IMPLEMENTATION OF LOW COST SWITCHED RELUCTANCE MOTOR DRIVE USING RT-LAB

Transcription

1 IMPLEMENTATION OF LOW COST SWITCHED RELUCTANCE MOTOR DRIVE USING RT-LAB Jignesh Makwana 1, Ambarisha Mishra 2, Pramod agarwal 3, S.P Srivastava 4 1 Research Scholar, Electrical Department IIT Roorkee, Uttrakhand, India, jigneshamakwana@yahoo.com 2 Research Scholar, Electrical Department IIT Roorkee, Uttrakhand, India,amb0097@gmail.com 3 Professor & Head, Electrical Department, IIT Roorkee, Uttrakhand, India, pramgfee@iitr.ernet.in 4 Professor, Electrical Department, IIT Roorkee, Uttrakhand, India, satyafee@iitr.ernet.in Abstract This paper demonstrates the implementation of low cost switched reluctance motor (SRM) drive and application of RT lab as real time hardware-in-loop (HIL) controller. Split DC converter and positioning sensing arrangement is developed for 500W 8/6 pole SRM. Control part is implemented using Opal RT Lab technology. Application and optimistic characteristics of proposed low cost drive are described. Experimental results of performance and efficiency for proposed SRM drive are presented which shows very low cost versus performance ratio compare to induction motor and permanent magnet motor drive. Index Term Converter, reluctance motor, RT Lab, electric drives *** INTRODUCTION The Switched Reluctance Motor (SRM) drive promises an impressive set of benefits over its competition includes high efficiency over a wide speed range and partial loads, highspeed capability, easy cooling with heat source only in the stator, ruggedness for high-temperature or vibration environments, and relatively simple mechanical construction. But sheer numbers of induction and brushless permanent magnet (PM) motors at work in industrial and commercial applications testify to their well-established manufacturing infrastructure and user acceptance. This has limited wide use of SRM - a technology that offers a practical alternative for various demanding applications. Perhaps today s growing demand for energy efficiency motivates the users and companies to look at SRM as an alternative comes from concern about magnet material cost in PM synchronous motors and a desire to move away from induction motors for overall efficiency and system cost. Current resurgence in demand is observed for SRM drive with a variety of platforms intended for industry, includes high speed applications such as screw compressors, blowers, and high-speed pumps and low-speed, high-torque areas (conveyors, feeders). SRMs can t run direct-on-line, thus require an associated power converter (drive) to complete an SRM drive system. SRM power converter topology differs from that of conventional ac drives in the arrangement of power switch and fly-back diode circuits. For smaller drives, use of power modules is a costeffective design route, but off-the-shelf modules are not available for SRM as for other motor technologies. As more applications become variable speed, the SRM option, whose cost is competitive with an equivalent inverter-fed induction motor, becomes viable across a growing range of applications. This paper presents the development of low cost SRM drive which includes development of split DC controller and open loop controller with position sensing arrangement. Fixed frequency PWM controller is developed and implemented using Opal RT Lab. A single phase induction motor achieved worldwide acceptance for general purpose motor drive in domestics and industrial application because its feature to run direct on AC lines without having costly converters. Beside large number of converter and control modules are readily available today for induction motor and brushless DC motors. While there are no such a converter and controller modules are available for the SRM which discourage the usage of the SRM technology which offers a high performance and efficiency. Low cost SRM drive presented in this paper is to show the performance of SRM to run directly on AC mains supply with low cost but reliable converter and position sensing arrangement without starting hesitation. Available 772

2 2. SWITCHED RELUCTANCE MOTOR Switched Reluctance Motor is a doubly salient and singly excited motor. Unlike conventional AC or DC motor which required either two winding or one winding and one permanent magnet to produce the torque SRM have only winding in the stator. The rotor has no windings, magnets or cage windings but is built up from a stack of salient pole laminations. Torque is produced due to force of attraction between magnetic field of stator winding and magnetic material of rotor. SR machines offer a wide variety of aspect ratios and salient pole topologies. Each application is likely to a better suited to a specific SR topology. Fig. 1 shows the geometry of four phase SRM having 8 stator pole and 6 rotor pole which denoted by 8/6 SRM in general. Generally, selection of higher number of phase and pole reduces the torque ripple, but it required more switching devices. Some important old references of the SRM are [1]-[5]. Fig. 2 shows the typical magnetic characteristics of the SRM which represent the number of magnetic curves relates flux linkage and phase current for unaligned to aligned position of rotor. Its shows the two saturation mainly due to pole corner saturation near unaligned position at lower current and due to saturation of yoke near aligned position at higher current. If magnetic saturation is neglected then the relation between fluxlinkage and current at an instantaneous position θ is a straight line whose slope represents an instantaneous inductance L. Thus Ψ = Li and, 1 w w Li 2 c f (5) dl Therefore torque T i N-m (6) 2 d Fig-1: Geometry of four phase 8/6 SRM When current is passed through the phase winding the rotor tends to align with the stator poles and it produces a torque that tends to move the rotor to a minimum reluctance position. The direction of torque generated is a function of rotor position with respect to energized phase, and is independent of direction of current flow through phase winding. Continues torque can be produced by intelligently synchronizing each phase s excitation with the rotor position. An equivalent expression of torque is, w T c i constant w f or T i Where w c and constant respectively. Mathematically, (1) (2) w f are co-energy and stored field energy w c di (3) and w f i d (4) Fig.-2: Typical flux linkage characteristics of SRM Variation of idealized phase inductance is shown in Fig. 3. To develop continuous torque in positive direction it is required to energize the phase only during their respective rising inductance period as shown in Fig. 3 which explains the necessity of position sensor to command the phase current. Different converter topology may be use to energize the phase of the SRM but most common is two switched per phase asymmetric converter shown in Fig. 4. There are number of converter topology is published in the literature to reduce the number of switches per phase and reduce the cost of converter and firing circuit [6]-[11]. Available 773

3 encoder, Hall Effect sensors or even many sensorless methods have been developed [12]-[16]. Low cost high speed position sensing arrangement with control scheme is shows in Fig 6. Toothed disk having teeth symmetrical to the rotor pole is attached on shaft and is in perfect synchronization with the rotor pole. Disk cuts the light emitted by the source which generates two digital pulses to decide commutation period of the phase. By combining high speed TTL logics individual commutation pulse can be generated for all phases which mixed with the PWM signal to achieve current control as shown in Fig 6. For easy and flexibility Opal RT Lab is used to implement controller part. Fig-3: Inductance profile of four phase SRM Fig-5: PWM and hysteresis current control Fig-4: Asymmetric bridge converter There are several methods to control the torque-speed and the position of the SRM. Hysteresis current control and PWM control are two low cost and simplest methods for easy implementation. In hysteresis control phase switch turned off and on according whether the current flowing through the winding is greater or less than the reference current, while in PWM control fixed frequency variable duty cycle scheme can be employed to regulate the current as shown in Fig POSITION SENSING ARRANGEMENT AND PWM CONTROL STRATEGY There are so many options for choosing position sensing scheme for the SRM drive including absolute or incremental Fig. 7 shows the commutation pulse C4 and C3 which decide the conduction period of phase 4 and phase 3 respectively while conduction period of phase 2 and phase 1 is decided by commutation pulse C2 and C1 which are logically invert of C4 and C3 respectively. Fig. 8 shows that commutation pulse is logically mixed with the fixed frequency PWM pulse and generated gate pulse are applied to the isolated MOSFET driver circuit shown in Fig. 9 to achieve the current and speed control of the SRM. Fig-6: Position sensing arrangement and control logic Available 774

4 Fig-7: Commutation pulse to decide on-off instant of phase diph dlph( ) Vph Rphiph Lph( ) miph (8) dt d One switched and one diode is associated with each phases. At any instant two phase are ON to maximize the torque and which also minimize the torque ripple. Alternative phases (1,3 and 2,4) are never going to conduct simultaneously. It also helps in balancing the capacitor C1 and C2. Fig. 10 shows the mode of operation of the converter. Mode 1 is phase energize mode and mode 2 is regenerating mode. When M1 is ON voltage across phase is V dc /2 and current is circulating through C1, M1 and Phase 1. At the instant of turning OFF M1; diode D1 comes in conduction and current circulate through the D1, Phase 1 and C2. Fig-9: One switched per phase split DC converter with AC mains Fig-8: Gate pulse; Commutation pulse combined with PWM pulse 4. SPLIT DC CONVERTER Unlike conventional AC and DC motor SRM cannot run with direct AC or DC supply. SRM require converter circuit to guide the current in appropriate phase with rotor position sensing arrangement. Fig. 9 shows split DC one switched per phase converter circuit for SRM. Simple diode bride with filter is added to AC to DC conversion. Here main aim is to obtain performance characteristics of 0.5KW SRM with 230 V AC mains. Assume that there is no magnetic saturation that means inductance is unaffected by the current. Also neglecting the mutual inductance for the simplicity voltage equation of the one phase is, V ph Where d ph d ph Rphiph Rphiph m (7) dt d V is the phase voltage equal to V 2 and ph Vdc 2V rms dc Mode 1 Mode 2 Fig-10: Mode of operation of split DC converter 5. OPAL RT-LAB TECHNOLOGY Real-time simulation of SRM drives on a CPU-based real-time simulator can produce accurate results, but can also have the undesirable effect of causing current overshoots because of model latency. To remedy this problem, an FPGA implementation is desirable because it offers a very low calculation time and I/O latency. Available 775

5 RT-LAB, from Opal-RT Technologies, is a real-time simulation platform that enables real time and HIL (hardware in loop) simulation of controllers, electric plants or both, through automatic code generation methods. The entire process occurs without the need for handwritten C code, enabling very rapid deployment of prototyped controllers or HIL-simulated plants. The process is notably very efficient when applied to I/O code because RT-LAB provides a set of simulink blocks that automatically configure common I/O functions, like analog input/outputs and time-stamping capable digital I/Os, with a 10 nanosecond resolution. Special interpolating models use this timing information to greatly increase simulation accuracy [17]. RT-LAB simulator is equipped with a user-programmable FPGA card. The FPGA card can be programmed with the Xilinx system generator blockset for simulink enabling implementation of complex sensor models like resolvers, Resolver-To-Digital and FM resolvers or even complex motor drives [18], [19]. contain the model of the actual controller while subsystem named SC_speed_control represent the model for user interface for online parameter control and monitor. Fig. 12 shows the modelling of controller and Fig. 13 shows user interface panel available for HIL speed control and monitor. Fig-11: Controller subsystem RT-Lab is used as real time hardware-in-loop controller in this implementation for easy and flexibility. Table I summarizes the characteristics of FPGA board used in this paper and Table II summarize the input output card used for analog output of phase current and gate pulse. Fig-12: Subsystem model of controller Table-1: Reconfigurable FPGA Boards Table-2: Input output configuration 6. HARDWARE IN LOOP CONTROLLER As shown in Fig. 6 two rotor position signals are applied to the analog input card of RT- Lab. Here logic operation is performed to mix the fixed frequency PWM control signal to control the current and speed of the motor. From analog output card four gate pulses are taken out and supplied to the MOSFET driver circuit as shown in Fig. 6. Duty cycle of the PWM pulse can be controlled in real time to control the motor speed. User interface is provided to control and record/observe the motor speed in real time. RT Lab allows to model a subsystem in MATLAB simulink environment with some own rules and perform automatic code generation and transfer of the simulink model for the FPGA implementation. Fig. 11 shows the subsystem modelled for the present controller. Subsystem named SM_speed_control Fig-13: User interface available for real time control and monitor Available 776

6 7. PERFORMANCE CHARACTERISTICS OF SRM DRIVE Fig. 14 to Fig. 22 shows the different performance plots for the projected SRM drive include speed-torque characteristics, efficiency, power-factor, no-load input power, noise analysis and vibration details. Fig.23 shows the commutation pulse with the phase current at no-load speed of 1100 rpm. Fig. 24 shows the phase voltage and phase current waveform for the motor speed of 880 rpm and load torque of 4 kg-cm. Fig. 25 shows the experimental setup for the proposed SRM drive. Fig-17: Steady state speed versus PWM duty cycle Fig-14: Speed torque characteristics Fig-18: No-load current versus speed Fig-15: Efficiency versus speed Fig-19: No load power input versus speed Fig-16: Efficiency versus load Fig-20: No load power factor versus speed Available 777

7 Fig-21: Noise performance versus speed Fig. 25 Experimental setup Fig-22: Vibration performance versus speed Fig-23: Commutation pulse and phase current at no load speed of 1100 rpm Fig-24: Phase voltage and current at 880 rpm and load of 4 Kg-cm 8. CONCLUSION Projected scheme shows impressive advantages over conventional motor drive regard in motor, converter and control electronics. Motor offer maintenance free robust performance with low manufacturing cost and low material cost. Rotor inertia is very low because of salient pole type construction lead to low weight and small size compare to conventional AC and DC motor. Stator is simple to wind; end turns are short and robust and have no phase-phase crossovers support low cast easy manufacturing steps and also easy to repair. In most applications the bulk of the losses appear on the stator which is relatively easier to cool. Because there is no any costly permanent magnet on rotor permissible rotor temperature is high compare to permanent magnet motor in cost effective way. Motor provide higher torque compare to commutator motor and induction motor at all speed. Furthermore starting torque can be very high without the problem of excessive inrush currents and extremely high speed is possible. Motor is fully resistant to environment contras to the permanent magnet motor. SRM Controllers add to the benefits, since they do not need a bipolar (reversed) device because torque is independent of direction of current. One switched per phase MOSFET controller is cost effective compare to inverter particularly for brushless permanent magnet machines. It offers fault tolerant operation with one or more faulty phase or even with shorted MOSFET. It is found experimentally that with one phase open motor is running with 80% of its full capacity and with one MOSFET shorted motor is running with less efficiency and Available 778

8 capacity because one phase is always remains on irrelative of rotor position which generate negative torque and required more starting current. In addition projected converter allows two-phase excitation at a time which reduces the ripple in the torque. Furthermore digital controller and MOSFET driver add to the benefit of low cost in simplest way. MOSFET driver circuit required only three isolated power supply while mostly used asymmetric bridge converter of SRM requires five. Low frequency PWM speed controller offers benefits over hysteresis controller that it does not required a single current sensor while hysteresis controller requires four current sensors for reference current and four individual controllers. Low frequency PWM control reduces the switching losses and acoustic noise thus increases the performance and efficiency in simple and cost effective way. Body mounted infrared positioning scheme add the benefits of cost with compare to costly encoders with reliable performance with CMOS and TTL logics for very high speed performance. Result dictates that the cost versus performance ratio of the proposed SRM drive is quite low. It s observed that proposed SRM drive gives rugged performance with 230V AC mains supply without starting hesitation. Use of RT-Lab is much time saving in developing a control model for the practical electric motor drives and offer great easy and flexibility. Counter part of the proposed drives is the level of acoustic noise production which prevents the use of SRM for the domestic application like fan and other continuous duty application. APPENDIX Motor Specifications: Duty Type continuous Motor Type 8/6 four phase SRM Output power 0.5 KW Phase voltage 150 V Number of turn per phase 310 turns per phase, Resistance per phase 4.5 ohm per phase, Stator outer diameter 90.8mm Rotor outer diameter 48.4mm Electronics specifications: Power switch: IRFP450A Diode: MUR1560 PWM frequency 1.66 KHz ACKNOWLEDGMENT Author is thankful to the electric department of Indian Institute of Technology Roorkee for providing required equipments for experimental setup. REFERENCES [1] Lawrenson, P.J., and AGU, L. A.: Theory and performance of polyphase reluctance machines, IEE Proc. B, Electr. Power Appl., 1964,111, pp [2] Lawrenson, P.J., Stephenson, J.M., Blenkinsop, P.T., Corda, J., and Fulton, N.N. : Variable-speed switched reluctance motors, IEEE Proc, 1980,Vol.127, Pt.B.No (4), pp [3] Harris, M.R., Andjargholi, V., Lawrenson, P.J., Hughes, A., and Ertan, B.: Unifying approach to the static torque of stepping-motor structures, ibid., 1975,122, pp [4] Krishnan, R., Switched Reluctance Motor Drives: Modeling, Simulation, Analysis, Design, and Applications, CRC Press, [5] TJE Miller, Electronics control of switched reluctance machines, Newnes, [6] Barnes, M., Pollock, C. (Nov. 1998). Power electronic converters for switched reluctance drives. IEEE Trans. on Power Electronics. 13: [7] [M. Barnes and C. Pollock, Selecting Power Electronic Converters for Single Phase Switched Reluctance Motors, Proceedings of IEE Conference on Power Electronics and Variable Speed Drives, London, September 1998, pp [8] G. Venkatesan, R. Arumugam, M. Vasudevan, S. Paramasivam and S. Vijayan, Modeling and Simulation of a Novel Switched Reluctance Motor Drive System with Power Factor Improvement, American Journal of Applied Sciences 3 (1): , 2006 [9] Vukosavic, S. and V.R. Stefanovic, SRM Inverter Topologies: A Comparative Evaluation, IEEE IAS, , [10] Krishnan, R. and P. Materu, Design of A Single-Switch- Per-Phase Converter for Switched Reluctance Motor Drives, IEEE Trans. on Industrial Electronics, 37(6), , Available 779

9 [11] Pollock, C. and B.W. Williams, Power Converter Circuits for Switched Reluctance with the Minimum Number of Switches, IEE Proc., Vol. 137, Pt. B, No. 6, , 1990 [12] Lyons J.P., MacMinn, S.R. and Preston, M.A., Flux/Current methods for SRM rotor position estimation, Conf. Rec. IEEE Industry Applications Society Annual Meeting, pp , 1991 [13] Jones S.R. and Drager B.T, Performance of a high-speed switched reluctance starter/generator system using electronic position sensing, Conf. Rec. IEEE Industry Applications Society Annual Meeting, pp , [14] DiRenzo M.T. and Khan W., Self-trained commutation algorithm for an SR motor drive system without position sensing, Conf. Rec. IEEE Industry Applications Society Annual Meeting, pp , October [15] M. Ehsani, I. Husain, S. Mahajan, and K. R. Ramani, New modulation encoding technique for indirect rotor position sensing in switched reluctance motors, IEEE Trans. Ind. Applicat., vol. 30, pp , Jan./Feb [16] M. Ehsani, I. Husain, and A. B. Kulkarni, Elimination of discrete position sensor and current sensor in switched reluctance motor drives, IEEE Trans. Ind. Applicat., vol. 28, pp , Jan./Feb [17] M. Harakawa, H. Yamasaki, T. Nagano, S. Abourida, C. Dufour and J. Bélanger, Real-Time Simulation of a Complete PMSM Drive at 10 us Time Step, Proceedings of the 2005 International Power Electronics Conf. (IPEC 2005), April 4-8, 2005, Japan. [18] C.Dufour, J. Bélanger, V. Lapointe, FPGA-Based Ultra- Low Latency HIL Fault Testing of a Permanent Magnet Motor Drive using RT-LAB-XSG, Simulation: Transactions of the Society for Modeling and Simulation International, SAGE Publications, Vol. 84, Issue 2/3, February/March 2008, pp [19] C. Dufour, J. Bélanger, S. Abourida, V. Lapointe, FPGA-Based Real-Time Simulation of Finite-Element Analysis Permanent Magnet Synchronous Machine Drives, Proceedings of the 38 th Annual IEEE Power Electronics Specialists Conference (PESC 07), Orlando, Florida, USA, June 17-21, BIOGRAPHIES Jignesh Makwana received the B.E and M.E degrees in Electrical Engineering from the Birla Vishvakarma Mahavidhyalaya, v.v.nagar, gujarat, India, and L.D. Engineering College, ahemadabad, gujarat, India in 2004 and 2006 respectively. He was a lecturer with the C.U. Shah College of Engineering and Technology from 2006 to 2008 and joined the R.K College of Engineering and Technology in Currently he is a research scholar in Electrical Department of Indian Institute of Technology, Roorkee, India. His fields of interest are electric machines, drives and power electronics. Ambarisha Mishra was born in He received B.Tech. (Electrical) from Uttar Pradesh.Technical University Lucknow, India, in 2007 and M.Tech.(Power Electronics & Drives) from National Institute of Technology Kurukshetra, India, in Currently he is pursuing PhD from in Electrical Engineering Department, Indian Institute of Technology Roorkee, India. His field of interest includes electric drives and power electronics. Pramod Agarwal received the B.E., M.E., and Ph.D degrees in Electrical Engineering from the University of Roorkee, India, in 1983, 1985 and 1995, respectively. He joined the erstwhile University of Roorkee, India in 1985 as Lecturer. He was a Postdoctoral Fellow with the Ecole de technologie superior, University of Quebec, Montreal, Canada. He is currently a Professor with the Department of Electrical Engineering, Indian Institute of Technology, Roorkee, India. He has developed a number of educational units for laboratory experimentation. His fields of specialization are electrical machines, power electronics, microprocessor and microcomputer controlled ac/dc drives, active power filters, multi-level inverters and high power factor converters. S. P. Srivastava received the bachelor's and master's degrees in Electrical Technology from I.T. Banarus Hindu University, Varanasi, India in 1976, 1979 respectively and the Ph. D degree in Electrical Engineering from the University of Roorkee, India in Currently he is with Indian Institute of Technology (IIT) Roorkee, India, where he is a Professor in the Department of Electrical Engineering. His research interests include power apparatus and electric drives. Available 780