Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

Switched reluctance motor for hybrid electric vehicles

AbstractThis paper discusses the design of a segmented rotor switched reluctance motor (SSRM) for hybrid electric vehicles (HEV). Some requirements of this application are high power density, high overload capability, wide range of eld weakening and high efciency over a wide range of loads and speeds. A segmented rotor SRM which satises these characteristics is designed. The designed motor has 4 phases, with 16 stator poles and 14 rotor segments. The performance of this machine is evaluated using nite element based simulation. Further, an excitation strategy for this motor to achieve high efciency over a wide range of loads is proposed. The simulation results indicate that the designed motor has high efciency and low weight.

of speeds is required, eld weakening ability of the machine is evaluated. A. Selection of motor topology

200 180 160 Torque (Nm) 140 120 100 80 60 40 0 2000 4000 6000 Speed (rpm) 8000 10000 12000

I. I NTRODUCTION Motors most widely used for hybrid electric vehicle (HEV) application are interior permanent magnet motors (IPM). This is due to high efciency, wide range of eld weakening and overload capability. However, due to rising cost of permanent magnets, much of the recent work has focused on using switched reluctance motor (SRM) for this application instead. A 3 phase 18/12 SRM was proposed for HEV application in [1]. Further, a 4 phase 8/6 SRM was designed for HEVs in [2]. While previous work has focused on maxmizing the torque output of SRMs, a high efciency over a wide range of loads is targeted here. The current paper proposes a 4 phase 16/14 segmented rotor type SRM for HEVs. The targeted vehicle is Toyota Prius. Toyota Prius uses a hybrid synergy drive. The engine operates only in the region where its efciency is high. The motor is expected to act as prime mover in the rest of the operating region of the vehicle [3]. For this reason, the motor must have high efciency over a wide range of speeds and loads. Other requirements of the application are high overload capability, high power density and wide range of eld weakening. The motor is designed according to these requirements. Further, an excitation strategy to achieve high efciency over a wide range of loads is proposed. In this scheme, only one phase conducts at a time at lower currents, while two phases conduct simultaneously at all times at higher currents. The performance of the designed 16/14 SSRM is also compared with the 18/12 SRM designed for HEV application in [1]. The results indicate signicant performance improvement. As operation in the constant power region over a wide range

Fig. 1. Required torque-speed characteristics of Toyota Prius [3]

TABLE 1 S PECIFICATIONS OF IPM MOTOR CURRENTLY USED IN T OYOTA P RIUS Motor Number of phases Stator poles/rotor poles Winding arrangement Stator OD Airgap Stack length Interior PM motor [3] 3 48/16 Full pitch, distributed 264 mm 0.73 mm 51 mm

The torque-speed characteristics required in this application are shown in Fig. 1. This is obtained from [3]. The motor in use for this application is an interior PM motor with 48 stator poles and 16 PM poles. It can be seen from Table 1 that the space available for the motor is constrained. Further, maximum torque of 200 Nm is required. In order to achieve such high torque, higher operating current density is maintained, and forced cooling is used [1]. At the same time, constant power operation upto the speed of 12000 rpm is required [3]. It was shown that increase in the width of constant power region

Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

of the motor increases its torque rating, thereby increasing its size. However, operation in constant torque region over a wide range of speeds increases power required during acceleration [4]. As a trade-off between motor size and acceleration power requirement, the motor is rated for 150 Nm at 4000 rpm. It is overloaded to obtain 200 Nm at lower speeds. Segmented rotor SRMs have higher torque output than conventional SRMs [5]. However, these machines require full pitch winding for their operation. Full pitch winding increases end winding volume as end turns of one phase overlap with those of the other. An alternative topology employing single tooth windings was proposed in [6]. The machine proposed there has 12 stator poles with 10 rotor poles. It was reported to have performance similar to SSRM with full pitch winding, but with lower end winding volume and copper loss. The cross section of 12/10 SSRM is shown in Fig. 2.
Airgap Stator C+ A+ A-

variation is not linear due to saturation of the magnetic circuit. The area between ux linkage - curreny diagrams at aligned and unaligned positions is proportional to the average torque output [8]. The ux - linkage current diagrams for 18/15 and 12/10 SSRM are shown in Fig. 4. The unaligned ux linkage can be expressed in terms of current as follows: u = Lu I (1)

Upto saturation, the aligned ux linkage can be expressed as a = Laus I (2)

After saturation, according to [8], the slope of the ux linkage - current line is approximately equal to unaligned inductance. Hence, aligned ux after saturation is expressed as: a = Laus I1 + Lu (I I1 ) (3)

CRotor segments

B+

B+

Where: Lu : Unaligned inductance Laus : Unsaturated aligned inducance I1 : Current at which magnetic circuit gets saturated at the aligned position For the 18/15 SSRM, Laus is lower than that of the 12/10 motor. This can be understood as follows: Suppose that both 18/15 and 12/10 machines have the same total MMF per phase, which is equal to NI. In case of the I 12/10 machine, there are 4 slots per phase, each carrying N 2 ampere conductors. Likewise, ampere conductors per slot for I 18/15 SRM is N 3 At low operating currents, magnetic circuit is not saturated. Hence, the reluctance of the iron parts can be neglected. The air gap ux density can be expressed in terms of ampere conductors per slot using Amperes circuital law as: Bg = o Ampere-conductors per slot 2g (4)

B-

Non-magnetic material C+ A-

B-

A+ C-

Fig. 2. Geometry of 12/10 SSRM

A segmented rotor SRM with single tooth winding is considered for HEV application here. The motor is designed with the same diameter as interior permanent magnet which is currently in use for this application (i.e. 264 mm). It was shown in [7] that SRM with higher number of rotor poles than stator poles have high torque output with low copper loss. However, such such machines have lower torque than other SRM topologies when operating current density is high. Increase in the number of stator poles along with the rotor poles improves torque at high current density [9]. Thus, initially an 18/15 SSRM with 3 phases is designed. This machine is similar in construction to the 12/10 SSRM of [6]. The 12/10 SSRM has multiplicity of 2, while the 18/15 SSRM has multiplicity of 3. However, a disadvantage of the 18/15 motor is lower torque per ampere than other SSRM topologies at lower operating currents. This is illustrated in Fig. 3. It can be seen from this gure that 18/15 SSRM has lower torque output than 12/10 SSRM at lower currents (loads). This can be explained as follows with the help of ux linkage - current diagrams. These diagrams are obtained by nite element based simulation. The motor is held stationary at its aligned and unaligned positions. At each of these positions, current is varied from 0 to the rated value, and the ux linkage is determined. At the unaligned position, the ux linkage varies linearly with current, while at the aligned position the

Where g is the air gap length. Due to lower ampere condcutors per slot, air gap ux density in case of 18/15 SSRM is lower than in 12/10 SSRM. This leads to lower aligned inductance i.e. Laus . Lower Laus leads to lower torque, as seen in Fig. 4. However, Fig. 4 shows that La of the 18/15 SSRM (after saturation) is higher than that of the 12/10 machine. Thus, at higher currents, torque output of 18/15 SSRM exceeds that of 12/10 SSRM. Thus, the limitation of using an 18/15 SSRM is that lower torque is obtained at lower currents, thereby necessitating increase in current to meet the required load. This leads to reduction in efciency at lower loads. But in this application, high efciency over a wide range of loads and speeds is desired. It is found that higher torque can be obtained at both high and low loads by using a 4 phase SSRM with single tooth winding. This motor has 16/14 poles, and its cross section is shown in Fig. 6. It is also derived from the 12/10 SSRM of [6]. The variation of torque with current is shown in Fig. 7 (a). Higher torque than 18/15 SSRM can be observed at lower currents. However, it is seen that torque output at higher

Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

Stator

120
18/15 SSRM
D+

A+

AB-

100
C-

D-

Rotor segments Air

B+

C+

80 Torque (Nm)
12/10 SSRM
C+ Stainless Steel frame B+

C-

60

DBAA+

40

D+

20

0 0

Fig. 6. Geometry of 16/14 SSRM

Fig. 3. Variation of torque with current for 12/10 and 18/15 SSRMs

150

100

1 Flux linkage (Wb) 0.8 0.6 0.4 0.2

18/15: aligned position

Torque (Nm)

4 phase SSRM (1 phase ON) 3 phase 18/15 SSRM

12.10: aligned

50

18/15: unaligned position

0 0
12/10: unaligned

2000

4000 6000 MMF (AT)

8000

10000

0 0

(a)
10 20 Current (A) 30 40
180 160 140 120 Torque (Nm)
4 phase SSRM (2 phases ON)

Fig. 4. Variation of ux linkage with current for 18/15 and 12/10 SSRMs) SSRMs

0.35 0.3 Flux linkage (Wb) 0.25

100 80 60 40

4 phase SSRM (1 phase ON)

0.2

20 0 0 2000 4000 6000 MMF (AT) 8000 10000 12000

0.15 0.1

(b)
0.05 A00 20 40 Current (A) 60 80

Fig. 7. Comparison of torque outputs of (a) 16/14 SSRM (with only one phase ON) and 18/15 SSRM (b) 16/14 SSRM with 1 phase ON and with 2 phases ON

Fig. 5. Flux linkage - current diagram of 16/14 SSRM

currents is lower than that of 18/15 SSRM. The ux linkage current diagrams for 16/14 SSRM machine is shown in Fig. 5. It can be seen that at higher currents, the unaligned ux linkage

approaches the unaligned ux linkage leading to reduction in torque. However, as seen in Fig. 4, in case of 18/15 SSRM there is sufcient margin between aligned and unaligned ux linkages. But an advantage of the 4 phase machine is that two phase

Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

A+ D+ DC-

ABD+ DC-

A+

AB-

0.7 C
B+ C+

B+ C+

0.6 B Flux linkage (Wb) 0.5 0.4 E 0.3 D 0.2 0.1 A00 20 40 Current (A) 60 80

C+ B+ BAA+ D+ D-

C-

C+ B+ BAA+ D+ D-

C-

(a)

(b)

Fig. 8. Flux paths of 16/14 Segmented rotor SRM in aligned position (a) Flux paths with one phase ON (b) Flux paths with 2 phases ON

Fig. 10. Comparsion of Flux-linkage current diagrams for 16/14 SSRM with one phase and 2 phases ON
A+ D+ AB-

DC-

B+ C+

C+ B+ DBAA+ D+

C-

(a)
A+ D+ AB-

DC-

B+

C+

C+ B+ DBAA+ D+

C-

The improvement in torque per ampere at higher currents due to simultaneous excitation of two phases can also be explained with the aid of ux linkage current diagrams. Area enclosed between the ux-current diagrams at aligned and unaligned positions is proportional to the average torque output of the motor. This area corresponds to area ABCA in Fig. 10 when only one phase is excited at a time. Area ADEA represents this area when 2 phases are excited simultaneously. When 2 phases are kept on at a time, peak phase current is 1 reduced by in order to maintain the same rms value. At 2 the operating point of 80 A rms, area ABCA is 8 J, while area ADEA is 10 J. From Fig. 7 (a), it is seen that 16/14 SSRM has higher torque output than 18/15 SSRM at lower currents. Also, as seen above, 2 phases can be kept on at a time in this machine to get higher torque output at higher currents. Thus, higher torque per ampere can be achieved by making only one phase conduct at lower currents, and 2 phases together at higher currents. II. F IELD WEAKENING ABILITY OF HIGH POWER DENSITY SRM In order to obtain high power density, current density in the coils is increased. It is found here that a high power density SRM has smaller eld weakening ability. In the eld weakening region, current turn ON position is advanced such that the current reaches its peak value, Ip at the start of the positive torque region. At maximum speed p , this advance is adv (in radians electrical). Therefore, di Lu Ip p = (5) dt adv The applied voltage V is taken to be equal to times the back emf at base speed (b ). Hence V = Lu dL La Lu = mIp b (6) d 2 La and Lu are aligned and unaligned inductances respecLa tively and m is the number of phases. Putting L = S , and u V = Eb = Ip b

(b) Fig. 9. Flux paths of 16/14 Segmented rotor SRM in unaligned position (a) Flux paths with one phase ON (b) Flux paths with 2 phases ON

are simultaneously in the torque producing zone. It is found that this feature can be used to improve the torque output at higher currents. The variation of torque in both cases (i.e. with only one phase excited, and with 2 phase ON simultaneously) is shown in Fig. 7. Improvement in torque output at higher currents can be noted. The aligned ux path with only one phase excited is shown in Fig. 8 (a), and that with two phases on is shown in g. 8 (b). The unaligned ux paths with one and two phases on are shown in Fig. 9. It should be ensured that coils of consecutive phases in adjacent slots carry current in opposite directions to obtain the ux pattern of Fig.8 (b). This can be achieved by using the following coil arrangement: A+ A- B- B+ C+ C- D- D+ A+ A- B- B+ C+ C- D- D+.

Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

from (5) and (6) the ratio of maximum speed to base speed is given as m(S 1)adv p = b 2 (7)
Current (A)

14 12 10 8 6 4 2 0 0 0.002 0.004 0.006 Time (s) 0.008 0.01

The value of S reduces with increasing current due to saturation. Equation (7) shows that a small value of S leads to a small ratio of maximum speed to base speed i.e. narrow constant power range. In case of high power density SRM, the value of operating current density being high, S is low (about 2 in the current study). Thus, a wide eld weakening zone as required by the application cannot be obtained by the conventional method. However, it is observed that operation in constant power region can be obtained by using the continuous conduction technique of [10]. In case of SRM, each phase has a positive torque region of duration 180 electrical, and a negative torque zone also of the same duration. During operation in the discontinuous conduction region, the phase current is made to decay to zero at the end of positive torque region. However, at high speeds less time is available for the current to rise. Thus, the peak value of current reduces, thereby reducing the torque and power. In order to address this limitation, continuous conduction technique was proposed in [10]. In this technique, phase current is not allowed to decay to zero at the end of positive torque region. Thus, the current rises to a higher peak, thereby increasing the torque. At the same time, due to phase conduction in negative torque region, copper loss also increases. The current waveforem during continuous conduction is shown in Fig. 11. This current waveform is obtained using MATLAB/Simulink. The torque-speed characteristics of the high power density SRM are shown in Fig. 12 (a), while the power speed characteristics are shown in Fig. 12 (b). These characteristics are obtained from MATLAB/Simulink based simulation. It can be seen that continuous conduction technique must be used to obtain the required constant power operation of the motor. In the simulation study, is chosen to be equal to 1. Another way to obtain constant power operation of the motor is to oversize the inverter i.e. choose a higher value of . However, this would increase the inverter VA rating of the drive. On the other hand, use of continuous conduction technique to obtain constant power operation reduces efciency [11]. An alternative is to decide the value of such that the motor operates in the continuous conduction region only over a small range of speeds. III. C OMPARISON OF 16/14 SSRM WITH 18/12 SRM
TABLE 2 C OMPARISON OF 16/14 SSRM AND TOOTHED ROTOR SRM Motor 16/14 18/12 (toothed rotor type) Stack length 90 mm 110 mm Copper loss 1.7 kW 1.9 kW Iron loss 2.281 kW 3.16 kW Active weight 18 kg 35 kg

Fig. 11. Current in one phase during continuous conduction

160 140 120 Torque (Nm) Continuous conduction 100 80 60 40 20 0 Discontinuous conduction

2000

4000

6000
Speed (RPM)

8000

10000

12000

(a)
70 65 60 Power (kW) 55 50 45 40 35 30 4000 5000 6000 7000 8000 9000 Speed (RPM) 10000 11000 12000 Disontinuous conduction Continuous conduction

(b) Fig. 12. Characteristics of high power density SRM (a) Torque-speed characteristics (b) Power-speed characteristics

A toothed rotor SRM with 18 stator poles and 12 rotor poles for designed for HEV application in [1]. The performance of the designed 16/14 SSRM is compared with 18/12 SRM. The toothed rotor SRM has 18 stator poles. Thus, the slot pitch of this machine is lower than that of 16/14 SSRM. Another point is that in case of toothed rotor SRM, coils are wound around all the poles. However, in the designed SSRM, coils are wound around alternate poles. Thus, end turn lengths are smaller in the toothed type SRM. In order to ensure that both machines have the same overall length, the stack length of toothed type SRM is increased. Comparison of 18/12 toothed type SRM and 16/14 SSRM at rated condition is given in Table 2. This table shows that not only are the net losses much higher in the

Fifth International Conference on Power and Energy Systems, Kathmandu, Nepal | 28 - 30 October, 2013

former, but also the machine weight. Iron loss of segmented rotor SRM is signicantly lower. This is due to short ux paths and lower iron weight. The variation of torque with copper loss of 18/12 SRM and 16/14 SSRM is shown in Fig. 13. It can be seen that the designed 16/14 SSRM gives higher torque output for the same copper loss over a wide range of loads. Also, as seen in Table 2, the iron loss of 16/14 SSRM is lower. This indicates that 16/14 SSRM has higher efciency than 18/12 SRM.
200 16/14 (4 phase) SSRM

160 Torque (Nm) 18/12 (3 phase toothed rotor) SRM

120

80

40

200

400

1000 600 800 Copper loss (W)

1200

1400

1600

Fig. 13. Torque vs copper loss of 16/14 SSRM and 18/12 (toothed rotor) SRM

An important issue in SSRM is the design of the rotor. The rotor segments are isolated. Embedding of the isolated rotor segments in aluminium block was proposed in [12]. However, it is found eddy currents induced in aluminium block are inductance limited. This causes signicant deterioration in efciency of the motor. Placement of rotor segments in stainless steel mitigates this limitation. IV. C ONCLUSION This paper discusses an SRM suitable for hybrid electric vehicles. The designed machine is a 4 phase 16/14 SRM with segmented rotor structure. In order to achieve high efciency throughout the operating range, one phase at a time must be excited at lower loads, while 2 phases can be made to conduct simultaneously at higher loads. Comparison with 18/12 SRM designed for hybrid electric vehicles shows that efciency can be increased and weight can be reduced by use of the segmented rotor topology. Further, it is found that high power density SRMs as required in HEV application have limited eld weakening ability. Continuous conduction must be used in order to obtain operation in the eld constant power region. R EFERENCES
[1] M. Takeno, A. Chiba, N. Hoshi, S. Ogasawara, and M. Takemoto, Power and efciency measurements and design improvement of a 50kW switched reluctance motor for Hybrid Electric Vehicles, IEEE Energy Conversion Congress and Exposition (ECCE), pp.1495-1501, 2011 [2] S. Wang, Q. Zhan, Z. Ma and L.Zhou; , Implementation of a 50-kW four-phase switched reluctance motor drive system for hybrid electric vehicle, IEEE Transactions on Magnetics , Vol.41, No.1, pp. 501- 504, 2005

[3] T.A. Burress, S.L. Campbell, C.A. Coomer, C.W. Ayers, A.A. Wereszczak, J.P. Cunningham, L.D. Marlino, L.E. Seiber and H.T. Lin, Evaluation of the 2010 Toyota Prius Hybrid Synergy Driver System, 2011 [4] M. Ehsani, K.M. Rahman and H.A. Toliyat, Propulsion system design of electric and hybrid vehicles, IEEE Transactions on Industrial Electronics, Vol. 44, No. 1, pp. 19-27, 1997 [5] B.C. Mecrow, J.W. Finch, E.A. El-Kharashi and A.G. Jack, Switched reluctance motors with segmental rotors, IEE proceedings on electric power applications, Vol. 49, No. 4, 2002 [6] B.C. Mecrow, E.A. El-Kharashi, J.W. Finch and A.G. Jack, Segmental rotor switched reluctance motors with single-tooth windings, IEE proceedings on electric power applications, Vol. 150, No. 5, 2003 [7] P.C. Desai, M. Krishnamurthy, N. Schoeld, and A. Emadi, Novel Switched Reluctance Machine Conguration With Higher Number of Rotor Poles Than Stator Poles: Concept to Implementation, IEEE Transactions on Industrial Electronics , Vol. 57, No. 2, pp. 649-659, 2010 [8] T. Miller, Switched Reluctance Motors and their Control, Monographs in electrical and electronic engineering, Magna Physics, 1993 [9] R. Vandana, S. Nikam and B.G. Fernandes, Criteria for design of high performance switched reluctance motor, International Conference on Electrical Machines (ICEM), pp. 129-135, 2012 [10] J.S. Lawler, J.M. Bailey, J.W. McKeever and P.J. Otaduy, Impact of continuous conduction on the constant power speed range of the switched reluctance motor, IEEE International Conference on Electric Machines and Drives, pp. 1285-1292, May 2005 [11] H. Hannoun, M. Hilairet and C. Marchand, Experimental Validation of a Switched Reluctance Machine Operating in Continuous-Conduction Mode, IEEE transactions on vehicular technology, Vol. 60, No. 4, pp. 1435-1460, 2011 [12] J. Oyama, T. Higuchi, T. Abe and N. Kifuji, Novel Switched Reluctance Motor with Segment Core Embedded in Aluminum Rotor Block, IEE Japan Transactions on Industry Applications, Vol. 126, No. 4, pp. 385 - 390, 2006.