1 Direct orque Control of Open-end Winding Induction otor Drive Uing the Concept of Imaginary witching ime for arine Propulion ytem Arbind Kumar, tudent ember IEEE B.G. Fernande K. Chatterjee Department of Electrical Engineering, Indian Intitute of echnology Bombay, Powai, umbai-4 76, India elephone: Fax: Abtract - hi paper propoe a control trategy for marine propulion ytem uing the open-end winding configuration of induction motor. It ue the concept of imaginary witching time for pace vector pule width modulation (PW) with direct torque control (). he imaginary witching time greatly implifie the algorithm. hi method alo eliminate the procedure for identifying the region for pace voltage vector. Alo, it doe not require the ector and angle identification of the voltage vector. wo independent two-level inverter feed power from both the end of the three-phae tator winding. hi applie a 3-level line voltage waveform to the machine. hi reduce the torque ripple with improvement in current pectrum. In order to retrict the zero equence current, two iolated dc ource are conidered. imulation tudie have been carried out for the propoed cheme. Experimental reult have been preented to validate the viability of the cheme. I. INRODUCION hough induction motor are already in ue for the auxiliary ytem for hip/ubmarine, it ha not been ued for main propulion motor. motor are preferred for thi purpoe due to their inherent peed control capability. But thee motor require regular maintenance and have low torque to weight ratio. Due to variou advancement in power electronic, it i now poible to ue the induction motor drive for a wide range of peed with a fat dynamic repone. herefore it i propoed that open-end winding induction motor with direct torque control can be an alternative for marine propulion ytem. hi will greatly reduce the maintenance and increae the energy denity and torque to weight ratio. With rapid development in the field of engineering, epecially in power electronic, a concept of electric warhip [-6] ha emerged. In electric warhip, mot of the mechanical equipment are replaced by their electrical counterpart. Recent literature ugget that many developed countrie have undertood the advantage of the electric warhip and are coming forward to upgrade, procure/commiion their marine ytem with thi technology. Induction motor drive are becoming popular for low and medium power in variou indutry application. However, a ingle -level inverter i not uitable to upply the power to the high rating motor. hi i due to the limitation of the device rating. ultilevel inverter can overcome thi problem. hee inverter increae the complexity in the control algorithm. Open-end winding configuration of induction motor fed by two eparate conventional inverter with IGB a witching device reduce thee problem. hi configuration ha been propoed for marine propulion ytem with direct torque control (). Direct torque control of induction motor wa propoed around two decade ago -. he cheme preented in  wa implemented for high power application uing an induction motor having open-end winding configuration . However, the technique preented in  reult in: orque and peed fluctuation which lead to acoutic noie and vibration. Higher ripple in the tator current that can caue power lo and hence heating of the machine. Ue of a three-phae reactor to reduce the zeroequence current, make the ytem bulky. ariou control technique for the inverter-fed induction motor drive with open-end winding are alo dicued in [-]. Direct elf Control technique ha been propoed for thi machine with open-end winding configuration [3-5]. Both end of the tator winding are connected to two three-level inverter. he ue of a three-level inverter at both end improve the performance of the machine but will lead an increae in cot and complexity. Alo a reactor ha been ued on the AC/ ide to reduce the flow of zero equence current. In [6-7], the open-end winding configuration ha been propoed for high power electric vehicle/hybrid electric vehicle (E/HE) propulion ytem. In , pace vector pule width modulation technique i ued /5/$. 5 IEEE 54 Authorized licened ue limited to: INDIAN INIUE OF ECHNOOGY BOBAY. Downloaded on October 4, 8 at 7:33 from IEEE Xplore. Retriction apply.
2 to control the output voltage of both the inverter connected at both end of the motor winding. It i to be noted that the witching frequency capacity of both the inverter i ame. In  a cheme i propoed for direct torque control of induction motor with openend winding configuration. he reference voltage vector i controlled by pace vector PW technique. hi method reduce the witching frequency of one of the inverter by clamping it for one ampling interval. But thi control trategy i complex due to identification of the region ince the entire region i divided in 4 mall region. In order to reduce the complexity in identifying the region, a impler method i propoed in . But in thi method, entire region i further divided into even maller region. In order to implify the control trategy, a method for open-end winding configuration i propoed with PW uing the concept of imaginary witching time. It doe not require any ector or ub-ector identification. It greatly implifie the control trategy. Alo, the witching frequency of both the inverter can be ame. he propoed control trategy maintain all the advantage of all electric hip (AE) uch a improved urvivability, reduced ignature etc. . In addition to thi it generate three level voltage, which again decreae the torque ripple and improved current harmonic pectrum. II. OPEN-END WINDING CONFIGURAION OF I.. FED BY WO-EE INERER A chematic of the open-end winding configuration of induction motor drive i hown in Fig.. A two level inverter IN feed three end of tator winding R Y B and the other three end R Y and B are fed by another two level inverter called IN. IN and IN are connected to eparate dc ource of magnitude dc /. ource are derived from a bank of batterie a thee are ued in many application like on-board hip/ ubmarine propulion ytem. hi eliminate the diadvantage of zero equence current and the requirement of zero equence reactor or iolating tranformer i eliminated. In cae of general ue, iolated dc ource are required, which can be derived from two iolating tranformer. dc IN G B Y B R R Y INDUCION OOR Fig. Open-end winding configuration of the Induction motor Drive ince there are two independent two-level inverter, the total poible generated vector will be 3 x 3 (64). However in actual practice there i only one zero voltage vector located at the centre of the hexagon of Fig.. here are ix mall voltage vector, G IN dc ix medium voltage vector and ix large voltage vector denoted by, and repectively and remaining overlap to thee voltage vector () 5 3() 5() 6 35 () 6() 5 () Fig. Generated oltage ector 4 he ix mall voltage vector form the inner hexagon of Fig.. mall voltage vector, are achieved by operating IN in witching mode clamping IN to either at or 7 (both zero voltage vector for IN). edium and large voltage vector are formed when both the inverter are in witching mode. For example, to achieve vector 4, IN i witched to poition and IN i witched to poition 4. III. CONCEP OF IAGINARY WICHING IE If the reference voltage vector i lying in the firt ector a hown in Fig. 3, it can be bet contructed by it two nearet voltage vector, and zero voltage vector ( or7 ) in ome equence for a pecified ( ) α ( ) ( ) ( ) 7 Fig. 3 Contruction of reference voltage vector time a dicued in [-3]. otal time during which the active voltage vector are applied i known a the effective time. Power tranfer from inverter to motor take place in thi duration only. hee time are calculated by comparing the volt-econd of reference voltage vector with the applied voltage vector i.e. in(6 α ). a () in6 55 Authorized licened ue limited to: INDIAN INIUE OF ECHNOOGY BOBAY. Downloaded on October 4, 8 at 7:33 from IEEE Xplore. Retriction apply.
3 in( α). a () in6 where a, ampling time,, & are the duration for which, & or7 are applied, α i the angle of with repect to d-axi and 3. If equation () & () are implified in term of intantaneou phae value correponding to it reference voltage vector, they can be re-written a: a b or a b (3) and b c or b c (4) where a b c a, b & c are defined a the three imaginary witching time. he value of thee time could be negative a it depend on the reference tator phae voltage a, b & c derived from the reference voltage vector. I. PROPOED CONRO RAEGY he reference tator flux can either be derived from the reference peed or it can be controlled independently, while the poition of the reference flux pace vector can be derived from the torque error and actual rotor peed. orque error i proportional to lip peed a explained in  and adding it with actual rotor peed give the ynchronou peed of the reference flux pace vector. he actual tator flux pace vector i derived from the motor model itelf. he error between thee two tator flux vector generate the fictitiou imaginary time reference vector. he d-q component of imaginary time vector can be determined from the following -: tator voltage equation can be written a: d Ri dt Under the condition of negligible tator reitance, it can be implified a: or j ( j ) (5) Comparing real and imaginary part of (5) give: (6) (7) where t i the ampling time. herefore imaginary witching time in d-q tationary reference frame are calculated a follow: ( ) (8) ( ) (9) j j or () hu a new concept of imaginary time vector given by equation () i directly reponible for calculating the actual witching intant of the inverter. In any cae, magnitude of imaginary time vector cannot be more than the ampling time. Component of imaginary time vector can be converted into three-phae by imply uing two-tothree phae tranformation which give the imaginary witching time a, b & c. From imaginary witching time, actual time are determined a explained in  for two-level inverter. hee time are determined in each ampling interval and accordingly the witching intant for the PW inverter are generated. Hence torque and flux error are compenated in each ampling interval. hi improve torque and flux waveform. After each ampling interval, actual tator flux vector i corrected by the error and it trie to attain the reference flux pace vector. Flux error i minimized in each ampling interval. peed of the reference tator flux take care of torque demand becaue it i the addition of lip peed, derived from torque error, and actual rotor peed. he d-q component of imaginary time vector are obtained from equation (8) & (9) and compenation time equivalent to tator reitance voltage drop are added. hi method eliminate the requirement of ector or angle identification which further reduce the complexity. In order to implement thi concept of imaginary time vector, for open-end winding configuration, 56 Authorized licened ue limited to: INDIAN INIUE OF ECHNOOGY BOBAY. Downloaded on October 4, 8 at 7:33 from IEEE Xplore. Retriction apply.
4 it i equally divided into two halve, imilar to the voltage vector, a hown in figure () 5 3() 5() () 6() 5 () Fig.4 Dividing the imaginary time vector in to equally half time vector for IN and IN he imaginary time vector IN IN i reponible for generating either or a deired for inverter. imilarly generate the ame equence or for inverter. hi method doe not require any ector or angle identification of voltage vector or imaginary time vector. Hence it further implifie the control trategy. he overall block diagram of the propoed control trategy i hown in figure 5. he electromagnet torque of the open-end winding configuration of induction motor i calculated in a ame manner a for normal induction motor uing the tandard torque equation a: te 3 P( i i ) / where P i the number of pole, & are the tator d-q axi flux and i & i are the d-q axi tator current in the tationary reference frame. he load torque of the marine propulion ytem depend on urface condition, water denity, linear peed, rotational peed, etc.. It can be implified a follow: It ha two component:- ω r ω r e PI PI he firt component at nominal condition i 5 percent of rated load torque . It i aumed contant throughout the range for the purpoe of implicity. he econd one i proportional to the uare of angular peed . herefore the load torque of the marine propulion ytem can be expreed a: tl K K ω r Here the parameter are choen a: K.367 & 4 K 3.8. IUAION REU imulation tudie have been carried out for the propoed method uing AAB-IUINK. Induction motor parameter ued for imulation are given in APPENDIX. In Fig. 6, tator D-Q axi flux i plotted while in figure 7, reult have been hown for the tarting of the motor with a reference peed command of 9 rpm at loaded condition and alo at t ec a command of udden top i applied. Figure 8 how the phae R voltage i.e. between R and R. It how that three-level voltage are generated by two two-level inverter. tator Q-axi flux e tator D-axi flux Figure 6 D-Q axi tator Flux ω lip e & dc Generation of IN & IN witching ignal to 6 AC ource Ioltating ranformer IN to 6 IN Open end winding Induction otor peed encoder ω r 7 to 7to Fig.5 chematic of the propoed method 57 Authorized licened ue limited to: INDIAN INIUE OF ECHNOOGY BOBAY. Downloaded on October 4, 8 at 7:33 from IEEE Xplore. Retriction apply.
5 Phae Current (amp) ime (ec) 7.5 orque (N-m) Elec tromagnetic torque oad torque ime (ec) 8 peed (rpm) ime (ec) Figure 7 Phae current, Electromagnetic torque and peed Fig D-Q axi tator flux II. CONCUION Figure 8 Phae voltage I. EXPERIENA EUP AND REU he prototype of the ytem i built and teted on. kw induction motor. wo emikron B6C/6/45-F inverter are connected at both the end of the induction motor tator winding. wo iolating tranformer are ued to generate two ource. 3F47A ezdp tarter kit i ued to implement the control trategy. At preent, the reult are for the open loop configuration. Figure 9 how phae voltage between R & R' and phae current at R & R for the ame phae. he nature of phae voltage in thi figure validate the imulation reult that i hown in Figure 8. It alo confirm the generation of three level voltage that improve the performance and reduce ripple in the electromagnetic torque. Figure how the tator d-q axi flux in the tationary reference frame and thi i the actual flux in the DP environment. hi confirm the imulated reult of Figure 6. A control trategy for marine propulion i propoed. It atifie the requirement of low harmonic and reduced torque ripple becaue thi method produce the three-level inverter with leer complexity compared to the multi-level inverter. It i uitable for high power application. Alo the propoed control trategy i very imple a it doe not require dividing the operating region into ub-ector/ector. Determining the ector and angle of voltage vector i eliminated. Both the inverter operate at ame witching frequency. It indirectly generate the threelevel voltage by uing the two -level inverter at both end of the tator winding. Due to the three level voltage generation, torque and current waveform are uperior to that of baic direct torque control cheme. he propoed trategy i alo uitable for battery operated propulion ytem uch a ubmarine. It eliminate the requirement of zero equence reactor or iolating tranformer. he performance of the propoed control trategy improve toward full load a both the inverter are operated and equally loaded. In cae of light load condition, both the inverter will be operational which lead to an increae in witching loe. In order to limit thee loe, the author have propoed [9-] clamping method in which one of the inverter i clamped for entire witching period to a particular voltage vector. hi trategy will greatly reduce the maintenance cot due to the advantage aociated with the induction motor. Direct torque control trategy will improve the peed control capability and dynamic repone. here are two inverter connected acro both the end of the tator winding. hi ha another advantage of controlling the induction motor by uing only one inverter in the light load condition or in emergency. APPENDIX Fig 9 From top to bottom: Phae oltage (5/div, Phae Current een from R (.5 A/div), Phae Current een from R' (.5 A/div) and X axi m/div. Parameter of. kw,, 43 rpm, 4-pole, 3-phae induction motor ued for imulation/experimental tudie: R 7.83 Ω Rr 7.55 Ω.475H r.475 H 58 Authorized licened ue limited to: INDIAN INIUE OF ECHNOOGY BOBAY. Downloaded on October 4, 8 at 7:33 from IEEE Xplore. Retriction apply.
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