in par- nets are embedded in epoxy without any supporting plate, there
allel at high speed to get more current. The results obtained by the is no proper magnetic flux returning path except air in this config-
neuro-fuzzy controller were well fit the results by manual switch- uration. Hence, the design is undoubtedly under-performing due to
ing. When the stages of the generator are connected in parallel lack of adequate magnetic flux for both stator coils.
at low speeds, the efficiency is the highest among the other One of the most comprehensive work up to date was carried out
configurations. This shows that the generator designed is much by Park et al. [85] in 2012, where they simulated and constructed
more suitable for low wind speed applications as compared to an axial flux permanent magnet generator that was 93% efficient
already available machines. and could generate 100 W at 80 rpm and was capable of generating
In a study, Dranca et al. [82] compared the above two configu- more power at higher wind speeds. This work is arguably the most
ration where one was coreless without an iron core and the other comprehensive attempt to calculate and account for various losses
design was slot-less with an iron core. Their numerical simulation and measure them using a test setup. They made use of a coreless
provided very similar figures for power generation capability of stator leading to zero cogging torque increasing the generator out-
1.8KW with total joule loses, permanent magnet joule losses very put. However, despite their lack of detail of the stator and rotor coil
similar except a 58 W iron loss associated with the design with and magnet placement, closer view indicates that there are many
the iron core. They carried out the simulation at 780 rpm which deficiencies that leads to voltage cancellation due to lack of gap
generated them 1.5KW power was not a typical value associated between coils and magnets. This will be discussed in the next sec-
with typical direct drive permanent magnet generators. As pointed tion of this article.
out earlier, any shaft speeds of over 70 rpm is not realistic for prac- Ishikawa et al. [86] proposed a novel combined axis flux and
tical applications of permanent magnet direct drive generators. 3D radial flux hybrid design that is expected to have nine times more
representation of their design is shown in Fig. 9. power density than a similar volume axial flux design. However,
In the work of Praglowska et al. [83], their design which was a their work failed to indicate the rpm for the generator. Due to
coreless two rotor NdFeB N40 magnet with 21 coils produced the stator core present in the design, it is also likely to suffer from
power output of 1800 W at an rpm of about 200. They further higher cogging torque due to the hybrid nature of axial flux as well
claim that their generator is capable of generating up to 3-4KW as radial flux design. It is questionable whether such a proposal has
with an rpm of 200. Looking at the size of N40 magnets with much any validity in wind energy based applications.
less magnetic flux and the mere size of 10x20x40 mm, it is highly Daghigh et al. [87] presented a dual rotor coreless stator design
unlikely such a machine is capable of producing such a high out- with a outer diameter of almost 105 cm and an rpm of 200 gener-
put. Their generator was tested using a DC motor that is not even ating 30KW. Even though the work is only based on simulation,
rated to be higher than 2KW. A closer analysis of the data pre- some expectations of the simulation where the stator and rotor
sented reveal a more reasonable power output of about 80 W at gap of 1.5 mm is almost impossible to be achieved even with very
about 100 rpm. rigid design material for the stator for such large diameter of
105 cm. Such a design capable of generating 30KW will have many
kilowatts of various energy loses making such a practical design
not feasible in the context of an axial flux design.
Many research attempt to model the magnetic flux in the gap
between rotors and stators in AFPMSG by finite element analysis.
However, due to the complexity of the designs, it is not a very time
efficient approach. Work presented by Ikram et al. [88], use analyt-
ical approach that relies on few simplifications or assumptions
about the model. The computation magnetic flux density compo-
nents at no load is achieved by solving Maxwell’s equations using
the boundary conditions. They use the following assumptions to
obtain the analytical solutions and simplify the derivation of the
analytical method: There is no magnetic saturation and the rotor
back iron has infinite permeability, the permanent magnets have
uniform magnetization and have constant relative recoil perme-
ability, and eddy currents are negligible. Magnetic field by the indi-
vidual rotor is calculated by neglecting one side’s permanent
magnets when calculating the magnetic flux density due to the
other side’s magnets. The permeability of the coil region is consid-
ered the same as that in the air gap region in the calculation of
magnetic field due to upper and lower rotors magnets. The sum-
mation of magnetic fields due to both rotor permanent magnets
is considered as the net magnetic field in the machine. Fig. 9 shows
Fig. 9. 3D representation of the three-phase coreless axial flux permanent magnet their initial model of the 1.0 kW, three phase, Y-connected, double-
generator (AFPMG). [82]. sided AFPMSG. The AFPMSG has two disc-shaped rotor yokes with
277
Fig. 10. Exploded view of the 3-D FEA model of the AFPMSG. [88].
permanent magnets placed on it. Their model with an exploded

view is shown if Fig. 10.
Direct driving of axial flux generators allows low power gener-
ators to be more effective in harnessing low wind speeds. However,
when wind speeds are low, the amount of power generated is
indeed low. An innovative design of counter rotating two rotors
have been presented by Kutt et al. [89] to make use of low wind
speed by effectively doubling the rotor speeds by driving the two Fig. 11. Physical prototype of counter rotating rotor axial flux generator [89].
rotors using two propellers in opposite directions. This effectively
increases the theoretical efficiency to 64% in harnessing wind
energy. Yet, the design is mechanically challenging and was The NdFeB based surface mounted topology was unable to develop
inspired by works of [90–93]. In the designs of [90,91], rotor is dri- the nominal electromagnetic torque, and also its torque ripple was
ven by one wind propeller and the stator is driven by another pro- higher and the efficiency lower than the spoke type generator.
peller. Due to the rotor and the stator were rotating in opposite Lee et al. [96] published an article based on their work on a dual
direction, the relative rpm of both were twice the typical rpm axial flux permanent magnet generator where their focus was to
experienced at the same wind speeds for a single turbine. [92,93] use stator core and then to reduce cogging torque by permanent
used differential planetary gear system for more equilibrated tor- magnet skew and stator displacement. The stator displacement
ques between both rotors. In the design of [89], they eliminated design was to reduce the cogging torque and was achieved by plac-
the mechanical gear box to lower inefficiencies associated with ing the upper and the lower stators in a staggered pattern. The
gearboxes in low wind speeds. However, despite this approach, work appears only to be a mathematical analysis and a simulation
their design resulted in very low efficiencies. Prototype of their hence their success of 91% efficiency could not be verified with a
generator with counter rotating turbines are shown in Fig. 11. physical design. The capability of power generation was stated as
Minaz et al. [94] developed a simulation model that uses three 1.32KW for the design. This was one of the few attempts where a
rotors and double stators based coreless axial flux generator to small wind generator is attempted with a core being aware of
maximize the power generation. They claim to have achieved high the fact that it affects the turbines ability to respond to slow winds.
power density despite not having cores in both stators. 18 coils and Hence, they undertook efforts to minimize torque. They claim to
48 magnets have been used in the machine and according to the succeed in coming up with a design capable of generating.
simulation results at 500 rpm, the machine generates approxi- All the research discussed above had one common aspect; the
mately 1.8 kW power. The observed current wave forms were sinu- generators used for wind power harnessing were all horizontal axis
soidal and no distortion was observed in harmonic analysis up to devices. In horizontal axis designs, there should be other mechan-
500 rpm. Yet, the claim of generating 4.5KW at 1500 rpm is very ical capabilities to respond to changing wind directions as only the
unrealistic as these are not achieved by wind speeds. At such high direct facing winds contribute to the maximum power generation.
speeds, temperature increases significantly weakens the magnets However, there exist vertical axis wind generators also known as
and the efficiencies. savonious rotor designs that respond to any direction. They also
Asefi et al. [95] undertook a study to compare the economic/ result in low starting wind speeds [97,98]. Their design did not
practical benefit of dual rotor axial flux generator using both ferro- incorporate permanent magnets unlike many common approaches
magnet and more expensive rare-earth magnets. Their goal was to but relied on excitation coils to deliver magnetic field. This also
see whether there would be cost-benefits to the more inexpensive resulted in using a core resulting in cogging torque but they man-
design of the ferromagnet based generator design. They used bicy- aged to minimize it by adding half slot pitch shift.
cle spoke type design for ferromagnets and ’sectors’ for NdFeB in Some researches have utilized magnetic levitation in achieving
both simulation and the physical fabrication. They found that the least amount of resistance from bearings in realizing vertical axis
spoke type Ferrite topology was a viable alternative to the conven- wind generators. Since the vertical axis design requires bearings
tional NdFeB generator and had lower total harmonic distortion to support the entire weight of the turbine, the generator and other
(THD) in its open circuit voltage, cogging torque and torque ripple. support rails, the bearings tend to degrade fast and also result in
The total weight of the ferromagnet design was 16% more active resistance to movement at low wind speeds. [99] discusses their
material mass and its induced voltages were more unbalanced. work in 2017 where they used NdFeB magnets to levitate the tur-
278
bine, generator and other support rails in achieving almost friction- which is undoubtedly a very inefficient design as well as the tur-
less bearing. In this design, power is generated using axial flux gen- bine rpm of 1000 was very unrealistic in using wind power.
erator. Fig. 12 shows their implementation of magnetic levitation. This Related work section is concluded with the most notable
However, their coils used in the generator were circular in shape research reported so far assessing the best design for axial flux gen-
erator as reported by Wirtayasa et al. [100]. The research modelled
9 different models of axial flux generators with different rare-earth
magnet shapes and coil placements as shown in Fig. 13. They kept
many parameters constant in order to assess the best configura-
tion. For a fair comparison process, they used five parts of each
generator that were kept constant including the axial length of
the airgap, the volume of the permanent magnet,the total axial
length of the generator, the outer and inner diameter of the stator
core and the number of the stator winding turns. Along with the
dimension being kept constant, each generator was also simulated
with the same magnitude of armature current (Ia) which resulted
in the same magnitude of armature reaction voltage. In selecting
the most effective generator, there were several constraints for
each parameter including the magnetic flux density not exceeding
Fig. 12. Magnet Placement of NdFeB Magnets to achieve magnetic levitation[99]. 2T; Vt were to be 15 V, THD was to be 8% according to the standard
Fig. 13. Nine different patterns of the AFPMGs between single-side and double-side topologies with surface mounted-permanent magnet method.[100].
279
Table 7
Comparison of axial flux generators at low RPM.
Paper Title RPM Power g Type

Performances compar. of axial-flux perm. magnet generators for small-scale vertical-axis wind turbine [100] 835 438 W 92% Simulation
Design, Implem. and Ana. of a MAGLEV W. [164] 100 1.4 W - Physical
Comparative Analysis of Coreless Axial Flux Permanent Magnet Synchronous Generator for Wind Power Generation [165] 100 40 W - Physical
Design, Prototyping, and Analysis of a Low Cost Axial-Flux Coreless Permanent-Magnet Generator [69] 100 negligible - Physical
Electromagnetic Analysis of an Axial Flux Permanent Magnet Generator [166] 100 negligible - Simulation
Electromagnetic Design of a New Axial Flux Gen. [71] 300 65 W - Simulation
Prototype of an Axial Flux Permanent Magnet Generator for Wind Energy Systems Applications [80] 600 340 W 70% Physical
Design Optimization of Direct-Coupled Ironless Axial Flux Permanent Magnet Synchronous [87] 200 30KW Simulation
Characteristic Analysis on Axial Flux Permanent Magnet Synchronous Generator Considering [85] 150 300 W 94% Physical
Model of Coreless Axial Flux Permanent Magnet Generator [83] 200 3KW - Physical
Comparative Design Analysis of Axial Flux Permanent Magnet Direct-Drive Wind Generators [82] 780 1.5KW 95% Simulation
Axial-flux PM Wind Generator with A Soft Magnetic Composite Core [79] 300 19 W 20% Physical
EN 61000–2-2, power output was to be 300W and the efficiency

was made close to 100%. Based on all the constraints above, the
generator type IX constructed with rectangular magnets was
selected as the best configuration. Table 7 summarize many proto-
type designs both physical and simulations that generates power
at low RPM. Generating power at low RPM highlights the opti-
mized designs where slow moving wind energy can be effectively
harnessed. It should be stated that some of these designs have no
merit at low RPM however, there inclusions highlight the limita-
tions of many design aspects that should be avoided.
The next section on Optimized design of dual axial flux generator
through analysis and simulation will discuss our approach to identi-
fying and optimizing the most crucial parameters to increase the
generator efficiency substantially. There is also another trend in
the world where researchers often uses established knowledge in
Neural Networks and classification techniques to increase the effi-
ciency of varying aspects of power generation [101–105,108,106,
107,109–111]. This review will not be a complete treatment if
many other attempts are not listed for the benefit of the readers.
It is essential that many developments in axial flux generators
are a result of large number of design paradigms that attempted Fig. 14. Dual rotor axial flux generator with labelled parameters for optimisation.
to enhance the multiple aspects of power generation. These
attempts have modelled many aspects of large scale and small
scale power generation as well as axial flux related machine design The major design parameters in a dual rotor axial flux generator
even with variety of core materials to ascertain the best practice can be stated as the coil shape, coil thickness, number of turns in
[112–166]. any individual coil, number of coils in the stator, the coil gap
between each coil in the stator configuration, the type of connec-
tion of coils which is star or delta, the type of magnet (strength)
3. Optimized design of dual axial flux generator through and its shape and dimensions, number of magnets in each rotor
analysis and simulation and the gap between the magnets and finally the airgap between
rotors and stator which is extremely crucial for an efficient design
From the comprehensive survey of many axial flux generator for given materials [167–171,86,172]. This research undertaking
designs, many designs fail to live up to be feasible for practical has established simulation results that show drastic reduction in
small-scale low wind speed deployments. This is mainly due to generated power when the air gap increases from even 1 mm to
the fact that most of the designs expect rpm of over 200 to gener- 2 mm. The goal of this work is to use a typical DRAFG design which
ate reasonable amount of power. Looking at the most of the complies with both Fleming’s right-hand rule and Lenz’s Law to
designs in stators and rotors of axial flux generators, it is quite maximize the power generation and then to optimize the parame-
obvious that this is mainly due to cancellation of voltage and hence ters that were stated above. This work also has introduced one
power due to coil shapes, magnet shapes and their close proximity extra constraint; for practical power generator with low rpm, the
to each other in each revolution. These deficiencies will be high- design should be adequate to generate around 100 W with an
lighted in this section and the best design parameters will be rpm value of around 100.
drawn at the end of the article. Some of these design parameters As mentioned above, many parameters that lead to inefficien-
are highlighted in Fig. 14. cies in dual axial flux generators will be first assessed using first
This work describes a reasonably practical design that can gen- principals of electromagnetic voltage induction and once a practi-
erate around 100 W of power at 100 rpm. This is very close to real cable design is reached, multiple simulations will be attempted to
life wind energy applications as higher rpms are not very practica- optimize many prominent parameters for the chosen design. The
ble for wind energy. Another important aspect of this design con- simulation will be attempted using Cedrat Flux 12.1 of the basic
sideration is that stator material such as PCBs are not practicable design with an objective of generating 100 W at 100 rpm to car-
for generation of more than 10 W and their flexible nature makes ryout finite element analysis of the magnetic flux distribution of
it inherently unsuitable for precision. the design.
280
Fig. 15. (a) An image captured from our DRAFG simulation (b) one ‘wedge’ shaped coil enlarged along with the four adjoining magnets from both rotors to explain the EMF.
3.1. Preliminary work lation. For the dimension of the magnet shown in Fig. 16, the mag-
netic field is given by:

The basic rules to implement the design is the Fleming’s right-
BðxÞ ¼ Bpr arctan pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
bc ffi arctan pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
bc
hand rule and Lenz’s Law. Fleming’s right-hand rule provides the 2x 4x2 þb2 þc2 2ðaþxÞ 2 2
4ðaþxÞ þb þc2
direction of the current flow whenever a conductor is moving For N52, the remnance value Br is about 1:3T. As shown in the
inside the magnetic field. This theory applies whenever the rotor diagram, the B is essentially a function of distance x from the cen-
is rotating between the stator where it creates an induced current tre of the magnet’s surface as shown in Fig. 16. However, when two
in the coils which increases the output power of the DRAF genera- rotors, each containing 16 magnets on each plate is facing each
tor. Lenz’s Law states that when the current is induced, it creates a other, the above calculation does not provide any useful means
magnetic field around the current flow. In order to create signifi- to calculate the varying magnetic field and hence the total flux.
cant amount of electrical energy, significant amount of mechanical In order to calculate the overall effect, the model has to be
energy is also required in the dual rotor axial flux generator. extended to what is shown in Fig. 17. This diagram clearly shows
We can carry out a simple analysis to determine an approxi- a section of the two-rotor magnetic plate along with the stator con-
mate value of the EMF that could be possible with our design using
fundamental theory. Using Fleming’s right hand rule and Len’s law,
we can clearly see that for a ‘wedge’ shaped coil as seen on Fig. 15
(a), when the magnets rotate, only the brown ’shoulders’ of the coil
(shown in Fig. 15(b)) produce any current as the magnetic flux is
only sweeping the coils at zero degrees there by not producing
any current.
3.2. Magnetic field due to permanent magnets
In order to analyse the emf generated by dual axial flux gener-

ator, one wedge shaped coil can be enlarged as seen on 15 (b). Here Fig. 16. Determining the magnetic flux density of a typical rectangular magnet.
the magnetic field strength is B, the angular velocity of clockwise
rotating rotors is x, and the area of one shoulder of the coil is A
(these shoulders are the only part of the coil that produces emf)
and the number of turns in each shoulder is N, then the peak elec-
tromotive force generated without any losses for each coil is:
E¼AN2-B
The total amount of power generated from the complete gener-
ator with 12 coils at peak disregarding losses is: ETotal ¼ 24ANB-.
For 100RPM with 100 turns of coil, which is the target of the
simulation, the angular velocity will be.
ETotal ¼ 24ANB- ¼ 24 A 100 B 2p 100
If the length of the coil shoulder is 7 cm and the width of the
section is 1 cm then the above ETotal can be expressed in terms of
magnetic flux density as ETotal ¼ 1055:5B.
The magnetic field strength B depends on the air gap between
the coil and the magnets from both sides and the type of material
used (Neodymium-Iron-Boron N52 in our experiment). B is a value
that is difficult to calculate but can easily be modelled using the
properties and the dimension of the magnets as done in the simu- Fig. 17. Construction of a dual rotor axial generator.
281
and the current sheet with peak value of K b located at the mean
axial position of yc ¼ Y22 [174]. The flux density at the mean axial
position using Fourier analysis is

b n l coshðln Y c coshðl ðY 2 yÞÞ cos l x ;

Bnarm ðxÞ ¼ K ð6Þ
0
sinhðln Y 2 n n
Fig. 18. Coordinate system of the magnetic flux density (This is a cross sectional
view of machine looking inward radially). where the linear current density function is
X
K ð xÞ ¼ b n sinðun xÞ:
K ð7Þ
taining two coils. It also shows the magnetic path for four magnets
The peak current density is
and how the path is closing the loop through the air gap.

Price et al. [173] describes how an analytic expression for airgap
b n ¼ Ni 4 sin nxc p
K ð8Þ
magnetic flux density is determined using the coordinate system of xc np 4Rm
Fig. 18 [173]. The circumferential and axial directions are repre-
sented by x and y coordinates respectively. Using the analytical
work of both Price et al. and Brumby et al., the space harmonic flux 4. Simulation
densities at position y due to magnets on the rotors 1 and 2 are
found to be [173,174], Our simulation of DRAFG design is implemented using Cedrat
! Flux 12.1 and Solidworks 2013 software. Solidworks is mainly used
bJ n l sinhðun lm Þ
Byn1 ðxÞ ¼ 0
coshðun ðY 2 yÞÞ cosðun xÞ ð1Þ to model the desired shape of the design in 3-dimensional geome-
un sinhðun Y 2 Þ try and is then imported into Cedrat Flux 12.1 to simulate the mesh
computation at each node. Many parameters of physics, mesh
!
bJ n l sinhðun lm Þ design and geometry are needed to be considered so that the sim-
Byn2 ðxÞ ¼ 0
coshðun ð yÞÞ cosðun xÞ ð2Þ ulation runs properly. The design has three phase axial generator
un sinhðun Y 2 Þ
that consists of 4 poles (coils) and four different alternating current
Where Y 2 ¼ lg þ 2lm and un ¼ 2pn=k, k ¼ 2pRm =p. circuits for each phase. The material used for each coil is copper
Here, Rm is the mean core radius of the radial slice, p represents and the permanent magnets used at each rotor were Neodymium
Magnets (N52). Besides that, other parameters such as magnet
the number of pole pairs and bJ n is the equivalent current sheet due
specifications, coil specifications, rotations per minute, coil and
to permanent magnets 1 or 2. The total flux density for each slice of
magnet gaps and air gap between stator and rotors were also taken
the machine is determined by the superposition of Eq. (1) and Eq.
into consideration in the design process..
(2). The magnets of each radial slice are modelled as an equivalent
Based on the well-understood theory as outlined in the Related
current sheet:
work, a design of dual rotor axial flux generator has been decided.

bJ n ¼ 4Br npsm Axial flux is chosen over radial flux because axial flux can be easily
sin ð3Þ
sp l0 lrec 2sp manufactured due to its volume and round shape of rotors and sta-
tor. The proposed design of DRAFG will have one stator in between
where sm and sp are the magnet and pole pitches, respectively, for the two rotors. Each rotor will have sixteen rectangular Neody-
the radial slice under analysis. The terms Br and lrec are the remnant mium permanent magnets and the stator will have twelve coils.
flux density and permeability of the permanent magnets. The total We have the desire to experiment with lower number of coils
flux densities due to magnets on rotors 1 and 2 for a single radial and lower number of magnets if those configurations can still gen-
slice are the sum of space harmonics: erate the desired output power of 100 W at 100RPM. The system is
X an AC generator with 3 phases and four pole per phase. The stator
By1 ðxÞ ¼ Bym1 ðxÞ ð4Þ
is coreless type that will have air between each coil to reduce cog-
n¼1
ging torque effect. The speed of rotor rotation for the experiment is
X from 100 rpm up to 400 rpm. The target output power is approxi-
By2 ðxÞ ¼ Bym2 ðxÞ ð5Þ
mately 100 watts that any household can use for simple tasks with
n¼1
an estimated induced current of 4 Amperes at 25V output. .
The design phase is performed in three steps. The first step is to
3.3. Magnetic field due to armature reaction perform dimension calculation and 2D drafting of stator and rotor
for DRAFG using Qcad 3.15. It is important to attempt 2D draft
This section discusses the magnetic field due to armature reac- before designing the 3D model of DRAFG in order to avoid miscal-
tion which complicates the magnetic field generated by the place- culating the number of permanent magnets used, find the correct
ment of permanent magnets on both rotors. Fig. 19 shows the angle for each position of coils and magnets, determine the size
coordinate system used in the expressions for the armature reac- of air gap between coils and to find the best size for each coils
tion. According to the expressions of [173,174] current sheet K and magnets that will fit in the proposed design. The first and sec-
ond drawings are as shown in Fig. 20 and Fig. 21 while the dimen-
sion for each drawing is as given in Table 8. This 2D draft is then
used to generate 3D drawings for stator and rotor. The final step
is to import each part into the simulation software and use the
toolbox to perform mesh analysis and obtain the results.
Cedrat Flux 12.1 is a software that is capable of computing finite
element electromagnetics for both 2D and 3D model. It is a power-
ful software that is also able to optimize any electromagnetic
Fig. 19. Cross sectional view showing the coordinate system for armature (looking devices using physics analysis. Firstly, the application chosen is
inward radially). Transient Magneto Analysis to perform movement of object at y-
282
Fig. 20. 2D draft of magnet and coil placement on a 40 cm rotor and stator.
Fig. 22. Complete volume assignment.
Table 9
Proposed DRAFG’s rotor and stator specification.
Specifications Description
Rotor diameter 40 cm
Num. permanent magnets 32 units (16 at each rotor)
Fig. 21. 2D draft of more accurate coil and magnet positioning based on real coils Type of permanent magnet N52 (Neodymium)
and magnets in hand. Remanence (T) for N52 1.43T
Permeability for N52 1.05
Rotor rotation Counter clockwise
Magnet size (LxWxH) 6.34x2x1.5 (cm)
Table 8
Stator diameter 40 cm
Dimension used on two different drawing.
Number of coils 12 coils (4 poles)
Specification First drawing Second Coil airgap 2 cm
drawing Coil shape Trapezoidal
Number of turns 100 turns
Rotor disc (cm) 40 40
Coil diameter 1 mm
Stator disc (cm) 40 40
Coils to air ratio 80% coils 20% air
Num. Permanent magnets 16 16
Current flow direction Counter clockwise
Length of permanent 9 cm 6.3419 cm
Coil length 31 m
magnet
Coil resistance 0.67X
Width of permanent 2 cm 2 cm
Air gap coil to PM 10 mm
magnet
Number of coils 12 12
Length of coil 13 cm 13.0717 cm
Width of coil 0 cm 0.7946
Arc of coil 7.3908 7.3908
rectly, an electrical circuit needs to be constructed and linked to
Air gap between coils 1.98 but not 1.98 each coil as shown in Fig. 23. Here, each coupling has a fixed cop-
parallel per resistance value of 0.67X which is due to the 1 mm diameter
and the 100 turns in each copper coil. We will experiment with dif-
ferent external resistor values to produce different output power,
induced current and voltage. There are four cases of study that
axis. The 3D model created in Solidworks are imported into Cedrat has been conducted in the experiments. The output power,
Flux 12.1 and each of its volume is defined accordingly to its func- induced current and the voltage generated will be tabulated, anal-
tion. The volume assignment created in Cedrat Flux 12.1 is shown ysed and discussed thoroughly. The focus of the experiment is to
in Fig. 22. find the best design that is capable of producing 100W of output
The pre-processing is a simulation phase to test the magnetic power at low rpm.
field and operate the DRAFG similar to hardware testing. The rotors As shown in Table 10, we experimented with three different
containing the permanent magnets will rotate in y-axis and the external resistor values; 10X, 1X and 0.1X. The purpose was to find
stator containing the coils will have fixed position. Each movement which resistance value will result in reasonable amount of voltage,
of magnets will be computed to obtain any readings such as volt- low current and high output power. As can be seen from the table,
age induced, current induced and power output produced at each we find that the 10X is suitable for further experimentation in our
coil. The magnetic field and the flux flow also can be analysed for simulation. Here, the output power was limited due to the air gap
each rotation of the rotor. of 10 mm which substantially weakened the magnetic flux sweep-
ing the stator. Usually having a larger gap between rotor and the
4.1. Simulation results stator will allow much more tolerance in designing a practical gen-
erator. We then moved onto reduce this gap. One of our goals in
Several dimensions are set as default value and the output will this experiment is to simulate a versatile design that is compact
be based on the rotation speed and the external resistor value. and uses minimum amount of resources such as copper and mag-
Table 9 shows the specification that has been considered for stator nets so that a generator could be developed at the lowest cost.
and rotor in the experiment. In order to simulate the DRAFG cor- Keeping this in mind, we ran few simulations where the diameter
283
Fig. 23. Circuit constructed in Cedrat Flux 12.1.
Table 10 Table 12
Power generated at different RPM with different external resistances of 10X, 1X and The impact of generating power at 100 rpm and 200 rpm when the air gap is reduced
0.1X from 5 mm to 2 mm (shown on the right-most column)
100 RPM 200 RPM 300 RPM 400 RPM Speed 100 RPM 200RPM
Voltage 5.4 V 10.8 V 15.9 V 21 V 10X 1-phase 3-phase 1-phase 3-phase 5 mm
Current 2.1A 4.11A 6A 8.19A Voltage 3.97 V 15.88 V 8.96 V 35.84 V
Power 11.34 W 44.4 W 95.4 W 172 W Current 1.49A 4.47A 2.98A 8.94A
Voltage 1.89 V 3.66 V 5.19 V 6.66 V 1X Power 5.92 W 70.98 W 26.07 W 320.4 W
Current 5.79A 11.52A 17.16A 22.77A Voltage 5.01 V 20.04 V 9.99 V 40 V 4 mm
Power 10.94 W 42.2 W 89.1 W 151.65 W Current 1.52A 4.56A 3.24A 9.72A
Voltage 0.67 V 1.5 V 2.4A 3.09 V 0.1X Power 7.61 W 91.38 W 32.36 W 388.8 W
Current 7.05A 14.1A 21A 27A Voltage 5.35 V 21.4 V 11.02 V 44.08 V 3 mm
Power 4.73 W 21.15 W 50.4 W 83.43 W Current 1.65A 4.95A 3.51A 10.53A
Power 8.82 W 105.93 W 38.68 W 464.2 W
Voltage 6V 24 V 12.27 V 49.08 V 2 mm
Current 1.8A 5.4A 3.67A 11.01A
Power 10.4 W 129.6 W 45 W 540 W
from Fig. 24, the 5 cm drop in the diameter somehow resulted in

dimensions that lead to cancelling of voltages leading to a very
poor power output. We discontinued the 35 cm diameter and
moved back to 40 cm diameter to systematically analyse the effect
of reducing the airgap on the output power. Next, the simulations
were attempted using multiple air gaps starting from 5 mm and
lower.
The design is performing as expected and the reduction in the
airgap has increased the power output. However, keeping in mind
that our goal is to produce 100 W at 100 RPM, we did not rush into
lower the airgap drastically as a tiny airgap demands precision
workmanship. We would be content with a design that produces
our power output goal with the biggest airgap.
Looking at the results of Table 12, power outputs at 3 mm and
2 mm, we realized that 100 W is attainable at 100 RPM. We wanted
to see whether we could still attain 100 W if we reduce the number
of magnets so that production cost for such a generator could still
be lowered. Table 12 indicates that this indeed was not the case as
Fig. 24. 35 cm diameter rotor and stator simulation in Cedrat 12.1.
the total power was reduced by almost 95%. The rotor with 40 cm
diameter and with 12 magnets is shown in Fig. 25. It is possible
Table 11
that magnet placement has resulted in cancelling of voltages gen-
Power generated using a rotor and stator with 35 cm diameter at 5 mm air gap
configuration shown in Fig. 24.
erated in the coils.
The next set of experiments were conducted to decide how
Speed 100 rpm 200 rpm
much power could be generated at different rpms when the air
1-phase 3-phase 1-phase 3-phase gap was lowered. The results are shown in Table 12. In this, it is
Voltage 0.03 V 0.09 V 0.11 V 0.33 V very clear that reduction in the air gap increases the generated out-
Current 0.011 A 0.033 A 0.027 A 0.081 A put as expected for 40 cm diameter rotor and stator combinations.
wer 0.0003 W 0.0029 W 0.003 W 0.026 W It is also evident that by increasing the rpms of the rotors, the
power can be substantially increased. According to this Table 13,
of the rotors and stator were reduced to 35 cm as shown in Fig. 24. highest amount of power that can be generated at 100 rpm was
This essentially reduces the size of the coils as well as the size of when the air gap was only 2 mm. Next, we would observe whether
the magnets that can be mounted on the rotors. However, the the power output can be further increased at our final air gap at
power output was negligible as shown in Table 11. As can be seen 1 mm.
284
The first simulation was run with an air gap of 1 mm when 12

magnets were used. As outlined in the previous section, reducing
the number of magnets will substantially lower the cost of a prac-
tical design. As can be seen in Table 13, the generated power was
only 5 W. This is again due to currents cancelling out when the
magnets are configured in a 12-magnet configuration. We ran
our final simulation with 16 magnets and an airgap of 1 mm. The
results in Table 14 shows that the system is capable of generating
almost 150 W at 100 RPM as desired, which would result in a prac-
tical system even with substantial loses. The final coil and magnet
configuration of the successful design is shown in Fig. 26. It is also
important to look at the waveforms of 3-phase current and volt-
ages to ensure the generator is capable of supporting a reasonable
load.
5. Conclusion
Developing an optimized DRAFG has a great impact on small

scale power generation when lower rpm and low noise require-
ments are paramount. Our design which had a goal of generating
100 W at 100 rpm was realized with a diameter of 40 cm with 12
coils and 16 N52 magnets. We have also realized that changing
Fig. 25. The coil and the magnet placement when there are only 12 magnets. the diameter even by 5 cm using the same magnets and coils can
change the configuration drastically reducing the power by
twenty-fold due to currents cancelling each other. This points out
Table 13
Power generation at 1 mm Gap with 12 Magnets.
that any DRAFG design should best be simulated first using realis-
tic component dimensions and properties in order to avoid any
Speed 100 rpm
inefficiencies associated with coil and magnet gap variations. We
1-phase 3-phase also observed that the airgap is very crucial in determining the
Voltage 4.40 V 17.6 V final power output and hence kept it to a minimum. This allows
Current 0.1 A 0.3 A the magnetic flux to flow through the tiny air gap with the least
Power 0.44 W 5.24 W reluctance generating the most amount of power. Looking at many
designs in the Related work section, many inefficiencies were asso-
Table 14 ciated with the shape of magnets such as using cylindrical magnets
Power generation at 100 and 200RPM at 1 mm with 16 Magnets. or cylindrical coils. Choosing large magnets with small coil shoul-
Speed 100 rpm 200 rpm ders should also be avoided for efficient implementations. Cur-
rently, few manufacturers in China are exporting many models of
1-phase 3-phase 1-phase 3-phase
low-power generators using axial design that produces up to
Voltage 5.81 V 23.24 V 13.47 V 53.88 V 2KW of power. These are specially geared towards wind generation
Current 2.12 A 6.36 A 4.21 A 12.63 A
Power 12.31 W 147.8 W 56.71 W 680.5 W
around buildings and highways at very low costs. It is feasible that
the new technology that is developed here will lead to more effi-
cient and more productive designs in future.
Declaration of Competing Interest
The authors declare that they have no known competing finan-

cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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1988.
288
Inas Jawad Kadhim received her B.Sc. and M.Sc. degree Brendan Halloran received his Bachelor of Engineering
in Electrical/Communication Engineering in 2003 and degree majoring in mechatronics at the University of
2005 from the University of Technology, Baghdad, Iraq. Wollongong, NSW, Australia in 2016, where he is cur-
She received her Ph.D. degree in Electrical and Compu- rently pursuing his PhD in distributed processing for
ter Engineering from the University of Wollongong, robotic vision with the School of Electrical, Computer
Australia in 2020. She is currently a lecturer in the and Telecommunications Engineering. His research
department of Electrical Power Engineering in the interests include probabilistic graphical modelling,
Electrical Engineering Technical College, Middle Tech- control theory, and SLAM.
nical University. Her research interests include infor-
mation hiding and image processing.
289

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Neurocomputing 509 (2022) 272–289

Contents lists available at ScienceDirect

could be the typical winds available under certain climate condi-

U-core C-core E-core

Slot opening Trapezoidal tooth Parallel tooth

Solutions Case 1 Case 2 Case 3

Fig. 6. (a & c) trapezoidal teeth (b & d) parallel teeth [5].

permanent magnets placed on it. Their model with an exploded

Paper Title RPM Power g Type

EN 61000–2-2, power output was to be 300W and the efficiency

3.2. Magnetic field due to permanent magnets

In order to analyse the emf generated by dual axial flux gener-

Fig. 22. Complete volume assignment.

Fig. 23. Circuit constructed in Cedrat Flux 12.1.

from Fig. 24, the 5 cm drop in the diameter somehow resulted in

The first simulation was run with an air gap of 1 mm when 12

Developing an optimized DRAFG has a great impact on small

Declaration of Competing Interest

The authors declare that they have no known competing finan-

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