Stator Yoke
Concentrated Winding PM Stator Rotor
Figure 2.9. Multi-stage PMAFM
2.3 Transverse flux machine
As mentioned above, the electrical machine can be categorized into LFM and TFM considering the relative direction between the rotor movement and the magnetic flux path. In LFM, the main magnetic flux path lies within the same plane as the rotor movement while the magnetic flux in the TFM flows in planes that are perpendicular, in other words transverse to the direction of the rotor movement. For a better view, part of the basic PM excited TFM (PMTFM) is linearized and illustrated in Fig. 2.10.
Copper winding
Rotor yoke PM Stator
Movement
Figure 2.10. Part of a linearized one-phase PMTFM
The PMs with alternating magnetization direction indicated with the green and red color respectively are mounted on the surface of the rotor yoke. The stator consists of many separate cores which are used to conduct the magnetic flux from PMs indicated by the dotted line. The winding with simple structure and high filling factor is placed in the slots of the stator and fed with alternating current. The magnetic flux flows through the PM stator cores, the air gaps, the PMs and the rotor yoke.
Because of the special structure, the TFM has some outstanding advantages.
Some of them are summarized as follows:
• High torque density
• Simple winding structure
• Decoupling of the magnetic loading and the electrical loading
Due to the special structure of TFM, the magnetic and electrical circuits are decoupled, which do not compete for the same space as in the LFM. Thus, a large number of poles can be achieved without increasing the external diameter, and the electromagnetic design is simplified. In addition, the winding used in TFM has usually simple structure and high filling factor which lead to lower cop-per losses and higher rated electrical loading. In conclusion, TFM can achieve high efficiency and outstanding torque density through increasing the number of poles and electrical loading.
Despite the above advantages, there are two considerable drawbacks preventing the wide applications of TFM:
Low power factor
The power factor of TFM is generally low because of the large amount of leak-age flux which comprises the slot leakleak-ages, the leakleak-ages flux between the phases, the end winding leakages and the fringing fluxes. It is caused by a large number of isolated components in the magnetic circuit and many sides directly facing the air. In addition, the power factor reduces with increasing pole number. The reduced power factor causes a higher rating and costs of the drive inverter. As a result, for a given dimension, a high number of poles leads to small pole pitch, high leakage flux and consequently low power factor. Therefore, a compromise has to be made between the torque and the power factor.
2.3 Transverse flux machine
Complex mechanical construction
The mechanical structure of the TFM is more complicated than the conventional RFM because a large number of isolated and small components are needed to form the magnetic circuit. As a result, the manufacturing costs and assembly difficulties are enhanced, and the mechanical integrity becomes worse.
It should be noted that at least two phases are needed to guarantee the contin-uous operation of TFM. For instance, a three-phase TFM can be obtained by stacking three one phase TFMs together along the axial or peripheral direction.
Furthermore, either the PMs or the stator cores must be shifted to achieve a 120 electrical degree among the phases. For the TFM in Fig. 2.11, the stator cores are shifted while the PMs are aligned.
In order to reduce the leakage flux among the phases, a small gap or barrier between the cores of adjacent phases are necessary. Because of the electromag-netic independence among the phases without considering the leakage flux, it is only necessary to analyze one pole pair model of one phase.
Copper Winding
Rotor yoke Stator
Movement Stator
Stator
Figure 2.11. Part of a linearised three-phase PMTFM
In general, the TFM can be classified considering the type of excitation (elec-trical or PM excitation), the arrangement of the PMs (surface mounted or flux concentration), the placement of PMs (positive or passive rotor), the position of rotor (inner or outer rotor), the distribution of stator segments (single or double sided stator) and the configuration of the magnetic circuit (axially or circumfer-entially shifted phases).
TFM
Reluctance TFM
PM excited TFM
Flux concentration
Surface mounted PM
Single-Sided Double-Sided
Single-Sided Double-Sided A reluctance TFM (RTFM) and a single-sided PMTFM are illustrated in Fig. 2.12 where all components of the stator are placed on one side of the ro-tor. One of the advantages of RTFM is the absence of PM, which results in low costs and better mechanical robustness. Generally, the RTFM has a rotor with large weight and can offer lower torque density compared with the PMTFM.
Compared with the TFM in Fig. 2.10, the PMTFM in Fig. 2.12 can utilize the generated magnetic flux from all PMs at the cost of increased weight and re-duced available winding space.
Rotoryoke Stator
(a) RTFM
Copper Winding
PM Stator
(b) PMTFM
Figure 2.12. Reluctance TFM and TFM with surface mounted PM
A higher magnetic flux density in the air gap can be achieved by reducing the effective air gap and concentrating the flux from the PMs. For instance, a single-sided and a double-single-sided TFM with flux concentration are illustrated in the Fig. 2.13 where the PMs with inverse circumferential magnetization directions are embedded in the rotor.
Although a higher torque density can be realized with the flux concentration structure, the machines are difficult to be manufactured, and the mechanical