Figure 2.8 Ga-Rampuru wind generator

The following chapters describe the steps taken by the author to investigate the performance of a synchronous permanent magnet machine constructed using recyclable loudspeaker magnets.

Chapter 3. Generator Design

3.1 A brief background

This chapter will detail a simple procedure undertaken to design the wind generator from recyclable materials. Permanent magnet machines are preferred for this application as they reduce the excitation losses significantly and hence a substantial increase in the efficiency of the machine. In addition, permanent magnet machines are simple to construct and maintain [10].

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.

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

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), д 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:

(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.

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.

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].

A low carbon steel core with low permeability will be used in this design as it was found in the recyclable materials found in the village. This type of steel is cheap and mostly available. Compared with other better and expensive steel such as silicon, cobalt, etc. this type of steel has a very high core loss. The steel saturation flux density Bsat is estimated from the BH curve to be 1.5T.

3.6.3 Rotor Diameter (D)

The rotor diameter must be greater than the rotor yoke height (Hry), shaft radius (Rshaft) and the radial magnet length (Lm) [10].

D = 2 Hry + 2 Rshaft + 2Lm (Eq. 3.8)

In this design, D is restricted by the magnet arc radius of 25mm. Therefore D will be 50mm.

3.6.4 Rotor and Stator Yoke heights

The minimum rotor yoke height Hry is the same as the stator yoke height Hsy. The height should be large enough to avoid saturation. This also has advantages of reducing core loss and reluctance.

The minimum yoke heights are given by [10]:

(Eq. 3.9)

3.6.5 Airgap Length

The airgap length has a minimum value limited by the manufacturing tolerances; this value is typically in the range of 0.3mm to 1mm. In this design 0.5mm is chosen to be the airgap length.

3.6.6 Generator Length

The generator length is estimated to be 95mm; this is approximated from flux required to give the output voltage of the generator.

3.6.7 Airgap Flux Per Pole

In a radial machine, the flux per pole is given by:

(Eq. 3.10)

where B is the average airgap flux density, D is the rotor inner diameter, L is the generator length, Kst is the lamination stacking factor and p is the pole pairs.

For this design the average flux density per pole Bgav is equal to the peak flux density Bg since the magnet arc is close to 180 degrees. Therefore the peak airgap flux is estimated to be 0.5T at the airgap and Kst is assumed to be 0.97.

The airgap flux and the lamination stacking factors were estimated from the following dimensions of the loudspeaker magnet:

· Magnet arc = 180 mechanical degrees

· Inner radius = 8mm

· Arc radius = 25mm

· Magnet radial length = 4mm

· Area of one pole = 706.8 мm2

From the above magnet dimensions, the flux per pole in the machine is then estimated to be 1.16 mWb this value is calculated from equation 3.10.

3.6.8 Windings

The stators of most synchronous generators are wound with three distinct and independent windings to generate three-phase power [14]. A simple layer winding was used in this design. Slot per pole per phase was chosen to be 1 and the winding pitch was full pitch.

A. Types of winding

The preferred type of winding is distributed winding as it reduces harmonics and makes better use of the stator or rotor structure. The mmf induced in the airgap is not sinusoidal, to get a pure sinusoidal mmf the number of slots have to be infinity. This means that the distributed winding is expected to have some harmonics.

Induced voltage for the distributed windings is:

(Eq. 3.11)

Kw is the winding factor and depends on the winding arrangements and has a value less than unity. Distribution factor Kd and a short pitch factor Kp reduces the winding voltage magnitudes but also reduces certain harmonics in EMF and MMF waveforms.

(Eq. 3.12)

Distributed winding configuration has one slot per pole per phase and its winding factor is equal to 1.

B. Winding arrangement

Single layer winding, where each slot contains one coil side, will be used in this design as it is economical to manufacture and has simpler end connection. Emf and mmf can be modified to reduce harmonics. In a three phase system even harmonics do not appear due to the winding symmetry, the only visible harmonics are the belt harmonics.

C. Winding Pitch

Short pitch is the most commonly used type of winding pitch. It reduces the distorting harmonics and produces a truer sinusoidal wave. The length of the end connection is also reduced thereby saving copper and reducing copper loss in the coil.

The drawback of short pitch winding is that the induced emf in it is smaller than in a full-pitch coil. The reason is that the total flux linking the short-pitch coil is smaller than that of the full-pitch coil.

3.6.9 Number of turns

The number of turns per pole is estimated to be 60 turns from equation 3.11.

The design parameters discussed will be modelled in FEMM in the next chapter to induce the output voltage and flux of the generator.

Chapter 4. Modelling the design in FEMM

4.1 Introduction

In this chapter, the two pole generator geometry discussed in chapter 3 will be modelled in FEMM to analyse the output induced voltage and the flux of the generator. The lua-script in FEMM is run and the rotor is rotated 360 electrical degrees, for the lua-script refer to appendix C1.

Initially, a choice was made of three typical commercial magnet grades. Neodymium-iron-boron NdFeB was chosen from the rare-earth magnet group. Alnico6 was chosen from the Alnicos and the last type was barium ferrite from the ferrite or ceramic group. Then the machine will be modelled using different types of commercial magnets to investigate the performance of the generator.

The author then proceeded to investigate the magnetic properties of the loudspeaker magnet. This was done so that the parameters can be modelled in the finite element package.

Finally a design using the loudspeaker magnets was modelled to explore the recycled generator output.

4.2 Two pole geometry

Table 4.1 below summarizes the generator specifications that were discussed in chapter 3. These parameters will be modelled in FEMM to view the output induced rms voltage and the flux.

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