Planning of mobile complete set for a rural wind generator
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
|
3.4987
|
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.
Chapter 8. Recommendations
Based on the above conclusions, the following recommendations
were drawn:
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-');
plot(time*1000, flux_link_C,'g-');
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)
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