Torque Improvement in Partitioned Stator Wound Field Switched Flux Machine By Using Assisted Ferrites

Abstract

Wound field (WF) synchronous machines without rare-earth permanent magnet (PM) have been attractive recently due to lower cost than PM machines [1]. The wound-rotor synchronous machines suffer from brushes and slip rings which are essential for DC field excitation [2]. However, they can be eliminated in the wound-stator synchronous machines in which both DC winding and AC windings are placed in the stator, e.g., the WF switched flux (WFSF) machine [3], [4]. Due to the magnetic gearing effect in the stator-excitation machines [5], [6], the partitioned stator WFSF (PS-WFSF) machine having separated DC and AC windings in two stators and hence a higher total slot area and $>19$% larger torque is proposed and analyzed in [7], e.g., the 12/10-stator/rotor-pole PS-WFSF machine shown in Fig. 1(a). In this paper, to further improve the torque density of the 12/10-stator/ rotor-pole PS-WFSF machine shown in Fig. 1(a), short-circuited ferrites are introduced to reduce the inner stator tooth saturation and improve the air-gap field density. The developed 12/10-stator/rotor-pole PS-WFSF machine with assisted ferrites can be obtained as shown in Fig. 1(b). They have similar machine topologies, i.e., two stators wound by AC windings and DC winding, respectively, and a sandwiched rotor consists of several modulation iron pieces. The only difference is that the short-circuited ferrites are introduced in the proposed topology. As shown in Fig. 1(c), flux density $B_{1}$ can be produced in both the inner stator tooth and the inner air-gap due to the WF magnetomotive force (MMF). However, an opposite flux density $B_{2}$ can be generated in the short-circuited magnetic circuit for the ferrite MMF, as shown in Fig. 1(d), whilst the corresponding inner air-gap flux density is $B_{3}$. Due to the opposite directions of $B_{1}$ and $B_{2}$, as shown in Fig. 1(e), the inner stator saturation and hence the iron loss will be decreased with a reduced flux density $B_{1}$'$- B_{2}$', where $B_{1}$' $> B_{1}$ and $B_{2}$' $> B_{1}$. Moreover, as shown in Fig. 1(e), the inner air-gap flux density can be improved from $B_{1}$ in the machine without ferrite to $B_{1}$' $+ \quad B_{3}$' in the machine with ferrite, where $B_{3}$' $< B_{3}$. Therefore, both the open-circuit phase flux-linkage and the average electromagnetic torque can be improved in the proposed PS-WFSF machine with assisted ferrites. This can be evidenced by the 2D FE predicted results shown in Fig. 1(f) and Fig. 1(g). As shown in Fig. 1(f), the fundamental phase flux-linkage when the AC coil number of turn is $N_{ac}=1$ can be improved by 2.33%, from 0.272mWb to 0.278mWb. However, since the on-load inner stator tooth saturation in the PS-WFSF machine without ferrite is stronger than the open-circuit one due to armature reaction, the average electromagnetic torque can be more effectively improved by 3.76%, from 1.47Nm to 1.53Nm, when both the DC winding copper loss $p_{cuf}$ and the AC windings copper loss $p_{cua}$ are 30W and the machines operate at BLAC mode under zero $d -$axis current control, i.e., $i_{d}=0$, due to negligible reluctance torque [7]. Moreover, as shown Fig. 1(h), a higher total copper loss and hence a stronger saturation in the machine without ferrite will result in more effective torque improvement in the its counterpart with ferrites. Both the proposed 12/10-pole PS-WFSF machine with assisted ferrites and its counterpart without ferrite are built and tested to validate the FE predicted results. The machine components are shown from Fig. 2(a) to Fig. 2(d), i.e., Fig. 2(a) for the shared outer stator, Fig. 2(b) for the shared cup rotor, Fig. 2(c) for the inner stator with ferrites and Fig. 2(d) for the inner stator without ferrite. The comparison between the measured and 2D FE predicted phase-back-EMFs is given in Fig. 2(e) and Fig. 2(f), respectively. It can be observed that the 2D FE predicted results agree well with the measured results in both machines. The comprehensive comparison including loss and efficiency between the proposed topology with ferrites and its counterpart without ferrite will be given in the full paper, together with the investigation of the average torque separation by DC winding and ferrites by using frozen permeability.