F B

Where

equivalent current (A)

Hc = coercivity (Am1)

= permeability of free space = 4n x 10" A machine which relies on a field winding for excitation is likely to be limited to around 50 kAm-1, giving a shear stress an order of magnitude lower.

A slow physical velocity may be converted to a high speed flux change by a process known as magnetic gearing. Figure 3.5a shows a single stator pole, split into 3 teeth, interacting with five individual rotor poles, each a surface mounted permanent magnet. If the rotor moves by one rotor pole to the right, as in Figure 3.5b, the direction of flux flow through the pole can be seen to reverse. In non-toothed designs, the rotor would have to move a distance five times this value, Figure 3.5c and Figure 3.5d. This toothed design has therefore increased the rate of change of flux five fold. However, the rotor is only interacting with three stator poles at any one time, reducing the maximum value of flux by a factor of 3/5 (compare 'c' and 'e'). The net magnetic gearing of the split rotor and stator poles is hence threefold.

Figure 3.5: Magnetic Gearing

Recent developments have resulted in a new class of machine, known as Variable Reluctance Permanent Magnet machines (VRPM). There are many topologies of these machines [63, 64, 65, 66, 67, 68], all bound by the same principles of operation. Alternative return paths are offered to the magnetic flux through the interaction of rare earth PMs and small pitch iron teeth. Force is developed as the path of minimum reluctance is sought. These machines are capable of shear stresses unmatched by other electrical machines, which has led to a general reduction in the size of machine. The proceeding sections detail common features associated with this family of machine plus two proposed topologies.

These advantages are further amplified if rare earth magnets are used. Neodynium Iron Boron (Nd-Fe-B) has a remnant flux density of around 1.2 T, compared with 0.4 T for ferrite magnets. Until recent years, the price of these materials has compromised the advantages they offer, yet a gradual drop in their price gives the potential for them to be economical.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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