4 Novel Axial Wang

A Novel Axial Field SRM with Segmental Rotor: Concept, Design and Analysis

Wang Bo Department of Mechatronics

Engineering Kyungsun University

Busan, Korea [email protected]

Dong-Hee Lee Department of Mechatronics

Engineering Kyungsun University

Busan, Korea [email protected]

Jin-Woo Ahn Department of Mechatronics

Engineering Kyungsun University

Busan, Korea [email protected]

AbstractA novel axial field switched reluctance motor

(AFSRM) with single teeth stator and segmental rotor is

introduced in this paper. The stator and rotor of the proposed

motor are disk type: the stator poles are composed of excitation

poles and auxiliary poles, the rotor is made up of a series of

discrete segments. Because of the axial field structure and novel

excitation source, the proposed motor can provide much higher

output torque within reducing the copper volume. The basic

operation concepts and design rules are introduced and in order

to calculate the characteristics of the proposed structure

detailed, the finite element model is established base on the

software of Maxwell 12-3D, which is aimed at flux linkage,

inductance, output torque, radial force and axial force.

Compared with the conventional 12/8 SRM in the same

parameters, the proposed motor can provide higher output

torque and more efficient, which is much suitable for the low

speed and high torque applications.

KeywordsAxial Field SRM; Single Teeth; Segmental Rotor

I. INTRODUCTION The conventional radial magnetic geometry of switched

reluctance machines (SRMs) has been effectively fixed for over 20 years. The basic structure consists of a series of stator poles which are magnetically connected together by a core back, and a series of rotor poles, with the magnetic circuit completed by a rotor core back. The windings are wounded on the stator poles and there is no winding or permanent magnets on the rotor. SRMs have some advantageous features such as fail safe, robustness, low cost, and possible operation in high temperatures or in intense temperature variations. However, during the application of electric vehicles the SRMs should be work as a traction motor which will be fixed up into the

wheels, thence the axial length of the motor and the output performance will be particularly requested [1].

The axial magnetic field disk type induction motor (DSIM) is different from the traditional radial magnetic field rotating machines, whose stator and rotor cores are laminated into disk shape and the two iron cores take the relative position in space. Because of the special structure, the axial length of this kind of motor can be reduced compare with the radial motor. Simultaneously, the special axial structure can accept higher current density before oversaturation, which can be used to provide higher output torque and overload capacity as well as the higher power to weight ratio. The disk type motor has many different structure forms which can be applied on many occasions, especially for the low speed and high torque applications. Fig.1 shows the conventional structure of the stator and rotor of the SRMs and DSIMs [2].

Fig.1. Conventional structure of SRM and DSIM

In this paper, a novel axial field disk type SRM with single teeth and segmental rotor is introduced. The proposed motor combines the features both of the SRMs and DSIMs: the stator and rotor cores are laminated into disk shape, the stator tooth are formed into two groups, double-width teeth, which are wounded by the coils, and standard-width teeth which are unwounded but used to provide the circuit for the flux path, the rotor is made up of a series of segmental rotor blocks, which are embedded into the aluminum body. The fundamental magnetic design concepts and design rules are derived with the mechanical model, characteristics of the proposed structure are

978-1-4799-1007-6/13/$31.00 2013 IEEE

analyzed with the case of flux linkage, inductance, output torque, radial force and axial force. Compared with the previous researches, the proposed motor can provide much better output performances within reducing the copper loss. Fig.2 shows the structure diagram of the proposed motor with 3D model.

Fig.2. Structure diagram of the proposed motor

II. FUNDAMENTAL MAGNETIC DESIGN CONCEPTS

The innovation of the proposed motor is that: the stator and rotor cores of the proposed motor are laminated into disk shape which taking the relative position in space. The stator poles are composed of excitation poles and auxiliary polessingle teeth structure and the segmental rotor cores are embedded into the body of the rotor which is casting by the material of aluminum.

To reveal the fundamental points of the magnetic design the geometry is simplified as much as possible, so a rectilinear geometry is chosen to present before progressing to a full rotating machine design. Fig.3 shows the rectilinear diagram of the proposed motor, the windings are wounded on the exaction pole in aligned position.

Fig.3. Simple rectilinear model of the proposed

structure

A. Dimensions Design

Base on the previous research, the fundamental magnetic design concepts are shown in Fig.3, t and are the width of the teeth and the length of the slot, respectively. The ratio between t and will severely affected to the MMF of the proposed

motor. If the ratio of t/ is too small, which means the stator pole takes more width of the stator pith, there will be not enough space for the coils, and meanwhile there will be not enough excitation for the output performance. Vice versa, the stator pole will be oversaturation because of high flux density which is provided by the extra coils. So followed the previous researches, the ratio of t/ should be defined between 0.6-0.7, which is much suitable [3].

Fig.4. Magnetic flux distribution of proposed motor The flux density distribution of each part of the motor

should be uniform, thus with the references to Fig.3 and 4, the dimensions for the machine are chosen using the following criteria, which mainly followed the principium of that:

(a) The distance between the adjacent stator and rotor poles should be the same, x=y, which will be minimizing the unaligned permeance without compromising the aligned inductance as shown in Fig.3.

(b) The overlap length between the stator tip and segmental rotor is t/2, in order to ensure that the air gap flux density corresponds to the tooth flux density as shown in Fig.4.

(c) The vertical height of the segmental rotor t/2 is equal to half of the width of the stator pole t, so that the magnetic flux density in the rotor and stator will be the uniform as shown in Fig.4.

B. Single Teeth Design

However, consider about the previous researches, there is a serious disadvantage of the multi tooth stator SRMs. The machine had substantially longer end-windings, which reduced the electric loading and made it impractical for applications which combined a short lamination stack length with a large pole pitch. Thence, the rotor structure should only permit adjoining teeth to be magnetically linked, in this way the magnetic flux can only enclose a single stator slot, in order to reduce the copper volume and form into the short flux path.

Fig.5-(a) shows the previous work design in rectilinear form: It can be seen that the coil spans three stator poles and occupy two rotor segments. If the shaded region was taken off, and connects the rest parts together, then the coils will just span one stator which is twice width of the previous stator pole, and meanwhile the coil would span only one segmental rotor. In the resulting, single teeth and the single teeth winding arrangement, which are shown in Fig.5-(b).

(a) Multi tooth stator (b) Single teeth stator

Fig.5. Rectilinear representation of single teeth The single teeth structure is made up of two groups:

double-width teeth, which is wounded by the coils and standard-width teeth which is unwounded. The unwound teeth still have a function which acts as return path for the magnetic flux. Excitation of one single phase now excites two adjacent slots and the phase permeance is the sum of the two slot permeances. The tooth pitch of the wound stator teeth must be equal to the rotor pole pitch, so that the permeance variation of these two slots with respect to rotor position is in phase. The flux path of the novel single tooth is shown in Fig.6 at aligned and unaligned position compare with the conventional multi tooth structure [4]:

(a) Aligned position (b) Unaligned position

Fig.6. Flux path of the novel single teeth For clarity a complete set of design rules for the single teeth

structure design are given below:

(a) Only one winding can be wounded on one exaction pole, so that the adjacent stator will be wounded the same turnings to ensure the MMF of each phase.

(b) The distance between the adjacent single stator pole and segmental rotor should be the same, which ensures that neither the rotor nor stator contribute unnecessarily to the unaligned permeance, as shown is Fig.6-(a), x=y.

(c) The width of the unwounded pole t/2 should be equal to the vertical height of yoke of the stator t/2, which is half length of the wounded pole t, to ensure that the flux density in the stator is uniform.

(d) The vertical height of the segmental rotor t/2 should be equal to the width of the unwounded pole t/2, which ensures the flux density between the rotor and stator is uniform.

C. Optimization of the Tips

The tip of the pole should be considered, because the tips will influence the overlap area of the stator and rotor, which will be indirect effect the inductance and torque, so it is very important to appropriate choose the tips. Generally, there are two kinds of tips, one is angled tips and the other one is the squared tips which are shown in Fig.7 and 8.

Fig.7. Flux density of the angled tips

The tips are the most sensitive parts of the motor, which is much easier to be oversaturation because of the small dimension. And during the processing, the mechanical strength and processing accuracy also should be considered. Fig.7 shows the angled type tips with the angle from 10 degree to 80 degree by each step 5 degree, and the flux density of the tip is shown by the Y axis.

Fig.8. Flux density of the squared tips

Fig.8 shows the squared type tips with the depth d from 0.5mm to 5mm by each step 0.5mm. It can be easily seen that the flux density will be reduce with the depth of the square tips increase, so combine with the optimized simulation and empirical formulas, the radial depth of the stator tips should be choose the square tip and the depth is 3mm. Fig.9 shows the flux distribution of the stator tips by the proposed depth within 3mm and the maximum flux density is 2.17T.

Fig.9. Flux distribution of the stator tips

The major dimensions of the prototype machine are tabulated in Table 1.

Table 1: Dimensions of the prototype machine Number of phases 3

Number of stator slots 12

Number of segmental rotors 10

Stator outside diameter(mm) 104

Rotor outside diameter(mm) 106

Stack axial length(mm) 36

Air gap length(mm) 0.25

Stator tooth width:

Tip of pole(degree)

Double-width teeth: 30.25

Standard-width teeth: 18.5

Stator tooth width:

Body of pole(degree)

Double-width teeth: 20

Standard-width teeth: 8

Segmental rotor width(degree) 30.25

Number of series turns/phase 10

Coil span 1 single teeth pith

Effective wire diameter(mm) 2.836

Slot fill-factor 0.3

III. CHARACTERISTICS ANALYSIS FOR THE PROPOSED DT-SRM

For the performance estimation proposed motor, some characteristics such as inductance, output torque and axial force are very important. These characteristic curves reflect to the performance of the proposed DT-SRM. Due to the particularity of the structure of proposed motor which use the axial field, the magnetic field distribution in the motor is complex, so 3D FEM is used to analyze characteristics, which includes magnetic flux distribution, inductance, flux linkage, output torque, radial force and axial force.

A. Magnetic Flux Distribution

The finite element analysis is base on the Maxwell 12-3D model. Fig.10 shows the simulation model with rotor and stator, respectively.

Fig.11-(a) shows magnetic flux distribution at aligned position by the front view. It can be clearly seen that, the flux starts from the exaction pole and go through the air gap, then the flux path will be separated into two paths, both of the two paths pass the segmental rotor cores, then re-through the air gap to get the unwounded poles, finally the two paths will be closed at the yoke of the stator. Fig.11-(b) shows the distribution by the top view, it can be clearly seen that the tips are the parts which are in high level of saturation. The closed magnetic circuit is different from the conventional radial motor because of the axial field excitation.

(a) Segmental rotor (b) Single teeth stator

Fig.10. 3D simulation model of rotor and stator

(a) Front view (b) Top view

Fig.11. Magnetic flux distributions of the proposed motor

B. Flux Linkage and Inductance Characteristics

Fig.12 shows the flux linkage and inductance profiles for the proposed motor with various rotor positions and currents. It can be clearly seen that the two characteristic curves change obviously for different rotor positions with the same phase current. The flux linkage curve is similar with the inductance curve because the flux linkage relate to the product of inductance and current, the maximum and minimum value appear at aligned and unaligned position, respectively. At the same time, the inductances decrease because that core saturation increases with the increasing of the phase current.

(a) Flux linkage (b) Inductance

Fig.12. Characteristics of flux linkage and inductance

C. Output Torque Characteristics

The torque is proportional to the square of the current and change ratio of inductance with respect to rotor position. Therefore, torque profiles are determined by inductance profiles. According to the previous analysis, the change rate of the inductance is rapidly from the aligned position to unaligned position, so the output torque will get the peak value of during the two regions. At the same time, the overlapping area of the poles is supported by the axial field segmental rotors, so there will be rapid transformation between the peak value of Max and Min output torque as it is shown in Fig.13. And the torque curve of the proposed motor is similar to the conventional disk type inductance motor.

Fig.13. Characteristics of output torque

D. Radial Force Characteristics

Fig.14-(a) and (b) show the radial force of X-axis and Y-axis with various rotor positions and currents, respectively. The force is formed by the change of the magnetic flux density between the stator and rotor, so there will no force at aligned position in the ideal state. However, during the operating, the flux mainly closed by the axial field, so it can be seen the peak value of the radial force is less than 0.2 Newton which is almost can be ignored compare with the axial force.

(a) Force on X-axis (b) Force on Y-axis

Fig.14. Characteristics of the radial force

E. Axial Force with Uniform and Non-uniform Airgap

There will be an electromagnetic force at the axial direction between the stator and rotor because of the effect of the axial field magnetic flux, meanwhile the axial force will cause the

vibration of the rotor during the operating. Therefore, the axial force should be considered at two situations as Fig.15 shows: for the uniform air gap which means the air gaps D1=D2=D3=0.25mm, and for the non-uniform air gap, the air gap D1 is constant because of the mechanical strength of the shaft, but for D2 and D3, one will be increase and the other one will be decrease, but D2+D3=0.5mm should be constant.

Fig.15. Air gaps of the axial force by the vibration

(a) For the uniform air gap

Fig.16. Axial force for uniform air gap

During the operating, give the current to one phase which are the opposite poles on stator. Fig.16 shows axial force profiles with various rotor positions and current. Obviously, the peak value of the axial force is much bigger than the force on X and Y axis, because the mainly magnetic flux go through by the axial direction, and the force will be increase by the increasing current. The Max and Min value appear at the aligned and unaligned positions, respectively.

(b) For the non-uniform air gap

For the non-uniform air gap, the axial force between the operate excitation poles of one phase is different because of the different air gaps which is caused by the vibration. The operate poles of one phase span four segmental rotor piths, so for one phase, it should be calculate 180 degrees. Fig.17-(a) shows the axial force for non-uniform air gap situation from 0.1mm to 0.4mm with rated current. The current is given to excitation pole independently, so that the force almost liner changed relates to the air gap. The Max and Min force appear at aligned and unaligned positions, because the two positions are the magnetic flux gets to Max and Min point. Fig.17-(b) shows the axial force at simultaneously excitation for one phase, because the two poles of one phase state on the opposite position which span four segmental rotors, so it should be calculate 180 degrees to show the performance. And the force for the two poles are different, because of the air gap is different which is caused by the vibration.

(a) Independent excitation

(b) Simultaneously excitation

Fig.17. Axial force for non-uniform air gap

F. Comparison

By the simulation results, there is a comparison between the proposed motor and a conventional radial field 12/8 SRM. Obviously, the two motors are in same design dimensions and excitation source, the output torque of the proposed motor is much better than the conventional 12/8 SRM within reduce the copper volume.

Table 2: Dimensions of the prototype machine Parameters 12/8 SRM 12/10 DT-SRM

Number of phases 3 3

Number of stator poles 12 12

Number of rotor poles 8 10

Outside diameter (mm) 105 104

Stack axial length (mm) 35 36

Air gap (mm) 0.25 0.25

Arc of stator (degree) 14 30.25/18.5

Arc of rotor (degree) 16 30.25

Number of series turns 5 10

Coil span 3 1

Length of coils (mm) 1878.59 1697.52

Output torque (N.m) 1.69 1.97

IV. CONCLUSIONS A novel axial field SRM, combing the single teeth stator and

segmental rotor has been designed, analyzed and compared. The axial field design enables a large increase in the flux linking each turn of the machine, thereby creating a large increase in torque density. The proposed motor delivers 16.57% more output torque than the conventional 12/8 SRM.

The design of single teeth stator offers an advantage with coils spanning single teeth, due to the short length of the end-winding. This makes the concept particularly suitable for machines of a relatively short axial length. Because of the single teeth design, 9.63% copper volume will be saved compare with the conventional 12/8 SRM.

The concept of the segmental rotor forms the magnetic flux into short flux path which can reduce the core loss of the rotor, and the segmental cores are embedded into the aluminum body which will reduce both the weight of the rotor and the rotational inertia.

Finally, the simulation analysis and comparison can be used to verify that the proposed motor is much effective and suitable for the application of low speed and high torque.

There is still some subsequent researches should be continued, such as the optimization of the axial force which can be saved by the mechanical methods or double stator one rotor structure. And the motor is processing, the control part will be shown after the experiment to supply the previous work.

ACKNOWLEDGMENT This research was financially supported by the Ministry of

Education, Science Technology (MEST) and National Research Foundation of Korea(NRF) through the Human Resource Training Project for Regional Innovation

REFERENCES [1] [1] SANADA, M., MORIMOTO, S., TAKEDA, Y., and MATSUI,

N.:Novel rotor pole design of switched reluctance motors to reduce theacoustic noise, IEEE Conference on industry applications, Rome,October 2000

[2] Metin Aydin,Surong Hung and Thomas A.Lipo.Design and 3D Electromagnetic Field Analysis of Non-slotted and Slotted T0-RUS Type Axial Flux Surface Mounted Permanetn Magnet Disc Magnet Disc MachinesIn Proceedings of IEEE Electric Machines and Drives Conference,IEMDC01PP.645-651

[3] Federico Caricchi,Fabio Giulii Capponi,Fabio Crescimbini and Luca Solero,Experimental Study on Reducing Cogging Torque and No-Load Power Loss in Axial-Flux Permanent-Magnet Machines With Slotted Winding, IEEE Trans IndApplicat ,2004vol40,pp1066-1075

[4] NEACOE, C., FOCGIA, A., and KRISHNAN, R.: Impact of pole tapering on the electromagnetic torque of the switched reluctance motor. 1997 IEEE International electric machines and drives conference record, WA1/2.1-3, Milwaukee, WI, USA, 18-21 May 1997 WU,

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