advertisement

www.ijemr.net ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 Volume-6, Issue-1, January-February-2016 International Journal of Engineering and Management Research Page Number: 30-34 Performance Characteristics of Induction Motor under a Balanced Load by Star- Delta Starting Oti Stephen Ejiofor1, Akpama E.J.2, Nnadi Damian Benneth3 Department of Electrical Engineering, University of Nigeria, Nsukka, Enugu State, NIGERIA 2 Department of Elect./Electronic Engineering, Cross River State University of Technology, Calabar, NIGERIA 1,3 ABSTRACT The purpose of this work is to investigate an induction motor model using its physical parameters so as to examine the characteristic performances: phase currents curves, motor speed curve, electromagnetic torque curve and the torque-speed curve of the induction motor using SIMULINK - a tool box extension of the MATLAB program. This performances are to be correlated to the Star-Delta starting Scheme so as to observe the associated response. Keywords---- Induction, Performance, Star-Delta I. INTRODUCTION Induction (Asynchronous) machines are known to be superior to their DC counterparts and most widely used in industries because of their ruggedness, robustness, reliability, low cost, output power per weight, high efficiency and good self-starting capability [1]. The main aspect which distinguishes induction motor from synchronous motors is that induction motors are capable of producing torque at any speed below synchronous speed [2]. The three-phase induction motors, which are widely used in industrial and commercial applications, are capable of producing torque at any speed below synchronous speed. In [3], Nyein Nyein Soe carried out a work on the dynamic simulation of small power induction motor based on mathematical modeling. The dynamic simulation is one of the key steps in the validation of the design process of the motor drive systems and it is needed for eliminating inadvertent design mistakes and the resulting error in the prototype construction and testing. Nyein’s paper demonstrates the simulation of steady-state performance of induction motor by Matlab program using a 3-Ф, 3hp induction motor which was modelled and simulated with 30 Simulink model. A work on the simulation of an induction machine performance under an unbalanced source voltage conditions was also presented in [4] where they compared the performance of the machine under balanced and unbalanced conditions. Munira Batool [5] also looked at the mathematical modelling and speed-torque analysis of 3-Ф induction motor using Matlab/Simulink. In his work, he presented the speed-torque characteristics of an induction motor which were calculated on the basis of a mathematical model. The approach is in compliance with the IEEE standard test procedure for polyphase induction rotors and generators. Induction motor tests using Matlab/Simulink and their integration into electric machinery were as well carried out in [6, 7]. His work describes Matlab/Simulink implementation of three phase induction motor tests, namely; DC, No load and blockedrotor test performed to identify equivalent circuit parameters. These simulation models are developed to support and enhance electric machinery. II. PRINCIPLE OF OPERATION OF INDUCTION MOTOR The equivalent circuit of the induction motor is obtained by combining the stator figure (1) and rotor figure (2) equivalent circuits as shown in the figure (3) that follows, to form the equivalent identical to that of a two winding transformer. Copyright © 2016. Vandana Publications. All Rights Reserved. www.ijemr.net ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 determined through a series of calculations. Performing these calculations can help the designer provide a motor that is best suited to a particular application. The performance of the three phase induction motor from the parameters of its equivalent circuit is evaluated in the usual way. From the complete equivalent circuit, the mechanical torque developed is given by; Tmech 1 3V 2 = ∗ = R2' ÷ S 2 2 ' ' ω s R1 + R2 ÷ S + X 1 + X 2 [ ( )] ( ) (1) At low values of slip: Tmech ≈ 1 ωs ∗ 3V12 ∗S R2' (2) At high values of slip: Tmech ≈ 1 ωs ∗ (X 3V12 1 1 + X2 ) 2 = R21 S (3) The maximum pullout or breakdown torque developed by the motor is: Tmax = 1 * 2ωs R + 1 [R 2 1 3V12 ( + X 1 + X 2' )] 2 (4) The maximum torque is independent of rotor resistance, but the value of the rotor resistance determines the speed at which the maximum torque is developed. III. The equivalent circuit of a three-phase induction motor as shown in the figure (3) above consists of the fixed stator, a three-phase winding supplied from the threephase mains and a turning rotor. There is no electrical connection between the stator and the rotor. The currents in the rotor are induced via the air gap from the stator side. Stator and rotor are made of highly magnetizable core sheet providing low eddy current and hysteresis losses. The magnetic field generated in the stator induces an emf in the rotor bars. In turn, a current is produced in the rotor bars and shorting ring and another magnetic field is induced in the rotor with an opposite polarity of that in the stator. The magnetic field, revolving in the stator, will then produce the torque which will “pull” on the field in the rotor and establish rotor rotation. In the design of the induction motor, operational characteristics can be 31 BALANCE AND UNBALANCED LOAD STATES The analysis of an induction machine is always carried out with the assumption that there is symmetry. That is, the source voltages in the three phases are balanced and the single phase loads connected to the system are also balanced. But in practice, there is however, a possibility on accident short circuit’s between coils, that the three phase winding may not remain symmetrical. Copyright © 2016. Vandana Publications. All Rights Reserved. www.ijemr.net ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 Also unbalanced phase voltages do exist due to the presence of unbalanced load on the system are also balanced. Voltage unbalance comes in diverse ways; single phase under-voltage, two phase under-voltage unbalance, three phase under-voltage unbalance, single phase overvoltage unbalance, two phase over-voltage unbalance, three over-voltage unbalance, unequal single phase angle displacement, and unequal two-phase-angle displacement. For a balanced, pure sinusoidal three phase supply, the sum of the three phase voltages is zero; as a result the zero sequence voltage will be zero. In order to analyze 3 − φ induction machine, it is usually assumed that the source voltage is a balanced three phase network as shown in figure 4(a). The three-phase winding of an induction machine is usually symmetrical as a result of proper design and construction. In the case of the unbalanced state, there is however, a possibility on account of accidental short circuits between coils etc., that the three-phase winding may not remain symmetrical in which case an unbalanced system of figure 4(b) is obtained. IV. EQUIVALENT CIRCUIT OF INDUCTION MOTOR USED Mathematical model of an induction motor is usually done in the arbitrary rotating reference frame, from which other reference frames are realized. The two commonly used reference frame are the stationary reference frame and the synchronously rotating reference. , The former is realized by substituting the variable , where and the later is realized by substituting is the synchronous speed of the motor in electrical radians per second. Details of the derivation of the conventional synchronous motor can be found in [8], and the equivalent circuit is shown below: Figure 5: q-axis Equivalent in the Arbitrary Reference frame 32 Figure 6: d-axis Equivalent in the Arbitrary Reference frame Figure 7: Zero Sequence Equivalent Circuit V. MOTOR PARAMETER The three phase induction motor parameters used for this study are presented in the table below. It is a 4pole, 60-Hz, 3-phase induction motor, and its reactance parameters are expressed in ohms as shown in the table below. Table1. Induction motor Parameters used Parameter Value Rated Power 500 hp Rated phase voltage 230V Rated Speed 1773rpm Rated Torque 1980Nm Nominal current 93.6A Stator resistance per phase, r s 0.262 ohms Stator leakage reactance per phase, 1.206 ohms X ls Magnetizing reactance, X m 54.02 ohms Rotor leakage reactance referred to 1.206 ohms the stator, X' lr Rotor resistance referred to the 0.187 ohms stator, r' r Moment of Inertia (J) 11.06 Kg.m2 VI. MATLAB SIMULATION OF STARDELTA STARTING Copyright © 2016. Vandana Publications. All Rights Reserved. www.ijemr.net ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 In the course of this work, this major simulation on Star-delta starting was carried out by starting the motor in star and afterwards it is changed ptheta_e 1 s pFds 1 s pFqs 1 s pFdr 1 s pFqr 1 s theta_e Fds Fqs Fdr Clock [ib] Fqr [wr] pwr 2*pi*60 we fcn Constant Vab ia [ia] 1 s [ic] [iar] [ia] y [ibr] vao timer Va Vbc ib [ib] ic [ic] Te [Te] iar [iar] ibr [ibr] icr [icr] Switch vbo timer2 Switch2 Figure 10: Rotor currents in the three phases for the stardelta starter [wr] Switch1 [Tl] [icr] [Te] vco timer1 Vb Vca To Workspace [Tl] Tl wr Step theta_r Vc 1 s Embedded MATLAB Function Figure 8: SIMULINK model for Star-Delta Starter. over to delta. The changeover to delta occurred 1.5 seconds after start-up and it was loaded with the rated load of 1980Nm after 3 seconds. The SIMULINK model is shown in figure 8 below: VII. SIMULATION RESULTS OF STARDELTA STARTING Figure 9: Stator currents in the three phases for the stardelta starter 33 Figure 11: Plot of electromagnetic torque for the star-delta starter Figure 12: Plot of Load Torque for the star-delta starter Copyright © 2016. Vandana Publications. All Rights Reserved. www.ijemr.net ISSN (ONLINE): 2250-0758, ISSN (PRINT): 2394-6962 not accelerate insulation deterioration or shorten motor life span. IX. Figure 13: Plot of rotor speed (star-delta starter) VIII. DISCUSSION OF THE SIMULATION RESULTS Figure 9 which was started with a star-delta starter has much reduced starting phase current amplitude of about 800A. If it was a DOL scheme it would have given about twice that quantity which makes this method a safer one. Also, figure 10 shows the rotor currents for the star-delta starter. The rotor starting current is about the same magnitude with that of the stator current because of transformer action and also because it is referred value to the stator. After the start-up, the frequency of the rotor current becomes relatively low because the speed of the motor is now close to the synchronous speed. At the time interval when the motor is moving at synchronous speed it can be seen that the rotor currents are all equal to zero. Figure 11 shows the electromagnetic torque for the star-delta starter. The motor shows a pulsating torque during start-up, but on the average, this torque is positive and helps to overcome the inertia torque. When the load is applied, as observed in figure 12, the motor reacts to produce an almost steady torque after a while to overcome the load torque. Figure 13 shows the motor speed for star-delta starter. Whenever the motor is loaded the speed drops and if it is unloaded the speed increases, this is not so obvious in these figures however, the reason for this is because when the induction motor is loaded, its speed drops so that more emf will be induced in the rotor, consequently more current and torque would be produced to counter this load. Also from the result, it was observed that an induction motor under balanced load (i.e) balanced voltages will result in; decreased heating at rated horsepower load, in which under extended operation may 34 CONCLUSION This research result has shown that there is an appreciable success in the performance of an induction motor under balance load as demonstrated using star-delta starting. The induction motor operational characteristics for star-delta may differ with the direct online scheme only during transients and start-up periods but remain the same subsequently. The results prove that, the operational performance of an induction machine can be studied using simulated result from MATLAB® without going through the rigorous analytical method. This is just joining the fact that this work has demonstrated the elegance of Matlab in the performance characteristics of an induction motor more so with the Matlab Embedded function developed. REFERENCES [1] M.C. Richard, “Electrical Drives and their control”. Oxford University Press: New York, 1995. [2] O.I. Okoro, “Dynamic Modelling and Simulation of Squirrel-Cage Asynchronous Machine with Non-Linear Effects”. Journal of ASTM International, 2(6), 2005, 1-16. [3] N.S. Nyein, “Dynamic Modelling and Simulation of 3 − φ Small Power Induction Motor”. International World Academy of Science, Engineering and Technology, 2000, pp. 18. [4] E.J. Akpama, O.I. Okoro., and E. Chikuni, “Simulation of the Performance of Induction Machine under Unbalanced Source Voltage Conditions”. Pacific Journal of Science and Technology, 11(1), 2010, pp. 9-15. [5] M. Batool. “Mathematical Modelling and Speed Torque Analysis of 3 − φ Induction Motor using MATLAB”. Second Edition, McGraw Hill, 1995, pg. 9-17. [6] C.O. Nwankpa, “Induction Motor tests using MATLAB”. IEEE Transactions on Education, Vol. 48, 2005, pp. 1. [7] O.I. Okoro., ‘Dynamic and thermal modelling of induction machine with non linear effects’, Ph.D. Thesis, University of Kassel Press, Germany, September 2002. [8] Chee Mun Ong. “Dynamic Simulation of Electric Machinery using MATLAB/SIMULINK”. Upper Saddle River, New Jersey, 1997, pp. 229. Copyright © 2016. Vandana Publications. All Rights Reserved.