ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 8, February 2013 A Novel Method for Starting of Induction Motor with Improved Transient Torque Pulsations Nithin K.S, Dr. Bos Mathew Jos, Muhammed Rafeek, Dr. Babu Paul Abstract- In this paper a new strategy for starting of induction motor is proposed. This scheme depends on the initial switching instances at which each phases of motor terminals are connected to the supply. The performance analysis of the motor has been carried out using a d-q axis induction motor model. A mathematical model is developed in matlab/simulink for a 415 V, 4.7A, 1435rpm, 50 Hz three phase induction motor. The method is shown to be very effective in the elimination of transient torque pulsations and current components during the starting. The simulation results presented in this paper show the effectiveness of the method developed. Index Terms-Induction Pulsations, Modeling. J ids, iqs idr, iqr rs, Ls rr, Lr Te TL Vds, Vqs Lm wr Po Motor, Soft Starter, Torque NOMENCLATURE Moment of inertia in Kg-m2 2-phase stator currents in Ampere Transformed two phase rotor currents in Ampere 2-phase stator resistance in ohms and inductance in Henry 2-phase stator resistance in ohms and inductance in Henry referred to stator in ohms Electromagnetic Torque in N-m Load Torque in N-m 2-phasee stator voltage in Volts mutual inductance in Henry rotor speed in rad/sec Number of poles I. INTRODUCTION Three phase induction motors have been the workhorse for many industrial and manufacturing processes from the capacity of several kilowatts to thousands of kilowatts as the driving units. It is a high efficiency electrical machine when working closed to its rated torque and speed. The starting methods of three phase induction motor [1] are generally classified into four basic categories: Direct on line starting, electromechanical reduced voltage starting, solid state reduced voltage starting and variable frequency drive starting. DOL starting of ac motors is the cheapest way but it may present difficulties for the motor itself and the loads supplied from the common coupling point because of the voltage dips in the supply during starting. An uncontrolled starting may cause a trip in either overload or under-voltage relay, resulting in starting failure. Furthermore the number of starts per day is limited to a few attempts. Electromechanical reduced starting comprises of auto transformer starting, star-delta starting, resistance or reactor starting. All these methods have draw backs[2][3] such as need for frequent inspection and maintenance, non simultaneous switching of motor phases to the supply, failures in the moving parts due to the large number of switching etc.VFD is more expensive than any other method mainly because of the converter and inverter section. Reduced voltage starters or so called soft starters are often employed as effective and low cost means of reducing high starting currents and torque pulsations through the use of thyristor based voltage control. These are cheap, simple, and reliable and compact in size and are increasingly employed in industries nowadays. This circuit is a feasible solution to the starting problem of large ac motors in applications where the starting torque requirement of the load is low. An Induction motor produces severe torque pulsations depending on the initial switching instants to all the three phases to the supply [4]. These are often large and vary from positive to negative values. The mechanical subsystem determines the amount of torque pulsations in shaft at starting instant. These may cause shocks to the driven equipment and damage in mechanical system components such as shafts, couplings and gears. In this paper a new control strategy is proposed for minimizing the torque pulsations which is based on the switching instants of each phases of motor terminals given to the supply and a mathematical model is developed in MATLAB/SIMULINK for a 415 V, 4.7A, 1435rpm, and 50 Hz three phase induction motor. II. MATHEMATICAL MODEL Starting with the mathematical model of the induction motor in terms of abc axes quantities, all space-angle and, hence, time varying inductances are eliminated by applying the three phase to two phase transformations; to the d-q axis frame fixed to the stator[5]. The dynamic equations for the induction motor are given below. 2 1 dids rs r Lm rs Lm r Lm L 0 0 0 L Ls Ls Ls Lr Ls dt s s V 2 i diqs r Lm rs r Lm rr Lm ds 0 1 0 0 ds Vqr dt 1 L L Ls Ls Ls Ls Lr iqs 1 di s s 1 i r L L L r dr dr s m r m r m r 0 0 0 0 (1) Lr 0 dt Ls Lr Ls Lr Ls iqr diqr L r L L 1 r r m s m r m r 0 0 0 Lr dt Ls 2Ls Lr Ls Lr Where σ = 1- (Lm /LrLs) Vds and Vqs are given by 219 ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 8, February 2013 determine the current variation and the transient torque 1 1 1 V 2 2 rs Vds pulsations at the starting of induction motor. So by controlling 2 Vys V 3 the switching instants of the supply the torque pulsations can 3 3 0 qs be reduced to a minimized level. Also the current can be 2 …………… 2 Vb s (2) maintained within the acceptable limit during starting. and Extensive simulations were carried out with different sin(t ) Vr combinations of initial switching instances. It is found that V V sin(t 2 / 3) improved starting torque pulsations are obtained by giving Y m y ..…………………….. (3) and B phases simultaneously at the starting instant t=0 and R Vb sin(t 2 / 3 phase with a time delay of t1. So till the switching instant of the third phase only two phases are available at the motor The electromagnetic torque is given by terminals. This seems to be a two phase supply to the motor, results a reduced voltage at the starting instant hence the 3 Po current is controlled. Gradual variation in flux also reduces L ( i iqs i i ) Te = m dr qr ds …………………… (4) 2 2 the torque pulsations. So the motor gets smooth acceleration And the speed is governed by and high starting torque with minimized torque pulsations and reduced starting current. This is found to be better technique dr Te TL ………..……………………..(5) which can put a great effect on the induction motor applications. dt J IV. SIMULATION RESULTS The mathematical model and the proposed control scheme is implemented in Matlab. Switching instants of three phases are given as follows. Three input voltages are shown in fig 2. Switching instants of three phases are Y,B, phases t= 0 R phase t1=0.0133333sec Fig 1: Schematic Diagram of Starter for Induction Motor Dedicated software is developed in matlab/simulink for modeling the drive system. The matrix equation (1) is derived by the proper transformations of the quantities for which the rotor reference frame is fixed in the stator. These differential equations are used to develop a suitable system matrix for the entire model. Here rs, rr, Ls, Lr are the corresponding stator and rotor resistances and inductances respectively. Matrix voltage equations can then be written as in (2) and (3) shown below and (2) shows the transformation to d-q axis components. After adding the torque balance equation for dynamic operation, the model is brought into the state space form for simulation. Matlab function for measuring the speed in radians as well as in rpm is also developed. III. PROPOSED CONTROL SCHEME Fig 1 shows the schematic of the starting circuit for induction motor. Three switches are connected to each phase of the supply. This scheme depends on the switching instants of each phase of the motor terminals given to the supply. The switching instants of all the three phases to the supply Fig .2: Three Phase Input Voltages Given To The Motor Terminal The simulation is started with a delay of 2 sec for better understanding. So all the waveforms in the above figures are started at instant 2 sec. The voltage waveforms in the new scheme are given in fig 2. Torque profile for normal starting scheme is given in fig 3. It is observed that starting of induction motor with small firing angles, the torque pulsations are severe. Also the starting current is very much large. But the time taken to attain the steady state is less compared to profiles with large firing angles. In the case of large firing angles, the starting torque pulsations are lesser. But the time taken to attain the steady state is much larger [6]. Torque pulsations are large even for this case. 220 ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 2, Issue 8, February 2013 REFERENCES [1] R.M. Hamouda, A.I.Alolah, M.A.Badr, M.A. Abdel- halim , ‖A Comparative Study on the Starting Methods of Three phase wound-rotor Induction Motor’’,IEEE Transactions on Energy Cpnversion.Vol.14, No.4, December 1999. [2] A. J. Williams and M. S. Griffith, ―Evaluating the effects of motor starting on industrial and commercial power systems,‖ IEEE Trans. Ind. Applicant., vol. IA-14, pp. 292–299, July/Aug. 1978. [3] J. Nevelsteen and H. Aragon, ―Starting of large motors—Methods and economics,‖ IEEE Trans. Ind. Applicat., vol. 25, pp. 1012–1018, Nov./Dec. 1989. Fig.3: Torque Wave Forms for Firing Angle 00 [4] W.S.Wood, F. Flynn, A. Shanmugasundaram. ‖Transient Torques in Induction motors due to Switching of the supply,’’ Proc. Inst. Elect.Eng., Vol.112, no.7, pp.1348-1354, July1965. [5] K. Sundareswaran, Bos Mathew Jos,‖ Analysis , Simulation and Performance Comparison of AC Voltage Controller Fed Three wire and Four wire connected Induction Motor Drives‖,IEEE Indicon 2005 Conference, Chennai, India, 11-13 December 2005. [6] K Sundareswaran, Bos Mathew Jos,’’ Comprehensive Study on Starting Performance of Thyristor Controlled Induction Motor Drives’’, IEEE Indicon 2005 conference. AUTHOR’S PROFILE Fig.4: Torque Characteristics for the Proposed Scheme In the proposed scheme the torque pulsations are minimized to a large extent as shown in fig 4. The starting current is controlled and amplitude variations are reduced to a permissible level (fig 5). So this method is well suitable for starting of squirrel cage induction motor. V. CONCLUSION In this paper a new control strategy is presented to improve the electromagnetic torque pulsations at starting. Torque profile is seemed to be improved with minimized torque pulsations. In order to obtain a better torque profile, the system is properly unbalanced by giving only two phases of supply to the motor model at the starting instant and then third phase is applied. So this method will efficiently minimize torque pulsations at starting, hence avoid shocks to the driven equipment and reduce damage in mechanical system components such as gears, couplings and shafts. This technique can be implement in industries for large induction motors. APPENDIX Induction motor details Three phase, 415V, 4.7A, 1435rpm, 50Hz Stator resistance, rs= 4.5 Ω Rotor resistance, rr=4.5 Ω Magnetizing inductance, Lm=1.296H Stator inductance, Ls=1.3381H Rotor inductance, Lr=1.3385H Moment of inertia, J=0.00605 kgm2 Nithin K.S received the B.Tech degree in electrical and electronics engineering from Govt. Engineering College Idukki, Mahatma Gandhi University, Kottayam, and Kerala, INDIA in 2011. He is currently M.Tech scholar in Power Electronics at Mar Athanasius College of Engineering, Kothamangalam, and Mahatma Gandhi University. His areas of interest include power electronic applications for electric motor drives. Dr. Bos Mathew Jos received B.Tech Degree from Mahatma Gandhi University, Kottayam, Kerala, INDIA in 1993 and M.Tech and Ph. D Degrees from National Institute of Technology Tiruchirappalli, INDIA in 2004 and 2010 respectively. Since 1998, he has been with Mar Athanasius College of Engineering Kothamangalam, Kerala. Currently, he is an Associate Professor of Electrical and Electronics Engineering Department. His research interests are Power Electronics and Drives. Muhammed Rafeek has received the B.Tech degree in electrical and electronics engineering from College of Engineering Munnar, Cochin University of Science and Technology, Kalamassery, Kochi, Kerala, INDIA in 2010. He is currently M.Tech scholar in Power Electronics at Mar Athanasius College of Engineering, Kothamangalam, and Mahatma Gandhi University. His areas of interest include power electronic applications for electric motor drives. Dr. Babu Paul received B.Tech Degree from Mahatma Gandhi University, Kottayam, Kerala, INDIA in 1990 and M.Tech from Govt. Engineering College Thrissur in 2003 and Ph.D from IIT Bombay in 2011. Since 1998, he has been with Mar Athanasius College of Engineering Kothamangalam, Kerala. Currently, he is an Associate Professor of Electrical and Electronics Engineering Department. 221