TWO-PHASE INDUCTION MACHINE WITH TOROIDAL WINDING AND IMPROVED PERFORMANCE

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1 TWO-PHASE INDUCTION MACHINE WITH TOROIDAL WINDING AND IMPROVED PERFORMANCE TWO-PHASE INDUCTION MACHINE WITH TOROIDAL WINDING AND IMPROVED PERFORMANCE PhD Student Eng. Ana-Maria MIHAI 1, Prof. Eng. Alecsandru SIMION 1, PhD Eng. Sorin MIHAI 1, Assoc. Prof. Eng. Leonard LIVADARU 1, Assis. Prof. Eng. Adrian MUNTEANU PhD 1 1 Technical University of Iasi, Faculty of Electrical Engineering, REZUMAT. În această lucrare e se propune o solutie originală în scopul ameliorării performanţelor maşinii asincrone bifazate, prin eliminarea armonicii spatiale de ordinul trei din spectrul câmpului din întrefier. În acest scop, este utilizată tehnica înfăşurării în inel şi o metodă de repartizare a bobinelor în crestături. Simulările bazate pe MEF,, precum şi încercările experimentale demonstrează superioritatea acestei soluţii propuse asupra performanţelor maşinii bifazate "clasice", precum şi asupra maşinii de inducţie trifazate. Cuvinte cheie: proiectare, analiza MEF, înfăşurare toroidală, maşina de inducţie bifazată. ABSTRACT. This paper advances an original solution for the improvement of the two-phase induction machine performance by eliminating the third order harmonic c from the air-gap magnetic field spectrum. For this purpose, toroidal coils are used and a special placement in the stator slots is designed. The fem based simulations and the experimental tests prove the superiority of this solution over the classic two-phase machine performance as well as over the three-phase machine. Keywords: design, FEM analysis, toroidal winding, two-phase induction machine. 1. INTRODUCTION Although considered classic, the induction machine continues to be the main electro-mechanic conversion system, used in almost all domains, due to its advantages against the other electrical machines: reduced complexity; reduced production price; high safety during exploitation; high technical performances (high starting torque, high efficiency); stability in operation, exploitation, maneuvering and basic maintenance. In addition, a main characteristic of the actual development stage in the fabrication of the electrical machines is the permanent enlargement of their application domains and as an immediate consequence, the growth in the number of electrical machines types [7]-[11]. Considering that the two-phase induction machine offers higher dynamic characteristics due to the elimination of the magnetic coupling between phases, it can be successfully used in high speed electric drives. Thereby, for particular applications, when the twophase induction machine can be a better solution in exploitation, the execution of a superior model is preferable [9], [14]. 2. BASICS ON TWO-PHASE MAGNETIC FIELD The magnetic field created by the fundamental: The two-phase induction machine is the machine with a minimal number of windings that works based on the rotating field. It has two stator windings, 9 electrical degrees shifted in space [9], [14]. The two symmetrical currents corresponding to the phases (A-X and B-Y), create an air-gap magnetic field with the general expression: Bδ rez ( α, t) = Bδ AX ( α, t) + Bδ BY ( α, t) = (1) B1max sin( ωt pα) The shape of the air-gap field is trapezoidal (Fig. 1.a) which determines, actually, the presence of some odd harmonics of higher order besides the fundamental. The most significant is the third order harmonic which contributes to the performance decrease of the twophase machine compared to the three-phase one [14]. The performance developed by the two-phase induction machine is always inferior to the three-phase counterpart. The main reason is the presence of the 3rd order harmonic that creates a backward field and respectively, an opposite torque. Among the known attenuation methods of the high order harmonics are: the sinusoidal stator winding method, the shorted-pitch winding method and the method which involves a special positioning of the coils in the slots. The proposed solution in this paper refers to an original coils assignation method which takes into account the attenuation of the third order harmonic. It Buletinul AGIR nr. 4/212 octombrie-decembrie 1 137

2 CONFERINŢA NATIONAL NAŢIONALĂ CONFERENCE DE ACŢIONĂRI OF ELECTRICAL ELECTRICE, DRIVES ediţia XVI, CNAE SUCEAVA consists of doubling the number of coils from certain slots and therefore the change of the number of slots per pole and per phase. For example, for the presented model in this paper, it is changed from q=6 to q=8 slots per pole and per phase, according to the principle presented in Fig. 1.b, Fig. 2 and Fig. 3.b. To be more specific, the method consists in doubling the coils positioned in the corners of a square, as it can be seen in Fig. 1.c. A-X B-Y Z 1=24 A-X B-Y Z * 1 =32 Z 1= a. classic two-phase winding b. original optimized two-phase winding c. cross section Fig. 1. Air-gap flux density wave of a symmetrical two-phase induction machine with 24 slots A B X Y B X Y A a. classic two-phase winding b. original optimized two-phase winding Fig. 2. The two-phase toroidal winding scheme for 2p=2 Since the principle consists in doubling the coils which are placed in the corners of a square, the winding will be denoted as two-phase quadratic winding. Table 1 a. b. Fig. 3. The explanatory of the original solution, a) classic twophase winding, b) original optimized two-phase winding A very important element in making this modification is the winding type. Usually, the induction machines have drum stator windings. In order to obtain the proposed solution, it is necessary to use a toroidal stator winding (individual ring coils placed in each stator slot), Fig. 3. This solution can be applied also for the drum winding, but the method is valid only for one pair of poles windings (2p=2), and respectively for certain numbers of stator slots. Table 1 shows the number of stator slots for which the proposed solution is applicable, respectively the number of slots per pole and per phase (q) are shown, before and after applying the mentioned procedure of coil distribution (q*). Number of stator slots for which the proposed solution is applicable Z q q * Z 1 * * the total number of the effective slots (Z 1* ), the number of the slots per pole and per phase (q 1*), after applying the technique of coil distribution; 3. FEM-BASED ANALYSIS Constructive features of the induction motor: The FEM study has had as main purpose the analysis of the air-gap flux density in order to put in view the influence of the proposed solution on the content in high order harmonics. As consequence, magneto-dynamic analyses have been performed for the 2 138

3 TWO-PHASE INDUCTION M MACHINE WITH TOROIDAL WINDING AND IMPROVED PERFORMANCE three situations:, two-phase classic winding and two-phase quadratic winding. Fig. 4. Induction motor with toroidal winding The construction of the analyzed induction motor with toroidal winding is presented in Fig. 4, and Table 2 shows the main geometrical parameters. Inner stator diameter, D i Outer stator diameter, D e Rotor diameter, D r Main Geometrical Parameters Length of the magnetic circuit, l 7 mm 134 mm 69,4 mm 7 mm Number of stator slots, Z 1 24 Number of rotor slots, Z 2 18 Number of turns of ring coils 37 Table 2 Results obtained from simulation One of the first results of the simulation offers the visualization possibility of the current densities established in the conductors, respectively the flux density repartition in different constructive elements of the machine (Fig. 5). In order to obtain truthful results, the current density corresponding to the stator turns has been maintained constant (8A/mm 2 ). Fig. 6 presents in a comparative way the air-gap flux density waves and Fourier analysis corresponding to rated operation, for the three situations: three-phase winding, two-phase classic winding and two-phase quadratic winding. Table 3 presents the amplitude of the main harmonics for this the three situations. The results reveal two major conclusions: the fundamental is higher in value in comparison with the two-phase classic winding but as well as with the three-phase one. At a first sight, this is a questionable result. The explanation derives from the structure of the new winding: it has 8 more ring coils placed in the corner slots and consequently the field is stronger; the 3rd order harmonic has a very small value, practically it is eliminated. Of great importance in performance evaluation is the torque-slip characteristic, Fig. 7.a. As it can be seen, the two-phase quadratic solution is superior both on startup and breakdown values. Harmonic order Air-Gap Flux Density Main Harmonics Threephase winding Two-phase classic winding Table 3 Two-phase quadratic winding Fundamental,718 T,76 T,77 T 3rd order,5 T,98 T,14 T 5th order,61 T,56 T,57 T 7th order,29 T,2 T,27 T a) Current density in conductors b) Flux density color map Fig. 5. Current density in conductors and the flux density color map, s=,5 Buletinul AGIR nr. 4/212 octombrie-decembrie 3 139

4 CONFERINŢA NATIONAL NAŢIONALĂ CONFERENCE DE ACŢIONĂRI OF ELECTRICAL ELECTRICE, DRIVES ediţia XVI, CNAE SUCEAVA mm -1 mm -1 mm a) b) two-phase classic winding c) two-phase quadratic winding Fig. 6. Air-gap flux density wave and Fourier analysis corresponding to rated operation 5. output torque, T 2 [Nm] a) Simulation 4. EXPERIMENTAL RESULTS Fig. 7. Torque versus slip characteristics slip, s b) Experimental results The current variation with the load for the three situations is shown in Fig. 9. To achieve this experimental model, we started with a real machine having the following parameters: number of poles 2p=2, rated output power PN=,55kW, rated supply line voltage UN=38 V, star connection, rated current IN=1.6 A, rated frequency f=5/6 Hz, rated rotor speed n=285/346 rot/min. The existing drum winding was replaced with a toroidal one. The final form of the model used for the experimental tests and for drawing the operating characteristics is presented in Fig. 8. The experimental tests have been performed on a bench by monitoring electrical and mechanic quantities, with analog and virtual instrumentation (data acquisition card (DAQ) and the Lab-VIEW environment). It should be noted that the comparative analysis was performed for approximately equal current densities. Fig. 8. Modified induction motor with toroidal winding experimental model Fig. 1 presents the input power for different load torque values. The characteristics prove that the quadratic twophase winding has a superior performance as regards both output torque and efficiency. For example, the 4 14

5 TWO-PHASE INDUCTION M MACHINE WITH TOROIDAL WINDING AND IMPROVED PERFORMANCE difference in efficiency is of 3%, (ηqbif=.75, ηtrif=.72), which is significant (Fig. 11). Current, I f [A] output torque, T 2 [Nm] output torque, T2 [Nm] Fig. 9. The current variation with output torque input power, P 1 [W] Fig. 1. Variation of input power with the output torque T2=f(P1) efficiency, η [%] output torque, T 2 [Nm] Fig. 11. Efficiency characteristics The mechanical characteristics obtained experimentally (Fig. 7.b) shows that: - the experimental characteristics are extremely close to the ones obtained in the simulation, with the observation that, in the experimental tests, the mechanic characteristics means output torque and in simulation, the electromagnetic torque. - the use of the original solution proposed in order to improve the two-phase induction machines performance is justified by the fact that the values of the maximum torque are obtained on identical energetic performances or even superior to the three-phase induction machines. 5. CONCLUSIONS The proposed solution to improve the two-phase induction machine performance is justified by the fact that the third order harmonic is totally eliminated both for rated operation and for high slip values (starting situation). It should not be overlooked the fact that the solution with shorted-pitch leads to a pronounced decrease of the fundamental. In addition, the proposed original quadratic solution offers a dual influence upon the two-phase induction machine behavior: practically annihilate the third order harmonic and determine an increasing of the fundamental amplitude. With the transition from q=6 to q=8, the number of the effective slots is increased from Z1=24 to Z1*=32, which means that the proposed solution for improving the two-phase induction machine performance allows a voltage growth with an immediate effect in the growth of the pull-out torque values. BIBLIOGRAPHY [1] Nakamura, R., Kamyia, K., Chiba, A., Asama, J., Fukao, T., Stator Design of a Multi-Consequent-Pole Bearingless Motor with Toroidal Winding. Proc. 29 IEEE Energy Conversion Congress and Exposition, pp [2] Mirza, N. I., Toroidal stator winding through computer controlled equipment. Proc Electrical Electronics Insulation Conf. and Electrical Manufacturing & Coil Winding Conf., pp [3] Ji-Young Lee, Byoung-Kuk Lee, Jung-Jong Lee, Jung- Pyo Hong, A Comparative Study of Switched Reluctance Motor with Conventional and Toroidal Winding. Proc. 25 IEEE Int. Conf. on Electric Machines and Drives, pp [4] Tao Sun, Ji-Young Lee, Jung-Pyo Hong, Magnetic Field Analysis using Magnetic Equivalent Circuit for the Initial Design of Toroidal Winding Switched Reluctance Motor. Proc. of the 8th Int. Conf. on Electric Machines and Systems, vol. 3, pp , Sept. 25. [5] Ferreira, F.J.T.E., Cistelecan, M.V., Almeida, A.T., Baoming, G., Simple strategy to recovery energy during stopping period in large high-inertia line-fed induction motor driven systems. Proc. of the 18th Conference on Buletinul AGIR nr. 4/212 octombrie-decembrie 5 141

6 CONFERINŢA NATIONAL NAŢIONALĂ CONFERENCE DE ACŢIONĂRI OF ELECTRICAL ELECTRICE, DRIVES ediţia XVI, CNAE SUCEAVA Electrical Machines, Vilamoura, Portugal, pp. 1-6, 6-9 Sept. 28. [6] Stefanovic, V., Miller, J.M., Toroidally Wound Induction Motor-Generator with Selectable Number of Poles and Vector Control. US Patent 6,876,176 B2, Apr.5, 25. [7] Pyrhönen, J., Jokien, T., Hrabovcová, V., Design of rotating electrical machines, Ed. John Wiley & Sons, 28. [8] Gieras, J. F., Advancements in Electric Machines, Ed. Springer Verlag. Rockford, Illinois, U.S.A., March 28, ISBN [9] Boldea I., Nasar, S.A., The Induction Machine HandBook. CRC Press LLC, Boca Raton, London, New York, Washington, 22. [1] Popescu, M., Miller, T., McGilp, M., Rasmussen C.B., Effect of MMF Harmonics on Single-Phase Induction Motor Performance A Unified Approach. Industry Applications Conference, 27 IEEE, pp , Sept. 27. [11] Kocabas, D.A., Novel Winding and core design for maximum reduction of harmonic magnetomotive force in AC motors, IEEE Trans. Magnetics, Vol. 45, No. 2, pp , Feb. 29. [12] Dongsen, S., Baoming Ge, Daqiang, Bi., Winding design for polephase modification of induction machines. IEEE Energy Conversion Congress and Exposition. Atlanta, pp , Sept. 21. [13] Mihai, S., Simion, A., Livadaru, L., Munteanu, A., Induction Motor with Switchable Number of Poles and Toroidal Winding. Advances in Electrical and Computer Engineering, vol.11, pp , 211. [14] Simion, Al., Electrical Machines - Synchronous Machine. vol. 2. Iaşi, Ed. Gh. Asachi, 23, p.152. About the authors PhD Student, Ana-Maria MIHAI, m_ana1985@yahoo.com Received the B.Sc. and M.Sc. degrees in electrical engineering from the Technical University of Iaşi, Romania, in 29 and 21, respectively. She is currently a Ph.D. student in the Electrical Engineering domain, at the same university. Her main research interest is design and simulation of electrical machines. Professor Alecsandru SIMION, asimion@iota.ee.tuiasi.ro Received the B.Sc. and Ph.D. degrees in electrical engineering from the Technical University of Iaşi, Romania, in 1968 and 1976, respectively. He is currently a Professor with the Department of Electrical Machines at Electrical Engineering Faculty from the Technical University of Iaşi, Romania. He has published over 23 papers in conference proceedings and 1 books. His technical interests are electric machines and drives, simulation and design. He is the holder of 12 patents. PhD. Sorin MIHAI, mihai_sorin_d@yahoo.com Received the B.Sc. and Ph.D. degrees in electrical engineering from the Technical University of Iaşi, Romania, in 27 and 21, respectively. He is currently as an energetic engineering at a Romanian Company. His area of concentration is energy audit, designing and execution of low and medium electrical voltage network, complex energy balance, consulting and technical support for data analysis centralized by energy management applications and utilities. Assoc. Prof. Eng. Leonard LIVADARU, PhD., livadaru@ee.tuiasi.ro Received the B.Sc. and Ph.D. degrees in electrical engineering from the Technical University of Iasi, Romania, in 1985 and 23, respectively. He is currently Associate Professor with the Department of Electrical Machines at Electrical Engineering Faculty from the Technical University of Iasi, Romania. He has published over 14 papers in conference proceedings and 5 books. His technical interests are electric machines, simulation, design and optimization based FEM. Assis. Prof. Eng. Adrian MUNTEANU, amunteanu@ee.tuiasi.ro Received the B.Sc. and Ph.D. degrees in electrical engineering from the Technical University of Iaşi, Romania, in 24 and 28, respectively. He is currently an Assistant Professor with the Department of Electrical Machines at Electrical Engineering Faculty from the Technical University of Iaşi, Romania. He has published over 2 papers in conference proceedings. His technical interests are electric machines, simulation, design and optimization based on finite element method