Construction of a Low Cost Asymmetric Bridge Converter for Switched Reluctance Motor Drive

Transcription

1 Construction of a Low Cost Asymmetric Bridge Converter for Switched Reluctance Motor Drive E.Afjei 1, A.Siadatan 2 and M.Rafiee 3 1- Department of Electrical Eng., Faculty of Electrical & Computer Eng., Shahid Beheshti University G.C., Tehran, IRAN e-afjei@sbu.ac.ir 2- Department of Electrical Eng., Faculty of Electrical & Computer Eng., Shahid Beheshti University G.C., Tehran, IRAN a_siadatan@wtiau.ac.ir 3- Young Researchers Club, West Tehran Branch, Islamic Azad University, Tehran, IRAN rafiee.mehran@ieee.org Abstract- The presented paper introduces a low cost converter for Switched Reluctance Motor (SRM) control. Many types of converters are implemented to drive SRM. Generally Asymmetric Bridge Converter (ABC) is selected in which two switches are used per phase to excite the phase winding. The upper transistor in ABC faces a problem in gate-source voltage which prevents SRM phase to be excited properly. Some methods are used to solve the problem which are usually increases the cost and also decreases the efficiency of the converter circuit. The problem is basically related to the custom Mosfet drivers. In this paper, a low cost ABC is presented in order to solve the problem. In the converter, a different type of Mosfet driver is used. The constructed ABC circuit is then tested in the laboratory on a 3hp 6 by 4 SRM and the operational results are obtained. Keywords: Converter, Switched Reluctance Motor, Mosfet driver. I. INTRODUCTION SRM is a special type of motor which has many advantages. The stator and rotor poles in this motor are salient. The rotor has no winding which means the motor structure is so simple and low cost. On the other hand, lighter rotor allows the motor to achieve higher speed and efficiency. This type of the motor is able to operate in harsh environment and less maintenance is require for SRM. Although SRM was invented many years ago but since its drive circuit includes power electronic devices and switches, it is not implemented for many of years till last decades when power electronics improved and power switches were built. In SRM drive the rotor pole position information is needed. A lot of direct and indirect methods are presented in order to rotor position detection and SRM drive [1-5]. After position estimation, desired command pulses are produced which have to be transferred to the phase windings of the motor. Converters are used in this transition. Many converter topologies are presented so far. ABC and a converter with N+1 switches in N phases SRM and bifilar converter are presented in [6], [7]. Different types of C-dump converters are studied in [8]. A converter with voltageboosting capacitors is proposed in [9]. In this paper, a corrected ABC is utilized in order to reduce the cost and also size of the board. A custom problem of driver circuits is also solved. The driver circuit is then constructed and tested on three-phase 6 by 4 SRM This study is organized as follows: in section II, the specification and simulation of the selected 6 by 4 threephase SRM is presented. Section III explains custom ABC and the problem existing in it. In section IV, the corrected ABC is introduced and the constructed circuit is tested in laboratory and operational results are obtained. The conclusions are presented in section V. II. THE SELECTED SRM SPECIFICATION In order to design a converter circuit, a three-phase 6 by 4 SRM is selected which is shown in Fig.1. Fig.1. Selected three-phase 6 by 4 SRM In the motor structure, each phase contains two salient stator poles which are located opposite to each other and switched on together. There are no windings on the rotor poles. The motor specification is presented in table.1. The rotor pole arc is 32 while the stator pole arc is 28. The motor has about 3hp. Magnet CAD package [10] uses Finite Element Method (FEM) which is performed to find solution for partial differential and integral equations. Two common methods are implemented to solve magnetic field problems. One of them uses electric vector potential and the other uses

2 magnetic vector potential whose partial differential equation is: J A x x A y y A z z 1 A is the magnetic vector potential. TABLE I: THE SPECIFICATION OF THE 6 BY 4 THREE-PHASE SRM Stator core outer 170mm diameter Stator core inner 145mm diameter Stator pole arc 28 Air gap Rotor core outer diameter Rotor shaft diameter 0.25mm 35mm 15mm Rotor pole arc 32 Number of turns per pole 150 In Magnet CAD package electric vector potential (T) is used: J T 2 By using Maxwell's equation, equation (3) is obtained: H J T 3 And H T 0 4 The (H-T) vector can be obtained by: H T Ω 5 Where Ω is a magnetic scalar potential. By knowing: E B 6 t And E 1 T B σ t μ μ T Ω t μ μ T t Ω t 7 This finally reduces to the following two scalar equations: T μσ T t μσ Ω t 8 And it's obtained that: Ω 0 9 The three-phase 6 by 4 SRM is simulated in Magnet by 3A phase current. The flux density is shown for different values of rotor position in Fig.2. The maximum value of flux density is about Tesla in fully aligned position. pulse and gives it to the gate of the Mosfet. Mosfet drivers are used to adequately switch the Mosfets. (a) (b) III. CUSTOM ABC STRUCTURE The rotor position of the SRM is the most important parameter in SRM drive. A converter is used to excite the stator phase windings by the controller command pulse. One leg of a usual ABC is shown in Fig.3. N-Channel Mosfets are used as power switches of the phase. Schottky diodes are used in order to discharge the phase winding energy when the phase is turned off. The command pulses are generated by the controller. Each Mosfet needs a Mosfet driver which gets the command (c) Fig.2: The magnetic flux density in custom 6/4 SRM: (a): unaligned, (b): half aligned and (c): fully aligned position

3 Usually, in order to solve the problem, the upper Mosfet driver is supplied by an independent battery, isolated DC source or switching power supply and then coupled to the transistor gate in the form of Fig.6. Fig.3. One leg of usual ABC In Fig.4 a Mosfet driver is coupled to the transistor. It seems that the circuit is correct but in fact this circuit will not operate adequately. PH1 Fig.6. Mosfet driver with isolated voltage source PH1 Fig.4. One leg of ABC with Mosfet driver By using this configuration, the problem is solved and the upper transistor gate-source voltage is corrected (Fig.7) and the transistor switches on and off appropriately. But by this topology, each leg of the converter or in other word each phase of the SRM requires an independent isolated power supply for the upper Mosfet driver which highly increases the circuit cost, size and also complexity. The problem is getting worse in SRMs with more phases or in multilayer SRMs. In the lower transistor, the generates a pulse on the transistor gate which is exactly the gate-source voltage of the Mosfet so the transistor is switched on and off correctly. But the upper source is not grounded with the same ground of the Mosfet driver or in other word the source is floated because it's coupled to the phase winding and its voltage is the sum of winding and drain-source voltage of the lower Mosfet. As a result the upper gate-source pulse is in the form of Fig.5. With this gate-source pulse the transistor is not able to be switched on. Fig.7. gate-source voltage of the upper transistor with isolated voltage source for Mosfet driver Fig.5. Gate-source voltage of the upper transistor IV. NEW ABC STRUCTURE The novel presented structure is based on using a half bridge Mosfet driver in each phase of the SRM in different form. In this paper, Hip2500 Mosfet driver is placed in the circuit as it's illustrated in Fig.8. It solves the problem of the float ground in the upper transistor and the problem of increasing the size, cost and complexity of the circuit as well.

4 In the constructed circuit an Atmega16 is implemented as a controller. The drive circuit and Mosfet drivers are all supplied with a 12V power supply. A LM7805 voltage regulator is utilized to provide 5V from the 12V for microcontroller. Different waveforms of the circuit are illustrated in Fig.11. In Fig.11.a the output pulse of the microcontroller is shown. Fig. 11.b depicts the output pulse of the Mosfet driver. Output pulse of Mosfet driver versus voltage of the phase winding is illustrated in Fig.11.c. Fig.8. One leg of ABC with half bridge Mosfet driver The Hip2500 Mosfet driver corrects the gate-source voltage as it's presented in Fig.9. (a) Fig.9. gate-source voltage of the upper transistor in converter the corrected structure The constructed driver circuit is shown in Fig.10 during the test in the laboratory. (b) (c) Fig.11. Different waveforms of the ABC circuit: (a): output pulse of the microcontroller. (b): output pulse of Mosfet driver. (c): output pulse of Mosfet driver versus voltage of the phase winding Fig.10. Constructed driver and controller circuit in the laboratory

5 V. CONCLUSION This paper presented a corrected structure of ABC for SRM drive. In the presented circuit, half bridge Hip2500 Mosfet driver was used to excite the phase transistors. The new structure corrected the gate-source voltage of the transistor. The upper transistor source was not grounded in custom ABC and the source of the transistor was floated. But in the corrected ABC, a boot strap circuit was used which corrected the gate-source voltage of the transistor. The presented ABC was so simple. The custom ABC required isolated DC supply in each phase which increased the cost of the driver board and made it complex as well. The custom and corrected ABC were both constructed and tested on a three-phase 6 by 4 SRM in the laboratory and the results were obtained. REFERENCE: [1] A.Moraveji, A. Siadatan, E. Afjei, M. Rafiee and E. Zarei Ali Abadi, DSP Sensorless Controller of Switched Reluctance Motor- Generator Approaching to AM Modulation, IEEE Ind, 1992, pp [2] A.Najafi, E. Afjei and H. Khalili, Rotor Position Detection in SRM Drive Utilizing Phase Shift Variations in a Formed Resonant Circuit, IEEE Ind, 2007, pp [3] P.P. Acarnley, Stepping Motors: A Guide to Modern Theory and Practice, Peter Peregrinus Ltd, [4] I.Hosein and M.Ehsani, Rotor Position Sensing In Switched Reluctance Motor Drivers By Measuring Mutually Induced Voltages, IEEE Trans Ind.App, vol. 30, no. 3, 1994, pp [5] E. Afjei, H. Moradi Cheshmeh Baygi, and H. Nouri, Detecting the Rotor Position by Employing Pulse Injection Technique And Digital Pulse Width Modulation Decoder in Switched Reluctance Motor, IEEE Ind, 2010, pp [6] R.Krishnan, Switched Reluctance Motor Drivers: Modeling, Simulation, Analysis, Design and Application, CRC Press, [7] R. Krishnan Electric Motor Drives Modeling Analysis and Control Printice Hall [8] T.J.Miller, Switch Reluctance Motor Drive, Ventura, CA; Intertec Communications Inc, [9] Dessouky, Y.G., Williams, B.W. and, " A novel power converter with voltage-boosting capacitors for a four-phase SRM drive", IEEE Transactions on industrial electronic, vol. 45, no. 5,1998, pp [10] Magnet CAD Package: User Manual, Infolytica Corporation Ltd., Montreal, Canada, 2006.