International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:03 40

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1 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 4 Improvement Torque Ripple of Switched Reluctance Motor Enhance Decay Time and Dwell Angle Dr. Abdal-Razak Shehab Dept. o Electrical engineering, Kufa University Abdul razzaq.aljuburi@uokufa.edu.iq Abstract This paper presents the performance of a 6/4-pole configuration switched Reluctance Motor (SRM) drive with fixed turn-off angle control scheme. Turn-off and on angles (dwell angle) plays an important role in developing electromagnetic torque in SRM and leads to stable or unstable operation of the drive. However, this study analyze the effect of decreased decay time for form of wave current by adding resistance in the return path, so that give us a flexibility to tuning the overlap angle (on and off angles) without developing negative torque, finally lead to reduce the torque ripple. The presented method is capable of minimizing SRM torque ripple as shown in addition to accurate average torque control compared with conventional current or voltage control. Torque ripple are intolerable for many high performance applications, especially at low speeds. The advantage of this method is very simple, inexpensive and effective to improvement torque ripple control scheme are emphasized. A 6/4 SRM modeled, simulation results for loaded and unloaded machine shows that the proposed scheme has good performance and practical application prospects. Index Term Modeling, performance analysis, simulation, switched reluctance motor, decay time. 1. INTRODUCTION The name of switched reluctance has now become the popular term for this class of electric machine. the switched reluctance motor (SRM) has been used not only for low performance application such as fans, pumps, and hand tool but also for high performance application such as centrifuge,electrical vehicles and spindle. As a result the manufacturing cost of (SRM) is relatively low because of the simplicity of construction and it can be operate at very high speed without mechanical problems. In SRM, only the stator presents windings, the rotor is just made of steel laminations without conductors or permanent magnets. The very simple structure reduces and avoids greatly its cost and losses heat effectively. Motivated by this mechanical simplicity together with the recent advances in the power electronics components, many studies have been developed recently [14, 19]. The switched reluctance motor is very simple machines in respect to the mechanical construction but it need to power electronic device in the convertor to drive it thisproblem complex the control and using this machine because of the cost of the power electronic devices and the connection between them and the machine coil are applied to the fault and it expensive this reason limits the using of (SRM) and make it for the high performance application such centrifuge and electrical vehicles not suitable [2]. SRM drives have been used for aerospace systems, marine propulsion systems, linear drives, mining drives, hand held tools and home utilities applications. The SRM is also suitable for variable speed as well as servo type applications [11]. The limit of its using because its complexity in control system and we can use the typical theory for its operation and the other reason is its nonlinearity. In (SRM) motion is produced as a result of the variable reluctance in the air gap between the rotor and the stator. When a stator winding is energized producing a single magnetic field, reluctance torque is produced by the tendency of the rotor to move to its minimum reluctance position. The implication of machine operation and its salient feature are inferred from the torque expression. The torque expression requires a relationship between machine flux linkage or inductance and the rotor position. [6] An average torque will result due to the combined instantaneous values of electromagnetic torque pulses of all machine phases. The machine produces discrete pulses of torque and, by proper design of overlapping inductance profile; it is possible to produce a continuous torque. In actual practice, it will result in reduced power density of the machine and increased complexity of control of the SRM drive. [6] To ensure instantaneous torque production it is essential that the desired current comes on at the instant of increasing inductance. From a practical point of view, the current cannot instantaneously rise or fall in a resistor, inductor (RL) circuit. This necessitates advanced application of voltage for starting the current and advanced commutation to bring the current to zero before a negative sloping inductance profile is encountered. Syeda Fatima Ghousia in [9] proposed the precise dwell angle control method for multi-level and multiphase excitation method to reduce torque ripple and vibrations in SRM. The impact of dwell angle parameters is analyzed in detail in order to get optimized values. Yusuf OZOGLU [1] In this study dynamic analysis of SRM with conventional pole shape and SRM with modified pole shape, both incorporate driver circuits, have been made and improvement in torque ripple and improvement in efficiency have been achieved. In Paper [17]-[16], authors optimize the phase

2 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 41 current waveform to study the torque ripple. In paper [15], the direct torque control is used to reduce the torque ripple in line with the nonlinear relation between torque and clockwise excitations of the stator phases results in a clockwise rotation of the rotor due to a positive torque generation. 2. EQUIVALENT CIRCUIT An elementary equivalent circuit for the SRM can be derived neglecting the mutual inductance between the phases as follows. The applied voltage to a phase is equal to the sum of the resistive voltage drop and the rate of the flux linkages and is given as: V = R S i + dλ(θ,i).. (1) where Rs is the resistance per phase, and λ is the flux linkage per phase given by: λ = L(θ, i)i (2) where L(θ, i) is inductance dependent on the rotor position and phase current. The phase voltage equation, then, is d{l(θ, i)i} v = R s i + = R s i + L(θ, i) di dθ dl(θ, i) + i = R s i + L(θ, i) di + dl(θ,i) dθ dθ ω m i.. (3) In this equation, the three terms on the right-hand side represent the resistive voltage drop, inductive voltage drop, and induced e.m.f., respectively, and the result is similar to the series excited dc motor voltage equation. The induced e.m.f., e, is obtained as: e = dl(θ,i) ω dθ m i = K b ω m i. (4) where K b may be construed as an e.m.f. constant similar to that of the dc series excited machine and is given here as: K b = dl(θ,i) dθ. (5) Note that the e.m.f. constant is dependent on operating point and is obtained with constant current at the point. From the voltage equation and the induced e.m.f. expression, the equivalent circuit for one phase of the SRM is derived and shown Figure (1). Substituting for the flux linkages in the voltage equation and multiplying with the current results in instantaneous input power. [5] p i = vi = R s i 2 + i 2 dl(θ,i) + L(θ, i)i di.. (6) v e Here, the last term is physically uninterruptible; to draw a meaningful inference, it may be cast in terms of known variables as in the following: d (1 L(θ, 2 i)i2 ) = L(θ, i)i di + 1 dl(θ,i) i2.(7) 2 Substituting the above into Eq. (6) P i = R s i 2 + d (1 L(θ, 2 i)i2 ) + 1 dl(θ,i) i2.. 2 (8) Where p i is the instantaneous input power. This equation is in the familiar form found in introductory Fig. 1. equivalent circuit for single phase electromechanics texts, implying that the input power is the sum of the winding resistive losses given by R si 2, the rate of change of the field energy given by p[l(θ, i) i2 ], and 2 the air gap power, p a, which is identified by the term [i 2 pl(θ,i)]/2 where p is the differential operator, (d/). Substituting for time in terms of the rotor position and speed, with t = θ. (9) ω m in the air gap power results in: p a = 1 dl(θ,i) i2 = dl(θ,i) i2 dθ = 1 dθ 2 dl(θ,i) i2 ω dθ m (1)

3 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 42 The air gap power is the product of the electromagnetic torque and rotor speed given by: p a = ω m T e... (11) from which the torque is obtained by equating these two equations as T e = 1 dl(θ,i) i2 (12) 2 dθ This completes development of the equivalent circuit and equations for evaluating electromagnetic torque, air gap power, and input power to the SRM both for dynamic and steady-state operations. 3. TOPOLOGYES of DRIVE 3.1 Asymmetric Half Bridge Converter : The asymmetric half bridge converter is the most used converter, each machine phase is connected to an asymmetric half bridge consisting of two power switches and two diodes. And Figure (2) illustrated its circuit: For off state : =L di/ + irs + Eb.. (14) i(t) = Eb R [1 e trs L ] So that to decreasing the decay time period we must accelerate the rate change for the waveform of current by adding a resistance in the derenergized for each phase in the return path to supply. From equations 13 and 14 and for specified time we can calculate the value of resistance which satisfied that. 4. SIMULATION of SWITCHED RELUCTANCE MOTOR CONTROL 4.1 States for improvement performance in SRM There are several parameters effect the average electromagnetic torque developed in each phase like angles (dwell), inductance and current (last two dependent on each other). Let us discuss what will be happen if we add a resistance in period of denergized return path of each phase in the drive circuit as shown figure (3). Fig. 2. symmetric half bridge Phase current in asymmetrical half bridge converter is controlled by selecting from three possible states : (1) Both switches in a phase leg are on, and phase is energized from power supply (magnetizing stage). Fig. 3. single phase drive circuit Asymmetric converter for SRM with freewheeling diode and adding control resistance regeneration capability. (2) Both switches in a phase leg are off. Phase current commutates to the diodes and decays rapidly (demagnetizing stage). (3) Only one of the switches is off. The voltage across winding is near zero and phase current decays slowly (freewheeling). The equations of Asymmetric Half Bridge Converter are : For on state : V=L di/ + irs + Eb *. (13) i(t) = (v E b) [1 e tr s L ] R * Eb: is the back e.m.f. and its found in dynamic state. while in static state we can neglect it. The equation of current in symmetric converter after adding control resistance is:- Vs = R T i + L di + E b Where R T = R S+R C Where supply voltage equal zero i(t) = E b R Theoretically :- Let us take t=4.6 τ, E b =1V, R s =.5Ω, i(t) = 5 A t [1 e τ] R T = = Ω 5

4 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 43 Where R T = R S + R C R C = = Ω 4-2 Simulation results In terms of conventional control, build the simulation model of the whole SRM dynamic system in SIMULINK, as Fig. 4 shows, and Fig. 5 shows simulation result. In observing the derives waveforms we can note that the SRM operation speed band can be divided into two regions according to the converter operating mode( current controller and voltage ved). After adding the control resistance and simulate we can see clearly that adding control resistance in the decay period without change the dwell angle have little effected to the average and torque ripple as shown in fig. 4, and simulation results as in fig. 5&6 in both cases with no load and loaded machine the current waveform, magnetic flux and the average torque which increase from (29.31 N.m) to (29.99 N.m) with little improve in torque ripple. The second state after adding resistance and change in dwell angle( by trial and error we got on=39 and off =77) as shown in figure 7, so we do that adding a control resistance and in the same time varying in dwell angle to decrease the overlap between adjacent phases as a result we must get a decrease in the torque ripple compared with that done by turn on angle at 4 o as a result we got a real decreased in the torque ripple and the average torque will be (55.8 N.m), the overall improvement of ripple as percentage about (2%) as compared with conventional control (29.31 N.m) as shown in table below. method Reference torque N.m average ΔT Torque conventional With additional resistor in driver With resistor and change the on angle ripp le% To have a better general view of the torque ripple, define the torque ripple coefficient [12]: Torque Ripple = T ins.(max. ) T ins. (min. ) T avg. Where Tins.(max.), Tins.(min.) are the maximal value and minimal value of total torque, Tavg. is the average value of total torque. 5. CONCLUSIONS The summation of separated torque for each phase with adjusted firing angle give us the total electromagnetic torque (useful torque if the lost torque neglected)as shown in fig.4. As we see the motor will be in the steady state after (.25 sec), in this simulation we can also control the firing angle as advance (in or off) for separately phase which they influence the all operation as shown in fig.(5). From equations and this simulation we can emphasis that the torque ripple in this motor depend upon more than one parameter (angle of firing,form of current and flux) and these parameter internally depends upon each other s. In SRM drives, both the average torque and torque ripple are affected by the turn-on and turn-off angles and by the current waveforms, and their characteristics change as a function of the motor speed. It is clear from simulation results that our suggestion to improving torque ripple is effective easy and inexpensive and convenient at no-load and full-load as compare with the conventional control (without suggested addition resistance) as shown in figure (5) and figure (7). In many applications, electric vehicle drives for instance; it is highly desirable to have highest torque/ampere ratio and lowest torque ripple and this over a widest speed range possible. The SRM torque characteristic can be optimized by applying appropriated pre-calculated turn-on and turn-off angles in function of the motor current and speed (this may a future work).

5 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 44 CONVERTER TL Scope2 2 G A1 A2 B1 A1 A2 B1 mm <Flux (V*s)> <I (A)> V+ B2 B2 <Te (N*m)> 24 V V- C1 C2 C1 C2 <w (rad/s)> -K- Switched Reluctance Motor w sig alf a 4 Turn-on angle (deg) beta 75 Turn-off angle (deg) Mean Position_Sensor Mean Value Display Discrete, Ts = 1e-6 s. pow ergui Switched Reluctance Motor? More Info Fig. 4. block diagram circuit representation used in simulation(conventional& control resistor) Data sheet parameters for SRM: Stator resistance (ohm)=.5 Inertia(Kg.m.m)=.5 Friction(N.m.s)=.2 Inintial speed and position[w(rad/s)theta(rad)=[,] Magnetization characteristic table(mat file): 'srm6/4-6kw.mat' Rotor angle vector used in MAT file (degrees): [ ] Stator current vector used in MAT file(a): :25:45 Fig. 5. simulation result with On=4 off=75 at no-load.

6 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 45 Fig. 6. simulation result with On=4 off=75 and control resistor at no-load.6 <Flux (V*s)> <Flux <I (A)> (V*s)> <Te <I (A)> (N*m)> <Te (N*m) speed(r.p.m.) Fig. 7. simulation result with On=39 off=77 and control resistor at full-load

7 International Journal of Engineering & Computer Science IJECS-IJENS Vol:13 No:3 46 REFERENCES [1] Bae,H.K. and R.Krishnan,"A study of current controllers and development of novel current controller for high performance SRM drives", IEEE Ind. Appl. Soc.Ann. Mtg. Conf. Rec., [2] R. Krishnan" Switched reluctance motor drives modeling simulation, analysis, design and application", 21. [3] Bae,H.K., B.S. Lee, P. Vijayraghavan and R. Krishnan," A linear switched reluctance motor; converter and control" COT/Rec. IEEE Ind. Appl. Soc. Ann. Mtg., Oct., ,1999. [4] T J.E. Miller," Switched reluctance motor and their control", Clarendron press, Oxford, [5] D.A. Torrey, X.M. Niu, EJ. Unkauf,"Analytical modeling of variable reluctance machine magnetization characteristics", IEEE Proceedings-Electric Power Application, vol. 142, No. 1, January [6] John G. and A. R. Eastham," Speed control of SRM using slide mode control strategy", Conf. Rec. IEEE IAS Ann. Mtg., Oct. 1995, pp [7] J. D. Lewis, H. R. Bolton, and N. W. Phillips, Performance enhancement of single and two phase SR drives using a capacitor boost circuit, in European Power Electronics and Applications Conf. Rec., vol. 3, pp , [8] C. H. Choi, S. H. Kim, Y. D. Kim, and K.H. Park, A new torque control method of a switched reluctance motor using a torquesharing function, IEEE Trans. On Magnetics, vol. 38, no. 5, September 22, pp [9] Syeda Fatima Ghousia (212), Impact Analysis of Dwell Angles on Current Shape and Torque in Switched Reluctance Motors, International Journal of Power Electronics and Drive System (IJPEDS) Vol.2, No.2, June 212, pp. 16~169. [1] Yusuf Ozoglu "Implementation of dynamic analysis of SRM with method pole geometry" Journal of Electrical& electronics Engineering Volume-1, No.1, pp [11] Iqbal Husain and Syed A. Hossain. Modeling, Simulation, and Control of Switched Reluctance Motor Drives, IEEE Trans on Industrial Electronics 25,52(6): [12] ISHIKAWA Himki, WANG Daohong and WAITOH Haruo. A new switched reluctance motor drive circuit for torque ripple reduction, IEEE PCCO Saka, 22,2(2): [13] S.Paramasivam and R.Arumugam. A Hybrid controller Design and Implementation for Switched Reluctance Motor Drives, Journal of The Institution of Engineers, 25, 45(5): [14] Pavol Rafajdu and Ivan Zrak. ANALYSIS OF THE SWITCHED RELUCTANCE MOTOR (SRM) PARAMETERS, Journal of ELECTRICAL ENGINEERING, 24, 55(7-8): [15] Youn-Hyun Kim1, Sol Kim2, Jae-Hak Choi, et al. Direct torque control of switched reluctance motor for minimizing torque ripple, Journal of Applied Electromagnetics and Mechanics, 28, 28(1-2): [16] Inanc N. Phase current modulation of switched reluctance motor to minimize torque ripple, Electric Power Systems Research, 22, 61(1): [17] D.S. Schramm, B.W. Williams and T.C. Green, Torque Ripple Reduction of Switched Reluctance Motors by Phase Current Optimal Profiling, Proceedings of Power Electronics Specialists Conference, 1992, vol. 2, pp: [18] Venkatesha, L and Ramanarayanan, V. Torque ripple minimisation in switched reluctance motor with optimal control of phase currents, Power Electronic Drives and Energy Systems for Industrial Growth, 1998, Vol. 2, Issue, 1-3 Page(s): [19] Alexey Matveev. Development of Methods, Algorithms and Software for Optimal Design of Switched Reluctance Drives, Technische Universiteit Eindhoven, 26.