Characteristic analysis of a less rare earth hybrid PM assisted synchronous reluctance motor for EVs application

Characteristic analysis of a less rare earth hybrid PM assisted synchronous reluctance motor for EVs application Characteristic analysis of a less rare earth hybrid PM assisted synchronous reluctance[.]

Characteristic analysis of a less-rare-earth hybrid PM-assisted synchronous reluctance motor for EVs application

Wenye Wu, Xiaoyong Zhu, Li Quan, Deyang Fan, and Zixuan Xiang

Citation: AIP Advances 7, 056648 (2017); doi: 10.1063/1.4975661

View online: http://dx.doi.org/10.1063/1.4975661

View Table of Contents: http://aip.scitation.org/toc/adv/7/5

Published by the American Institute of Physics

Characteristic analysis of a less-rare-earth hybrid

PM-assisted synchronous reluctance motor

for EVs application

Wenye Wu, Xiaoyong Zhu, Li Quan, Deyang Fan, and Zixuan Xiang

School of Electrical and Information Engineering, Jiangsu University,

Zhenjiang 212013, China

(Presented 3 November 2016; received 23 September 2016; accepted 8 November 2016;

published online 3 February 2017)

Low-energy permanent magnet (PM) such as ferrite is usually adopted in a PM-assisted reluctance (PMAREL) motor to enhance the output torque and reduce costs However, the relatively low magnetic energy product and remanence in such PMs may lead to the risk of demagnetization By using two types of materials of rare-earth NdFeB and non-rare-earth ferrite PM, a new less-non-rare-earth hybrid PMAREL motor is proposed in this paper, where the output torque and the power factor can be improved obviously, and meanwhile the risk of irreversible demagnetization in ferrite PMs can be reduced sig-nificantly due to the existence of NdFeB PMs To verify the validity of the proposed motor, the operating principles of the motor and the positive interaction influences between the two involved types of PMs are analyzed Moreover, by using the finite ele-ment method, the torque characteristics and anti-demagnetization capabilities are also investigated in details Both the theoretical analysis and simulated results confirm the

advantages of the proposed motor © 2017 Author(s) All article content, except where

otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ) [http://dx.doi.org/10.1063/1.4975661]

I INTRODUCTION

Recently, the price of rare-earth materials has suffered dramatical rise and fall, which has renewed the interest in research of non-rare earth motors.1 In particular, a type of PM-assisted synchronous reluctance motors employing ferrite PMs is becoming an attractive candidate in variable speed appli-cation such as electric vehicles (EVs), owing to its specific advantages, including robustness, high overload capability, wide constant power speed range and low cost.2 4

The type of the PM-assisted reluctance (PMAREL) motors is often characterized by a com-plex rotor structure with multi-flux barriers per pole, which makes it possible to boost a desired magnetic anisotropy, so as to increase the reluctance torque component And a proper volume of ferrite PMs is inserted in its rotor flux barriers with magnetization direction along the negative

q-axis, which can provide a great advantage in terms of increasing power factor and lowering stator

current, when compared with the traditional synchronous reluctance (REL) motors and induction motors.5,6

Such typed of the ferrite PMAREL motors has shown a price advantage and several merits,

however, the main defect is its low remanence (B r ) and coercivity (H c ) The low B rimplies a lower

magnetic loading, which limits the torque output capabilities Moreover, the very low H c value makes it susceptible to demagnetization especially when the motor operating in field-weakening and short-circuit conditions.7,8 To overcome these inherent deficiencies, in Refs.9 11, different rotor features such as U-shaped topology, V-shaped topology and conceptual spoke-type topology have been designed to lessen the demagnetization risk of ferrite PMs

In this paper, by combining the non-rare-earth ferrite PM with rare-earth NdFeB PM materials, a new hybrid PMAREL motor is proposed for the potential application of EVs Since the flux barriers and the two types of PMs are artfully designed, the proposed new topology can not only improve the high output torque but also enhance the operating point of ferrite PMs To highlight its unique features,

056648-2 Wu et al. AIP Advances 7, 056648 (2017)

the electromagnetic performances, including torque behaviors, the operating points of the two PMs and the anti-demagnetization capabilities are investigated in details Furthermore, the protective effects of NdFeB PMs in the ferrite operating points are particularly addressed

II MOTOR TOPOLOGY AND OPERATING PRINCIPLES

A Motor topology

Figure1shows a topology of the proposed motor, and its main design specifications are listed

in TableI In this motor, three layers of flux barriers are stacked from the rotor’s outer diameter

to the inner diameter for improving saliency ratios Iron ribs are set along the flux barriers, which cut down the flux barriers into several segments In each rotor poles, the rectangular-shaped ferrite PMs and NdFeB PMs are arranged to the flux barriers, with radial and tangential magnetization directions, respectively For the cost and demagnetization consideration, the size of ferrite PM’s pieces is designed larger than that of NdFeB PMs The inherent characteristics of the PMs are given

in Table II It can be found that both remanence and coercivity of NdFeB PMs are far superior

to ferrite PMs, and the remanence of ferrite PMs is nearly 0.4T, less than half of the value of NdFeB PMs

B Operating principles

Figure2(a)and(b)shows the magnetic flux path in a one-third model of the proposed motor

It can be seen that there are two different rotor paths for the flux One is a high permeability path

of d-axis path, where the flux lines flow in the rotor iron paths And the other is a low permeability paths referred as the q-axis, where the flux lines are blocked by multilayer flux barriers.1 , 3 , 5 As a

result, the d-axis inductance L d is far larger than the q-axis inductance of L q, which is similar to the traditional REL motor In addition, since the permeability of the PMs is almost equal to that of air,

FIG 1 The topology of the less-rare-earth hybrid PMAREL motor.

TABLE I The main specifications of the proposed motor.

TABLE II The specifications of the two types of PMs.

FIG 2 Operating principles of the less-rare-earth hybrid PMAREL motor (a) Magnetic flux path (b) The flux in one rotor pole (c) Vector diagram of REL motor and PMAREL motor.

the great difference between the d-axis inductance L d and the q-axis inductance L qis remained after adding the assisted PMs of ferrite and NdFeB to rotor flux barriers, which still keeps a high saliency

ratio and high reluctance torque When the PMs are magnetized opposite to the q-axis, an additional flux linkage along the negative q-axis is produced In particular, a part of the flux flows to the stator,

which yields an additional contribution to the total electromagnetic torque

Figure2(c)shows the motor’s vector diagram, where the black and red lines represent the vector diagram of the REL and the proposed PMAREL motor respectively For the proposed PMAREL motor, due to the effect of the PM flux linkage Λpm , the flux linkage L q I qis compensated along the

negative q-axis As a result, the flux linkage vector Λ REL is moved out of the current vector to the flux linkage of ΛPMAREL , and the corresponding voltage vector V REL is rotated towards the current

vector to V PMAREL, which can reduce the power factor angle from the ϕRELto ϕPMAREL Therefore, a higher power factor can be achieved compared with that in REL motor

III TORQUE ANALYSIS

The torque versus current phase angle ( β) of the proposed motor is investigated under the rated

conditions with a rated current (I r ) of 21A and a rotor speed (n r) of 1600rpm As is shown in Figure3, there are three components in the torque production of the motor: the reluctance torque, the

NdFeB-PM torque and the ferrite-NdFeB-PM torque As the current phase angle β varies from 0◦to 90◦, the curves

of reluctance torque and total torque present a similar variation trend, where the peak value of the

056648-4 Wu et al. AIP Advances 7, 056648 (2017)

FIG 3 Torque characteristics of the less-rare-earth hybrid PMAREL motor (I r = 30A and n r= 1600rpm).

reluctance torque is 19.8Nm at β of 65◦, and accounts for approximately 64.1% of total torque As expected, since the high saliency ratio is kept in the proposed motor, the reluctance torque is still a dominant component in the total torque Meanwhile, the PM torque nearly offer 35.9% of the total torque, although only a small amount of rare earth permanent magnet are used

IV DEMAGNETIZATION ANALYSIS

To investigate and compare the demagnetization characteristics of the proposed motor, the opera-tion points variaopera-tion of the two PMs under the rated current and speed with β of 55◦(MTPA operating point) is shown in Figure4(a)and(b) As is shown, both ferrite PMs and NdFeB PMs exhibit a regular fluctuation in their flux density Figure4(c)shows the operating points of NdFeB and ferrite PMs in

a sketch of B-H curve at the different of β It can be seen from the figure that both ferrite PMs and NdFeB PMs are nearly kept at their high operating points even when the maximum current ( β=90◦)

is applied Considering that the irreversible demagnetization often occurs when the PM’s flux den-sity is lower than that of its knee point Therefore, it can be concluded that the operating points of the two types of PMs in this motor are not sensitive to its armature reaction even under the rated conditions

To explore the interaction between NdFeB PMs and ferrite PMs, simulations are carried out with and without NdFeB PMs inserted in the rotor As shown in Figure4 (d), when the motor operates without NdFeB PMs, the ferrite PMs are directly exposed to a large demagnetizing current As a result, the operating point of ferrite gets lower than its knee point when β is greater than 30◦ And when β reaches 90◦, it even deteriorates to the value of 0.15T, which is lower than the threshold value

of 0.2T And the irreversible demagnetization will occur in the motor On the other hand, when a small amount of NdFeB is added to the rotor flux barriers, the ferrite operating point presents a relatively slower declining trend with the increase of β, and it remains above its knee point of 0.2 T when the demagnetizing current reaches its maximum ( β=90◦) So, the added NdFeB PMs can provide a significant protection for the ferrite PMs, where the ferrite operating points are increased above its knee point even under the maximum demagnetizing current operating conditions

In the potential EV application, peak currents may be applied during vehicle acceleration and uphill drives More critically, during short-circuit faults, a larger current than its rated current will be

developed against the magnet flux in q-axis of the motors.12,13Therefore, to confirm the capacity of demagnetization of the hybrid PMAREL motor, a rated current amplitude of 30A and an overload

current amplitude of 40A (133% of the peak current) are applied in d-axis by a high β of 90◦ The demagnetization performance of the PMs for the two conditions is illustrated in Figure 4(e) and Figure4(f) It is shown that, as a current of 30A is applied, the flux density of the whole ferrite PMs

is still higher than its knee point, which means that no irreversible demagnetization has occurred to ferrite PM’s bodies When the current increases to 40A with the maximum β of 90◦, only a partial demagnetization occurs at the edges of ferrite PM’s bodies in the first layer of flux barriers And yet, the ferrite PMs in the second and third layers are not affected by the overload current It indicates

FIG 4 (a)-(b) The fluctuation of PM’s flux density (I r =21A, n r=1600 rpm and β=55 ◦ ) (c) The operating points of NdFeB and

ferrite PMs in a sketch of B-H curve, according to different β value in rated current (I r =21A) and rated speed (n r=1600rpm) (d) The protective effects of NdFeB PMs in the ferrite PMs in their operating points operates in rated current and rated speed.

(e)-(f) The demagnetization performance of rotor in a rated current amplitude (I peak=30A) and an overload current amplitude

(I peak=40A) as the motor operates in rated speed.

that the proposed motor possesses a considerable capability of resistance to demagnetization even in overload conditions

V CONCLUSION

This paper investigates the characteristics of a new less-rare-earth hybrid PMAREL motor The results show that, a less amount of NdFeB PMs is added to a ferrite PMAREL motor, which not only yields an additional contribution to the total torque, but also provides the ferrite PMs an effective protection for its operating points And meanwhile, the risk of irreversible demagnetization in ferrite PMs is reduced significantly due to the existence of NdFeB PMs It demonstrates that the proposed hybrid PMAREL motor has a potential application prospect in the EVs

ACKNOWLEDGMENTS

This work was supported in part by National Natural Science Foundation of China under Grant

51477069, 51377073, in part by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and in part by the Project of Innovation of Postgraduate of Jiangsu Province

of KYZZ16 0335

1G Pellegrino, T M Jahns, N Bianchi, W Soong, and F Cupertino, The Rediscovery of Synchronous Reluctance and Ferrite Permanent Magnet Motors: Tutorial Course Notes (Springer, Switzerland, 2016).

2 A Vagati, B Boazzo, P Guglielmi, and G Pellegrino, IEEE Trans Ind Appl.50, 1768 (2014).

3 N Bianchi and H Mahmoud, IEEE Trans Energy Convers.31, 800 (2016).

4 H Huang, Y S Hu, Y Xiao, and H Lyu, IEEE Trans Magn.51, 1 (2015).

5 N Bianchi, E Fornasiero, and W Soong, IEEE Trans Ind Appl.51, 3600 (2015).

6 M Barcaro and N Bianchi, IEEE Trans Ind Appl.50, 979 (2014).

056648-6 Wu et al. AIP Advances 7, 056648 (2017)

7 M Fasil, N Mijatovic, B B Jensen, and J Holboll, IEEE Trans Magn.51, 1 (2015).

8 M Kimiabeigi, J D Widmer, R Long, Y Gao, J Goss, R Martin, and B Mecrow, IEEE Trans Ind Elec.63, 113 (2016).

9 S Morimoto, S Ooi, Y Inoue, and M Sanada, IEEE Trans Ind Elec.61, 5749 (2014).

10 H Cai, B Guan, and L Xu, IEEE Trans Ind Elec61, 5741 (2014).

11 D G Dorrell, M F Hsieh, and A M Knight, IEEE Trans Magn.48, 853 (2012).

12 G Pellegrino, A Vagati, P Guglielmi, and B Boazzo, IEEE Trans Ind Elec.59, 803 (2012).

13 K I Laskaris and G Kladas, IEEE Trans Ind Elec.57, 138 (2010).