Traction motors for electric vehicles and hybrid electric vehicles require wide power band characteristics. In order to increase the power band, field weakening control is utilized in interior permanent magnet synchronous motors. However, the effectiveness of the field weakening control is not high for use in traction motors, and the efficiency in high rotation speed ranges is low. In order to solve the problem, various variable flux reluctance motors have been proposed [1, 2]. However, they are not practical due to 2 sets of coils: armature coils for generating a AC rotating magnetic flux, and field coils for controlling the DC magnetic field. In order to solve the problem of the previous variable flux reluctance motors, we propose a current superimposition variable flux reluctance motor (CSVFRM) shown in Figure 1(a) [3, 4, 5]. The CSVFRM has only 1 set of coils, and AC and DC voltages are superimposed using a 6-phase inverter.
The CSVFRM that we have developed has 10 salient poles and 12 slots. 10 salient poles are equal to 20 poles in a permanent magnet synchronous motor. Therefore, eddy current losses in high speed rotation ranges are expected to be large. In this paper, in order to decrease the eddy current losses of the 10-pole-12-slot CSVFRM, a CSVFRM that consists of 8 salient poles and 12 slots (8-pole-12-slot CSVFRM) is proposed. The operational principle is described, and its characteristics are compared as that of the 10-pole-12-slot CSVFRM.
2 Current superimposition variable flux reluctance motor with 8 salient poles
2.2 Operating principle
The coil connection of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs is the same with each other, and is shown in Figure 2. Although a 6-phase inverter is used to drive the CSVFRM, the A and D, B and E, and C and F phases can also respectively correspond to the U, V, and W phases of a conventional 3-phase motor. A positive DC voltage (+Vdc) is applied into the A, C, and E phases, and a negative DC voltage (-Vdc) into the B, D, and F phases. 3-phase vector control is applied to the CSVFRM because the proposed motor has 2 sets of 3-phase coils.
In the 10-pole-12-slot CSVFRM, when a positive DC voltage is input into the A, C, and E phases, and a negative DC voltage into the B, D, and F phases as shown in Figure 2, a 6-pole-pair field magnetic flux is created. The 6-pole-pair magnetic flux is then modulated by the 10 salient poles of the rotor and a 4-pole-pair modulated magnetic flux is created. At the same time, a 3-phase AC voltage is superimposed onto the DC voltage as shown in Figure 2 and the 4-pole-pair rotating magnetic flux due to the AC current is synchronized with the modulated magnetic flux from the DC current. On the other hand, in the 8-pole-12-slot CSVFRM, a 3-pole-pair field magnetic flux is modulated by the 8 salient poles of the rotor and a 5-pole-pair modulated magnetic flux is created. At the same time, the 5-pole-pair rotating magnetic flux due to the AC current is synchronized with the modulated magnetic flux.
3 Characteristics comparison
3.1 Models comparison
10-pole-12-slot and 8-pole-12-slot CSVFRMs are compared. The shape of the stator of both CSVFRMs are the same and are shown in Figure 1. The stator diameter is ϕ10 mm and the stator stack length is 83 mm. The number of coils per slot of both CSVFRMs is 20 turns and the coils are connected as shown in Figure 2, where the phase resistance is 0.018 Ω.
3.2 Conditions comparison
The rotation speed-torque (N-T) characteristics of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs are computed. In this study, the supplied DC voltage is 24 V. A load is changed from 0.5 Nm to 10 Nm under current leading angles of 0, 20, 40, 60, and 80 degrees. Finite element analysis under vector control was conducted and its control diagram is shown in Figure 3. A 3-phase vector control is used to drive the CSVFRMs. In this time, an imaginary 3-phase current is created as shown in Figure 3.
In the 8-pole-12-slot CSVFRM, the phase angle of the EMF of the A and D phases is different although both phases correspond to the U phase. In this simulation, the center value of the phase angles of the A and D phases is adopted to the phase angle of the U phase.
In the CSVFRM, the relationship between a DC current Idc and AC current amplitude Iac can be controlled, and this influences on the N-T characteristics. The effective value of the phase current of the CSVFRM Iu is shown in (1).
If the CSVFRM is driven so that the copper loss would be minimum, n in (2) is 1.
In this study, n = 2 is selected so that a high output power and power factor would be obtained.
3.3 Results comparison
The motor efficiencies of both CSVFRMs are high in high rotation speeds. The maximum motor efficiency of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs is more than 90% and 88%, respectively. In addition, the motor efficiency of the 8-pole-12-slot CSVFRM is entirely lower than that of the 10-pole-12-slot CSVFRM. This is because the phase current of the 8-pole-12-slot CSVFRM is higher than that of the 10-pole-12-slot CSVFRM. Namely, the torque density of the 8-pole-12-slot CSVFRM is lower than that of the 10-pole-12-slot CSVFRM, and the motor efficiency of the 8-pole-12-slot CSVFRM is decreased due to large phase currents.
In this way, the motor efficiency of the 8-pole-12-slot CSVFRM is lower than that of the 10-pole-12-slot CSVFRM. This is because the torque density of the 8-pole-12-slot CSVFRM is lower than that of the 10-pole-12-slot CSVFRM. In the next section, the cause of the low torque density is discussed.
4 Cause of the low torque density
In Figures 6 and 7, the phase current of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs under a load of 10 Nm is 73 Arms, and 90 Arms, respectively. In this section, the cause of the phase current is discussed.
The magnetic flux density of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs are shown in Fig. 10. The maximum magnetic flux density of the stator yoke of the 10-pole-12-slot and 8-pole-12-slot CSVFRMs is 1.66 T and 1.97 T, respectively. The stator yoke of the 8-pole-12-slot CSVFRM is magnetically saturated.
In order decrease the magnetic flux density of the stator yoke of the 8-pole-12-slot CSVFRM, the stator yoke width is increased as shown in Fig. 11, where the width of the stator yoke is equal to that of the stator tooth. The number of coil turns is decreased from 20 to 11.5 due to the expanded stator yoke. The phase current under a load of 10 Nm is computed using the new stator.
The phase current under a load of 10 Nm is 89.8 Arms, which is almost the same with the previous 8-pole-12-slot CSVFRM (90 Arms). This is because the number of coil turns is decreased from 20 to 17.5. From the magnetic flux density shown in Figure 11, it is observed that the stator yoke is not magnetically saturated.
Assuming that the space factor is ignored, the 8-pole-12-slot CSVFRM with the new stator and 20-turn coils is computed under a load of 10 Nm. The phase current is 78 Arms, and is a little larger than the 10-pole-12-slot CSVFRM (73 Arms). The magnetic flux density is shown in Figure 12. From Figure 12, the small difference of the phase current is the magnetic saturations in the salient pole in the rotor.
In the CSVFRM, the torque density is increased if the rotor salient pole width is equal to the stator slot open. This is because the saliency increases. In order to reduce the magnetic flux density in the rotor salient pole of the 8-pole-12-slot CSVFRM, firstly, the stator shape is changed so that the stator tooth width would be equal to the stator slot open. In addition, the rotor salient pole width is increased. In this CSVFRM, the rotor salient pole width is equal to the stator tooth width and stator slot open. However, the phase current under a load of 10 Nm is increased to 98 Arms, and the magnetic flux density is shown in Figure 13. From Figure 13, it is observed that the stator tooth, stator yoke, and rotor salient pole are magnetically saturated. The increase of the rotor diameter is needed to reduce the magnetic flux density.
In this way, there are more design constraints in the 8-pole-12-slot CSVFRM compared with the 10-pole-12-slot CSVFRM. In particular, the yoke width of the 8-pole-12-slot CSVFRM must be larger than that of the 10-pole-12-slot CSVFRM. Due to this, the coil slot area of the 8-pole-12-slot CSVFRM is small, and the power density also decreases.
This paper proposed a current superimposition variable flux reluctance motor with 8 salient poles. The operational principle of the 8-pole-12-slot CSVFRM was described. The 8-pole-12-slot CSVFRM was compared with the 10-pole-12-slot CSVFRM in terms of the motor efficiency, phase current, and iron losses. The motor efficiency of the 8-pole-12-slot CSVFRM was lower than those of the 10-pole-12-slot CSVFRM. This was because the torque density of the 8-pole-12-slot CSVFRM was low due to the magnetic saturation. The magnetic saturation of the 8-pole-12-slot was due to design constraints.
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About the article
Published Online: 2017-12-29
Citation Information: Open Physics, Volume 15, Issue 1, Pages 857–861, ISSN (Online) 2391-5471, DOI: https://doi.org/10.1515/phys-2017-0102.
© 2017 Kazuaki Takahara et al.. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0