A novel 6-5 segmental rotor type switched reluctance motor C

Abstract — A novel 6/5 segmental rotor type switched reluctance motor (SRM) is proposed in this paper. Different from conventional structures, the rotor of proposed structure is constructed from a series of discrete segments, and the stator is constructed from two types of stator poles: exciting and auxiliary poles. Moreover, in this structure short flux paths are taken and no flux reversion exists in the stator. Compared with conventional SRM, the proposed structure increases the electrical utilization of the machine and reduces the core losses, which may leads to higher efficiency. To verify the proposed structure, finite element method (FEM) is employed to get static and dynamic characteristics of conventional 12/8 and proposed SRM. Finally, the validity of the proposed structure is verified by the analyzing results.

I.

I NTRODUCTION

Switched Reluctance Motor (SRM) is a double-salient and single-excited motor, in which windings are located on the stator and no windings or permanent magnets are located on the rotor. These mechanical structures cause the SRM to have many advantageous characteristics, such as good fault tolerance, robustness, low cost and applicability in harsh environments, such as high temperatures or high speeds [1]. With these advantages, SRM has gained more attention recently and has been treated as a good candidate for the electric motor drive application.

Nowadays, there is greater persity in the electrical design of SRMs. The majority continue to use simple concentrated windings because of their simplicity and short end-windings. However, there have been developments using full pitched windings to increase the electrical utilization of the machine, which improving torque capability at the expense of increased end-winding length [2-4].

In this paper, a novel 6/5 segmental rotor type SRM with short flux path and no flux reversion in the stator is proposed. Different from conventional structures, the rotor of proposed structure is constructed from a series of discrete segments and the stator is constructed from two types of stator poles: exciting and auxiliary poles. Compared with conventional SRM, the proposed structure increases the electrical utilization of the machine, decreases the magneto-motive force (MMF) requirements and reduces the core cost.

II. C ONCEPT OF 6/5 SEGMENTAL ROTOR TYPE SRM A novel segmental rotor type SRM is proposed in Fig. 1. Different from conventional structures, this structure has six stator poles and five rotor poles. The rotor is constructed from

a series of discrete segments, each of which is embedded in aluminum rotor block and magnetically isolated from its neighbor. The stator has two types of stator poles: exciting and auxiliary poles. The exciting poles are wound by the windings, while the auxiliary poles are not wound by the windings, which only provide the flux return path.

Fig. 1. Concept of 6/5 segmental rotor type SRM

Fig. 2 (a) shows the magnetic paths of the proposed structure when phase A is excited at the aligned position. The magnetic paths of proposed structure with phase B and C energized are shown in Fig. 2 (b) and (c), respectively. As shown in the figure, the magnetic flux flows down from the exciting pole, through the rotor segments and returns via the adjacent auxiliary poles. All the conductors in each slot only couple with the ?ux driven by their own magneto-motive force (MMF) with very little mutual coupling between one slot and another, which increases the electrical utilization of the machine and decreases the MMF requirement. Meanwhile, in the proposed structure short flux paths are taken and no flux reversion exists in the stator core, which may lead to lower core losses.

III. D ESIGN OF 6/5 SEGMENTAL ROTOR TYPE SRM To verify the proposed structure, a prototype of the novel structure is designed to compare with a conventional 12/8 SRM, which is designed for cooling fan application (12V, 500W, and 2800rpm). The following assumptions are made to enable this comparison.

a) The SRMs are designed for the same application so that the external stator dimensions and shaft diameters are the same. The stack lengths are also equal so that the both machines roughly have the same volume.

b) The wire gauge is the same to keep the same conductivity. c) The air-gaps are the same.

A Novel 6/5 Segmental Rotor Type Switched Reluctance Motor: Concept, Design and Analysis

Zhenyao Xu, Jin-Woo Ahn

Dept. of Mechatronics Eng., Kyungsung University, Korea

E-mail: zhenyao87@6ec734780740be1e640e9a45

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2013 International Conference on Electrical Machines and Systems, Oct. 26-29, 2013, Busan, Korea

978-1-4799-1447-0/13/$31.00 ?2013 IEEE

d) The flux density should be kept lower than 1.8 Tesla to satisfy the actual material requirement.

e) The slot factor should be kept the same or lower than 12/8 SRM.

The assumptions are reasonable design criteria that can be used to compare the two machines. TABLE I shows the detailed parameters of both SRMs. Fig. 3 shows general machine arrangement for proposed SRM based on the parameters in TABLE I.

Fig. 2. Magnetic flux path in proposed SRM. (a) Phase A energized. (b) Phase

B energized. (c) Phase

C energized.

TABLE I

P ARAMETERS OF

12/8 AND PROPOSED

SRM

Parameter 12/8 SRM Proposed SRM

Number of Phase 3 3 Outer Radius (mm) 52.5 52.5 Yoke thickness of Stator (mm) 5 10 Outer Radius of Rotor (mm)

31

26 Length of air gap (mm) 0.25 0.25 Inner radius of rotor (mm) 24

— Radius of shaft (mm) 4 4

Length of stack (mm) 35 35

Stator tooth width (mm) 7.6

20.5/10.5

Stator pole arc (°) 14 54/30

Rotor pole arc (°) 16 66

Number of turns/phase (N) 20 12

Wire diameter (mm) 2.836 2.836 Slot factor 0.3 0.26 Estimated turn length (mm) 2100 1750

Resistance per phase (m ?) 7.0 5.9

(a) (b)

Fig. 3. Machine arrangement of proposed SRM. (a) Stator with windings. (b)

Rotor.

IV. A NALYSIS OF 6/5 SEGMENTAL ROTOR TYPE SRM SRM has very high nonlinear magnetization characteristics. Hence, to verify the proposed structure, finite element method (FEM) is employed to get the characteristics of conventional 12/8 and proposed SRM.

A. Static Characteristics

1) Magnetic Flux Distribution and Magnetic Vector

Fig. 4 shows the magnetic flux distribution of the proposed SRM with phase A excited at aligned and unaligned position, respectively. Fig. 5 shows the magnetic field vector of the proposed motor with three phase windings energized, respectively. From the figures, it can be seen that all the conductors in each slot only couple with the ?ux driven by their own magneto-motive force (MMF) with very little mutual coupling between one slot and another. Moreover, in the proposed structure short flux paths are taken and no flux reversion exists in the stator core, which coincide with the concept of the proposed structure.

(a) (b)

Fig. 4. Magnetic flux distribution of proposed SRM. (a) Aligned position.

(b) Unaligned position.

(a) (b) (c) Fig. 5. Magnetic field vector of proposed structure. (a) Phase A energized. (b) Phase B energized. (c) Phase C energized 2) Magnetic Flux Density SRM is normally designed to operate in the saturated region. Fig. 6 shows the flux density of conventional 12/8 and

proposed SRM, when the motor operates in the full load condition (peak excitation current is 110 A). As shown in the figure, the maximum flux densities in both structures are almost the same and less than 1.8 T, which satisfies the designed requirement as described in Section III.

(a) (b)

Fig. 6. Magnetic flux density of 12/8 and proposed structure. (a) 12/8 SRM

(b) Proposed SRM

3) Inductance and Torque Characteristics

A switched reluctance motor is one in which torque is produced by the tendency of its moveable part to move to a position where the inductance of the excited winding is

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maximized. The torque of a switched reluctance motor, which is related to the rotor position and phase current, can be expressed as,

()θ

θd i dL i T ,212

= (1)

As shown in (1), the torque is proportional to the square of the current and the rate of change of the inductance respect to rotor position. Therefore, torque profiles are determined by the inductance profiles. Figs.7 and 8 show the inductance and torque profiles of 12/8 and proposed SRMs, respectively. TABLE II shows the comparison of simulated average torque for different current levels. As shown in TABLE II, the proposed SRM has higher average torque than conventional 12/8 SRM at all current levels. However, the superiority becomes smaller and smaller with the current increase. It is because the proposed structure is easier to be saturation than 12/8 SRM as shown in Fig. 7.

-4

Rotor Position (deg.)

I n d u c t a n c e (H )

-4

Rotor Position (deg.)

I n d u c t a n c e (H )

(a) (b)

Fig. 7. Inductance profile of 12/8 and proposed SRM. (a) 12/8 SRM.

(b) Proposed SRM

Rotor Position (deg.)T o r q u e (N .m )

Rotor Position (deg.)T o r q u e (N .m )

(a) (b) Fig. 8. Torque profile of 12/8 and proposed SRM. (a) 12/8 SRM. (b) Proposed

SRM

TABLE II

C OMPARISON OF SIMULATE

D AVERAG

E TORQUE

B. Dynamic Characteristics

In order to further verify the validity of the proposed structure, dynamic analysis is executed for 12/8 and proposed SRMs. 1) Asymmetric Converter Due to having many advantages, such as capability of independent control for each phase and four switching modes, asymmetric converter is adopted for the proposed SRM. Fig. 9 shows the topology of the asymmetric converter circuit.

Fig. 9. Asymmetric converter

2) Copper Loss

The copper loss is proportional to the square of the phase RMS current and phase winding resistance. Hence, according to the current waveform, the copper loss can be calculated as,

ph phrms cu R mI P 2

= (2)

∫=

T m phrms dt I T I 0

21 (3)

c

ph

A l R σ= (4)

where, m is the number of phase, I phrms is the RMS current of

phase, R ph is the phase resistance, T is the time of one period, I m is the phase current, l is the length of the winding, σ is the conductance of the winding, A c is the cross-sectional area of

the winding. 3) Core Loss The core loss is composed of hysteresis loss, excessive loss

and eddy-current loss. It can be calculated as,

e c h losses P P P P ++= (5) ()2m h h B

f k P = (6)

()5

.1m e e fB k P = (7)

()2

m c c fB k P = (8)

in which, P h , P c , P e are the hysteresis loss, excessive loss, and eddy-current loss, respectively; k h , k e and k c are the coefficient of hysteresis loss, excessive loss, and eddy-current loss, respectively. In the analysis, M19_29G is used as the stator and rotor material. From the datasheet, k h , k e and k c are given as 0.01642, 0 and 0.0000409, respectively. 4) Windage and Friction Loss

In the dynamic analysis, the windage and friction losses can

be estimated as 2~3% of rated output power, 500W. In this

paper 2.5% is adopted. 5) Simulation Results Based on the analysis in 1) ~ 4), an external circuit is built and both of motors are simulated with the external circuit at

rated condition, as described in Section III. Meanwhile, full

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voltage control method is used to control the two types. For the 12/8 SRM, the turn on and off angle are 2.4° and 18.4°, respectively, while for the proposed SRM, the angles are 0° and 24°, respectively. Fig. 10 shows the transient currents in the two motors. From the figures, it can be seen that the peak value of currents in 12/8 SRM is about 110A, while it is about 125A for the proposed motor. Furthermore, at the end of the conduction cycle, the current has an upwarp, which is caused by the saturation of the core. It can be removed by regulate the turn on and off angles. However, the current has a little upwarp at this position may make the motor high efficiency. Fig. 11 shows the core loss of the two motors when the motors are operating at rated condition. Compared with Figs. 10 (a) and 11 (a), Figs. 10 (b) and 11 (b), it can be seen

that the core losses are mainly produced at commutation region. Furthermore, the core loss is non-uniform in 12/8 SRM, while it is uniform in proposed SRM, it can be explained in Figs. 2 and 12. In Fig. 12, the rotor is rotating at counter-clock direction, “-” stands for the current flow into the paper, and “+” stands for the current flow out the paper. When the phase current changes from Phase B to C, it has one third flux reversal in the stator yoke and one second flux reversal in the rotor yoke. The phenomenon is the same when the phase current changes from Phase C to A. Therefore, the core loss is almost same in these two commutation regions. However, when the phase current changes from Phase A to B, it will be a

little different from the current changing from Phase B to C, there will be two third flux reversal in the stator yoke and one second flux reversal in the rotor yoke. Because the length of flux reversal path is increased, the core loss is also increased in this commutation region compared with the other two commutation region, as shown in Fig. 11 (a). But for the proposed SRM, due to short flux and no flux reversal in the stator, the core loss is uniform and smaller, as shown in Fig. 11 (b).

Time (ms)C u r r e n t (A )

Time (ms)C u r r e n t (A ) (a) (b) Fig. 10. Current of 12/8 and proposed SRM operated at rated condition. (a) 12/8 SRM. (b) Proposed SRM.

Time (ms)C o r e l o s s (W )

Time (ms)

C o r e l o s s (W )

(a) (b) Fig. 11. Core loss of 12/8 and proposed SRM operated at rated condition. (a) 12/8 SRM. (b) Proposed SRM.

TABLE III shows the dynamic characteristics comparison of 12/8 and proposed SRM. From the comparison results, it can be seen that, the proposed structure improves the electrical utilization of the machine and reduce the core loss, which reaffirms the novelty and advantages of proposed structure.

(a) (b) (c)Fig. 12. Magnetic flux path of 12/8 SRM with rotor rotating at counter-clock direction. (a) Phase A energized. (b) Phase B energized. (c) Phase C

energized.

TABLE III D YNAMIC CHARACTERISTICS OF 12/8 AND PROPOSED SRM Parameter 12/8 SRM Proposed SRM Rated voltage (V) 12 12 Rated speed (rpm) 2800 2800 Rated torque (N.m) 1.7 1.7

Friction and windage loss (W)

12.50 12.50 Copper loss (W)

70.72 59.66 Core loss (W) 18.76 9.74 Output power (W) 497.98 496.88 Efficiency (%) 83.00 85.85 V. C ONCLUSIONS In this paper, a novel 6/5 segmental rotor type SRM is proposed. The operation principle is illustrated. The design consideration is also expounded. Characteristics, including static and dynamic, are analyzed. A comparison between proposed and conventional SRM is also executed. The

comparison results verify the validity of the proposed structure. Further, prototype motor of proposed structure is being manufactured; detailed experimental results will be published in the future paper. A CKNOWLEDGMENT This work was supported by the Ministry of Education (MOE) through the Human Resource Training Project for Regional Innovation and BK21 Plus. R EFERENCES [1] D. H. Lee, Z. G. Lee, J. N. Liang, J. W. Ahn, “Single-phase SRM Drive with Torque Ripple Reduction and Power Factor Correction,” IEEE Trans. on Industry Applications , Vol. 43, No. 6, pp. 1578-1587, Nov.-

Dec. 2007. [2] B.C. Mecrow, “New Winding Configurations for Doubly Salient Reluctance Machines,” IEEE Trans. on Industry Applications , Vol. 32, No. 6, pp. 1348-1356, Nov.-Dec. 1996.

[3] J. Oyama, T. Higuchi, T. Abe, and K. Tanaka, “The Fundamental Characteristics of Novel Switched Reluctance Motor with Segment Core

Embedded in Aluminum Rotor Block,” Journal of Electrical Engineering & Technology , Vol. 1, No.1, pp. 58-62, 2006. [4] B.C. Mecrow, J.W. Finch, E.A. EI-Kharashi and A.G. Jack, "Switched Reluctance Motors with Segmental Rotors," IEE Proc. of Electr. Power Appl., Vol. 149, No. 4, pp. 245-254, July 2002. 585

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