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:
В· Heat
В· Radiation
В· 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] .
В
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.
В
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 i 2 R will be caused by the eddy current and will appear as heat in the core [13] . p> The average eddy current loss is:
(Eq. 3.3)
where P e is the eddy current loss in watts (W), k e is the constant that depends on the conductivity of the magnetic material, f is the frequency in hertz (Hz), Оґ is the lamination thickness in meters, B m is the maximum flux density in tesla (T) and V is the volume of the magnetic material in cubic meters (m 3 ) [14] .
The eddy current losses can be reduced by [13] :
В· Using a high-resistivity core material
В· 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.
В
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. p> By testing various ferromagnetic materials, Charles Steinmetz proposed that hysteresis loss can be expressed as [14] :
(Eq. 3.4)
where P h is the hysteresis loss in watts, k h 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. <В
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. br/>
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] . p> A low carbon steel core with low permeabi...
