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Nr 62 Prace Naukowe Instytutu Maszyn, Napędów i Pomiarów Elektrycznych Politechniki Wrocławskiej Nr 62 Studia i Materiały Nr 28 2008 switched reluctance motor , rolling rotor, simulation model, laboratory tests Antero ARKKIO* Adam BIERNAT**, Bogdan BUCKI**, Grzegorz KAMIŃSKI**, Andrzej SMAK**, Paweł STASZEWSKI** SIMULATION MODEL AND LABORATORY TESTS OF SWITCHED RELUCTANCE MOTOR WITH ROLLING ROTOR In the report measurements of Rolling Rotor Switched Reluctance Motor (RRSRM) laboratory model for large torque/small speed applications are presented. Laboratory model shortly described is constructed on base of the best design determined after calculations of several different motor constructions. The static torque, no-load, load and thermal characteristics of prototype motor are presented. Laboratory measurements are verified by simulated characteristics. 1. INTRODUCTION Rolling rotor electrical machines [5] based on reluctance principles can be considered as a volume saving alternative [4] to conventional frequency converter-induction motor-gear box torque delivering system. The lack of excitation in the rotor gives the flexibility in adaptation of motor shape to specific requirements. Since the torque produced depends on the current and the actual rotor position motor operation requires strict phase excitation strategy. Thus no load and load tests are crucial to evaluate RRSRM performance while parallel investigation based on simulation model allows for easy comparison of different control strategies and to determine one that best suit peculiar application requirements. 2. LABORATORY MODEL OF RRSRM No load and load tests were carried over RRSRM model [11] which construction is presented on Fig. 1. Motor stator contains 5 salient poles with concentrated winding. __________ * Helsinki University of Technology, Otakaari 5a str., 02015 Espoo, FI, [email protected] ** Warsaw University of Technology, pl. Politechniki 1, 00-661 Warszawa, PL, [email protected] 53 Fig. 1. Construction of tested RRSRM. 1 – stator, 2 – concentrated winding, 3– rotor iron, 4 – torque transmitting “fork” system; one sided crankshaft system is invisible. Rys. 1. Konstrukcja badanego RRSRM. 1 – stojan, 2 – uzwojenie koncentryczne, 3– rdzeń wirnika, 4 – system “fork” przeniesienia momentu; jednostronny system „crankshaft” jest niewidoczny. Rotor made of ring iron sheets, mounted on bearings of one sided crankshaft system is able to perform rolling movement on smooth internal stator surface filled with epoxy material. Non-slippery rotation [7] is secured by special rubber rings attached to rotor surface. Sequential activation of motor phase winding leads to development of reluctance type force due to minimization of the energy stored in the air gap. Force developing between rotor and stator extorts successively rotor roll and crankshaft rotation. Gear factor depends on crankshaft eccentricity ε and rotor diameter dr as follows: gf dr (1) For the test purpose high torque produced is transmitted outside with help of simple “fork” type mechanical system (chosen point of rotor performs movement of hypocycloidal shape [8, 9]). In the prototype simple one key per phase drive system is adapted as shown on Fig.2. Code disc mounted on crankshaft end together with sensor works as an absolute shaft position encoder. Mechanical mounting secures change of sensor position during motor operation resulting in change of phase “switch on” angle. RRSRM is a torque delivering machine, operating with low rotational speed [10], thus simple single switch system was adapted. Low switching frequency makes possible to use freewheeling phase current to produce useful torque for adequate switch “on” angle. 3. SYMULATION MODEL OF RRSRM Electrical equation of the machine can be described as follows: (i, ) di (i, ) d Ri u (t ) , i dt x dt (2) 54 (i, ) di di Ld (i, ) – transformation voltage in the phase winding, i dt dt (i, ) d d k E (i, x) – rotation voltage in the phase winding, Ψ – flux dt dt were: linkage, – angle of rotation, R – phase winding resistance, u(t) – supply voltage. Fig. 2. Drive system adapted for test of RRSRM Rys. 2. Układ napędowy badanego silnika RRSRM Fig. 3. RRSRM phase inductance as a function of rotor angle position and phase current Rys. 3. Indukcyjność fazy silnika w zależności od położenia kątowego wirnika i prądu fazowego Torque produced in the machine can be calculated as follow: i (i, )di 0 M e (i, ) . (3) In consequence to be able to simulatemachine behavior in dynamic state it is necessary to know the flux for all rotor positions and for all current densities. In case of laboratory model both measurements of flux or phase inductance as a function of rotor angle and phase current can be performed. A set of characteristics of phase inductance is presented on Fig. 3. 55 Due to the specific motor construction typical mechanical equation should be modified [2] by adding gravity influence if rotor is unbalanced and noticeable preload linked with anti-slip rubber rings and possible inaccurate crankshaft mechanical fitting: J d D M g ( ) M 0 M f ( ) M e (i, ) dt (4) were: D – rubber ring dumping coefficient, M g mg cos(t g ) – torque of gravity force origin, m – unbalanced mass, g – gravity factor, g –gravity force phase shift), M0 – preload linked with rubber rings compression, M f M f max (1 cos(t f ) – torque of inaccurate crankshaft fitting origin. 4. MEASUREMENTS Experimental results are limited to the only one type of phase switch “on/off” strategy, and one type of torque transmission system due to hardware limitations. Having simulation results compared with measurements and thus satisfactory level of similarity confirmed for extended testing of RRSRM behavior simulation model can be used [3]. The exemplary measurement results are presented below. Fig. 4. Static synchronous torque of RRSRM Rys. 4. Moment synchronizujący RRSRM Measurement of static synchronous torque on the crankshaft end was preformed for 1st and 3rd phase for two values of current – 1.6 A, and 2 A. Results of measurements are presented on Fig. 4. Differences result of inaccurate crankshaft mechanical fitting. Synchronous static torque produced on torque end of the shaft can be calculated based on transmission factor gf of RRSRM assuming non-slippery roll of rotor. Measurement of static synchronous torque on the crankshaft end was preformed for 1st and 3rd phase for two values of current – 1.6 A, and 2 A. Results of measurements are presented on Fig. 4. Differences result of inaccurate crankshaft mechan- 56 ical fitting. Synchronous static torque produced on torque end of the shaft can be calculated based on transmission factor gf of RRSRM assuming non-slippery roll of rotor. Fig. 5. Experimental dependence of rotational frequency of crankshaft end and motor current on supply voltage in respect to switch “on” angle for 72 deg switch “on” span Rys. 5. Zależność częstotliwości obrotowej wału i prądu od napięcia zasilania w funkcji kąta załączenia pasma dla kąta przewodzenia 72 Fig. 6. Trajectory of electromagnetic torque projected on electromagnetic torque plane captured for 5 subsequent crankshaft full 360 deg rotations Rys. 6. Trajektoria pracy silnika na powierzchni odzwierciedlającej moment elektromagnetyczny zarejestrowana podczas symulacji dla 5 pełnych obrotów wału Fig. 7. Temperature rise of phase 3 winding during load test. Thermal sensors placed in the frontal and groove part of winding Rys. 7. Przyrost temperatury uzwojenia 3 pasma fazowego podczas obciążenia. Czujnik temperatury został umieszczone na dnie żłobka i w czole uzwojenia No load tests were performed for three different values of supplying voltage (20 V, 25 V and 30 V). Exemplary results of measurements of rotational frequency of crankshaft end and mean value of motor current for fixed angular “switch on” span and different supply voltages are presented on Fig. 5. Due to high torque ripples for the well advanced and delayed switch “on” angles (close to –30 deg and +40 deg) witch can be regarded as a torque pulses swaying of shaft rotational speed can be noticed 57 both during experiment and simulation. Additionally a phenomenon of shaft sway of electromechanical resonant nature can be observed. Simulated trajectory of electromagnetic torque produced in one chosen phase during several full 360 deg rotations of crankshaft end is shown on Fig. 6. Different trajectory path for each subsequent full shaft rotation evokes chaotic behavior. During tests the motor was cooled only in natural way, no forced cooling in any kind was adapted. Thermal test was performed for 9.5 Nm load, angular switch “on” span 72, supply voltage 30 V. Test results are presented on Fig. 7. 5. CONCLUSIONS Analysis of measurements allows checking correctness of motor design and formulating several remarks that help to improve motor construction: a) over sized winding grove cross section (linked with assumed current density) resulting in comparatively low winding temperature gives the room for making the stator and frame motor thinner; b) to avoid appearance of torque originating in inaccurate crankshaft fitting demands of very high quality of mechanical treatment should be secured; c) application of modified system of torque reception from rolling rotor is necessary to assure smooth shaft rotation. d) simple “one key per phase drive system” impose supply voltage level/switch “on” phase control strategy for lowest torque ripple level operation; e) in case of battery operation requirements and torque ripples permissible the switch “on” span control strategy is recommended. The satisfactory similarity between simulation results laboratory measurements allows using the simulation model for testing different control strategies [1, 6] and motor dynamic behavior forced by operation cycles and load change. REFERENCES [1] BIERNAT A., Regulacja momentu w silniku reluktancyjnym z toczącym się wirnikiem. [Torque control of Rolling Rotor Switched Reluctance Motor]. International Symposium on Electrical Machines – SME, 2003, p. 297–305 [2] BIERNAT A., Identyfikacja wpływu parametrów węzłów mechanicznych na dynamikę przełączalnego silnika reluktancyjnego z toczącym się wirnikiem. [Identification of mechanical nodes parameters influence on dynamics of Rolling Rotor Switched Reluctance Motor]. Prace Naukowe Politechniki Warszawskiej, rok 2001, Elektryka z. 116, pp. 49–60 [3] BIERNAT A., KAMIŃSKI G., Dynamika przełączalnego silnika reluktancyjnego z toczącym się wirnikiem, Prace Naukowe Politechniki Warszawskiej, 2001, Elektryka Nr. 117, p. 5–16 [4] BORZIAK J.G., ZAJKOW M.A, NANII W. P., Elektrodwigatieli s katiaszczimsia rotorom, Kijew Tehnika, 1983; [5] KAMIŃSKI G., Silniki elektryczne z toczącymi się wirnikami, WPW 2003 58 [6] REINERT J., ENSLIN J.H.R., SMITH E.D. Digital Control and Optimization of a Rolling Rotor Switched Reluctance Machine, IEEE Transactions on Industry Applications, Vol. 31, No. 2, March/April 1995 [7] SCHON R., Elektrische Wälzmaschinen, Elektrotechnik und Maschinenbau. No3, 1961 [8] VIVIANI A., Experimental and theoretical study of hypocycloidal motors with two-harmonic field windings, IEEE Transactions on Power Systems, 1980, p. 292–300 [9] WILLIAMSON A.C., SPOONER E., BELARBI M., Improvements in hypocycloidal machines, Proceedings of the International Conference on Electrical Machines, Pisa, Italy, Sept. 12–14, 1988, Vol. 3, p. 351–356 [10] Wójcik M., Silniki wolnobieżne o wirniku toczącym się, Przegląd Elektrotechniczny nr 2, 1978 [11] Rolling-rotor electrical machine, Grant N0 40125/06 conducted in collaboration by Helsinki University of Technology and Warsaw University of Technology, sponsored by finish Academy of Technology Sciences TEKES, WARTSILA Oy and ABB/Finland, 2005–2007