Transverse Flux Machine TFM

In a TFM the main flux flow is in a direction perpendicular to the direction of travel. Figure 3.7 shows three views of a linear PM excited TFM. The translator consists of surface mounted PMs in a configuration that forces the flux to vary both axially and circumferentially. There are stator coils mounted either side of the translator with a series of iron yokes channelling the flux in such a way that alternate rotor poles excite the same coil[63, 64]. The result is an inherently three dimensional flux path, and flux from all the magnets contributes to linking one or other of the coils at all times. As such the TFM is the ultimate VRPM machine, producing very high shear stresses, values -2

approaching 200kNm- have been reported [63].

approaching 200kNm- have been reported [63].

Transversal Flux Generator
Figure 3.7: Transverse Flux Machine

The mounting of the stator yokes in combination with the three dimensional flux path present various problems with the support structure of this machine. The typical three dimensional structure does not allow the ferromagnetic elements to be laminated and the necessary small pitch often demands a large number of individual parts. Furthermore, inherent in this machine is the presence of cogging torque: the tendency of the magnets to align themselves with the path of least reluctance. It has been reported as 30 kNm-2 in one machine [64].

Figure 3.8: Equivalent circuit of TFM

Consider the magnetic circuit shown in Figure 3.8, where Sg and Sm are the reluctance of the gap and magnet respectively and NI is the mmf source from current in one of the coils. If the magnets in the TFM were replaced with air, and viewing the machine from the angle shown in Figure 3.7C , it can be seen that this circuit represents a simplified equivalent circuit of the machine with I amps flowing through the N coil turns. The flux density in the airgap, due to current excitation only, in a machine made of infinitely permeable iron is deduced from the equivalent circuit and given by equation (3.7).

B NI

If the rotor magnets are replaced with an equivalent current sheet, when the machine is viewed from the direction shown in Figure 3.7B, and the magnets and teeth are misaligned, the force resulting in the interaction of the current sheet and the flux due to the coil current is shown in Figure 3.9.

Each of the forces will be given by equation (3.8). The resulting shear stress, incorporating one of these forces acting per rotor pitch, is given in equation (3.9).

Where wm L

Transverse Flux Motor
Figure 3.9: Force generation in the TFM

NI.tm.Br.L

width of magnet (m)

axial length (m into plain of paper)

2BgtmBr wmWr

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