Курсовая работа: Planning of mobile complete set for a rural wind generator
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