Design and Analysis of a DC Field Multitooth Switched Reluctance Machine by Using

Soft-Magnetic-Composite Material

Christopher H. T. Lee, K. T. Chau, Fellow, IEEE and Chunhua Liu, Member, IEEE Department of Electrical and Electronic Engineering

University of Hong Kong Hong Kong, China htlee@eee.hku.hk

Abstract—This paper innovatively implements the dc field winding into the multitooth switched reluctance (MSR) structure to form a new DC field multitooth switched reluctance (DC-MSR) machine. The independent DC field winding can effectively achieve field excitation for efficiency optimization. Soft-magnetic-composite (SMC) material which is favorable for the complex structure is also applied into the proposed machine. In addition, the characteristics and performances of the proposed machine are analyzed by finite element method with various excitations.

Keywords—Multitooth switched reluctance (MSR) machine, soft-magnetic-composite (SMC) material, doubly-fed machine, DC field winding, flux control, finite element method (FEM).

I. INTRODUCTION There is an accelerating pace on the development of electric

vehicles (EVs) due to the increasing demands on the protection of the environment. Shortly, in order to increase the market penetration of EVs, the EV motor has to offer high efficiency, high power density, high controllability, wide speed range, and maintenance-free operation [1]-[5]. In general cases, permanent magnet (PM) machine can offer much higher torque than those with the magnetless one [6]-[10] and thus PM machines are more favorable in the domestic market for high-torque application environment. However, the prices of PM materials have risen rapidly such that more attentions should be drawn again on the magnetless machines [11]-[15].

In order to resolve the profound problem of low torque density for magnetless machine, the multitooth switched reluctance (MSR) structure which offered the flux-modulation effect for high-torque application was proposed [16]-[17]. Nevertheless, this type of machine still suffers from the drawbacks of uncontrollable flux and complex manufacturing structure. In recent year, the concept of independent dc windings which could offer the flux controllability was proposed [18]-[22] and this was also applicable to the MSR topology. In addition, the soft-magnetic composite (SMC) materials which were favorable for the complex structure

topology [23]-[25] were widely studied and could also be implemented in the MSR structure.

In this paper, by purposely incorporating the independent DC field winding into the MSR machine; a new DC field multitooth switched reluctance (DC-MSR) machine is proposed. The proposed DC-MSR machine adopts the 18/16-pole structure and by utilizing the controllable dc field winding, the proposed machine can operate with two modes, namely MSR operation mode and DC operation mode. Furthermore, in order to get round the problem of the complex manufacture issue, the proposed machine is purposely designed with the SMC material – ATOMET EM1. In Section II, the machine design will be discussed. Then, two operation modes of the proposed machine will be described in Section III. The machine analysis approach is presented in Section IV. Section V will give the machine performance analysis to prove the validity of the machine design. Finally, the conclusion will be drawn in Section VI.

Fig. 1. Proposed 18/16-pole DC-MSR machine.

II. MACHINE DESIGN Fig. 1 shows the topology of the proposed DC-MSR

machine which consists of an outer stator with 6 salient poles,

each fitted with 3 tooth and results with equivalent stator teeth of 18. This ends up with the inner rotor with 16 salient poles and it adopts two kinds of windings, namely armature winding and DC field winding.

The design of the pole-pair arrangement for the proposed DC-MSR machine is based on the following criteria [17]:

⎪⎩

⎪⎨

⎧

±=

=

=

iNN

NNN

miN

ser

stspse

sp

2

2 (1)

where Nsp is the number of stator poles, Nst the stator teeth, Nse the equivalent stator poles, Nr the rotor poles and i is any integer. By selecting Nsp = 6, Nst = 3, and i = 1, this results with Nr = 16 and ends up with the proposed structure of the 18/16-pole DC-MSR machine. The topology with large number of poles increases the complexity in practical manufacturing with the conventional silicon steel material. Thus, SMC material which utilizes the powder metallurgy techniques is purposely applied to the proposed machine.

SMC material can be pressed in a die into any desired shape easily and hence it is particularly suitable for the machine with complex structure topology [23]-[25]. Particularly, ATOMET EM1, one of the SMC materials, which is produced by Quebec Metal Powders (QMP) is chosen for the proposed machine. The B-H curve of ATOMET EM1 is shown in Fig. 2 [26]-[27]. Shortly, the major demerit of SMC material comes from its relatively poor torque density and the torque density from the SMC machine is generally lower than those from the traditional steel machine. Therefore, in order to reach to a satisfactory torque level, SMC material should be applied to the machine structure with high-torque characteristics and the proposed machine topology which offers high torque density in nature must be favorable for the SMC material application.

Fig. 2. B-H curve of ATOMET EM1.

Hence, the proposed machine inherently achieves the

following advantages: • Without installation of any PM materials, DS-MSR

machine takes definite merit of cost benefit.

• By adopting the SMC material, the problem of manufacturing difficulty can be resolved.

• MSR structure can offer higher torque density such that the SMC machines can also reach to satisfactory torque level.

• With the implementation of the independent dc field excitation, all of the torques producing zones is utilized. In addition, the airgap flux density can be controlled effectively for efficiency optimization.

• When there is any open circuit fault on the DC field excitation, the proposed machine can switch to the MSR mode to achieve the same rated torque.

III. MACHINE OPERATION ANALYSIS Without the DC field excitation, the proposed machine can

operate with the conventional conduction scheme which the unipolar armature current, i+ is fed during the increasing period of the self-inductance, L such that the corresponding reluctance torque, Tr+ is positive at all occasions as shown in Fig. 3(a), and this is known as the MSR operation mode. However, for this operation mode, only half of the period is utilized and thus the torque performance is degraded. The corresponding electromagnetic torque can be written as:

θddLiTr

2

21= (2)

By purposely incorporating with the DC field winding, the proposed machine can be operated at another operation mode, known as DC operation mode which can fully utilize all of the torque producing zones. Different from MSR operation mode, at this mode, the bipolar conduction scheme is applied instead. When the DC flux linkage, ΨDC is increasing, the positive armature current, i+ is applied which produces the positive torque, TDC+. Meanwhile, the negative armature current, i- is applied when ΨDC is decreasing and the torque produced, TDC- will also be positive [13]-[14]. The operating waveform of DC operation mode is shown in Fig. 3(b). It shows that both zones are utilized and thus the torque performance can be improved and the corresponding electromagnetic torque equation can be written as:

−+−+ +++= rrDCDCDC TTTTT (3) where TDC+ and TDC- are the DC torque, and Tr+ and Tr- the reluctance torque. It should also be mentioned that the electromagnetic torque at this mode is mainly contributed by DC torque, TDC; meanwhile, the reluctance torque, Tr pulsates with zero averaged value in each cycle [14].

To support the DC operation mode, a control circuit with the bipolar converter topology is preferred. As known, there are two types of bidirectional converter topologies available, namely full-bridge converter and half-bridge converter with split capacitors. In this paper, the half-bridge converter is chosen because this topology can minimize the number of power devices and hence reducing the complexity of the control circuit [28]-[31]. The half-bridge converter circuit is adopted for simulation as shown in Fig. 4.

(a)

(b)

Fig. 3. Principle of proposed machine operation: (a) MSR operation mode. (b) DC operation mode.

Fig. 4. Half-bridge convertor circuit for bipolar current operation.

IV. MACHINE ANALYSIS The time-stepping finite-element-method (TS-FEM) is

applied to obtain the performances of the machines. To perform the TS-FEM analysis, three sets of equations are established. First, the electromagnetic field equation of the machines is governed by [32]-[34]:

tA

yB

xB

vJyAv

yxAv

xrxry

∂∂+⎟⎟

⎠

⎞⎜⎜⎝

⎛∂

∂−

∂∂

−−=⎟⎟⎠

⎞⎜⎜⎝

⎛∂∂

∂∂+⎟

⎠⎞

⎜⎝⎛

∂∂

∂∂Ω σ: (4)

where Ω is the field solution region, A the magnetic vector potential, J the current density, σ the electrical conductivity, and the Brx, Bry remnant flux density. Second, the armature circuit equation of the machine during motoring is given by:

∫∫Ω Ω∂∂++=

e

dtA

sl

dtdiLRiu e

(5)

where u is the applied voltage, R the winding resistance, Le the end winding inductance, l the axial length, s the conductor area of each turn of per phase winding, and Ωe the total cross-sectional area of conductors of each phase winding. Third, the motion equation of the machine is given by:

λωω −−=∂∂

Lem TTt

l (6)

where Jm is the moment of inertia, ω the mechanical speed, TL the load torque, and λ the damping coefficient.

TABLE I. KEY DATA OF PROPOSED MACHINE

Item Value

Rotor outside diameter 160.0 mm Rotor inside diameter 40.0 mm

Stator outside diameter 280.0 mm Stator inside diameter 161.2 mm

Air-gap length 0.6 mm Stack length 80.0 mm

No. of stator poles 6 No. of stator teeth 3

No. of equivalent stator poles 18 No. of rotor poles 16 No. of AC phases 3

No of turns per armature coil 80 Rotor and stator material SMC: ATOMET EM1

With the help of TS-FEM, the no-load magnetic field

distribution of the proposed machine with DC field current of

1000 A-turns is shown in Fig. 5. It should be also noted that the DC field current can be independently controlled to achieve the flux regulation effect. The key data of the proposed machine is as shown in Table I.

Fig. 5. Magnetic field distribution with dc field current of 1000 A-turns.

V. MACHINE PERFORMANCE ANALYSIS By performing TS-FEM, the performances of the machine

can be calculated and analyzed. First, the proposed machine is conducted with the dc field current of 1000 A-turns and 2000 A-turns, respectively. The flux linkage and no-load EMF are then calculated as shown in Fig. 6 and Fig. 7, respectively. It can be seen that the flux linkages of the 3 phases hold the same pattern with no phase shift or distortion with each other and this confirms that the setting of DC field windings match the pole-pair arrangement of the proposed machine. In addition, it should also be noted that by increasing the dc field current, both the flux linkage and no-load EMF can be strengthened accordingly. Therefore, this validates that the proposed machine can offer the flux control easily by regulating the dc field current independently in order to achieve the efficiency optimization.

(a)

(b)

Fig. 6. Flux linkage of the proposed machine: (a) with DC field current of 1000 A-turns. (b) with DC field current of 2000 A-turns.

(a)

(b)

Fig. 7. No-load EMF of the proposed machine: (a) with DC field current of 1000 A-turns. (b) with DC field current of 2000 A-turns.

(a)

(b)

Fig. 8. Airgap flux density of proposed machine: (a) MSR operation mode. (b) DC operation mode.

(a)

(b)

Fig. 9. Steady torque performance of proposed machine: (a) MSR operation mode. (b) DC operation mode.

Second, the airgap flux density distributions of the machines are calculated and compared as shown in Fig. 8. At DC operation mode; the proposed machine is conducted with the DC field winding of 2000 A-turns. As expected, it is obviously that the airgap flux density at MSR operation mode is much lower than that at DC operation mode but both of them are within the acceptable range.

Third, the steady torque performances of the machines are shown in Fig. 9. It can be observed that the average steady torque of the proposed machine at MSR operation mode and DC operation mode are 6.38 Nm and 18.41 Nm, respectively. Comparing with MSR operation mode, the percentage of the increment of steady torque at DC operation mode is about 188.5 %. In addition, to further evaluate the torque performance, the torque ripple factor is defined as following:

%100minmax ×−

=avg

T TTT

K (7)

where Tmax, Tmin, and Tavg are the maximum, minimum and average torque of the machine, respectively.

By utilizing the torque ripple factor, the torque ripples (the peak-to-average value) at MSR operation mode and DC operation mode are found to be about 48.1 % and 32.9 %, respectively, which are all in the acceptable range. Undoubtedly, both the steady torque and torque ripple are improved at DC operation mode due to the fact that all the torque producing zones are utilized. Meanwhile, at MSR operation mode, only half period of the self-inductance is used and the torque performances are relatively degraded.

TABLE II. MACHINE PERFORMANCE COMPARISON

Item MSR mode DC modeRated speed 500 rpm 500 rpm Peak-to-peak EMF at 1000 A-turns N/A 110 V Peak-to-peak EMF at 2000 A-turns N/A 126 V Airgap flux density 0.75 T 1.55 T Rated torque 6.38 Nm 18.41 Nm Torque enhancement at DC mode N/A 188.5 % Torque ripple at rated load 48.1 % 32.9 %

VI. CONCLUSION In this paper, a new DC-MSR machine which offers two

different operation modes, MSR and DC modes is presented. To cope with the problem of manufacturing complexity, the machine is adopted with SMC material and the performance is analyzed by the TS-FEM. This paper quantitatively compares two operation modes and the comparison results are summarized in Table II, and concluded as below: 1. The proposed DC-MSR machine can successfully offer

two operation modes, namely MSR and DC modes. 2. Due to the utilization of all torque producing zones, DC

mode has superior performance compared with MSR mode. Meanwhile, MSR mode can act for fault-tolerant operation when there is open-circuit fault on the dc field winding.

3. There exists the independent DC field winding which can offer the flux controllability for the machine. Therefore, the efficiency can be optimized by purposely

strengthening and weakening the flux distribution effectively.

4. Multitooth structure is particularly favorable for the high-torque low-speed operation environment and this structure can perfectly implement with the SMC material which has the major demerit of low-torque density.

5. For the proposed machine, both operation modes can reach to a satisfactory rated torque level even SMC material is applied.

ACKNOWLEDGMENT This work was supported and funded by a Grant

HKU710612E from the Research Grants Council, Hong Kong Special Administrative Region, China.

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