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Apr 22, 2015 - DESIGN and control of electric machines for hybrid electrical vehicle (HEV) and pure electrical vehicle (EV) is a hot topic in current research [1].
IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015

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A Novel High Energy Density Double Salient Exterior Rotor Permanent Magnet Machine Lei Gu1 , Wei Wang1 , Babak Fahimi1 , and Morgan Kiani2 1 University 2 Texas

of Texas at Dallas, Richardson, TX 75080 USA Christian University, Fort Worth, TX 76129 USA

In this paper, a double salient exterior rotor permanent magnet machine is proposed and analyzed. This machine introduces the use of permanent magnet (PM) into a switched reluctance machine and has the same envelope as that of Toyota Prius thirdgeneration (P3G) 60 kW interior PM synchronous machine (IPMSM). Simulation results show that the continuous torque and peak torque of the proposed machine are 97% and 30.4% more than that of P3G IPMSM under the same current density. Index Terms— Double salient, exterior rotor, interior permanent magnet synchronous machine (IPMSM), Prius, torque density.

I. I NTRODUCTION ESIGN and control of electric machines for hybrid electrical vehicle (HEV) and pure electrical vehicle (EV) is a hot topic in current research [1]. Permanent magnet (PM) machine, especially interior PM synchronous machine (IPMSM), which featured with high energy density, high efficiency, and field weakening capability, is widely employed for existing EV/HEV [2]. However, the fluctuating price of rare-earth PM makes machine arts with less or even no usage of rare-earth PM more attractive. Induction machine (IM) is a good alternative for no use of magnets. However, it has some demerits, such as relatively low efficiency and low power density as compared with PMSM. While comparing IM with switched reluctance machine (SRM), the efficiency of SRM could reach 95% at high speed region. SRM is another alternative of high-speed application for its simple structure, ruggedness, and wide speed range. However, power density of traditional SRM is not high enough to meet the requirement. In [3], an SRM is designed to compete with the Prius third-generation (P3G) IPMSM. Almost the same amount of torque could be acquired by boosting the current density to 23.9 A/mm2 , however, the thermal problem is not addressed there. In [4] and [5], a new double stator SRM (DSSRM) with high power density was analyzed, which shows the advantage of short flux path. In [6] and [7], exterior rotor and inner rotor structure were compared and the former was adopted by taking advantage of its high torque capability. A double salient PM (DSPM) machine was analyzed and optimized, which illustrates a good torque density capacity in [9]. In this paper, inspired by DSSRM and DSPM, a new type of double salient exterior rotor PM (DSER-PM) machine is proposed and analyzed, and the results show that it has a high power density as compared with P3G IPMSM. The rest of this paper is organized as follows. In Section II, performance of the newly proposed DSER-PM machine and operational principle is shown. A performance comparison between the proposed machine and P3G IPMSM is listed

D

Manuscript received May 25, 2014; revised September 5, 2014; accepted October 22, 2014. Date of current version April 22, 2015. Corresponding author: L. Gu (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2366081

Fig. 1.

DSPM synchronous machine. TABLE I D IMENSION OF DSER-PM M ACHINE AND P3G IPMSM

in Section III. Finally, the conclusion is summarized in Section IV. II. M OTOR D ESCRIPTION AND A NALYSIS Fig. 1 shows the structure of the proposed three-phase DSER-PM machine. This machine houses 12-stator pole and 8-rotor pole configuration. Eight pieces of NdFeB PMs are embedded in the middle of the rotor segments, and the total volume of magnets is the same as that in P3G IPMSM. PMs marked with the same color are tangentially magnetized in the same direction. Nonmagnetic stainless steel blocks are used to mount the rotor segments. The dimension of this motor is listed in Table I with a comparison with that of P3G IPMSM. To demonstrate the advantages of the proposed machine, it is designed with the same envelope with that of IPMSM.

0018-9464 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015

Fig. 4.

Self-inductance of three-phase winding.

which is approximately proportional to the machine speed. However, the heat issue is not a challenge, which will be explained later. The variation of flux linkage will determines the current excitation pattern to generate positive torque, which will be explained in the following section. Fig. 2. No load flux linkage at different rotor positions. (a) Position a. (b) Position b. (c) Position c. (d) Position d. (e) Position e.

Fig. 3.

Three-phase flux linkage without current excitation.

B. Inductance The inductance profile is different from that of traditional SRM or PMSM. Since the magnets are embedded in the middle of the iron poles, and the permeability of magnet is almost the same as that of air, the same is true with the nonmagnetic stainless steel blocks. The position, where a magnet is aligned with the middle of slot opening, denotes as the unaligned position u 1 . When nonmagnetic stainless block locates at middle of slots opening, this position is the unaligned position u 2 . Aligned positions are defined as where the rotor steel is aligned with the middle of slot opening. The equivalent air gap at unaligned position u 1 is L u1 = L m + 2L g

(1)

the equivalent air gap at unaligned position u 2 is A. Flux Linkage of Three-Phase Windings Fig. 2 shows no load flux lines at five specific rotor positions. The flux linkage of each phase varies through rotor position. Taking phase A, which is shown as the yellow winding in Fig. 2, as an example, the corresponding flux linkage of phase A is shown in Fig. 3 as rotor rotates from position a to e. Magnetic saturation is not considered here. At position a, c, and e, four magnets are aligned with the stator teeth; at position b and d, four stainless blocks are aligned with the stator teeth. The net flux linkage across phase A is 0 at position a, c, and e. At position b, the flux circulation is limited by the air gap in the rotor, and the polarity of magnets on each side differs. The flux line circulates through the neighbor teeth, and the net flux linkage reaches to a local maximum value F1 . While position d is different from position b since the polarity of magnets on each side is the same, flux lines circulate through the top of the pole, and the net flux linkage reaches a local maximum value F2 . For next half period, the flux linkage will repeat but the direction is reversed, as shown in Fig. 3. The flux linkage of phases B and C are also included in Fig. 3. As can be predicted, there are two peaks and two valleys in a complete electrical period. This flux profile is different from that of traditional PMSM in which PMs are magnetized with opposite directions alternatively. Thus, the proposed machine works as a 12-slot 4-pole PMSM machine from the view of flux linkage. In Fig. 3, large portion of high amplitude low harmonics exist and causes higher iron loss,

L u2 = L ss + 2L g

(2)

and the equivalent air gap at aligned position is L a = 2L g

(3)

where L m denotes the width of magnet in the tangential direction; L ss is the width of nonmagnetic stainless steel block in the tangential direction; and L g is the length of air gap, which is much smaller than L m and L ss . The magnetic reluctance at aligned position and unaligned position are both very large, so the normalized inductance is quite small compared with traditional design, the variation of inductance is also small. The maximum inductance appears at the aligned position, where the rotor covers the tips of both sides of one slot. Figs. 4 and 5 show the three-phase selfinductance and mutual inductance, and the mutual inductance is relatively small and could be neglected. In other words, the three phases are completely decoupled from each other. Viewing from the inductance profile, this machine works like 16-pole SRM rather than an 8-pole one. C. Torque Generation Since the three phases are independent from each other, the voltage equation for each phase can be expressed as dφ d(φm + Li ) U = E + Ri = = + Ri dt dt d(φm ) d Li =ω + + Ri (4) dt dt

GU et al.: NOVEL HIGH ENERGY DENSITY DSER-PM MACHINE

Fig. 5.

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Comparison of self- and mutual-inductance.

where U denotes the terminal voltage, E means the back EMF of each winding, φ is the flux linkage for each phase, R is the winding resistance, i is the phase current, L is the self-inductance of winding, t is the time, φm is the winding linkage, θ denotes rotor position, and ω is the speed of machine. As can be observed in (4), the first part is the back EMF generated by the variation of flux linkage from PMs; the second part is the back EMF caused by the variation of flux linkage from the phase winding; and the last term is the voltage drop due to the winding resistance. Then, the power input to the system is P = Ui = ωi

dφm 1 1 di 2l dL + i 2ω + + Ri 2 . dt 2 dθ 2 dt

dφm 1 dL P =i + i2 . ω dt 2 dθ

(a) Three-phase current excitation. (b) Torque profile.

Fig. 7.

Efficiency map of DSER-PM machine.

(5)

At the right side of (5), the first two parts are the power output, which can be used to derive the torque generated by each phase; the third part is power stored in the phase winding; and the last part is the copper loss. The last two items do not contribute to the torque generation T =

Fig. 6.

(6)

The torque generated by each phase is shown in (6). The first item on the right hand denotes the reaction torque generated by the PMs; the second part is the reluctance torque, which is the same as that in traditional SRM. As can be predicted from (6), positive reaction torque will be generated if one injects positive current, while flux linkage is increasing and negative current while it is decreasing. In addition, similar to traditional SRM, reluctance torque can also be obtained since inductance varies through rotor position. However, the reluctance torque, compared with reaction torque, is quite small and not considered here. The difference between DSER-PM machine and traditional SRM is that the three phases could be excited simultaneously to generate more torque. To simplify the comparison, we compare the torque generation, while have the same current density as 19 A/mm2 . Fig. 6 shows torque profile, the average torque of the DSER-PM machine is around 270 N · m, which is around 97% higher than 160 N · m generated by P3G IPMSM. There are several factors that result in a higher power density in DSER-PM machine. First, this rotor structure does not need back-iron to constitute flux path, thus more space are available for copper and higher MMF can be injected within the same envelope. Second, the exterior rotor structure has a larger air gap diameter which contributes to the higher power density. Third, just like DSSRM, DSER-PM motor can always keep a short flux path, which is different from traditional SRM.

The short flux path leads to less MMF drop on the iron when saturation happens. Fig. 6 also shows the big torque pulsation of this structure, however, the torque pulsation can be improved through better magnetic design and current profile optimization. D. Efficiency Efficiency indicates the effectiveness of energy conversion. Losses, which contain copper loss, iron loss, and mechanical loss, will finally convert to heat and dissipate to the environment. A well-designed cooling system is extremely critical in PM machine since PM would be demagnetized in high-temperature environment. The analytical method to calculate copper loss could be expressed as Pcopper = k j 2ρV

(7)

and iron loss is given by Piron = C1 B f 1.5 + C2 B 2 f 2

(8)

where k is a correction factor corresponding to temperature, j denotes the current density, ρ is the electrical resistivity of copper, V is the total volume of copper, C1 and C2 are coefficient, B is the flux density, and f is the frequency. Table II compares the losses generated by the proposed motor and IPMSM, while they are working at the same speed and have the same amount of output torque. It shows that the loss of the proposed motor is much less than that of IPMSM, while the speed is lower than 8500 r/min. However, the loss is higher than that of P3G IPMSM when speed is higher than

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 3, MARCH 2015

TABLE II L OSS C OMPARISON B ETWEEN DSER-PM M ACHINE AND P3G IPMSM

Fig. 8.

Torque comparison under different current density.

8500 r/min. The reason is that at low-speed range, copper loss takes up a larger portion, and the current density needed to generate the same amount of torque for DSER-PM machine is much lower than that of IPMSM, thus less copper loss will be produced. However, iron loss is almost proportional to the square of frequency, so iron loss takes up a large portion at high speed. To improve the efficiency of this machine, the pancake structure could be optimized to make better use of copper and higher current density is not suggested for this design, since saturation becomes sever and the torque will not increase much but higher iron loss will be produced. The efficiency map of the whole speed range is shown in Fig. 7. This paper mainly focus on magnetic design, thermal design is not quantitatively investigated here. According to the report in [9], the cooling mechanism occurs via stator to a cooled case and via convection to the cooling oil then to the cooled case. The heat can also be transferred between stator and rotor through the oil film formed in the air gap. As in this case, four aspects could verify the effectiveness of this cooling system. First, as shown in Table II, the loss generated of DSER-PM machine is much less than that of IPMSM, while working at the same condition. Second, the end winding takes up a larger portion of the total copper, large portion of heat from copper loss can be dissipated to the coolant directly. Third, comparing with P3G IPMSM, though the proposed motor has a smaller stator surface; multiple-hole structure in the inner cooling jacket can be implemented to compensate the decreased stator surface. At last, and the formed oil film in the motor has larger surface to help transfer the heat from stator to rotor. In addition, cooled case helps not only to cool down PM directly but also to dissipate the heat from stator. Taking into all these into consideration, thermal problem will not be a challenge, and more detail will be discussed in the future. III. B RIEF P ERFORMANCE C OMPARISON B ETWEEN THE P ROPOSED DSER-PM AND P3G IPMSM Fig. 8 compares the torque generated of these two machines at different current density. Table III is a summary comparing the proposed DSER-PM machine and P3G IPMSM [2]. The DSER-PM machine shows clear advantage in terms of torque and power density.

TABLE III P ERFORMANCE OF DSER-PM M ACHINE AND P3G IPMSM

According to the report from Oak Ridge National Laboratory [2], the peak torque of the P3G IPMSM is 207 N · m and the current density is much larger than 20 A/mm2 , also the peak power can only be maintained for 18 s before the temperature reaches 150 °C. Assuming 10 A/mm2 is current density for the motor to work continuously. The continuous torque for IPMSM is around 95 N · m, while the continuous torque for DSER-PM machine is around 187 N · m, which is 97% more than that of Prius design. As for the peak power of DSER-PM machine, the current density is boosted to 19 A/mm2 rather than 28 A/mm2 of IPMSM to get the maximum torque 270 N · m, which is 30.4% higher than that of IPMSM. IV. C ONCLUSION In this paper, a DSPM machine is proposed and analyzed. This proposed motor is configured with exterior rotor in which PMs are embedded in the rotor segments. It has the same envelope with that of 2010 P3G IPMSM. Simulation results shows the continuous torque and the peak torque are 97% and 30.4% higher than that of IPMSM, respectively. Thus, this machine has a relatively higher torque density and power density. R EFERENCES [1] J. de Santiago et al., “Electrical motor drivelines in commercial allelectric vehicles: A review,” IEEE Trans. Veh. Technol., vol. 61, no. 2, pp. 475–484, Feb. 2012. [2] Evaluation of the 2010 Toyota Prius Hybrid Synergy Drive System, Oak Ridge Nat. Lab., Oak Ridge, TN, USA, Mar. 2011. [3] K. Kiyota and A. Chiba, “Design of switched reluctance motor competitive to 60-kW IPMSM in third-generation hybrid electric vehicle,” IEEE Trans. Ind. Appl., vol. 48, no. 6, pp. 2303–2309, Nov./Dec. 2012. [4] M. Abbasian, M. Moallem, and B. Fahimi, “Double-stator switched reluctance machines (DSSRM): Fundamentals and magnetic force analysis,” IEEE Trans. Energy Convers., vol. 25, no. 3, pp. 589–597, Sep. 2010. [5] W. Wang, C. Lin, and B. Fahimi, “Comparative analysis of double stator switched reluctance machine and permanent magnet synchronous machine,” in Proc. IEEE Int. Symp. Ind. Electron. (ISIE), May 2012, pp. 617–622. [6] M. D. Hennen and R. W. De Doncker, “Comparison of outer- and inner-rotor switched reluctance machines,” in Proc. 7th Int. Conf. Power Electron. Drive Syst. (PEDS), Nov. 2007, pp. 702–706. [7] J. W. Sensinger, S. D. Clark, and J. F. Schorsch, “Exterior vs. interior rotors in robotic brushless motors,” in Proc. IEEE Int. Conf. Robot. Autom. (ICRA), May 2011, pp. 2764–2770. [8] W. Chu, Z. Zhu, and Y. Shen, “Analytical optimisation of external rotor permanent magnet machines,” IET Elect. Syst. Transp., vol. 3, no. 2, pp. 41–49, Jun. 2013. [9] Report on Toyota Prius Motor Thermal Management, Oak Ridge Nat. Lab., Oak Ridge, TN, USA, Mar. 2011.