THREE-PHASE AC CHOPPER SUPPLIED FOR INDUCTION MOTOR

Transcription

1 International Journal of Engineering Inventions e-issn: , p-issn: Volue 7, Issue 8 [August 018] PP: THREE-PHASE AC CHOPPER SUPPIED FOR INDUCTION MOTOR Mohaed Shaban Electric & Electronic Dept., High Electric Institute, Suran, IBYA. ABSTRACT: In the paper the results of siulation researches of the induction otor, which is supplied fro three phase ac chopper at ultiple conducting of the power switches, are presented. In the achine odel the stator circuits are presented in natural coordinates, while the rotor cage is replaced the circuits in α β syste. In the researches the professional progras such as Matlab-Siulink, have been used. I. INTRODUCTION The ac chopper is the power electronic converter, which is included between the ains and the load. In such circuit the setting of the rs output voltage is possible by controlling of conduction tie of the power switches creating ac chopper. The ac choppers at single-tie and at ultiple conducting of the power switches in period of the supply voltage are differentiated. In the ac chopper at single-tie conducting of the power switches the switching frequency is equal the frequency of the ains while in the ac chopper at ultiple conducting of the power switches the switching frequency is uch higher than the frequency of the ains. Nowadays, ac choppers, in which thyristor SCR fulfill a role of the power switches, are used. In such Circuits the rs output voltage is setting by change thyristor firing angle; inial firing angle depends on the power factor cosϕ. Such ac choppers are usually eployed such as light control, heater control, and soft start. Properties of the thyristor ac chopper are already known. Inforation s about this circuit can find in any publications []. In second solution of the ac choppers the power switches full controlling (power) transistor, thyristor GTO, thyristor SCR with a switch off circuit) are used. At ultiple conduction each power switches creating ac chopper are turn on and off at frequency uch higher than the frequency of the ains. The setting of the rs output voltage is possible e.g. by changing conduction tie of the power switches at constant the switching frequency (change of chopper duty cycle). Thanks to such work lower haronic contents wavefor output voltage is obtained thus there is better circuit power factor. This solution of ac chopper, thanks own advantages, gives new possibilities using the ac choppers in driving circuits with the inductive achine in softstart. II. THREE-PHASE AC CHOPPER Three-phase ac choppers are analyzed basing on single-phase ac chopper. This paper describes syetrically three-phase ac chopper at ultiple conducting of the power switches (Fig.1). Three-phase ac chopper consists of three single-phase circuits; every phase of ac chopper contains two power switches (S1, S Fig.1). Each ac switch is a pair of inverse-parallel connected power transistors and diodes. Power switches are treated as ideal, i.e. during conducting state switches resistance equal zero (voltage drop in conducting switches is oitted) and in non-conducting state switches resistance tends to infinity. The switching frequency can be any, but ost often it is a ultiplicity of frequency of the supply voltage. Thanks to such work lower haronic contents wavefor output voltage is obtained [1]. The relation of switching frequency and frequency of the supply voltage is defined as (Fig.). T f P n (1) TP f Where: T Period of the supply voltage, T P Switching period, f Frequency of the supply voltage, f P switching frequency. 1 Page

2 Fig. 1 Three-phase ac chopper In switches S1, S are presented. The rs output voltage can be controlled by changing of chopper duty cycle, which is defined as Fig. wavefors of the phase output voltage u SA (t) and the switching algoriths of the power switches S1, S are presented. The rs output voltage can be controlled by changing of chopper duty cycle, which is defined as: t T on P For switching algoriths of the power switches S1, S, which is shown in Fig., one can prove [1] that the relative rs phase output voltage in ac chopper fro Fig.1 is defined as: U RMS U RMS ( r) () U z () Where: U aplitude of the supply voltage. z Fig. Wavefor of voltage in the circuit fro Fig.1 at ultiple conducting of the power switches at switching frequency f =500 Hz and duty cycle =0, 75. p Page

3 III. MATHEMATICA MODE OF THE INDUCTION MOTOR In siulation researches the atheatical odel of the induction achine, which schea is represented in Fig., is used. In the odel is accepted following assuptions: - Syetrical stator and rotor windings, - Monochroic electroagnetic field distribution, - inear saturation curve, - Oission of iron losses. In case of supplying of the induction achine fro circuit, which contains the power electronic converter, cofortably is to use the atheatical odel of achine, in which the stator is presented by natural co-ordinates as well as the rotor is presented by transfored co-ordinates. Such solution enables supplying the achine defored voltage and replaceent of the differential equations, which describe currents of the rotor bar cage, suppleentary circuits in axis. Fig. Matheatical odel of the induction otor. For the atheatical odel of the inductive otor as Fig. and for coordinates ( w 0), equations x describing the odel can be expressed: (4) Page

4 Where vector of the flux linkage is defined as: (5) And axiu values of inductances are defined as: s sr (6) IV. SIMUATION MODE AND RESUTS OF SIMUATION The siulation odel of the inductive otor supplied fro three-phase ac chopper at ultiple conducting of the power switches (Fig.1) in the for of Siulink block diagra is presented in Fig.4. In such odel the equations of achine (4)... (6) and the otion equation are used. The otion equation is defined as: dw p ( T e T ) (7) dt j The torque equation of a onoharonic inductive achine can be written as []: T e e s r p R ji I (8) The stator current space vector Is can calculate basing the instantaneous stator phase currents while the rotor current space vector Ir can calculate basing the axis coponents of the rotor current: I s ( isa a isb ai sc ) (9) I r I ji (10) r r The paper presented results of siulations for inductive achine: P 4kw U n sn 80, n 1450rp / in, cos n 0. 8 n I sn 6. 7A And paraeters: R T s.1, 0.5, 0N. R r 1.5, j 0.5kg. s r H 4 Page

5 Fig.4 Siulation odel in the for of Siulink block diagra. In the siulation odel (Fig.4), the inductive otor represents the Mat lab s-function (nae sil_1) basing the equations (4) and (7) for angle 0 (Fig.). The power switches of three-phase ac chopper are turn on and off by the transistor firing syste (UWT) depending on chopper duty cycle. The power switches represent eleent Switch of the Mat lab library Nonlinear [1]. Such solution perits to obtain wavefor output voltage as Fig.. The siulation researches carried out for range of duty cycle changing, i.e. for range 0 1. The wavefors of the stator current i sa and the torque T e during otor start up at duty cycle 1 are shown in Fig.5a. The wavefors of the stator currenti sa, the torque T e and the otor speed as well as the torquespeed characteristic during otor start up at switching frequency f p 500Hz and duty cycle are shown in Fig.5b. Fig.5 Siulation results: a) wavefors of the stator current sa i and the electroagnetic torque e T during otor start up at duty cycle 1) b) Wavefors of the stator current sa i, the electroagnetic torque e T, the otor rotational speed during otor start up at switching frequency duty cycle f p 500Hz, and 5 Page

6 The stator current beside first haronic contents high haronic. In Fig.6 the instantaneous steady state stator current and the results of FFT (Fast Fourier Transforation) carried out with the use of Mathcad progra are shown. In wavefor of the stator current occurs only soe of haronic. By right selection the switching frequency fop can eliinate a nuber of high haronic [1], because haronic about nuber appears ν = 1, n }1,n }1 }1, In Fig.7 the torque-speed characteristic for a few values of duty cycle ε is shown. This characteristic is calculated by average electroagnetic torque receiving fro siulation. Fig.6 Results of haronic analysis FFT of the stator current at switching frequency fp=500 Hz and duty cycle =0, Fig.7 Torque-speed characteristic at a few duty cycle V. CONCUSION The three-phase ac chopper at ultiple conducting of the power switches akes possible setting the rs output voltage as well as liitation of haronic contents in output voltage and current. By right selection of switching frequency fp and duty cycle ε lower haronic contents of the stator current is obtained. In the threephase ac chopper the power switches full controlling are used so it need using a icroprocessor controller, which can generate coplex transistors firing sequences. In the future, thanks own advantages, such circuit can take place of ac chopper using thyristors SCR in such circuit. REFRENCES [1] Revankar, G.N, Trasi, D.S., (1977) Syetrical Pulse Width Modulated A.C.Chopper I.E.E.E. Trans on Electron Contr. Instru. Vol.IECI-4, pp [] Jang, D., Choe, G., (1991) Asyetrical PWM Method for A.C. Chopper with Iproved Input Power Factor, I.E.E.E. - PESC Conf. Rec., pp [] Jang, D.H., Choe, G.H., Ehsani, M., (1995), Asyetrical PWM Technique with Haronic Eliination and Power Factor Control in A. C. Chopper, I.E.E.E. Trans. on Power Electronics, Vol. 10, Nr., pp [4] Choe, G., Park M. (1998), An Iproved PWM Technique for A.C. Chopper IEEE Trans. on Power Electronics, Vol.4, pp [5] ucanu, M., Ursaru, O., Aghion, C., (00), Single Phase AC Chopper with IGBT s, I.E.E.E. - Proc. of SCS, IASI, Vol. 1, pp [6] Ursaru, O., ucanu, M., Tigaeru,., Aghion, C., (004), PWM Sinusoidal Haronics Injection Control Strategy for a Single Phase AC Chopper, Buletinul Institutului Politehnic Iaşi, nr.1, 004. [7] Potapow W.: Silnik indukcyjny z koutatore tyrystorowy w obwodzie stojana w układzie regulacji prędkości obrotowej. Praca doktorska, Politechnika Śląska, Gliwice [8] Mozdzer A., Bose B. K.: Three-Phase AC Power Control Using Power Transistors. IEEE Transactions on Industry Applications, Vol. IA-1, No.5 Septeber/October 1976, pp Page