A Brushless doubly fed induction machine with flat plane rotary transformers

Transcription

1 A Bruhle doubly fed induction machine with flat plane rotary tranformer Fredemar Runco WEG Brazil Mauricio Ruviaro WEG Brazil Abtract - Thi paper preent and analye a 5kW bruhle doubly fed three-phae induction machine with it wound rotor circuit connected to flat plane rotary tranformer. It preent the advantage of ubtituting bruhe and lip-ring by rotary tranformer. In addition, it how rotary tranformer deign and preent the doubly fed induction machine operation. The teady tate model conider electrical circuit technique to provide information about current, power factor and efficiency on load. Equivalent circuit parameter ued on imulation were obtained through analytical calculation. INDEX TERMS: ALTERNATING CURRENT MOTORS, BRUSHLESS MACHINES, CIRCUIT ANALYSIS, CIRCUIT SIMULATION, EQUIVALENT CIRCUITS, INDUCTION MOTORS, PROTOTYPE, ROTATING MACHINES, ROTARY TRANSFORMER; NOMENCLATURE Induction machine parameter: V lm Stator winding line voltage (in volt). V lm Rotor winding line voltage (in volt). V m Stator winding ingle-phae voltage (in volt). V m Rotor winding ingle-phae voltage (in volt). I m Stator winding current (in ampere). I mm Magnetizing current (in ampere). I m Rotor winding current (in ampere). P m Power aborbed/delivered to grid (in kilowatt). P air-gap Power on the air-gap (in kilowatt). P haft Mechanical power on haft (in kilowatt). f m Stator winding electric frequency (in hertz). f m Rotor winding electric frequency (in hertz). f mec Mechanical frequency (in hertz). f yn Synchronou mechanical frequency (in hertz). R m Stator winding reitance (in ohm). X m Stator winding leakage reactance (in ohm). R mfe Stator iron reitance (in ohm). X mm Magnetizing reactance (in ohm). R mfe Rotor iron reitance (in ohm). R m Rotor winding reitance (in ohm). X m Rotor winding leakage reactance (in ohm). p m Number of pole pair. Slip of induction machine. Rotary tranformer parameter: V t Stator winding ingle-phae voltage (in volt). V t Rotor winding ingle-phae voltage (in volt). I t Stator winding current (in ampere). I tm Magnetizing current (in ampere). I t Rotor winding current (in ampere). S t Apparent power of rotary tranformer (in kilovoltampere). N t Number of turn at tator winding. N t Number of turn at rotor winding. a Tranforming ratio. R t Stator winding reitance (in ohm). X t Stator winding leakage reactance (in ohm). R tfe Stator iron reitance (in ohm). X tm Magnetizing reactance (in ohm). R tfe Rotor iron reitance (in ohm). R t Rotor winding reitance (in ohm). X t Rotor winding leakage reactance (in ohm). R ext External reitance (in ohm).. INTRODUCTION Three-phae induction machine i a popular motor for indutrial application and a largely ued generator in wind energy farm [] - []. In thi context doubly fed induction machine demand pecial attention regarding it feature on torque and peed controllability [] - [9]. Speed and torque can be controlled by rheotat or frequency converter via rotor winding. Connected to induction machine rotor circuit, the converter procee an amount of power proportional to rotor peed. Thi arrangement reduce converter power to a fraction of the total mechanical power, aving cot [] - [7], [8]. figure preent a doubly fed induction machine and it two terminal boxe.

2 Figure - Doubly fed three-phae induction machine The benefit of doubly fed induction machine ue are undeniable; neverthele, to take advantage of them it i neceary to provide electrical connection between rotor winding and tatic rheotat or frequency converter [] - [9]. Nowaday, the mot common way to acce rotor winding i by bruhe and lip-ring. However, the mechanical contact between moving lip-ring and tatic bruhe wear thee component and involve maintenance of them. Powder generated by bruhe wearing can be alo prejudicial for motor inulation. Additionally, any fault on electrical contact can generate park, limiting machine intallation only to non-exploive environment [] - []. Development of bruhle technologie i very intereting for reducing maintenance cot and expanding the ue of doubly fed machine to exploive atmophere [] - [], []. Many tudie conider the ue of two induction machine connected in cacade for obtaining bruhle device. One poibility conit in mounting two individual machine (each one with it own rotor and tator) on the ame haft with electrical connection between their rotor winding []. Another one i repreented by manufacturing a double winding tator and a pecial rotor cage able to join two different induction machine in one ingle frame [5] - []. The combination of two induction machine i effective at the view of eliminating bruhe and lip-ring, but introduce uperpoition of two different torque behavior. The reult i a device with an anomalou torque v. peed curve, in which ynchronou peed i determined by the combination of each machine number of pole [] - [6]. Only the combination of the induction machine with a device lacking in any torque would allow no change on ynchronou peed and on torque v. peed curve hape []. Since the eventie, there i made everal tudie in order to ubtituting bruhe and lip-ring by contactle energy tranfer ytem, a, for example, rotary tranformer [] - [8]. Initially, thi device wa developed concerning pacecraft application, where the lack of reliability and high rate of maintenance of bruhe and lip-ring are totally undeirable []. In [], Papatergiou and Macpheron propoe rotary tranformer a an alternative olution for contactle tranfer of energy acro the revolving frame of airborne electronic-canning radar. In [], Legranger et al. propoe the replacement of gliding contact of a wound rotor ynchronou machine by an axial rotary tranformer operating a contactle tranmiion power ytem. Depite of ome particularitie, all of thee uage for rotary tranformer involve application where the tranformer i ubmitted to frequencie of hundred of Hz [] - []. In [] - [], neverthele, Ruviaro et al. preent the ue of an axial three-phae rotary tranformer electrically connected to an induction machine rotor circuit. Working with induction machine rotor frequency, the rotary tranformer allow the acce to rotor circuit without any mechanical contact. By uing an appropriated drive, it i poible to control the induction machine to operate a a generator a well a a motor at almot any peed, except on ynchronicity. A well a conventional doubly fed induction machine, the olution preented in [] - [] i very convenient for ytem that mut generate contant frequency voltage by the ue of variable peed device, like wind turbine [] - [9]. Thi paper how main deign apect of a flat plane three-phae rotary tranformer integrated to a doubly fed induction machine and it operation. Figure and preent, repectively, the outide and inide part of a 5kW prototype deign under contruction at WEG Equipamento Elétrico S.A. All reult preented on thi paper are baed equivalent circuit parameter analytical calculation of thi prototype.

3 Rotary tranformer The three tranformer are hell-form with primary and econdary winding totally involved by the core. The option for inglephae unit deign ha the objective to reduce magnetizing flux unbalance on rotary tranformer ytem. The preence of air-gap introduce reluctance that change the magnetic circuit in comparion to conventional tranformer []. Like other device for contactle energy tranmiion [5], rotary tranformer ha high leakage/ magnetizing reactance ratio. Figure and table preent main dimenion of each ingle-phae rotary tranformer module. Induction machine l ta l tb Figure - Doubly fed three-phae induction machine with flat plane rotary tranformer. l tc. ROTARY TRANSFORMER DESIGN Rotary tranformer deign [] - [], [] - [8], different from conventional tranformer, ha the particularity of an air-gap to permit movement between primary (tator) and econdary (rotor) winding a can be oberved in figure. N t N t r td r tc r ta r tb Rotor Stator Figure - Single-phae rotary tranformer Table - Rotary tranformer dimenion r ta r tb r tc r td N t 7mm 8mm 55mm mm l ta l tb l tc a N t 5mm 6mm mm In the developed prototype, rotary tranformer core wa made of laminated ilicon teel. Lamination direction i longitudinal to the haft. Figure 5 preent three-phae rotary tranformer magnetizing flux behavior obtained via finite element imulation. Figure - Deign of three-phae flat plane rotary tranformer ytem.

4 Electric grid Tranformer Frequency converter Figure 5 - Rotary tranformer imulation by finite element method reource. The permanent alignment of rotor and tator winding reult in no lip between their magnetic field. A conequence, flat plane rotary tranformer produce no torque. Neverthele, the magnetic attraction between tranformer rotor and tator impact in axial force on haft. A conequence of thi phyic phenomenon, machine bearing mut be deigned to tand thi effort.. DOUBLY FED INDUCTION MACHINE OPERATION The doubly fed induction machine with rotary tranformer i the et of a three-phae induction machine with pm pole tator winding directly connected to the electrical grid and a three-phae rotary tranformer whoe tator winding can be hort-circuited or connected to rheotat bank or to electrical grid through a vector-controlled frequency converter [] - []. Electrical connection for the ue of converter are hown in figure 6. Thi configuration allow controlling torque, peed, power factor and current of induction machine by the converter connected to the tator winding of rotary tranformer. The frequency converter control the machine acting on amplitude, frequency and phae of voltage applied in tator winding of rotary tranformer [] - [6]. Induction machine Rotary Figure 6 - Grid connection of the doubly fed three-phae induction machine with rotary tranformer. When the tator winding of rotary tranformer i connected only to a reitive bank, it i poible to control torque, peed and current. However, power factor i not controllable [] - []. In the built prototype, all electrical connection were in Y. Neverthele, deign for connection in D i perfectly poible. The rotary tranformer permit to adjut it tator voltage (Vt) only changing the relation of turn (a) between the primary (Nt) and econdary (Nt) winding V = ( N N ).V = a. V () t t t t The only requirement i the ame voltage for induction machine (Vm) and rotary tranformer rotor (Vt) to avoid tranformer core aturation V = V () m t t The fundamental frequency of the air-gap induction wave generated by the induction machine tator winding induce a rotor winding current with electric frequency fm given by f m = fm pm fmec ()

5 Rotor winding of the induction machine i electrically connected to rotor winding of the tranformer; conequently their current have the ame electric frequency fm. Depite mechanical movement between rotor and tator tranformer, there i no lip between their magnetic flux. Current on rotor and tator winding are alo in frequency fm. The ynchronou mechanical frequency fyn i fyn fm = p () m The mechanical frequency of the haft of the machine i f = ) p (5) mec ( fm fm Equation (5) how that it i poible to control the peed of induction machine by changing the frequency f m of the voltage on tator winding of rotary tranformer [] - [9]. When the converter i connected to tator winding of the tranformer, a hown in figure 6, frequency, amplitude and phae of the voltage can be impoed on tranformer tator, allowing in thi way a complete control of the doubly fed induction machine. Thi control i not poible only at ynchronou peed, when electric frequency on rotary tranformer i null and it i impoible to tranmit energy between it rotor and tator. Thi energy tranference depend necearily of alternating current preence. Figure 7 how the frequency on induction machine tator and induced frequencie on induction machine rotor and tranformer winding. Induced electric frequencie are function of mechanical frequency or peed of machine haft. The ynchronou rotating frequency i repreented by f yn. f, f me, f mel te m. STEADY-STATE MODEL The teady-tate behavior i obtained through machine equivalent circuit []. Figure 8 preent the connection between winding of induction machine and the tranformer. From thi model it i poible to analyze the machine operating at teady-tate a motor and a generator. All parameter are reflected to the tator of the induction machine. R mfe Figure 8 - Equivalent circuit of doubly fed induction machine with rotary tranformer. 5. PROTOTYPE DATA ~ I mm jx m V ~ m V ~ m V ~ t Rm m R t R tfe jx jx t ~ I ~ m I m jx mm ~ ~ I t I t ~ I tm jx tm R mfe jx t R tfe R m R t Nominal data of the 5kW bruhle doubly fed three-phae induction machine prototype are hown in table. Table - Nominal data of the prototype V lm I m f m.p m P haft V lm S t 66V A 6Hz 6 5kW 8V 5kVA Table preent equivalent circuit parameter reflected to induction machine tator expreed in ohm. V ~ t R ext f e f yn.f yn f me f me= f te f mec Table - Equivalent circuit parameter in ohm at ºC R m X m R m X m R mfe X mm R mfe R t X t R t X t R tfe X tm R tfe f e Figure 7 - Current frequency in induction machine and rotary tranformer.

6 6. STEADY-STATE MODEL RESULTS By teady-tate model and equivalent parameter circuit preented in table III, it i poible to obtain performance curve of the prototype. Figure 9 diplay power curve of the doubly fed three-phae induction machine with rotary tranformer. Power (p.u.) - Power v. peed (P bae :77kW, n bae : rpm) P m P air-gap P haft Like a conventional induction machine, the minimum current i verified at ynchronou peed, when no active power i delivered on haft []. Figure and how that increaing external reitance to.r m, 5.R m and.r m (value referred to induction machine tator), it i poible to have higher tarting torque and lower locked rotor current. The mall decreaing on maximum torque value i conequence of tranformer magnetizing reactance []. Electromagnetic torque v. peed (T bae : 86N.m, n bae : rpm) Without Rheotat.R`m 5.R`m.R`m - - T m e (p.u.) Figure 9 - Power for tranformer tator winding hortcircuited. From to p.u. peed, the machine work a motor, tranforming electrical power in mechanical power on haft. From to p.u. peed, machine work a generator, converting mechanical power in electrical power injected on grid. Figure how the behavior of the divere current preented on the equivalent circuit from Figure 8. Current (p.u.) Current v. peed (I bae : A, n bae : rpm) I m I m I`m = I`t I`t I`t Figure - Electromagnetic torque for tranformer tator winding connected to external reitance. The increaing of external reitance increae magnetic flux on rotary tranformer, a can be oberved at Figure. Figure how electromagnetic torque behavior for external reitance value of.r m (from to. p.u. and.6 to. p.u.), 5.R m (from. to.6 p.u. and. to.6 p.u.),.r m (from.6 to.8 p.u. and. to. p.u.) and.r m (from.8 to. p.u.). In figure 5 and 6 i poible to oberve current and tranformer magnetic flux behavior Figure - Current for hort-circuited tranformer tator winding.

7 5 Current I m v. peed (I bae : A, n bae : rpm) Current v. peed (I bae : A, n bae : rpm) I m (p.u.).5 Current (p.u.).5.5 I m I m I`m = I`t.5 Without Rheotat R`ext =.R`m I`t I`t =.5 R`ext 5.R`m.R`m Figure - Induction machine tator winding current for tranformer tator winding connected to external reitance Magnetic flux (p.u.) Magnetic flux v. peed (Flux bae : mwb, n bae : rpm) Without Rheotat.R`m.5 5.R`m.R`m Figure - Magnetic flux in rotary tranformer for different external reitance connection Torque (p.u.) Electromagnetic torque v. peed (T bae : 86N.m, n bae : rpm) Figure - Electromagnetic torque for tranformer tator winding connected to variable external reitance Figure 5 - Current for tranformer tator winding connected to variable external reitance Magnetic flux (p.u.) Magnetic flux v. peed (Flux bae : mwb, n bae : rpm) Figure 6 - Magnetic flux in rotary tranformer for tator winding connected to variable external reitance. In figure 6, it i poible to analyze rotary tranformer magnetic flux behavior for external reitance changing. The connection of high external reitance impact in high magnetic flux during machine tarting. The conequence i axial attraction between rotor and tator. A the flat plane rotary tranformer i mechanically connected to induction machine haft, thi axial force act on it bearing. During deign tep, bearing pecification mut take account thi phenomenon. Table IV and V preent imulation reult for 5% to 5% load for motor and generator regime. In both cae, rotary tranformer tator winding i hort-circuited. Reult for motor and generator regime are cloe to each other. The main difference are related to peed, power aborbed or delivered to the grid and power factor.

8 Stray loe of.5% of power from grid and 5 kw of mechanical loe at rpm are conidered in efficiency calculation. Power factor verified for thi prototype i maller than tandard value for conventional 6 pole induction machine. Obviouly, thi reduction on power factor i explained by the inductive nature of rotary tranformer []. Table - Induction machine with rotor connected to rotary tranformer (motor operation) Simulation reult Motor Operation Load 5% 5% 75% % 5% V lm (V) T haft (N.m) P m (kw) P haft (kw) I m (A) I t (A) Efficiency (%) Power factor Speed (rpm) Table 5 - Induction machine with rotor connected to rotary tranformer (generator operation) Simulation reult Generator Operation Load 5% 5% 75% % 5% V lm (V) T haft (N.m) P m (kw) P haft (kw) I m (A) I t (A) Efficiency (%) Power factor Speed (rpm) CONCLUSION Subtituting bruhe and lip-ring i the greatet advantage of uing rotary tranformer in doubly fed induction machine. Avoiding mechanical contact between bruhe and lip-ring, motor and generator maintenance can be dratically reduced. Additionally, with the tudied device, the intallation of wound rotor machine on exploive environment become poible. Moreover, thi olution keep all the benefit inherent to the ue of induction machine rotor circuit for machine controlling. Flat plane rotary tranformer offer ome advantage on prototype contruction, but it repreent axial force preence on induction machine bearing. Steady tated model reult give good expectation about the bruhle doubly fed induction machine with flat plane rotary tranformer under contruction at WEG Equipamento Elétrico S.A. 8. ACKNOWLEDGMENT Author wih to thank WEG Equipamento Elétrico S.A. for the prototype building and the future ue of it tet facilitie. 9. REFERENCES [] M. Ruviaro, F. Rünco, N. Sadowki, I. M. Borge, Analyi and Tet Reult of a Bruhle Doubly Fed Induction Machine with Rotary Tranformer, IEEE Tranaction on Indutrial Electronic, Early Acce,. [] M. Ruviaro, F. Rünco, N. Sadowki, I. M. Borge, Deign and Analyi of a Bruhle Doubly Fed Induction Machine with Rotary Tranformer, in XIX International Conference on Electrical Machine (ICEM), Rome, Italy,. [] M. Ruviaro, Three-Phae Wound Rotor Aynchronou Machine Doubly Fed by Rotary Tranformer (in Portuguee), Mater diertation, Univeridade Federal de Santa Catarina, Brazil,. [] F. Rünco, Double-Fed in Cacade Bruhle Three- Phae Aynchronou Machine (in Portuguee), Mater diertation, Univeridade Federal de Santa Catarina, Brazil,. [5] F. Rünco, N. Sadowki, R. Carlon, A. M. Oliveira, P. Kuo-Peng, Performance Analyi of a Bruhle Double Fed Cage Induction Generator, preented at Nordic Wind Power Conference, Chalmer Univerity of Technology, Göteborg, Sweden,. [6] F. Rünco, Modeling, Project and Analyi of Bruhle Double-Fed Three-Phae Aynchronou Machine (in Portuguee), Doctoral thei, Univeridade Federal de Santa Catarina, Brazil, 6. [7] N. Patin, E. Monmaon, J.-P. Loui, "Modeling and Control of a Cacaded Doubly Fed Induction Generator Dedicated to Iolated Grid", IEEE Tranaction on Indutrial Electronic, vol. 56, no., pp. 7-9, Oct 9. [8] S. Shao, E. Abdi, F. Barati, R. McMahon, "Stator- Flux-Oriented Vector Control for Bruhle Doubly Fed Induction Generator", IEEE Tranaction on Indutrial Electronic, vol. 56, no., pp. -8, Oct 9. [9] F. Blazquez, C. Veganzone, D. Ramirez, C. Platero, Characterization of the Rotor Magnectic Field in a Bruhle Doubly-Fed Induction Machine, IEEE Tranaction on Energy Converion, vol, pp , Sep 9. [] R. Datta, V.T. Ranganathan, "Variable-peed wind power generation uing doubly fed wound rotor induction machine - a comparion with alternative

9 cheme", IEEE Tranaction on Energy Converion, vol. 7, pp., Sep. [] R.A. McMahon, P.C. Robert, X. Wang, P.J. Tavner, "Performance of BDFM a generator and motor", IEE Proceeding Electric Power Application, vol. 5, pp. 5, Mar 6. [] B. V. Gorti, G. C. Alexander, R. Spée, A. K. Wallace, Characteritic of a Bruhle Doubly-Fed Machine in Current-Fed Mode of Operation, in Proc. IEEE/IAS International Conference on Indutrial Automation and Control, pp. -8, 995. [] P.C. Robert, R.A. McMahon, P.J. Tavner, J.M. Maciejowki, T.J. Flack, "Equivalent Circuit for the bruhle doubly fed machine (BDFM) including parameter etimation and experimental verification", IEE Proceeding Electric Power Application, vol. 5, pp. 9, July 5. [] S. Williamon, A. C. Ferreira, A. K. Wallace, Generalied Theory of the Doubly-Fed Machine. Part : Analyi, IEE Proceeding Electrical Power Application, vol., n., pp. -, Mar 997. [5] S. Williamon, A. C. Ferreira, Generalied Theory of the Doubly-Fed Machine. Part : Model Verification and Performance, IEE Proceeding Electrical Power Application, vol., n., pp. - 9, Mar 997. [6] H. T. Ma, B. H. Chowdhury, Working toward frequency regulation with wind plant: combined control approache, IET Renewable Power Generation, Vol., I., pp. 8-6,. [7] G. M. Jokimovic, Double-fed Induction Machine- Dynamic Modeling uing Winding Function Approach, in Proc. IEEE International Electric Machine and Drive Conference, pp , 7. [8] T. J. E. Miller, Theory of the Doubly-Fed Induction Machine in the Steady State, in XIX International Conference on Electrical Machine (ICEM), Rome, Italy,. [9] G. Ofner, O. Koenig, G. Dannerer, R. Seebacher, Steady State Modelling of Doubly Fed Induction Generator for Mega Watt Cla Wind Turbine, in XIX International Conference on Electrical Machine (ICEM), Rome, Italy,. [] S. H. Marx, R. W. Round, A Kilowatt Rotary Power Tranformer, IEEE Tranaction on Aeropace and Electronic Sytem, vol. AES-7, iue 6, pp. 57-6, Nov. 97. [] J. Legranger, G. Friedrich, S. Vivier, J. C. Mipo, Comparion of Two Optimal Rotary Tranformer Deign for Highly Contrained Application, in Proc IEEE Electric Machine & Drive Conference (IEMDC), pp , 7. [] K.D. Papatergiou, D.E. Macpheron, "An Airborne Radar Power Supply With Contactle Tranfer of Energy Part I: Rotating Tranformer", IEEE Tranaction on Indutrial Electronic, vol. 5, no. 5, pp , Oct 7. [] K.D. Papatergiou, D.E. Macpheron, "An Airborne Radar Power Supply With Contactle Tranfer of Energy Part II: Converter Deign", IEEE Tranaction on Indutrial Electronic, vol. 5, no. 5, pp , Oct 7. [] K.D. Papatergiou, D.E. Macpheron, "Contact-le Tranfer of Energy by mean of a rotating tranformer", in Proc. IEEE International Sympoium on Indutrial Electronic, pp. 75-7, 5. [5] K.D. Papatergiou, D.E. Macpheron, Air-gap effect in inductive energy tranfer, IEEE Power Electronic Specialit Conference, pp. 9-97, 8. [6] T.A. K. Stuart, H. R.J. Shameddin, Rotary Tranformer Deign with Fixed Magnetizing and/or Leakage Inductance, IEEE Tranaction on Aeropace and Electronic Sytem, AES-, pp. 565, Sep 986. [7] J.P.C. Smeet, L. Encica, E.A. Lomonova, Comparion of winding topologie in a pot core rotating tranformer, preented at th International Conference on Optimization of Electrical and Electronic Equipment (OPTIM), Braov, Romania,. [8] Yu-Ting Huang, Chi-Jen Chen, Wen-Ben Shu, Finite Element Analyi on Characteritic of Rotary Tranformer, IEEE Tranaction on Magnetic, vol., pp. 866, Nov 99. [9] R. Mecke, Contactle Inductive Energy Tranmiion Sytem with Large Air Gap, in Proc. European Conference on Power Electronic and Application,. [] J. Legranger, G. Friedrich, S. Vivier, J. C. Mipo, Deign of a Bruhle Rotor Supply for a Wound Rotor Synchronou Machine for Integrated Starter Generator, in Proc IEEE Vehicle Power and Propulion Conference, pp. 6-, 7. [] C. Wm. T. McLyman, Tranformer and Inductor Deign Handbook, rd ed., Chapter 9, Ed. New York: Marcel Dekker Inc.,.. BIOGRAPHIES Maurício Ruviaro received the M.S. degree from the Univeridade Federal de Santa Catarina, Florianópoli, Brazil, in. He i currently an Electrical Engineer of Product Engineering Department at WEG Equipamento Elétrico S.A. - Energia, Jaraguá do Sul, Brazil. Hi work and reearch topic are electrical calculation of large induction and ynchronou machine. Fredemar Rünco received the Doctoral degree from the Univeridade Federal de Santa Catarina, Florianópoli, Brazil, in 6. He i currently the Manager of Development and Technological Innovation Department at WEG Equipamento Elétrico S.A. - Energia, Jaraguá do Sul, Brazil, and a Profeor with the Department of Electrical Engineering, Centro Univeritário Católica de Santa Catarina, Jaraguá do Sul, Brazil. He i the author or coauthor of nearly technical paper in journal and conference. Hi work and reearch topic are induction and ynchronou machine.