Учебная работа. Planning of mobile complete set for a rural wind generator

1 Звезда2 Звезды3 Звезды4 Звезды5 Звезд (Пока оценок нет)
Загрузка...

Planning of mobile complete set for a rural wind generator

Planning of mobile
complete set for a rural wind generator


Abstract

The aim of this thesis is to alleviate the chronic lack of
electricity supply in the rural South African areas by designing a portable
wind generator kit.

An extensive assessment on the rural village of Ga-Rampuru,
in Limpopo Province, was conducted, to investigate the present needs, as well
as the availability of resources both human and material that would be needed
to construct and assemble the system. From the inventory of recyclable
materials found during the investigation the author was more inclined to
suggest the design of a wind turbine that could be assembled and maintained by
the local artisans.

A two pole permanent magnet synchronous generator was
designed using standard commercial magnets, which were later replaced by
recyclable loudspeaker magnets that were found in the village. This was done to
compare the output of the generator in both cases. All the designs were
modelled in FEMM, a software package, to estimate the induced voltage and flux
of the generators.

Using standard commercial magnets the simulated voltage and
flux levels were 9.4, 5.1, 3.6V and 0.0489, 0.0186, 0.0175 Wb, respectively.
Assuming a generator current rating of 1 amp this would yield 36 watts at the
estimated average wind speed of 4 meters per second.

Then when these were substituted with recycled speaker
magnets the generator yielded a voltage of 3.5V and a flux of 0.0171Wb. The
estimated output power of the recycled generator was estimated to be 10.5W.
This compared well with the power output from the commercial magnets
generators.

From these preliminary results it is quite apparent that a
viable generator can be designed from the recyclable magnetic components. The
same design procedure as outlined in this thesis can be used to design larger
recycled generators with larger outputs. The design of this wind turbine will
obviously have a wide range of positive developmental benefits on the community
of Ga-Rampuru.

The next stage was practical construction to validate of the
simulation results. This however could not be realised in time.


Chapter 1. Introduction

 

1.1 The subject of the report

The aim of this thesis is to design a simple wind generator
kit that can be easily assembled and installed by rural artisans. The kit will
use recyclable materials that are found in the rural areas to ensure a cost
effective and environmentally sustainable solution.

 

1.2 Background to research and investigation of
rural electrification

“Electricity brings immeasurable benefits to human life. With
electricity, comes lighting and the ability to extend the daylight hours, to
study and to improve education. With electricity come cooling and heating and
the ability to store food and cooking. At its extended level, electricity
facilitates communications, transportation and production and paves the way for
the eradication of poverty, industrialisation and ultimately the growth of our
country’s economy”.[3]

Electricity is a basic necessity and access to it has a wide
range of positive developmental benefits for communities [1], yet,
in 2001 2.8 million South African households still had no access to electricity
[2]. The majority of these households are poor and live in remote
places which are located far from the central business districts and the
country’s electricity grid. And this makes it very expensive to connect them to
the country’s electricity grid.

As a national initiative to improve the quality of life in
South Africa, National Electrification Programme (NEP) aims to provide
universal access to all South Africans by 2012 [4]. Hence, this has
lead to the investigation of other safe, cost effective and environmentally
friendly alternative methods of electrifying rural areas in South Africa.

Renewable energy resources such as wind and solar, are the
fastest growing alternative means of providing a reasonable amount of energy at
the Ga-Rampuru is a small village located in Limpopo Province in
South Africa. The village is in a fairly rural mountainous area, which is
situated some 58 odd kilometres from Polokwane, the provincial capital city.
The area has sparsely populated households with some trading stores and
schools. Most of the people in the village are unemployed and rely on
agriculture for their subsistence.

People in the village have to travel long distances to
collect wood or to purchase fuels like liquid petroleum gas (LPG) and kerosene
to meet their cooking, lighting, refrigeration, infotainment and other needs.
Figure 1 illustrates a picture of an LPG refrigerator in one of the trading
store in Ga-Rampuru. This picture and others that will follow in this thesis we
taken by the author during a visit at Ga-Rampuru last June vacation.

The supply of these fuels is both expensive and
unpredictable. Additionally the problems related to the use of fuels such as
kerosene are incidences concerning burned houses and respiratory problem for
children who use kerosene candles for reading is well documented world wide [6].

The author paid a visit to the Provincial ESKOM office to
enquire about any plans to extend the grid to Ga-Rampuru village; and the
Electrification Manager guaranteed that ESKOM has plans to ultimately electrify
the whole country by 2012. However, further discussions with people from
Ga-Rampuru dismissed the ESKOM Manager’s promises as empty. They contended that
they had heard similar promises but they still lived in darkness.

It was the conclusion of the author that an alternative
solution to the problem had to be devised. Some means of generating electric
power to meet loads such as the refrigerator in figure 1, if only it could be
an affordable design. The best design would clearly be one that uses local
material and human resources.


1.2.2 Resource assessment

The author spent the next three weeks exploring the resources
available in Ga-Rampuru that would support the design and sustainable
construction of electricity generators.

To begin with Ga-Rampuru has two schools, namely Rampuru
primary school and Seokeng secondary school, all which constitute a total
population of roughly 1400 pupils. On average 30% of school leavers will
continue to tertiary education, some will migrate to urban centres in search
for jobs and a substantial number will remain in the village.

This village is endowed with adequate human capacity with
intermediate levels of education. These would constitute a source of trainable
technicians and potential consumers of locally manufactured products. There are
also local mechanics who fix cars and some electrical appliances. These people
will be easily trained as they have hands on experience.

Some of the people who left the village for jobs in the
cities come back to settle down in the village and build big houses like the
one indicated in Fig 2. This clearly indicates that this people can afford the
electricity tariffs if they were to be supplied with power.

Moving further around the village there was evidence of old
windmills used for pumping water. Figure 3 shows one of the windmills. These
windmills operate satisfactorily providing enough water to the villagers. The
presence of these windmills in this area is evidence that there is some wind
resource in the area.

Further investigations took the author to various waste-dump
sites and a range of disused old gadgets that could potentially be re-used, as
shown in appendix A, were discovered. These included cables from an old car,
loudspeaker magnets, drums and old machines that were used for grinding grain.

The other natural resource in the area (of course) is the sun
but from the inventory of recyclable materials found during the investigation
it is more inclined to suggest the design of a wind turbine.


1.3 Objectives of the report

In light of the above Background, the main objective of this
thesis is to design a small wind generator for Ga-Rampuru village using
recyclable materials found in this village. The idea is to build an easily
assembled and manufactured machine that can be build by the rural artisans.
This wind generator must of course be cost effective.

The resource assessment of Ga-Rampuru village is conducted in
order to investigate the present needs, as well as the availability of
resources both human and material that would be needed to construct and
assemble the wind turbine using recyclable materials. Furthermore, the resource
assessment analyses lead to an appropriate wind generator design specifically
for Ga-Rampuru village.

 

1.4 Method of investigation

The investigations were conducted in July 2006 at Ga-Ramrupu
village in Limpopo province. The author collected information regarding this
village in the following manner:

1.       The author grew up in Ga-Rampuru village and
therefore knows the problems and challenges that the villagers face on a
day-to-day basis living without electricity. This was an advantage in terms of moving
around the village doing the resource assessment analysis.

2.       One of the store owners in the village, Mr Morifi
was interviewed regarding the issues he faces in supplying power to his store,
especially to the refrigerator he has in store. The store owner mentioned that
he has to refill the petroleum gas (LPG) in his store every two weeks. He also
added that this is very expensive as there are also transport costs involved.

3.       Face to face interviews were conducted with some of
the villagers where many concerns and challenges were raised. Most of the
villagers said that it has been several years since they have been promised to
be electrified and nothing has been done to date.

4.       The author paid a visit to the Provincial ESKOM
office in Pretoria to enquire about any plans to extend the grid to Ga-Rampuru
village. The ESKOM Electrification Manager, Jack Bandile was interviewed in
this regard.

1.5 Plan of development

The report begins with a brief Background of the thesis and
introduction of the rural area for which the wind generator will be designed
for. Then, the remaining project researches are outlined as follows:

·        Chapter 2 reviews the design of a small
wind generator and after that a wind generator suitable for Ga-Rampuru village
is designed using recyclable materials that where found in this village.

·        Chapter 3 details the procedure undertaken
to design a permanent magnet synchronous generator for Ga-Rampuru village wind
turbine.

·        Chapter 4, the generator geometry
discussed in chapter 3 is modelled in FEMN using recyclable and commercial
magnets to analyse and estimate both machine designs.

·        Chapter 5 discusses the results found in
chapter 4.

·        Chapter 6 details all the steps that were
taken in an attempt to assemble a prototype of the wind generator.

·        Chapter 7 & 8 concludes the discussion
based on the analyses and finally presents recommendations.


Chapter 2. Design of the wind turbine prototype

 

2.1 Background on wind energy

Wind powered systems have been widely used since the tenth
century for water pumping, grinding grain and other low power applications [9].
Since then, this has lead to an investigation and attempt to build large wind
energy systems to generate electricity.

Wind energy has proven to be cost effective and reliable in
the past years. The main development of this technology has been with large
wind turbines in the industrialized world, but there is scope to deliver
decentralized energy service in the rural areas of developing countries [6].

Furthermore, wind energy is an attractive option to generate
electricity since it does not consume fossil fuels nor emit greenhouse gases.
The land on which the wind generators are build may also be used for
agricultural purposes such as ploughing the land or domestic animal gazing.

During its transition from the earlier day’s wind ‘mills’ to
the modern electric generators, the wind energy conversion systems (WECS) have
transformed to various sizes, shapes and designs, to suit the applications for
which they are intended for [5]. In this chapter, the main
components of a simple small wind generator will be investigated and a wind
generator suitable for Ga-Rampuru village will be designed using recyclable
materials found in the area.

The available wind resource is governed by the climatology of
the region concerned and has a large variability from one location to the other
and also from season to season at any fixed location [9]. Hence, it
is important that the wind generator is designed for a specific area; this will
ensure that the wind energy in that specific area is exploited to generate
maximum power from the wind.


2.2 Wind turbine basic principles

The wind generators are specially designed and build to
extract power from turning blades with the maximum efficiency and minimum
complexity [6]. The magnet rotor disk rotates as a result of the
force of the wind on the turbine’s blades.

A typical small wind generator consisting of blades, tower,
PM generator and the cabling is illustrated in figure 2.1. The main components,
which are common to most wind generators, will be discussed below.

Fig 2.1 Basic
features of a typical small wind generator [6]

 

2.2.1 The blades

Modern wind turbine rotors usually have two or three wooden
blades. A larger number of blades would create more turning force (torque), but
would not be capable of driving the PM generator fast enough to generate the
required voltage, because the rotor would turn more slowly [6]. The
rotor blades are designed in such a way that they extract the maximum power
from the wind.

Power supplied by the blades to the generator is [7]:

  (Eq 2.1)


whereis the air density (Kg/m3), C is the
dimensionless power coefficient and A the area swept by the blades in m3.

In equation 2.1
above, the power drawn from the wind is proportional to the cube of the wind
speed. This means that if the wind speed doubles, there is 8 times as much
power available from it [7].

A further
important parameter is the tip speed ratio. The tip speed ratio is defined as
the ratio of the tip of the blade to that of the undisturbed wind velocity entering
the blades [11]. The ratio is given by [7]:

                                           (Eq 2.2)

where R is the radius of the blades, ωr is the rotor speed in rad/s and W the wind speed (m/s).

Multi bladed
rotors operate at low tip speed ratios of 1 or 2, where else, one, two or three
bladed rotors operate at higher tip speeds of 6 to 10. The power coefficient in
equation 2.1 depends on tip speed ratio as shown in figure 2.2. For a
particular wind rotor design there exists a tip speed ratio which will produce
the maximum value of power coefficient [11].

Fig. 2.2 Power
coefficient Cp versus tip speed ratio [11]

 

2.2.2 Permanent magnet generator

Using permanent-magnet generators for small wind turbines is
very commonly used world wide. Usually an AC generator with many poles operates
between 10-100 Hz. Many configurations use surface mounted three phase
permanent magnet synchronous generators with a rectifier connected to the
generator terminals. [16]

A simple PM generator consists of the stator, magnet rotor
disk and a shaft. The magnet rotor disk is mounted on a bearing hub so that it
can rotate on the shaft due to the rotating blades of the wind generator.

The stator has coils of copper wire wound around them, which
are accommodated in the slots. Electricity is then generated when the magnets
on the rotor disks rotate past the coils embedded in the stator. The magnetic
field that is created induces a voltage in the coils [6].

2.2.3 Rotor design

There are two types of rotor configurations commonly used
world wide, these are the disk and the cup as shown in figure 2.3 below [20].

Fig. 2.3 Disk and cup rotor designs

The radius of the rotor primarily depends on the power
expected from the turbine and the strength of the wind regime in which it
operates [5].

2.2.4 Tower

The main function of the tower is to raise the blades and the
generator to a height where the wind is stronger and smoother than the ground
level. The wind speed increases with height because of the earth surface [9].
The tower should be high enough to avoid any obstacles such as trees, building,
etc. Practical considerations such as expense, safety and maintenance limit the
tower to between 10m to 20m [6] above ground level.

2.3 Design of a wind turbine for Ga-Rampuru
village

In this section a wind generator that is designed
specifically for Ga-Rampuru village will be discussed. The generator will be
designed using recyclable materials such as car brake plates, cables and drums
found in the village [See appendix A]; this will clearly ensure a cost
effective design. The wind turbine will be designed in such a way that the
local people can easily assemble and manufacture it themselves.

2.3.1 The drum

The output of the wind generator depend on the amount of wind
swept by the blades, therefore the wind extracting materials in a wind
generator are very significant. A plastic drum will be used in this design to
extract the wind since it can be easily shaped and carefully balanced to run
smoothly. Also, it is resistant to fatigue braking and has a very light weight.

The drum will be assembled as follows:

1.       The top and the bottom part of the drum will be cut
carefully by using a knife or pair of scissors to make a cylinder with open
ends.

2.       The cylindrical drum is then cut length-wise into
two equal halves.

3.       The two halves are then glued together similar to
the drum shown in figure 2.4.


Figure 2.4 An S-shaped drum

To prevent the over speeding of the drum, the permanent
magnet generator should always be connected to a battery or other electrical
load. If this is not done the wind turbine will become noisy and may vibrate so
much that some parts come loose and fall to the ground [6].

 

2.3.2 Magnet rotor disk

After a tour around the village neighbourhood dumpsites it
was discovered that there are many discarded loud-speakers that are no longer
in use in the village. These loud-speakers have permanents mounted to their
back. Since the PM generator requires magnets, these loud-speakers will be
recycled and the magnets on them will be used in this design. Figure 2.5 shows
one such magnet that was found in the village.

There are many factors such as heat, radiation and strong
electrical currents that can affect the strength of a magnet [8],
especially in such discarded state. These factors will be discussed later to
investigate exactly how much surface magnetic flux density these magnets loose
in the dumpsites.

And later on in this thesis the performance of a PM wind
generator designed using standard commercial magnets will be compared to a
generator using the recycled loudspeaker magnets as substitutes.

Designing a generator using the speaker magnets will pose the
following challenges due to their shape and strength:

·    How does one design a machine with these magnets?

·        Do they have to be smashed and aligned to
work?

·        Or should they be used the way they are?

·        How much flux density do these magnets
have, in other Word, can they give out any power when used in the generator
design?

·        Can different magnet types be used on one
machine? As this magnets are picked randomly in the rural area.

2.3.3 Rotor Disk

A cylindrically shaped rotor is preferred as it allows the
proper distribution of flux over the armature surface as the field coils are spread
over the periphery of the cylindrical rotor. Hence, a brake plate from an old
car like the one in figure 2.6 will be used as the rotor in this design to hold
and house the magnets.

2.3.4 Distribution cables

All the cabling that will be required in the construction of
the wind generator was found in an old car in the village [See figure 2.7].

 

2.3.5 Artist impression of the wind turbine

Figure 2.8 below shows the artist impression of the wind
generator designed exclusively for Ga-Rampuru village.

Figure 2.8 Ga-Rampuru wind generator


The following chapters describe the steps taken by the author
to investigate the performance of a synchronous permanent magnet machine
constructed using recyclable loudspeaker magnets.


Chapter 3. Generator Design

 

3.1 A brief Background

This chapter will detail a simple procedure undertaken to
design the wind generator from recyclable materials. Permanent magnet machines
are preferred for this application as they reduce the excitation losses
significantly and hence a substantial increase in the efficiency of the
machine. In addition, permanent magnet machines are simple to construct and
maintain [10].

The most common wind turbine systems are three blades
rotating on a horizontal axis coupled to an alternator to generate electricity,
which could be used to for battery charging. For a picture of a typical basic
wind turbine system refer to figure 2.1 in chapter 2.

A normal two- pole synchronous permanent magnet generator
will be designed and its performance will be analysed. Then recyclable
loudspeaker magnets found in the rural area of Ga-Rampuru village will be used
to substitute the standard commercial magnets in the generator. The performance
of the new generator will be analysed to understand the effect of the
loudspeaker magnets on the generator performance.

For this investigation, matching the refrigerator load in
chapter 1 will not be a priority.

This chapter will start with outlining the desired generator
Specification and then the generator will be designed thereafter. To design the
generator the permanent magnet properties will be discussed to understand their
effect on the generator performance and losses due to these magnetic materials
will also be investigated. And then, all the variables that are necessary to
construct and design a generator geometry will also be discussed.

Throughout this thesis the generator performance will be
tested under no-load conditions.


3.2 Generator specifications

In this thesis, a generator with the following specifications
will be designed and modelled in FEMM, a finite element package:

·        Output power = 36W @ 12V

·        Number of phases = 3

·        Number of poles = 2

The choice of the above dimensions of the generator was
influenced by the following consideration:

·        Induced output voltage, 12V is standard voltage
that is used in many applications. For example it is suitable to charge a
battery. Batteries are suitable to power a wide range of rural appliances and
instruments especially in remote areas of South Africa [11].

·        The generator must be easily assembled and
manufactured so that the rural artisans with little training can be able to
assemble this generator.

The following design procedure will be followed:

1.       A simple two-pole synchronous permanent magnet
generator will be designed using available standard commercial magnets such as
ceramics, alnicos and rare-earth magnets.

2.       The effects of the above magnets on the performance
of the generator will be investigated.

3.       The magnets from a loudspeaker that was randomly
picked in the village will then be used in the design and the performance will
also be investigated.

The designs above will be modelled using FEMM, a finite
element package. The main reason for using FEMM is to observe the output
induced voltage of the generator. This will be the method of how the
performance of the generator will be monitored.


3.3 Generator basic principle

The main function of a generator is to supply power to the
load, in order to do so; voltage has to be generated at the terminals. The
generator principle is based on Faraday’s law of induction [10]:

                                                (Eq. 3.1)

where e is the
instantaneous voltage, is the flux linkage and t is the time.

The law states
that for voltage to be induced in a winding, the magnetic flux has to change
relative to the winding. This means that the flux linkage is changing and the
conductor is fixed or stationary. The flux linkage is the total flux,, linking all conductors in a winding with N turns.
Therefore the flux linkage is given by:

                                                (Eq. 3.2)

To generate
voltage in practice, a mechanical motion and a source of magnetic flux must be
present. The mechanical motion can be linear or rotational, in this thesis the
motion is rotational and provided by the wind turbine. The source of flux is
permanent magnets.

 

3.4 Properties of permanent magnets

The use of permanent magnets in the construction of
electrical machines has lots of benefits. A PM can produce magnetic flux in the
airgap with no exciting winding and no dissipation of electric power [14].

Permanent magnets are known for their large hysteresis loop
and B-H curves. These curves are in the second quadrant of the loop called the
demagnetization curve; this is where the magnets operate. Demagnetization
curves of the PM materials are given is Fig 3.1

In all machines using permanent magnets to set up the
required magnetic flux, it is desirable that the material used for permanent
magnets have the following characteristics [12]:

a)       A large retentivity (residual flux density) so that
the magnet is “strong” and provides the needed flux

b)       A large coercivity so that it cannot be easily
demagnetized by armature reaction fields and temperature.

For analysis purpose, the magnet properties have to be known,
the remanence flux density Br and coercivity Hc. The
magnets are characterised by a large B-H loop, high Br and Hc.
Table 3.1 summarizes the properties of some of the standard commercial magnets,
these were estimated from figure 3.2 which indicate the demagnetization curves
of different permanent magnet materials.

Magnet

Type

Br
(T)

Hc (kA/m)

Rare-Earth

NdFeb32

1.22

900

Alnico

Alnico5

1.21

50

Ceramic

Ceramic8

0.4

260

Table 3.1 Magnets properties

Figure 3.1 Demagnetization curves for different PM materials


The remanence magnetic flux density Br is the
magnetic flux density corresponding to zero magnetic field intensity. High
remanence means that the magnet can support higher magnetic flux density in the
airgap of the magnetic circuit. While the coercivity Hc is the value
of demagnetizing field intensity necessary to bring the magnetic flux density
to zero in a material that is previously magnetized. High coercivity means that
a thinner magnet can be used to withstand the demagnetization field [10].

3.4.1 Types of magnets

There are three main types of magnets that can be found,
these are [10]:

1.       ALNICO (Aluminium, nickel, cobalt, etc.)

These type of magnets poses high magnetic remanent flux
density and low temperature coefficients. The coercive force is very low and
the demagnetization curve is extremely non-linear. Therefore, it is very easy
to magnetize and demagnetize ALNICO magnets.

2.       Ceramic or Ferrites (BaFe203 or SrFe203)

A ferrite has a higher coercive force than Alnico, but at the
same time has a lower remanent magnetic flux density. Their main advantage is
their low cost and very high electric resistance.

3.       Rare — earth (SmCO, NdFeb-Neodynium Iron Boron)

These are one of the strongest types of magnets available.
They poses high remanent flux density, high coercive force, high energy
product, linear demagnetization curve and low temperature coefficients. The
main disadvantage is the cost.

High performance rare-earth magnets have successfully
replaced Alnico and Ferrites magnets in all applications where the high
power-to-weight ratio, improved dynamic performance or higher efficiency are of
prime interest.


3.4.2 Factors affecting recycled magnets

The recycled magnets that will be used in this thesis were randomly
picked; therefore there is no indication on how long they have been in the
dumpsites. The following are the factors that can affect the strength of
magnets:

·   Heat

·        Radiation

·        Other magnets in close proximity to the
magnet

If a magnet is stored away from high temperatures, and from
the factors mentioned above, it will retain its magnetism essentially forever.
Modern magnet materials lose a fraction of their magnetism over time if
affected by the above factors [8].

 

3.5 Generator losses

The losses in a synchronous generator consist of rotational
loss (mechanical loss and magnetic loss) and copper loss in the armature
winding. The rotational loss and the field winding loss are subtracted from the
power to obtain the power developed by the armature. By subtracting the copper
losses in the armature from the developed power, we obtain the output power of
a synchronous generator.

In this section, the core loss will be discussed since they
are due to the magnetic flux variations.

 

3.5.1 Eddy current loss

This power loss occurs in a magnetic core when the flux
density changes rapidly in the core. Because core material has resistance, a
power loss i2R will be caused by the eddy current and will appear as
heat in the core [13].

The average eddy current loss is:

                       (Eq. 3.3)


where Pe
is the eddy current loss in watts (W), ke is
the constant that depends on the conductivity of the magnetic material, f is
the frequency in hertz (Hz), δ is the lamination thickness in meters, Bm is the
maximum flux density in tesla (T) and V is the volume of the magnetic material
in cubic meters (m3) [14].

The eddy current
losses can be reduced by [13]:

·        Using a high-resistivity core material

·        Using a laminated core, in transformers
and electric machines the parts that are made of magnetic core and carry
time-varying flux are normally laminated.

 

3.5.2 Hysteresis loss

During a cycle variation of current i, there is a net energy
flowing from the source to the coil-core assembly. This energy loss goes to
heat the core. The loss of power loss in the core owing to hysteresis effects
is called hysterisis loss.

By testing various ferromagnetic materials, Charles Steinmetz
proposed that hysteresis loss can be expressed as [14]:

                                   (Eq. 3.4)

where Ph
is the hysteresis loss in watts, kh is a constant that depends upon
the magnetic material and n is the Steinmetz exponent.

3.5.3 Core loss

The hysterisis loss and eddy current loss are lumped together
as the core loss of the coil-core assembly, and given by:

                                      (Eq. 3.5)


3.6 Design Variables

In the following section, all the parameters that are
necessary to design and construct a generator will be discussed and variables
such as generator diameter, length, etc. will also be calculated.

 

3.6.1 Speed of the generator

The annual mean wind speed at Ga-Rampuru is approximately
4m/s [11]. The rotor will rotate at the same speed as the wind
turbine; therefore this means that the rotor will rotate at:

        = 250 rad/s = 2387.3 rpm

The rotor speed
and the average frequency of the induced voltage are related by:

                   (Eq. 3.7)

Since a two-pole
machine will be designed, the frequency is calculated using equation 3.9 to be
39.79 Hz.

3.6.2 Rotor and Stator Core

A cylindrically shaped rotor will be appropriate for this
design as it allows maximum flux distribution over the armature surface as the
field coils are spread over the periphery of the rotor. This type of design
also accommodates the use of small cylindrical magnets [11].

A low carbon steel core with low permeability will be used in
this design as it was found in the recyclable materials found in the village.
This type of steel is cheap and mostly available. Compared with other better
and expensive steel such as silicon, cobalt, etc. this type of steel has a very
high core loss. The steel saturation flux density Bsat is estimated
from the BH curve to be 1.5T.

 

3.6.3 Rotor Diameter (D)

The rotor diameter must be greater than the rotor yoke height
(Hry), shaft radius (Rshaft) and the radial magnet length
(Lm) [10].

D = 2 Hry + 2 Rshaft + 2Lm                                       (Eq.
3.8)

In this design, D is restricted by the magnet arc radius of
25mm. Therefore D will be 50mm.

 

3.6.4 Rotor and Stator Yoke heights

The minimum rotor yoke height Hry is the same as
the stator yoke height Hsy. The height should be large enough to
avoid saturation. This also has advantages of reducing core loss and
reluctance.

The minimum yoke heights are given by [10]:

                                        (Eq. 3.9)

 

3.6.5 Airgap Length

The airgap length has a minimum value limited by the
manufacturing tolerances; this value is typically in the range of 0.3mm to 1mm.
In this design 0.5mm is chosen to be the airgap length.

 

3.6.6 Generator Length

The generator length is estimated to be 95mm; this is
approximated from flux required to give the output voltage of the generator.


3.6.7 Airgap Flux Per Pole

In a radial machine, the flux per pole is given by:

                                              (Eq. 3.10)

where B is the
average airgap flux density, D is the rotor inner diameter, L is the generator
length, Kst is the lamination stacking factor and p is the pole
pairs.

For this design
the average flux density per pole Bgav is equal to the peak flux
density Bg since the magnet arc is close to 180 degrees. Therefore
the peak airgap flux is estimated to be 0.5T at the airgap and Kst
is assumed to be 0.97.

The airgap flux
and the lamination stacking factors were estimated from the following
dimensions of the loudspeaker magnet:

·        Magnet arc = 180 mechanical degrees

·        Inner radius = 8mm

·        Arc radius = 25mm

·        Magnet radial length = 4mm

·        Area
of one pole = 706.8 μm2

From the above magnet dimensions, the flux per pole in the
machine is then estimated to be 1.16 mWb this value is calculated from equation
3.10.

 

3.6.8 Windings

The stators of most synchronous generators are wound with
three distinct and independent windings to generate three-phase power [14].
A simple layer winding was used in this design. Slot per pole per phase was
chosen to be 1 and the winding pitch was full pitch.

A.      Types of winding

The preferred type of winding is distributed winding as it reduces
harmonics and makes better use of the stator or rotor structure. The mmf
induced in the airgap is not sinusoidal, to get a pure sinusoidal mmf the
number of slots have to be infinity. This means that the distributed winding is
expected to have some harmonics.

Induced voltage for the distributed windings is:

                   (Eq. 3.11)

Kw is
the winding factor and depends on the winding arrangements and has a value less
than unity. Distribution factor Kd and a short pitch factor Kp
reduces the winding voltage magnitudes but also reduces certain harmonics in
EMF and MMF waveforms.

            (Eq. 3.12)

Distributed
winding configuration has one slot per pole per phase and its winding factor is
equal to 1.

B.      Winding arrangement

Single layer winding, where each slot contains one coil side,
will be used in this design as it is economical to manufacture and has simpler
end connection. Emf and mmf can be modified to reduce harmonics. In a three
phase system even harmonics do not appear due to the winding symmetry, the only
visible harmonics are the belt harmonics.

C.      Winding Pitch

Short pitch is the most commonly used type of winding pitch.
It reduces the distorting harmonics and produces a truer sinusoidal wave. The
length of the end connection is also reduced thereby saving copper and reducing
copper loss in the coil.

The drawback of short pitch winding is that the induced emf
in it is smaller than in a full-pitch coil. The reason is that the total flux linking
the short-pitch coil is smaller than that of the full-pitch coil.

The design parameters discussed will be modelled in FEMM in
the next chapter to induce the output voltage and flux of the generator.


Chapter 4. Modelling the design in FEMM

4.1 Introduction

The investigation that will follow focuses on the effect of
substituting standard commercial magnets with recyclable speaker magnets that
were collected from a dumpsite in the village, to compare the performance of
the generator in either case.

In this chapter, the two pole generator geometry discussed in
chapter 3 will be modelled in FEMM to analyse the output induced voltage and
the flux of the generator. The lua-Script in FEMM is run and the rotor is
rotated 360 electrical degrees, for the lua-Script refer to appendix C1.

Initially, a choice was made of three typical commercial
magnet grades. Neodymium-iron-boron NdFeB was chosen from the rare-earth magnet
group. Alnico6 was chosen from the Alnicos and the last type was barium ferrite
from the ferrite or ceramic group. Then the machine will be modelled using
different types of commercial magnets to investigate the performance of the
generator.

The author then proceeded to investigate the magnetic
properties of the loudspeaker magnet. This was done so that the parameters can
be modelled in the finite element package.

Finally a design using the loudspeaker magnets was modelled
to explore the recycled generator output.

4.2 Two pole geometry

Table 4.1 below summarizes the generator specifications that
were discussed in chapter 3. These parameters will be modelled in FEMM to view
the output induced rms voltage and the flux.


Quantity

Value

Frequency

39.79Hz

Poles

2

Connection

Y

Diameter of
Rotor

50mm

Machine Depth

15mm

Air gap length

0.5mm

Turns per phase

80

Stator slots

6

Steel Core

1020 steel

Table 4.1 Data of designed PM machine

The design is modelled in FEMM and is illustrated in figure
4.1 below.

Figure 4.1 The generator modelled in FEMM

 

4.3 Commercial magnets

To investigate the performance of the generator, the author
began by modelling the generator with standard commercial magnets with the
properties given in table 3.1. The output rms emf and flux of the generator is
tabulated in table 4.2 with different magnets that were used in the design.

Refer to appendix B for the graphs of the outputs. Matlab
soft ware was used to draw the output rms emf and the flux, matlab code
included in appendix C2.


Table 4.2 Generator output with commercial magnets

Magnet

Type

Flux (Rms)

EMF (Rms)

Rare-Earth

NdFeb32

0.0459

9.4262

Alnico

Alnoco6

0.0186

5.1619

Ceramic

Ceramic8

0.0175

3.6075

4.4 Recyclable magnet found in the rural area

The magnet that was used in this section was from a
loudspeaker that was found lying in one of the dumps at Ga-Rampuru village. To
start with the magnet shape was not of concern. The author aimed to investigate
the performance of the magnet on the speaker if used as it was found. The
properties of this magnet were investigated and a design was modelled using
these magnets. The magnet is shown below in figure 4.2.

4.4.1 Background on the characteristics of
loudspeaker magnets

For speaker applications, the amount of permanent magnet
required is directly proportional to the rated output power of the speaker. In
other words high power speakers are often made using the high-grade magnetic
types like the rare-earth. But since the speakers found in the dumpsite were
from low power appliances their typical magnets are normally from the ceramic
group type. In addition unlike Alnico magnets, ferrite or ceramic magnets are
not easily demagnetised magnetized and hence find wide application in such
appliances.

4.4.2 Properties of the loudspeaker magnet

According to its nameplate the speaker that used the magnet
in figure 4.3 had a 0.5W rms and an impedance of 8 ohm. The magnet type on the
loudspeaker is a ferrite [Refer to appendix D1]. The manufacturer of the magnet
on the speaker is traced in order to find the B-H properties of the magnet on
the speaker.

Appendix D2 indicates TDK datasheet for ferrite magnets FB
series. These notes were used to find the magnetic, physical and mechanical
characteristics of the magnet. The properties of the loudspeaker are summarized
in table 4.3.

Magnet

Type

Br
(T)

Hc (kA/m)

Ferrite

FB5N

0.43

214.9

Table 4.3 Summarized properties of the magnet speaker

4.4.3 Output EMF and flux of the recyclable
generator

The properties were modelled in FEMM, and the generator
outputs are tabulated in table 4.4. Refer to appendix B2 for the graphs of the
outputs.

Loudspeaker
Magnet

Flux (Rms)

EMF (Rms)

Ferrite

0.0171

Table 4.4 Generator output with the loudspeaker
magnet

 

4.5 The estimated output power of the generators

The output electrical power of a generator is given by:

                                      (Eq. 4.1)

where V is the
terminal voltage of the machine. The power factor is assumed to be unity for
these calculations since all the simulations and investigations are done at
no-load.

From the rated
power of the generator which is 36W. If the rated voltage is assumed to be 12 V
then the rated current of the generator can be calculated from equation 4.1 to
be 1A.

Table 4.2 and 4.3
above gives the results of the simulated induced voltages and flux obtained
from the generator with commercial and recycled magnets. Using the 1A above as
the rated current, the output power of the generator using commercial magnets
and recycled loudspeaker magnets is summarized in table 4.5 below. The output
power in all the cases is calculated from equation 4.1.

Magnet

Type

Output Power

Rare-Earth

NdFeb32

28.3W

Alnico

Alnoco5

15.5W

Ceramic

Ceramic8

10.8W

Ceramic

Speaker magnet

10.5W

Table 4.5 The output power of the generator


Chapter 5. Analysis of the generator outputs

In this chapter the author first began by analysing the
output power of the generator designed with commercial magnets and the one with
recycled loudspeaker magnets. The author then explored the factors that may
have affected the outputs from the recycled generator.

The terminal voltage induced from the recycled generator is
also explored to view if it can be used in any applications in the rural
village. This is done so that the voltage can be evaluated if it is useful or
not

Lastly the loudspeaker magnets are investigated to view how
they can be used in the recycled generator design; whether they should be
smashed and aligned to be re-used in the generator design or if they should be
used the way they are without being smashed.

5.1 The estimated output power of the generators

The output power of the generators is estimated from the
output induced voltages of the generators. Consequently, this means that the
higher the terminal voltage of the generator the larger the output power.

From the theory of magnets it is clear that the induced
voltage is directly proportional to the remanent magnetic flux density Br of a
magnet. In other words it is expected that rare-earth magnets which posses
higher Br will always induce high voltage when used in generators. Therefore it
can be said that the type of magnet used in a generator is very important as it
determines the output power of the generator.

As can be seen from the results, the induced voltage of the
generator with NdFeB magnets from the rare-earth magnet family is higher than
that with the AlNiCo and ferrite magnets. This was expected because of the
different B-H properties of these magnets.

The recycled generator in this thesis was designed using
loudspeaker magnet that is from the ferrite family. These types of magnets are
cheap and readily available, but their disadvantage is that they posses low
surface flux density. The induced voltage was therefore expected to be much
lower than the voltage induced in a generator with NdFeD magnets.

 

5.2 The rms output flux of the generator

The magnetic flux density in the gap of PM generators is
limited by the remanent magnetic flux density of PMs and saturation magnetic
flux density of ferromagnetic core. Hence, the simulated value of output flux
is directly proportional to the remanent magnetic flux. In addition, permanent
magnet machine cannot normally produce the high flux density of a wound pole
rotor.

5.3 Factors that may have affected the recycled
generator outputs

There are many factors that should be taken in consideration
with regards to the induced voltage from the recycled generator. Some magnetic
deterioration may have occurred after the magnets were thrown into the dump.
But, due to the magnet’s magnetic permanent properties, these magnets are
expected to still have some surface flux density when found in the dumpsites.

This is evidence that any permanent magnet that is found in
the dumpsites can be reused in a generator design to induce some voltage, of
course depending on their B-H properties.

The estimated properties of the speaker magnets that were
used in this thesis were found from the loudspeaker manufacture, clearly these
properties will not be the same as the properties of recycled magnets that were
found in the rural area of Ga-Rampuru. These recycled magnets have been affected
by different conditions such as temperatures, climates, etc.

The exact properties of the recycled magnets can only be
found by testing these magnets in the laboratory. For this thesis the author
was unable to take the loudspeaker magnets found in the rural area of
Ga-Rampuru to the laboratory.

 

5.4 Applications of voltage from the wind turbine

The induced voltage of the generator will vary with the wind
speeds experienced in this village. The generator can be connected to a battery
to store the power which can be utilized when there is little or no wind.

If more power is required, the voltage can be boosted by
using any economical booster that can convert the output of the recycled
generator to at least a minimum of 12V. The voltage from the booster can then
be put through a cheap electronic regulator that will only charge the battery
if the boosted voltage from the wind generator is sufficient to produce at
least 12V direct current.

To power the refrigerator in chapter 1, the store owner in
the village will have to purchase an inverter that will convert the DC voltage
to AC voltage. The inverter will convert the low-voltage from the battery (12V)
into mains-type 230V alternating current.

5.5 Design using speaker magnets

Finally, the author investigated how speaker magnets can be
used in the generator design, if they have to be smashed or used as they are.

As already investigated, loudspeaker magnets are commonly
from the ferrite magnets family. Ferrite and rare-earth magnets are by nature
very hard and brittle. Although they can be cut, drilled and machined this
should only be done by individuals who are experienced with ceramics. If the
magnets get over about 300 deg F, they will lose their magnetism permanently [17].

Therefore, it will be very difficult for rural artisans to
cut these magnets and use them. Due to limited time the author could not
investigate if these magnets can be used as they are in the machine.

In the next chapter the author attempts to assemble the wind
generator in the laboratory.


Chapter 6. Practical comparison of the generators

 

6.1 Introduction

The following chapter outlines the steps that were taken in
order to assemble the permanent magnet generator discussed in the previous
chapters. This is done in order to compare the practical outputs of the
generator with the simulated ones. The other reason is to investigate the
performance of recyclable magnets with irregular shapes.

This investigation will only concentrate on assembling the
generator part of the wind turbine system.

For the construction of the PM generator in this thesis two
options were considered, the first was to collect readily available off-shelf
materials to assemble a small generator. And the second was to convert an AC
induction motor to a PM generator. Both options are discussed in this chapter.

 

6.2 Materials required to assemble a PM generator

The main idea is to build a portable generator that is easily
assembled and constructed in the laboratory. The author first begins with
highlighting all the materials that are needed in the construction of this
generator. Figure 6.1 gives the schematic of how the generator will look like.

Figure 6.1 Basic wind generator design


From the generator illustrated above it is clear that the
following materials will be required in the construction of the generator:

·        Magnets

·        Stator and rotor

·        Rotor mounted on a rotating structure

·        Structure mound

In the following sections the author will outline steps taken
and the challenges faced in collecting these materials.

 

6.2.1 Magnets used in the generator

In the investigation of the performance of the generator, the
author was to begin by designing the generator using standard commercial
magnets, which were to be later substituted with recyclable magnets. The
recyclable magnets are picked randomly in the dumpsites of Ga-Rampuru village.

Finding commercial magnets for this investigation was a major
challenge since for this two-pole generator the author needed to purchase two
NdFeb32 magnets, two Alnico5 magnets and two ceramic8 magnets.

 

6.2.2 Stator and rotor

The rotor rotates with the structure mount while the stator
is fixed and mounted to a support structure. Since all these investigations
were to be carried out under controlled laboratory conditions a drive and a
frequency inverter which are readily available in the laboratory will be used
to rotate the rotor at the desired speeds.

The drive will rotate the rotor and the induced voltage from
the coils on the stator will be monitored by a voltmeter in the laboratory.
Figure 6.2 illustrated this type of drive.

The size of the rotor in this thesis was constrained by the
diameter of the recyclable speaker magnets. Therefore steel with this shape had
to be found or cut to this shape. After finding the relevant steel, the
cylindrical steel has to be drilled at the center.

 

6.3 Converting an induction motor into a PM
generator

Due to the challenges faced in gathering the materials needed
to assemble the generator the author then decided to find an alternative method
to investigate the performance of the generator using recyclable magnets. A
company called Magnetag that supplies motors and generators was approached and
after some negotiations the company was willing to donate an AC induction motor
to the author.

The idea was to convert this AC induction motor into a
permanent magnet generator. This was going to be done by stripping the motor
down and replacing the wound rotor with recyclable magnets. This looked like an
attractive option since recyclable magnets with any shape can be used in the
generator to explore its performance

The author was unable to complete investigating this option
in detail. This is strongly recommended for further work most probably at MSc
level.

6.4 Challenges faced during the construction of
the PM generator

The main challenge in the construction of this wind generator
was cost. For the laboratory investigation of the PM generator, a lot of
materials, like the magnets and coils on the stator were found to be very
expensive. This inadvertently gives more support for the use of recycled
materials.

There was a lot of machining needed for this project, the
rotor and the stator needed to be cut and shaped to the desired diameter and
drilled in the centre to fit on the mount structure. Time was the major
constrained since a lot of things were required to be done in the limited time
given for this thesis.

However the framework of how to proceed in constructing and
assembling the wind generator is already well laid out in this thesis.


Chapter 7. Conclusions

Based on the findings of the report, the following analyses
and conclusions were drawn:

 

7.1 There is an urgent need of electricity

Due to the number of people living without electricity in
rural South Africa it is clear that alternative means of supplying these areas
is essential. According to ESKOM, all house holds will eventually be
electrified, but the problem is, what is happening in the meantime? Where are
children’s medicines being stored? Therefore this makes the electrification
process extremely urgent.

7.2 Resource assessment

 

7.2.1Recyclable materials in the village

An extensive assessment on the rural village of Ga-Rampuru
was conducted. There are plenty of recyclable materials including old milling
machines that are not in use. These materials can be recycled to clean Ga-Rampuru
village.

7.2.2 Rural artisans who can assemble the wind
turbine

Since there are many local artisans who fix cars, electrical
appliances and do some mechanical work in this village, manpower should not a
problem. An engineer from the government or Non-governmental organization could
educate these local artisans on assembling the wind generator. This will have a
positive impact on Ga-Rampuru village as it will encourage people to work and
be creative. There are many old wind mills used for pumping water in Ga-Rampuru
village, most of these wind mills are working perfectly well supplying
sufficient water. This is a clear indication that there is a reliable supply of
wind in the village.


7.3 Simulation results

It has been shown that a reasonable amount of power can be
realised from a generator using recycled magnets from the dumpsites

 

7.4 Cost involved in the design

The overall cost of assembling this wind generator system
will be very cost effective since all the materials are recycled from the
village, and the entire system will be assembled by local artisans.

7.5 Validity of this thesis

Small power that can turn on small lamps will really be
appreciated in this village as children will be able to study after sunset.
This will clearly have a wide range of positive developmental benefits on this
community.

1.       For a more accurate recyclable wind turbine design,
all its components such as the drum, the tower, rotor disk and cables must be
explored in depth. The following must be considered:

·        Investigate how to extract maximum power
from the wind using the drum, and how to prevent the drum from over spinning.

·        How to use other irregular recyclable
magnets in the village in the generator design.

2.       Investigate how a permanent magnet generator
topology can be changed or re-designed to accommodate the design of a generator
with the shape of the loudspeaker magnets.

3.       Look in to how the magnets can be removed from the
speakers, since very strong clue is used to mount them, how this can be done in
a cost effective way.

4.       The axial flux permanent magnet topology should also
be looked into to compare it to the radial flux type.

5.       The exact costs of assembling and maintaining the
recycled wind turbine should also be incorporated in the design procedure.

6.       With the little output power generated in this
thesis, this project must definitely be taken further to alleviate the
electricity problems in South Africa.


References

1.       Socio-economic
rights project, “The right to affordable electricity” Copyright @ community law
centre 2002

2.       IDASA,
<#"1.files/image027.gif">

b)       Alnico FLux_RMS =0.0168

EMF_RMS = 5.1619

c)      

NdFeB FLux_RMS = 0.0459

EMF_RMS = 9.4262

2. Loud Speaker Magnet

FLux_RMS = 0.0171

EMF_RMS = 3.4987


Appendix B

Matlab code for sketching the output emf and flux of the
generators

% EMF calculation from FEMM

%By Maribini Manyage

clc

clear all; close all;

P = 2;

w = 1912; %mechanical speed in rpm

freq = (w*pi/30)*P/(4*pi); %frequency

XA = load(‘flux_link_A.txt’);

XB = load(‘flux_link_B.txt’);

XC = load(‘flux_link_C.txt’);

beta = XA(:,1); % angle between Is_r and d-axis [elec
degrees]

alpha = beta — beta(1,1); % Rotor position in [elec degrees]
from Zero

time = alpha*(pi/180)/(2*pi*freq);%*1000; %time

flux_link_A = 2*XA(:,2);

flux_link_B = 2*XB(:,2);

flux_link_C = 2*XC(:,2);

% Perform spline in order to differentiate flux linkage vs
time

pp_flux_A = spline(time,flux_link_A);

pp_flux_B = spline(time,flux_link_B);

pp_flux_C = spline(time,flux_link_C);

% extracting piecewise polynomial coefficients and derivation

[hgt,wdth] = size(pp_flux_A.coefs);

clear AA;

for k = 1:hgt

AA(k,:) = polyder(pp_flux_A.coefs(k,:));

end

dpp_flux_A = MKPP(time,AA)

[hgt,wdth] = size(pp_flux_B.coefs);

clear AA;

for k = 1:hgt

AA(k,:) = polyder(pp_flux_B.coefs(k,:));

end

dpp_flux_B = MKPP(time,AA);

[hgt,wdth] = size(pp_flux_C.coefs);

clear AA;

for k = 1:hgt

AA(k,:) = polyder(pp_flux_C.coefs(k,:));

end

dpp_flux_C = MKPP(time,AA);

%back emf

emf_A = ppval(time,dpp_flux_A);

emf_B = ppval(time,dpp_flux_B);

emf_C = ppval(time,dpp_flux_C);

figure(1);

plot(time*1000,flux_link_A,’r-‘);

hold on;

plot(time*1000, flux_link_B,’b-‘);

title(‘Flux linkage — under noload’);

xlabel(‘Time [ms]’),ylabel(‘Flux linkage [WbT]’)

grid;

figure(2);

plot(time*1000,emf_A,’r-‘);

hold on;

plot(time*1000, emf_B,’b-‘);

plot(time*1000, emf_C,’g-‘);

title(‘Back Emf — under noload’);

xlabel(‘Time [ms]’),ylabel(‘Back EMF [V]’)

grid;

x = length(flux_link_A);

FLux_RMS = norm(flux_link_A)/sqrt(x)

y = length(emf_A);

EMF_RMS = norm(emf_A)/sqrt(y)

Учебная работа. Planning of mobile complete set for a rural wind generator