Ironless Tubular PM machine

Significant structural savings can be made if the magnetic forces can be reduced or eliminated, which can be achieved by constructing a stator which contains no iron. These features are demonstrated in Figure 3.12.

Figure 3.12: Ironless tubular machine

The translator consists of opposed axially magnetised PMs separated by steel spacers, both mounted on a non magnetic shaft. This configuration, assuming that the iron does not saturate, results in the residual rotor forces being attractive. Without the spacers, strong repellent forces would be present. Radially magnetised or surface mounted magnets may prove more attractive for large diameters, but at small scale represent a more difficult structure to manufacture.

3.4.2.1 Surface Flux Density

A similar design with an iron core has been analysed [71] and optimised [72], which provides some relevant results. Using the notations given in Figure 3.12 and assuming that the airgap, g, is a specified design parameter restricted to its minimum obtainable size, there are three dimensional ratios which may be considered: wm/Wp, Rm/Ro and wm/Ro. The first of these, which for a given magnet width specifies the width of steel spacer, is said to behave independently and not influence the choice for the other two [72]. The choice of Wp is, by implication, not affected by the lack of iron in the stator, which effectively sets Ro to infinity. Within this research it was concluded that the value wm/Wp should be set for the condition of minimum torque ripple and a value of 0.6 and 0.7 was recommended [72].

For the same electrical and magnetic loadings, the ratio of thrust of a three phase machine compared to that of a two phase machine is 1.061 [72]. A 6% increase in performance can hence be expected from making a three phase machine when compared to its two phase equivalent, with an advantageous reduction in ripple force.

imaginary steel

Figure 3.13: Simplified flux flow through tubular machine

Ws/2 jWm/2 magnet spacer

Figure 3.13: Simplified flux flow through tubular machine

Figure 3.13 shows the simplified assumed flux flow through one magnet and two separate halves of steel spacers. The section consists of two half surface poles, separated by the centre line of the magnet. The pattern is repeated to make up a series of adjacent North and South poles along the surface, the polarity of which changes at the centre line of the magnet. Included here is the presence of an imaginary steel outer sleeve, which allows the flux density at the translator surface to be calculated. The flux flow is assumed to be parallel in the airgap and flow from the translator into the surrounding air through both the magnet and steel boundaries. The length of the airgap, lg, which gives the same reluctance as the ironless core is given by (3.15), see Appendix A.

In the actual airgap the flux density will rapidly reduce with distance from the surface. As a first estimate, it can be assumed that the decay of field is exponential with distance x, as shown in (3.16).

3.4.2.2 Reactive Force

Force is developed as a result of current flowing through the coils, which are situated in the magnetic field of the stator. The direction of this force is mutually orthogonal to the current and field strength and has a magnitude of their product multiplied by the length of conductor in the field.

Figure 3.14: Segment of copper in magnetic field

Figure 3.14 shows a segment of the copper coil over a surface pole with the mutually orthogonal elements shown. If the segment carries a peak current density of J Amps/m2 in the I direction then the force per m3 of this segment is given by (3.17).

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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