A Dual Stator Winding-Mixed Pole Brushless Synchronous Generator (Design, Performance Analysis & Modeling) M EL_SHANAWANY, SMR TAHOUN& M EZZAT Department (Electrical Engineering Department) University (Menoufya University) Address (Shebin El_Kom, Egypt) COUNTRY (EGYPT) en.ezzat@yahoo.com Abstract: - It is well known that critical loads are still excited either from conventional synchronous or induction generators. Conventional synchronous generators suffer from brushes and slip ring existence which reduce its reliability and increase the need for maintenance that conflict with the nature of these loads. Also, although induction generator is a brushless generator, it has several problems that still under research. This paper presents a new generator design suitable for these loads. A 3-phase conventional AC stator is used. Two rotor types, named salient pole and flux barrier rotors, are designed and used with this stator. The experimental results with these rotors are taken and compared with the theoretical ones. Key-Words: - Brushless Synchronous Generator, Dual Stator Winding, Mixed Pole, Modeling 1 Introduction Critical loads need a high reliable generation system. Conventional synchronous generators have brushes and slip rings in its rotor that increase its need for maintenance and of course reduce its reliability. Although the brushless version of synchronous generators has no brushes and slip rings, it has built in diodes in its rotor circuit and needs main and auxiliary exciters that increase its cost. Beside the increase in cost, the built in diodes are exposed to excessive heat so they need better cooling which adds to the cost. Induction generators are brushless generators, but they don't have the advantages of synchronous generators. So that there is a need for a reliable generator with the advantages of synchronous generators. The paper presents a new generator design which has both field and generation windings are wound in the stator side and its rotor has no windings or bars. wound with two sets of 3-phase windings. One of them is wound with 6-pole and the other one with 2- pole (Each winding has 55 conductor/slot). The 6- pole winding (Field winding) is connected as open delta with two reversed phases and the 2-pole winding (Generating winding) is connected as a star. Fig. (1) shows the used stator and winding arrangement. Winding arrangement 2 Machine Construction Construction of the proposed generator is the same construction as the brushless doubly fed reluctance machines (BDFRM) [1-8]. It consists of stator and rotor. The stator is made from silicon steel laminations in the same way of induction machine stator. Dual sets of three-phase windings with different pole numbers are wound in the slots in the same manner as in the self-cascaded induction machine [9]. In this paper, a standard stator of a 1hp 3-phase induction motor is used. This stator is A photo of the experimental stator Fig. (1) ISSN: 1792-5088 159 ISBN: 978-960-474-233-2
Different from the self cascaded induction machine, the rotor of the proposed generator is one of the reluctance types. In this paper two rotor types are used. One of them is a solid salient pole rotor with pole arc to pole pitch ratio equal to 0.5. The other rotor is a solid flux barrier rotor with pole arc to pole pitch ratio equal to 0.63 and each rib width equal to 1mm. These dimensions are chosen to achieve the highest possible generated voltage and both rotors are designed to fit the stator. Fig. (2) shows the used rotors. Flux density distribution at ө=0 Fig. (4) Salient pole rotor Flux barrier rotor Fig. (2) Flux Distribution at ө=60 o Fig. (5) 3 Theory of Operation Theory of operation of conventional synchronous machine depends on Faraday's law. Also, theory of operation of the proposed generator depends on the same law but in our case the field winding is wound on the stator side. Excitation of field winding results in stationary magnetic field in space. With rotor rotation the flux linking generating winding, also in the stator, varies with time which induces an emf in it. For more clearance in how the flux varies with rotor rotation, a finite element tool is used to show the flux variation. Fig s. (3 to 6) show the flux variation for two rotor positions only. Flux Distribution at ө=0 Fig. (3) Flux density distribution at ө=60 o Fig. (6) 4 Theoretical Analysis Voltage equations for the proposed generator can be written as follows: (1) (2) Where: *i F : is the field winding current (DC current). *i GABC : is the generating winding current (AC current). *r F : is the field winding resistance. *r G: is the generating winding resistance per phase. ISSN: 1792-5088 160 ISBN: 978-960-474-233-2
*V F : is the applied field voltage (DC voltage). *V GABC: is the generating winding terminal voltage (AC voltage). * : is the flux linking field winding. * : is the flux linking generating winding phases. To calculate the flux linkage, machine inductances must be calculated. The technique adopted here is the winding function analysis (WFA) technique. Inductances are calculated using the following formula that presented in [10]. (3) Fig s. (7&8) show the winding functions for a one phase of the 6-pole winding and a one phase of the 2-pole winding respectively. Inverse air gap function for salient pole rotor Fig. (9) Winding function of a 6-pole winding phase Fig. (7) Inverse air gap function for flux barrier rotor Fig. (10) Where: ad: is the inverse air gap function in the d-axis. aq: is the inverse air gap function in the q-axis. Based on equation (3), machine inductances are calculated using a MATLAB M-FILE and the following results are obtained. Winding function of a 2-pole winding phase Fig. (8) Where: q: is the slot per pole per phase divided by two. z sf : is the number of field conductors per slot. Z s : is the number of generation conductors per slot. Fig s. (9&10) show the inverse air gap function (g -1 ) adopted for salient pole and flux barrier rotors respectively. One can see that effect of slotting is taken into account for both air gap models. Mutual inductance with salient pole rotor Fig. (11) ISSN: 1792-5088 161 ISBN: 978-960-474-233-2
Mutual inductance with flux barrier rotor Fig. (12) From the above figures, it can be seen that mutual inductance obtained with the flux barrier rotor is higher than the obtained value with the salient pole one. So that it is expected that more voltage will be generated with the flux barrier rotor than the salient pole one. Self inductance of a 2-pole phase with the salient pole rotor Fig. (15) Self inductance of a 6-pole phase with the salient pole rotor Fig. (13) Self inductance of a 2-pole phase with the flux barrier rotor Fig. (16) One can see that self inductances either of 6-pole or 2-pole windings are not purely constant but they have AC components. From equations (1&2) and the calculated inductances a MATLAB SIMULINK model is built to compare the obtained results with the experimental ones (This will be done in the next section). Fig. (17) shows the SIMULINK model used. Field Currents 3-phase no-load Voltages In 1 Out1 In1 Out1 Field Voltage Out1 In 2 In 1Out2 Out3 In2 Out2 In3 Out3 Field Circuit Mutual between Generation and Field Generating Winding Self inductance of a 6-pole phase with the flux barrier rotor Fig. (14) In1 Out1In2 Phases Current In3 mutual between field and generating windings SIMULINK model Fig. (17) ISSN: 1792-5088 162 ISBN: 978-960-474-233-2
5 Results Fig s. (18&19) show the experimental and the simulated terminal voltage per phase for two phases at field current (I F ) of 2A and at 1500 r.p.m. Experimental terminal voltage for phases a&b (1500r.p.m,I F =2A,IL=0.5A, Salient pole rotor) Fig. (18a) Simulated terminal voltage for phases a&b (1500r.p.m, I F =2A,IL=0.78A, Flux barrier rotor) Fig. (19b) From the above figures, one can see that the generated phase voltages are displaced by approximately 120 o and its frequency is 100 Hz (Double the frequency obtained from the conventional synchronous generator at the same speed). Fig s. (20&21) show the experimental and the simulated no load voltage per phase. Simulated terminal voltage for phases a&b (1500r.p.m, I F =2A,IL=0.5A, Salient pole rotor) Fig. (18b) Experimental no load per phase voltage (1500r.p.m,I F =2A,IL=0A, Salient pole rotor) Fig. (20a) Experimental terminal voltage for phases a&b (1500r.p.m, I F =2A,IL=0.78A, Flux barrier rotor) Fig. (19a) Simulated no load per phase voltage (1500r.p.m,I F =2A,IL=0A, Salient pole rotor) Fig. (20b) ISSN: 1792-5088 163 ISBN: 978-960-474-233-2
Experimental no load per phase voltage (1500r.p.m,I F =2A,IL=0A, Flux barrier rotor) Fig. (21a) Simulated terminal voltage and phase current (1500r.p.m,I F =2A,IL=0.5A, Salient pole rotor) Fig. (22b) 100 V o ltag e in V o lts 50 0-50 -100 0.135 0.14 0.145 0.15 Time in seconds Simulated no load per phase voltage (1500r.p.m,I F =2A,IL=0A, Flux barrier rotor) Fig. (21b) Experimental terminal voltage and phase current (1500r.p.m,I F =2A,IL=0.78A, Flux barrier rotor) Fig. (23a) Fig s. (22&23) show the experimental and the simulated terminal voltage per phase and phase current at 2A and at 1500 r.p.m with resistive load. Simulated terminal voltage and phase current (1500r.p.m,I F =2A,IL=0.78A, Flux barrier rotor) Fig. (23b) Experimental terminal voltage and phase current (1500r.p.m,I F =2A,IL=0.5A, Salient pole rotor) Fig. (22a) From figures 20 to 23, one can find that the voltage waveforms approach the sinusoidal waveform with loading or it can be said that ripple voltages appear in no load is reduced with loading. ISSN: 1792-5088 164 ISBN: 978-960-474-233-2
Fig s. (24&25) show the steady state characteristics for both rotors. No load line voltage versus field current (1500r.p.m) Fig. (24) Terminal voltage per phase versus load current (IL) (1500r.p.m,I F =2A, Resistive load) Fig. (25) From the steady state characteristics, it can be shown that the flux barrier rotor gives a higher power than the salient pole type rotor. The low values of terminal voltages are due to the use of a standard stator. In conventional synchronous generator the field is wound in the rotor. If the field winding of the conventional generator is transferred to the stator side, there will be an expected increase in the stator slot depth and therefore high power can be obtained. 6 Conclusion A new brushless generator type suitable for critical loads has been presented. Experimental results on two rotor designs have been performed and the obtained results have been compared to the theoretical results and a good agreement has been achieved. The obtained performance gives a promise for a well design. This can be done in a next paper. References: [1] M. G. Javonovic, R. E. Betz, and J.Yu,The Use of Doubly Fed Reluctance Machines for Large Pumps and Wind Turbines, IEEE Transactions, vol. 3,Dec. 2002, PP. 1508-1516. [2] Y. Liao, L. Zhen and L. Xu, Design of a Doubly-Fed Reluctance Motor for Adjustable Speed Drives, Proceedings of the IAS Annual Meeting, Vol. 1, Oct. 1994, PP. 305-312. [3] G. Javonovic, A Comparative Study of Control Strategies for Performance Optimization of Brushless Doubly-Fed Reluctance Machines, J.Electrical Systems, vol.8, 2006, PP. 208-225. [4] R. Betz and M. Gavonovic, Control Aspects of Brushless Doubly Fed Reluctance Machines, Proceedings of the European Power Electronics Conference (EPE'99), Sept. 1999. [5] Feng Liang, Longya Xu and T.A, Lipo,D-Q Analysis of a Variable Speed Doubly AC Excited Motor, Electric Machines and Power Systems, Vol. 19, March 1991, PP. 125-138. [6] R. Betz and M. Gavonovic, Introduction to Brushless Doubly Fed Reluctance Machines- The Basic Equations, Tech. Rep. EE0023, Department of Electrical Engineering, University of Newcastle, Australia, April 1998. [7] R. E. Betz, M. G. Javonovic, Introduction to the Space Vector Modeling of the Brushless Doubly-Fed Reluctance Machine, Electric Power Components and Systems, vol.31, Aug. 2003pages, PP. 729-755. [8] E. M. Schulz and R. Betz, Optimal Rotor Design of Brushless Doubly Fed Reluctance Machines, IEEE Transactions, 2003, PP. 256-261. [9] A. R. W. Broadway, L. Burbridge, Self- Cascaded Machine: a Low Speed Motor or High Frequency Brushless Alternator, Proc. IEE, Vol. 117, July 1970, PP. 1277-1290. [10]Tang, Yifan, High Performance Variable Speed Drive System and Generating System with Doubly Fed Machines, PHD Thesis, The Ohio State University, 1994. ISSN: 1792-5088 165 ISBN: 978-960-474-233-2