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:
