LOSS-MINIMIZATION CONTROL OF SCALAR- CONTROLLED INDUCTION MOTOR DRIVES

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1 LOSS-MINIMIZATION CONTROL OF SCALAR- CONTROLLED INDUCTION MOTOR DRIVES Hussei Sarha, Rateb Al-Issa, ad Qazem Jaber Departmet of Mechatroics Egieerig, Faculty of Egieerig Techology Al-Balqa Applied Uiversity, Amma, P O Box 58, Jorda Abstract - The efficiecy of iductio motor drives uder variable operatig coditios ca be improved by predictig the imum flux that guaratees loss miimizatio I this paper, a loss-miimizatio scalar cotroller, based o determiig of modulatio idex of IGBT iverter-based AC-to-AC coverter, which correspods to imum flux, is proposed For this purpose, mathematical models for total power losses as a fuctio of magetic flux, imum flux as a fuctio of operatig speed ad load torque, ad modulatio idex as a fuctio of imum flux were aalytically derived Loss-miimized, scalar-cotrolled iductio motor drive system was modeled, simulated ad experimetally tested The results have validated the effectiveess of this system i miimizig the motor operatig losses, especially at light ad medium loads Also, the dyamic performace of the drive system has bee improved The proposed cotroller ca be implemeted i adjustable speed iductio motor drive systems with variable loads, operatig below rated speed Key Words - Iductio Motor Drive, Loss Miimizatio, Scalar Cotrol, Optimized Flux Nomeclature: Δ tot P = total power losses, W R = stator resistace, Ω R = rotor resistace referred to the stator, Ω ' I = stator curret, A I = rotor curret referred to the stator, A ' I = magetizig curret, A I = omial magetizig curret ΔP = magetic losses at omial operatig coditios, W ϕ = mag φ φ = ormalized (relative) flux, pu φ = operatig flux, Wb φ = omial flux, Wb f α = = ormalized (relative) frequecy, pu f f = operatig frequecy, Hz f = omial frequecy, Hz k = coefficiet depedig o the type of steel used i core costructio, usually = 3-5 T μ = = ormalized torque, pu T T = load torque, Nm T = omial load torque, Nm P ag = omial air-gap (electromagetic) power, W E = omial emf, V P = omial iput power, W I = omial stator curret, A P = omial output power, W ω = sychroous speed, rad/s s a, b = coefficiets of aalytical approximatio of magetizatio curve V = omial stator voltage, V I INTRODUCTION Iductio motors are the most used i idustry sice they are rugged, iexpesive, ad are maiteace free It is estimated that more tha 5% of the world electric eergy geerated is cosumed by electric machies Improvig efficiecy i electric drives is importat, maily for ecoomic savig ad reductio of evirometal pollutio [, ] Iductio motors have a high efficiecy at rated speed ad torque However, at light loads, motor efficiecy decreases dramatically due to a imbalace betwee the copper ad the core losses Hece, eergy savig ca be achieved by proper selectio of the flux level i the motor [3, 4] The mai iductio motor losses are usually split ito: stator copper losses, rotor copper losses, core (iro) losses, mechaical ad stray losses To improve the motor efficiecy, the flux must be reduced, obtaiig a balace betwee copper ad core losses May miimum-loss cotrol schemes based o scalar cotrol or vector cotrol of iductio motor drives have bee reported i literature [4] [8] Basically, there are two differet approaches to improve the efficiecy: loss-based model approach ad power measure based approach [6]-[8] I loss based model approach the loss miimizatio imum flux or power factor is computed aalytically, while i power measure based approach the cotroller searches for the operatig poit where the iput power is at a miimum while keepig the motor output power costat The proposed i literature various methods for loss-miimizig cotrol of iductio motors differ each other by approach, complexity, accuracy, covergece, etc [9, ] I this paper, a loss-miimizatio scalar cotroller of iductio motor drive system, based o calculatig the imal flux for loss miimizatio, is developed The, the validity of the proposed cotroller was examied by simulatio ad experimetal results Further, the effect of loss-miimizatio

2 scheme o the dyamic performace of the drive system was ivestigated II ALGORITHM OF THE MINIMUM-LOSS CONTROL The total power losses Δ Ptot i iductio motor ca be calculated by the followig equatio [, ]: a b B( b a) Cμ ( b ) = + k b ( b ) Cμ b a3 = k ( b ) (3) (4) Δ P = 3R I + 3R ' I ' + ΔP α () tot magϕ The magetic losses at omial operatig coditios Δ ca be approximately defied as: P mag Δ P mag = P Pag (3R I + 5P ) () The relatioship betwee stator curret I, rotor curret I ' ad magetizig curret is give by [8] I I ' + I (3) Also, the rotor curret I ' ca be represeted as: Pagμ I '= (4) E ϕ The omial air-gap (electromagetic) power is: P ag = T ωs (5) Accordig to aalytical aalysis provided i [3], the magetizig curret I ca be expressed as: k Cμ b a4 = (5) k ( b ) () has 8 roots, 6 of them are complex, oe real egative root, ad oe real positive root The complex roots ad the egative root have o physical meaig ad should be igored The remaiig real positive root will be the imal ormalized flux ϕ, which correspods to miimum power losses The imum value of cotrol commad, modulatio idex m, ca be defied as: m = (6) αϕ The last equatio shows how the magetic flux should be chaged to achieve eergy savig at differet operatig coditios Also, it is the cotrol algorithm of the proposed loss-miimizatio cotroller The imal value of modulatio idex is always The imal value of stator voltage V that miimizes the total power losses is: V = m V (7) I I a( ϕ ) + ϕ b( ϕ ) + ϕ (6) III MODELING AND SIMULATION OF OPTIMIZED DRIVE SYSTEM The coefficiets a ad b are selected so as most exactly describe the segmet of magetizatio curve, located i the iterval of imum flux variatio Substitutio of (3), (4), ad (6) ito () yields a( ϕ ) + ϕ μ k Δ Ptot = B + C + Dϕ α (7) where 3RI b( ϕ ) + ϕ ϕ B = (8) The drive system studied i this paper cosists of IGBTiverter-based AC to AC coverter, three-phase squirrel cage iductio motor ad cotroller I order to aalyze the system performace, all of these compoets should be modeled (mathematically described) Accordig to the aalysis doe i [5], the coverter will be modeled through its iput-output equatio based o the modulatio idex, ad the stadard statespace model will be used to model the iductio motor (6) represets the mathematical model of the cotroller The block diagram of the drive system studied usig MATLAB Simulik is show i Fig ' ) P ag ( R + R C = (9) 3E D = ΔP mag () (7) shows that the total power losses at specified frequecy ad load torque are a fuctio of ormalized magetic flux oly Takig the derivative of Δ Ptot with respect to ϕ i (7) ad equatig the derivative to zero yields ϕ + a ϕ + a ϕ + a ϕ + a = () 3 where b a = () b 4 Fig Model of imized drive system The parameters of modeled ad simulated iductio motor are give i table The MATLAB Simulik model of imal flux ϕ is show i Fig The model is used to defie the real positive root of () The coditio ϕ was cosidered i the <

3 model The required coefficiets i () were simulated accordig to () - (5) The relatioship betwee imal flux, frequecy, ad load torque is show i Fig 3 It is clear from Fig 3 that there is certai value of imal flux for each operatig poit origial ad imized systems are show i Figs 7 ad 8, which show that the proposed imizatio approach has sigificat effect at light loads Fig Model of imal flux ϕ TABLE I INDUCTION MOTOR DATA Parameter Value Stator resistace R 3 Ω Stator reactace X 5 Ω Mutual reactace X m 537 Ω Rotor resistace referred to the stator R ' 787 Ω Rotor reactace referred to the stator X ' 49 Ω Nomial voltage V /38V Nomial torque T 3634 N m Nomial output power P 55 kw Nomial stator curret I 5 A Nomial power factor cos ϕ 85 Nomial frequecy 5 Hz f Number of poles P 4 Nomial slip s 36 Sychroous rotatioal speed s 5 rpm Sychroous agular speed ω s 57 rad / s Nomial rotatioal speed 446 rpm Nomial agular speed ω 5348 rad / s Momet of iertia J 7 kg m Costat a 37 Costat b 3 Coefficiet k 4 I order to validate the effectiveess of the proposed cotroller, a motor loss model is required This model is show i Fig 4 The model was costructed accordig to (7)-() Examples of total power losses plots as a fuctio of stator voltage (flux) uder various load torques ad frequecies are show i Figs 5 ad 6, from which it ca be see that there is a imal value of stator voltage (flux) for each operatig poit, where the total power losses are miimum For quatitative aalysis, the total power losses i the imized system were compared with those i the origial (o-imized) system uder the same operatig coditios The origial system is cosidered that oe, i which the stator voltage (flux) remais costat ad equals the omial value for all operatig coditios Examples of total power losses plots i Fig 3 Optimal flux as a fuctio of load torque at variable frequecy Fig 4 Motor loss model Fig 5 Total power losses as a fuctio of stator voltage uder variable load torques ad frequecy f = 3Hz

4 Also, it was oticed i some cases, that there is a very small icrease i slip, as show i Fig This ca be explaied as a result of decrease i dyamic torque causig motor acceleratio due to decrease i magetic flux IV EXPERIMENTAL RESULTS Fig 6 Total power losses as a fuctio of stator voltage uder variable frequecies ad load torque T = Nm To verify the use of the proposed loss-miimizatio cotroller some experimets have bee carried out with the same drive system that was simulated The total power losses were experimetally obtaied for differet operatig coditios as the differece betwee the iput power ad the output power The iput power was measured for each operatig poit, while the correspodig output power was calculated as P out = Tω The experimetal results were compared with theoretical results Some of these results are represeted i Fig, which shows that there is some deviatio betwee experimetal ad theoretical results This is due to the assumptios ad approximatios doe durig loss model derivatio ad due to the losses i other compoets of the drive system Fig 7 Total power losses i origial system ad imized system uder variable frequecies (a) Load torque T = 5Nm ad (b) load torque T = 5Nm Effect of imizatio o the system dyamics has bee ivestigated Simulatio results show that the dyamic resposes of stator curret, electromagetic torque, ad agular speed had bee improved Examples of dyamic resposes of both imized ad origial systems are show i Figs 9, ad Fig 8 Total power losses i origial system ad imized system uder variable loads (a) Frequecy f = 5Hz ad (b) frequecy f = 3Hz

5 V CONCLUSIONS I this paper, the loss miimizatio i iductio motor drive systems through the magetic flux has bee derived ad examied The cotrol scheme utilizes modulatio idex of IGBT-iverter-based AC to AC coverter as the mai cotrol variable ad maipulates the stator voltage i order for the motor to operate at its miimum-loss poit The proposed cotroller ca be used i adjustable speed iductio motor drive systems operatig with variable loads The total power losses ca be decreased sigificatly at light loads However, i some cases, slip compesatio is required Simulatio results show that the dyamic performace of the imized system has bee improved REFERENCES Fig 9 Dyamic respose of electromagetic torque at load torque T = 5Nm ad frequecy f = 3Hz Fig Dyamic respose of agular speed at load torque T = 5Nm ad frequecy f = 3Hz Fig Experimetal ad theoretical power losses i the drive system for variable load torque ad costat frequecy 3Hz [] W Leohard, "Cotrolled AC drives, a successful trasfer from ideas to idustrial practice", CETTI 95, Brazil, Septemper -, pp -, 995 [] GCD Sousa, BK Bose, JG Clelad, "Fuzzy logic based o-lie efficiecy imizatio cotrol of a idirect vectorcotrolled iductio motor drive", IEEE Trasactios o Idustrial Electroics, vol 4, o, pp 9-98, April 995 [3] HR Aderse, CB Rasmusse, E Ritchie, JK Pederse, "Efficiecy compariso of electrical motors for eergy imized variable speed low power ad low cost household equipmet", 6 th Europea Coferece o Power Electroics ad Applicatios, EPE' 95, vol 3, pp , Seville, Spai, 995 [4] FC Li, SM Yag, "O-lie tuig of a efficiecyimized vector cotrolled iductio motor drive', Tamkag Joural of Sciece ad Egieerig, vol 6, No, pp 3-, 3 [5] H Sarha, R Issa, Modelig, simulatio ad test of iverter-iductio motor drive system with improved performace, Joural of egieerig scieces, Assiuit uiversity, vol 33, No 5, pp , 5 [6] F Abrahamse, JK Pederse, F Blaabjerg, "State-of-theart of imal efficiecy cotrol of low cost iductio motors drives", PEMC'96, vol, pp 63-7, Budapest, Hugry, Septemper -4, 996 [7] I Kioskeridis, N Margaris, "Loss miimizatio i iductio motor adjustable-speed drives", IEEE Trasactios o Idustrial Electroics, vol 43, o, pp 6-3, February 996 [8] Kioskeridis, N Margaris, "Loss miimizatio i scalarcotrolled iductio motor drives with search cotrollers", IEEE Trasactios o Power Electroics, vol, o, pp 3-, March 996 [9] F Abrahamse et al, "Efficiecy-imized cotrol of medium-size iductio motor drives', IEEE Trasactios o Idustry Applicatios, vol 37, No 6, pp , November/ December [] S M Yag, F C Li, "Loss-miimizatio cotrol of vector-cotrolled iductio motor drives", Joural of the Chiese Istitute of Egieers, vol 6, No, pp 37-45, 3 [] EI Epshtie, AC drives, Moscow, p 9, 98 [] S Lim, K Nam, "Loss-miimizig cotrol scheme for iductio motors", IEE Proc-Electr Power Appl, vol 5, No 4, pp , July 4 [3] VN Brodovsky, IS Ivaov, "Drives with frequecycurret cotrol, Eergy, Moscow, pp69, 975

Modeling and Simulation of Flux-Optimized Induction Motor Drive

Modeling and Simulation of Flux-Optimized Induction Motor Drive Research Journal of Applied Sciences, Engineering and Technology 2(6): 603-613, 2010 ISSN: 2040-7467 Maxwell Scientific Organization, 2010 Submitted Date: July 21, 2010 Accepted Date: August 20, 2010 Published