A Novel Converter Topology for 6 Phase

Switched Reluctance Motor Drives

Xu Deng, Barrie Mecrow and Shady Gadoue School of Electrical and Electronic Engineering,

Newcastle University, Newcastle upon Tyne, NE1 7RU England, United Kingdom

E-mail: x.deng4@ncl.ac.uk

Abstract—This paper presents a novel converter topology for 6- phase Switched Reluctance Motor (SRM) drives, which needs no additional energy storage elements with reduced number of components. Compared with an asymmetric half bridge converter, the novel converter reduces the number of switches, diodes, and connections between motor and converter to half. A dynamic model of a 12/10 SRM is developed in Matlab/Simulink environment. Simulation results show that compared to asymmetric half bridge converter the novel converter can produce the same torque ripple at low speed with less loss at high speed.

Keywords—switched reluctance motor; converter; torque ripple; switch loss

I. INTRODUCTION

The SRM system is a mechatronic device that has been developed for many years. With no windings or permanent magnets on the rotor and concentric windings on the stator, the SRM has the simplest structure of all electrical motors [1]. It has been used in many applications, such as aerospace, electric vehicles, high-speed drives, small automotive applications, cooling fans and pumps [2].

A Switched Reluctance Drive (SRD) is made up of an SRM, power converter and controller, of which the power converter is the key factor that affects the performance and the cost of SRD. A large number of converters are developed for SRMs such as the half bridge converter, full bridge converter, capacitive type converter, and magnetic type converter. Half bridge converter topologies have the widest use in SRM drives because of its phase independence and flexible control [3].

The best way to reduce torque ripple is to increase the phase number: to become comparable with three phase ac machines it is necessary to have six pulses per cycle, which would require a six phase SRM.

In [4] the authors apply Sinusoidal Pulse Width Modulation (SPWM) control to drive a 6 phase SRM by using a 3 phase full bridge converter and produce a torque dense drive with low torque ripple. Capacitive converters and magnetic converters can help phase winding to have a faster rate of current discharge [5-6].

This paper analyses the advantages and disadvantages of existing converters for SRMs and proposes a novel converter to overcome disadvantages while obtaining similar or better performance.

II. CONVENTIONAL SRM DRIVE TOPOLOGIES

Based on different applications and design ideas, a large number of converters have been considered for SRMs. These converters are designed according to the following rules: 1. Fewer switches, smaller voltage and current rating of

switches. 2. Less energy storage elements. 3. Fewer connections between the motor and converter. 4. Easy control and higher efficiency.

In general, smaller converter size and higher performance are the themes of this research.

A. Asymmetric Half Bridge Converter

Figure 1 shows a 6 phase asymmetric half bridge converter which has 3 operational modes for every phase. When S1 and S2 are switched on, phase terminal voltage is +Vs, and phase current increases rapidly. When S1 or S2 is switched on, a zero voltage freewheel loop is achieved, and phase current decreases slowly. When S1 and S2 are switched off, phase terminal voltage is -Vs, and phase current increases sharply. Because of its outstanding independence between every phase, asymmetric half bridge converter has a wide use in SRM drives [1]. However large number of switches and diodes are needed especially for multiphase SRMs, which will result in higher switches losses.

Figure 1. 6 phase asymmetric half bridge converter

B. Full Bridge Converter

Figure 2 shows a 6 phase SRM driven by a 3 phase full bridge converter, in which 6 phase windings are arranged in a delta connection [4]. The three phase full bridge converter produces 3 phase SPWM output waveforms to drive 3 pairs of anti-parallel phase windings. In this configuration an additional 6 diodes are used to convert the bipolar current waveform into two unipolar half waveforms, with unidirectional currents in every phase. Although only 6 switches are used in this configuration, 6 additional diodes are required.

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Figure 2. Full bridge converter for 6 phase SRMs

C. Capacitive Converters

Figure 3 shows a typical capacitive converter for a 6 phase SRM. This is called a capacitor dump converter and has many alternative configurations [5]. When S1 is switched on and Sa is switched off, the winding is energized by Cs. When S1 and Sa are switched off, Ca is charged by the energy stored in the winding. La, Da and Sa create a buck converter, which helps energy stored in Ca transfer to Cs when Sa is switched on. In this configuration, two additional energy storage elements are required, which increases the size of whole drive system.

Figure 3. Capacitor dump converter for 6 phase SRMs

D. Magnetic Converters

Figure 4 shows a traditional bifilar converter for a 6 phase SRM. In a bifilar converter [6], every phase has two windings, which are closely coupled primary and secondary windings. When S1 is switched on, primary winding is energized. When S1 is switched off, current flow transfers from the primary to the secondary, while the rest of the energy is fed back to the voltage source. The bipolar converter is quite easy to control because of its lower switch number. However additional secondary windings increase the size of machine.

Figure. 4 Traditional bifilar converter for a 6 phase SRM

In order to solve some problems caused by the above conventional converters, a new converter should be designed, which should use no additional energy storage element and reduce the number of switches and diodes, while improving or maintaining the drive performance.

III. PROPOSED DRIVE CONFIGURATION

Based on conventional SRM converters, a novel converter is designed for 6 phase SRM. Figure 5 shows the configuration

of the novel converter. This converter has no additional energy storage element and uses fewer switches and diodes when compared with other traditional converters.

Figure 5. Circle converter for 6 phase SRMs

In this novel converter, in order to obtain positive voltage during magnetization and negative voltage during demagnetization, every phase uses and shares 2 switches and 2 diodes with its adjacent 2 phases. For example, phase A shares its top switch and bottom diode with phase F, meanwhile, it shares its top diode and bottom switch with phase B.

Figure 6. Conduction order of a 12/10 poles 6 phase SRM

Figure 6 shows the conduction order of a 12/10 6 phase SRM. By using the novel converter to drive a 6 phase SRM, every phase has 3 operational modes. For example, when the top switch and bottom switch of phase A are on, winding A is energized by the voltage source. When the top switch and bottom switch of phase A are switched off, winding A is demagnetized in a negative voltage loop. However, phase A and phase B have some overlap in conduction, when phase A comes into demagnetization, phase B is still energized in a

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positive voltage loop, which means the bottom switch of phase A is not closed immediately and results in a zero voltage freewheel loop for phase A.

TABLE I. COMPONENT AND CONNECTION REQUIREMENT CAMPARISON BETWEEN THREE DIFFERENT CONVERTERS

Table.I shows the component and connection requirement comparison between an asymmetric half bridge converter, full bridge converter and the novel converter. Compared with the asymmetric half bridge converter, the novel converter reduces the number of switches, diodes, and connections between motor and converter to half. Meanwhile, compared with the full bridge converter with a delta connection, the novel converter reduces the number of diodes to half. Because of the circle winding connection method, the number of connections between motor and converter is double that of the delta connection.

IV. SIMULATION RESULTS

In order to compare the novel converter topology proposed in section III with the other two converters, a dynamic model of a 12/10 SRM is developed in a MATLAB/SIMULINK environment. This model uses the parameters of a 12/10 SRM prototype. Different control strategies are carried out and the following results are obtained to analyze the performance of the novel converter. Figs. 7-8 show the magnetic properties of the SRM prototype, which is chosen to be driven by the 3 different converters mentioned in section III. The performance of the different topologies will be investigated in both low and high speed regions of operations.

Figure 7. Relationship between rotor position, flux and current

Figure 8. Relationship between rotor position, current and torque

A. Low speed operation

To analyze the performance of the converters at low speed, the asymmetric half bridge converter and the proposed converter are controlled by a current chopping control strategy at 2000rpm to achieve the same average torque, so that their torque ripple can be compared [2].

Figure 9 shows the current and torque waveforms at 2000rpm. It can be noted that for the same output average torque, the novel and the asymmetric half bridge converters have almost the same torque ripple amplitudes.

(a) Asymmetric half bridge converter

0 5 10 15 20 25 30 35 00.1

0.20.3

0.40

10

20

30

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50

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Flux/(Wb)rotor position/(degree)

Cur

rent

/(A

)

05

1015

2025

30 0

10

20

30

-20

-10

0

10

20

rotor position/(degree)current/(A)

torq

ue/(N

m)

Converter Asymmetric Half Bridge Converter

Full Bridge Converter

Novel Converter

No. of Switches 12 6 6 No. of Diodes 12 12 6

No. of Connections between motor and

converter

12 3 6

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(b) Novel converter

Figure 9. Current and torque waveforms at low speed

B. High speed operation

To investigate the performance of the converters at high speed, 3 voltage control methods are employed. The three different converters are controlled to obtain the same driving voltage at 8000rpm, so that their total switches average losses can be compared. Figure 10 shows the drive signal, voltage; flux and current of phase A respectively driven by asymmetric half bridge converter, 3 phase full bridge converter and the novel converter. With appropriate control of switches, the three different converters can achieve the same driving voltage and the phase windings can generate the same flux and current waveforms [4].

(a) Asymmetric half bridge converter

(b)Full bridge converter

0

0.5

1

G_up

0

0.5

1

G_down

-1000

-500

0

500

1000Ua/(V)

0

0.05

0.1

0.15

0.2Flux_a/(Wb)

1 2 3 4 50

5

10

15Ia/(A)

0

0.5

1

G1

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0.5

1

G2

0

0.5

1

G3

0

0.5

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G4

-1000

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1000Ua/(V)

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0.2Flux_a/(Wb)

1.5 2 2.5 3 3.5 4 4.5 50

5

10

15Ia/(A)

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(c) Novel converter

Figure 10. Drive signal, voltage, flux and current of phase A

Figure 11 and Table. II show the instantaneous power loss waveform of one switch and the total switches average losses respectively for different topologies. As shown in Fig.11, the asymmetric half bridge converter has the smallest peak switching loss per switch (6 kVA). However, the asymmetric half bridge converter needs 12 switches, while the novel converter only needs 6, which means the novel converter has smaller total switches average losses as shown in Table. II.

(a) Asymmetric half bridge converter

(b) Full bridge converter

(c) Novel converter

Figure 11. Switch Conduction loss

TABLE II. TOTAL SWITCHES AVERAGE LOSSES OF THREE DIFFERENT CONVERTERS

Compared with the other two converters, the novel converter has the same VA rating as shown in Table. III.

0

0.5

1

G_up

0

0.5

1

G_down

-1000

-500

0

500

1000Ua/ (V)

0

0.05

0.1

0.15

0.2Flux_a/ (Wb)

1.5 2 2.5 3 3.5 4 4.5 50

5

10

15Ia/ (A)

Converter Asymmetric Half Bridge Converter

Full Bridge Converter

Novel Converter

total switch conduction losses

0.12KW 0.09KW 0.09KW

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TABLE III. VA RATING CAMPARISON BETWEEN THREE DIFFERENT CONVERTERS

V. CONCLUSION

This paper presents a novel converter topology for a 6 phase SRM, which does not need any additional energy storage elements and consists of only 6 switches and 6 diodes. Compared with the asymmetric converter, the number of switches, diodes and connections between motor and converter are reduces to half. Compared with full bridge converter with delta connection, the number of diodes reduces to half. Not using any additional energy storage element also reduces the size of the drive system.

By using this novel converter to drive a 6 phase SRM, similar or better performance can be achieved. The novel converter offers the same torque ripple as the asymmetric half bridge converter at low speed, same converter VA rating and lower losses at high speed.

REFERENCES [1] R. Krishnan, "Switched Reluctance Motor Drives: Modeling, Simulation,

Analysis, Design, and Applications," CRC Press, 2001.

[2] Miller T J E, Switched Reluctance Motors and Their Control, UK, Magna Physics Publishing and Clarendon Press. Oxford, 1993.

[3] Miller T J E, “Electronic Control of Switched Reluctance Machines”, UK, Reed educational and professional publishing Ltd, 2001.

[4] J. D. Widmer, B. C. Mecrow, C. M. Spargo, R. Martin, T. Celik , “Use of a 3 phase full bridge converter to drive a 6 phase switched reluctance machine,” IET International Conference, 2012.

[5] Jianing Liang, Guoqing Xu, Dong-Hee Lee, and Jin-Woo Ahn, “Classification of capacitive type converter topologies for SRM”, ICEMS, pp.1670-1675, 2010.

[6] H. Bagherian, M. Asgar, and E. Afjei, “A new C-dump converter for bifilar winding switched reluctance motor”, PEDSTC, pp.467-472, 2011.

Converter Asymmetric Half Bridge Converter

Full Bridge Converter

Novel Converter

peak VA rating 12.8KW 12.8KW 12.8KW

rms VA rating 8.6KW 8.6KW 8.6KW

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