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Nr 62 Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych Nr 62 Politechniki Wrocławskiej Studia i Materiały Nr 28 2008 electrical tram drives, field weakening, direct torque control with space vector modulation (DTC–SVM) Paweł WÓJCIK*, Dariusz SWIERCZYŃSKI**, Marian P. KAZMIERKOWSKI*, Michał JANASZEK*** DIRECT TORQUE CONTROLLED PWM INVERTER FED INDUCTION MOTOR DRIVE FOR CITY TRANSPORTATION In this paper an application of Direct Torque Control with Space Vector Modulation (DTC– SVM) controlled induction motor for tram drive is presented. Thanks to its advantages like: excellent dynamics, low torque ripples, insensitivity for motor parameters changes, constant switching and low sampling frequency, DTC–SVM is used in various applications. In proposed case DTC–SVM is used for tram traction drive based on PWM Voltage Sourced Inverter Fed Induction Machine. This method was chosen after comparison with Field Oriented Control (FOC), Switching Table Direct Torque Control (ST–DTC) and Direct Self Control (DSC). DTC–SVM combines advantages and eliminates drawbacks commonly used methods like FOC and ST–DTC. There are no hysteresis controllers, what gives possibility to reduce sampling and also switching frequency. It leads to reduce switching loses (important in high power applications). Constant switching frequency is ensured by using Space Vector Modulation strategy. In DTC–SVM linear PI regulators are used. Both stator flux and electromagnetic torque are controlled directly. High dynamics is achieved and also good stationary operation performance is kept. This advantages allow to implement DTC–SVM for traction drives. The paper presents parallel structure of DTC–SVM. Operating ranges, including field weakening region, are described. Some experimental results of the 75kW induction motor drive which illustrate its performance are attached. __________ * Warsaw University of Technology, Department of Electrical Engineering, Institute of Control and Industrial Electronics, Koszykowa 75, 00-662 Warsaw, Poland, [email protected], [email protected] ** Warsaw University of Life Science, Production Engineering Department, Nowoursynowska 164, 02-787 Warsaw, Poland, [email protected] *** Electrotechnical Institute, Department of Electric Machine–Tool Drives, Pozaryskiego 28, 04-703 Warsaw, Poland, [email protected] 356 1. INTRODUCTION Electrical traction drives for city transportation (tram, trolleybus, subway) have different requirements compared to conventional industrial drives. The main specifications are as follows: – robust and high starting torque, at zero stator frequency, – high dynamics of torque control to reduce torque in case of slip between wheel and rail, – wide speed operation range (0–150 Hz) including field weakening (65–150 Hz), – constant and low switching frequency to reduce inverter loses, – operation at wide range of DC link voltage changes (+/–50% of the rated value), – robust breaking in all situations including supply DC line voltage failure, – operation without mechanical motion sensor. Several control strategies have been used for induction motor drives: open loop V/Hz control [4], Field Oriented Control (FOC) [1], Direct Torque Control (DTC) [2], and Direct Self Control (DSC) [3]. The FOC method controls motor torque and flux indirectly using current loops and coordinate transformations. Therefore, FOC is more complex than DTC or DSC algorithms. However, DTC and DSC algorithms are based on hysteresis control which requires high sampling frequency. Moreover, inverter operates with variable switching frequency. In this work a DTC scheme with Space Vector Modulation (referred as DTC–SVM [5–10]) is applied for city tram induction motor drive 75 kW. The DTC–SVM scheme combines advantages of FOC system (constant switching frequency, SVM and PI controllers) and DTC algorithm (simple structure without current control loops and coordinate transformation, direct torque and flux control) and recently has been successfully used in industrial drives [11]. 2. DTC–SVM PRINCIPLES DTC–SVM can be implemented in various ways [6, 7, 8, 10]. The most reliable is parallel structure (see Fig. 1). This control scheme does not have any differentional algorithms, like in [6] or [7]. Instead of this in parallel structure of DTC–SVM reference voltage for space vector modulator is calculated by two linear PI regulators as two adjacent stator voltage components Usx and Usy. Usx is calculated by stator flux PI controller, whereas Usy is calculated by electromagnetic torque PI controller. Additionally, for high speed region, where stator flux magnitude has to be lowered, in parallel DTC–SVM structure field weakening algorithm can be applied easily. In DTC–SVM torque and flux are controlled directly. In traction torque control loop is used for speed regulation. Also field weakening algorithms are necessary for induc- 357 tion motor based traction drives. Therefore, presented algorithm is suitable for applications like: trams, trolleybuses, underground or fast city train. Fig. 1. DTC–SVM parallel structure Comparison between FOC, ST–DTC and DTC – SVM is shown in Table 1. Table 1. FOC, ST–DTC and DTC – SVM comparison FOC Advantages Disadvantages Modulator Constant switching frequency Unipolar inverter output voltage Low switching loses Low sampling frequency Linear PI controllers ST–DTC Coordinate transformation Current control loops Control structure depended on rotor parameters Structure independent on rotor parameters, universal for IM and PMSM Simple implementation of sensorless operation No coordinate transformation No current control loops No modulator Bipolar inverter output voltage Variable switching frequency High switching loses High sampling frequency DTC–SVM Structure independent on rotor parameters, universal for IM and PMSM Simple implementation of sensorless operation No coordinate transformation No current control loops Modulator Constant switching frequency Unipolar inverter output voltage Low switching loses Low sampling frequency Linear PI controllers 358 3. OPERATION IN FIELD WEAKENING REGION Induction machine operation range under nominal stator flux magnitude is limited. To achieve speed higher than base speed (maximum mechanical speed which is possible to achieve at maximum stator voltage magnitude and rated stator flux magnitude) flux weakening algorithm has to be applied. Tk T ωR ψS uS p Tk T ψS p uS ωR Sbase constant electromagnetic torque region constant power region Scritical ωS constant slip frequency region Fig. 2. Induction machine operation including high speed region Tk – breakdown torque, T – maximum torque, ΨS – stator flux magnitude, p – power, uS – stator voltage magnitude, ωR – slip angular speed Base speed is limited by DC link voltage, modulation strategy and rated stator flux. In constant torque region, the stator flux magnitude is constant. The simplest way of field weakening is stator flux magnitude reduction in inverse proportion to mechanical speed. This is known as 1/ω_r method. In flux weakening region the electromagnetic torque capability is decreased. In constant power region maximum torque is inverse proportional to mechanical speed. In constant slip frequency region maximum torque is equal to the critical torque, so is inverse proportional to square of the mechanical speed. It is shown in Fig. 2. Some results are shown in Fig. 3b. 359 4. EXPERIMENTAL RESULTS FOR 75 kW INDUCTION MOTOR Torque tracking performance is shown in Fig. 3. a) b) Fig. 3. Torque tracking performance: a) operation below base speed, b) operation at weaken flux a) From the top: reference torque 100 Nm/div, estimated torque 100 Nm/div, mechanical speed 500 rpm/div, phase current 200 A/div b) acceleration up to 900 rpm with 75 Nm (from the top: stator flux amplitude 0.4 Wb/div, mechanical speed 1000 rpm/div, estimated torque 100 Nm/div, phase current 200A/div) a) b) Fig. 4. Speed tracking (acceleration–steady state–speed reversal–steady state–deceleration): a) scalar control mode, b) DTC–SVM control mode From the top: mechanical speed 600 rpm/div, reference speed 600 rpm/div, phase current 200 A/div, estimated torque 100 Nm/div 360 Parameters of 75 kW induction motor used in this work are presented in Table 2. Table 2. Induction machine data PN IN fN ωN TN Usupply p 75 kW 160 A 68 Hz 2000 rpm 358 Nm 3 x 380 V 2 RS RR LS LR LM J 31 mΩ 31 mΩ 11 mH 11 mH 9 mH 15 kgm^2 Moment of inertia (J) consist not only inertia of the machine, but also additional inertia caused by one quarter weight of the tram. There are four motors in each tram (two motors per cart, and two carts per tram). All tests use this moment of inertia. 5. CONCLUSIONS In this paper DTC–SVM controlled PWM inverter–fed tram drive was presented. Induction motor chosen for the experiment is used in trams. Presented results confirm that DTC–SVM can operate in wide speed range including field weakening region, what is typical requirements for electrical traction drives. The drive performance in stationary and dynamic states is satisfactory at any speed. In high speed region because of stator flux magnitude decreasing also electromagnetic torque has to be limited. Electromagnetic torque is kept lower than breakdown torque. REFERENCES [1] BLASHKE F., The principle of fiels-orientation as applied to the Transvector closed-loop control system for rotating-field machines ’ in Siemens Reviev 34, 1972, pp. 217–220. [2] TAKAHASHI I., NOGUCHI T., A new quick-response and high efficiency control strategy of an induction machine ’ IEEE Trans. on Industrial Application, Vol. IA-22, no.5, Sept./Oct. 1986, pp. 820–827. [3] DEPENBROCK M., Direct Self Control of Inverter-Fed Induction Machines ‘ IEEE Trans. on Power Electronics, Vol. PE-3, no.4, Oct. 1988, pp. 420–429. [4] KAZMIERKOWSKI M. P., KRISHNAN R., BLAABJERG F., Control in Power Electronics Selected Problems ‘ Academic Press, 2002. [5] CASADEI D., SERRA G., TANI A., Constant frequency operation of a DTC induction motor drive for electric vehicle ‘ Proc. of ICEM Conf., Vol. 3, 1996, pp. 224–229. [6] SWIERCZYNSKI D., ZELECHOWSKI M., Universal structure of direct torque control for AC motor drives ‘ Przegląd Elektrotechniczny, No. 5/2004, pp. 489–492. [7] HOFFMAN F., JANECKE M., Fast Torque Control of an IGBT-Inverter-Fed Tree-Phase A.C. Drive in the Whole Speed Range – Experimental Result ‘ Proc. EPE Conf., 1995, pp. 3.399–3.404. 361 [8] XUE Y., XU X., HABETLER T. G., DIVAN D. M., A low cost stator flux oriented voltage source variable speed drive ‘ Conference Record of the 1990 IEEE Industry Applications Society Annual Meeting, Vol. 1, 7–12 Oct. 1990, pp. 410–415. [9] SWIERCZYNSKI D., ZELECHOWSKI M., Universal structure direct torque control for synchronous permanent magnet and asynchronous motors ‘ International XIII Symposium on Micromachines and Servodrives, Krasiczyn, Sep. 2002, pp. 333–340. (in Polish) [10] BUJA G. S., KAZMIERKOWSKI M. P., Direct Torque Control of PWM Inverter–Fed AC Motors – A Survey ‘ IEEE Transactions on Industrial Electronics, Vol. 51, Issue: 4, Aug. 2004, pp. 744–757. [11] Twerd Company, Torun, Poland (www.twerd.pl) NAPĘD FALOWNIKOWY Z SILNIKIEM INDUKCYJNYM STEROWANY METODĄ BEZPOŚREDNIEJ REGULACJI MOMENTU Z MODULACJĄ WEKTOROWĄ DLA TRANSPORTU MIEJSKIEGO W artykule przedstawiono zastosowanie metody Bezpośredniej Regulacji Momentu z Modulacją Wektorową (ang. Direct Torque Control with Space Vector Modulation DTC–SVM) dla napędów tramwajowych. Testowano typowy silnik trakcyjny o mocy 75 kW i momencie znamionowym 260 Nm. Przedstawione zostały wyniki badań w zarówno zakresie prędkości nominalnej, jak również w obszarze osłabiania strumienia. Zastosowanie metody DTC–SVM pozwala na bezpośrednią regulację momentu, redukcję tętnień prądu i momentu, a także zmniejszenie strat łączeniowych (w porównaniu z metodą bezpośredniej regulacji momentu z tablicą łączeń ST–DTC) poprzez utrzymanie stałej częstotliwości przełączeń tranzystorów. Ponadto, metoda DTC–SVM jest niewrażliwa na zmiany parametrów silnika, co daje jej przewagę nad metodami zorientowanymi polowo (ang. Field Oriented Control FOC). Dzięki swym właściwościom napęd tramwajowy stosujący metodę DTC–SVM spełnia wymagania stawiane nowoczesnym układom trakcyjnym.