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.
