Control of a Three Phase Induction Motor using Single Phase Supply

Transcription

1 Control of a Three Phase Induction Motor using Single Phase Supply G. R. Sreehitha #1, A. Krishna Teja *2, Kondenti. P. Prasad Rao #3 Department of Electrical & Electronics Engineering, K L University, Abstract- In Industrial applications, two forms of electrical energy are used: Direct Current (DC) and Alternating Current (AC). Usually constant voltage, constant frequency Single-Phase or Three-Phase AC is readily available. However, for different applications different forms, magnitudes and/or frequencies are required. This paper proposes how the Three-Phase inductive load is run by a Single-Phase supply by using Cycloconverter and a Scott-T connected Transformer. The controlling of a Three- Phase Induction Motor is done by Frequency variable method. Single-Phase to Three-Phase for motors offered by using high in performance, low on maintenance and is used to reduce of breakdown of electrical equipment, our range is also suitable for saving energy and require low maintenance. Keywords- Cycloconverter, Scott-T Transformer, Single-Phase to Three-Phase conversion. I. INTRODUCTION A Scott-T Transformer[1] is a type of circuit used to derive two-phase electric power with 90 0 phase shift[2] from a threephase source, or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source. The Scott three-phase transformer was invented by a Westinghouse engineer, C. F. Scott, in the late 1890 s to bypass Thomas Edison s more expensive rotary converter and thereby permit two-phase generator plants to drive Nikola Tesla s three-phase motors[3]. Two-phase motors draw constant power the same as threephase motors, so a balanced two-phase load is converted to a balanced three-phase load. However if the two-phase load is not balanced, the Scott-T transformer cannot fix this. Unbalanced current on the two-phase side causes unbalanced current on the three-phase side[1]. Frequency changers is an expanding field of power conversion technology. The increasing utilization of a.c motors in variable speed drives and the generation of electrical power from variable speed sources are examples of this field applications[4]. Cycloconverters are suitable for large a.c. machines because it has advantages: it has high efficiency owing to the simple construction of the main circuit, which consists, in its basic form, simply of an array of IGBT switches[5]. The application of a Cycloconverter is rather limited, because the control circuit is often very complex, and therefore expensive[6]. Andhra Pradesh, INDIA This converter consists of back-to-back connection of two full-wave rectifier circuits. Fig. 1 shows the operating waveforms for this converter with a resistive-inductive load. The input voltage, V s is an AC voltage at a frequency, f i as shown in Fig. 1b. For easy understanding assume that all the Switches (IGBT) are fired at α=0 firing angle, i.e. Switches act like diodes. Note that the firing angles are named as α P for the positive converter and α N for the negative converter. Consider the operation of the Cycloconverter to get half of the input frequency at the output. For the first cycle of V s, the positive converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees two positive half cycles as seen in Fig. 1c. In the next cycle, the negative converter operates supplying current to the load in the reverse direction. Note that when one of the converters operates the other one is disabled, so that there is no current circulating between the two rectifiers. Fig.1a. Single Phase Cycloconverter with Sinusoidal Pulse Width Modulation (Converter Consists of Back-to-Back Connection of two full-wave rectifiers). II. CYCLOCONVERTER ISSN: Page 431

2 Fig.2a. Positive Gate Pulse for Positive Conversion (for f/2). Fig.1b. Input Voltage to the Cycloconverter having f = 50 Hz Frequency. Fig.2b. Negative Gate Pulse for Negative Conversion (for f/2). Fig.2. Control Pulses for Cycloconverter (α=0 firing angle). Fig.1c. Output Voltage of the Cycloconverter having f/2 = 50/2 Hz Frequency Fig.1d. Output Voltage of Cycloconverter having f/4 = 50/4 Hz Frequency. Fig.1. Single Phase Cycloconverter with R-L load. To get one-fourth of the input frequency at the output, for the first two cycles of V s, the positive converter operates supplying current to the load. It rectifies the input voltage; therefore, the load sees 4 positive half cycles as seen in Fig. 1d. In the next two cycles, the negative converter operates supplying current to the load in the reverse direction. ISSN: Page 432

3 The frequency of the output voltage, V o in Fig. 1d. is 4 times less than that of V s, the input voltage, i.e. f o /f i = 1/4. Thus, this is a step-down Cycloconverter. On the other hand, Cycloconverters that have f o /f i > 1 frequency relation are called step-up Cycloconverters. Note that step-down Cycloconverters are more widely used than the step-up ones. The frequency of V 0 can be changed by varying the number of cycles the positive and the negative converters work. It can only change as integer multiples of f i in 1f-1f Cycloconverters. With the above operation, the 1f-1f Cycloconverter can only supply a certain voltage at a certain firing angle α. The dc output of each rectifier is: (1) where V is the input rms voltage. Then the peak of the fundamental output voltage is ( ) (2) Equation 2 implies that the fundamental output voltage depends on α. For α = 0, where. If α = (π/3), then. Thus varying, the fundamental output voltage can be controlled. Constant operation gives a crude output waveform with rich harmonic content. With different 's, the less are the harmonics. III. SCOTT T TRANSFORMER Assuming the desired voltage is the same on the two and three phase sides, the Scott-T transformer connection consists of a centre-tapped 1:1 ratio main transformer, T1, and an 86.6% (0.5 3) ratio teaser transformer, T2. The centre-tapped side of T1 is connected between two of the phases on the three-phase side. Its centre tap then connects to one end of the lower turn count side of T2, the other end connects to the remaining phase. The other side of the transformers then connect directly to the two pairs of a two-phase four-wire system. Fig.3. Scott-T Transformer (2ø to 3ø). Fig.4. Shows the characteristics of a Three Phase Induction Motor with the input voltage 220V and frequency 50Hz. The main transformer of a Scott-T having and teaser transformer having Fig.4a. shows one of the three phase voltages at output of the Scott-T transformer. Here input is always equal to output voltage magnitude, because the transformer ratio is 1:1. Fig.4a. Scott-T Transformer Circuit in MATLAB Simulink. ISSN: Page 433

4 Fig.4b. One of Three Phase voltage Waveform (Output of Scott-T Transformer). Fig.4c. Electromagnetic Torque waveform with 220V/50Hz Single Phase supply. Fig.4d. Rotor Speed waveform with 220V/50Hz Single Input supply. Fig.4e. Stator Three Phase Current waveform with 220V/50Hz Single Phase supply. Fig.4. Performance of Three Phase Induction Motor with 220V/50Hz Single Phase Input supply. ISSN: Page 434

5 IV. 1-ø TO 3-ø CONVERSION WITH CYCLOCONVERTER & SCOTT-T TRANSFORMER This paper proposed a circuit for industries to run three phase induction loads with single-phase supply. This is done by MATLAB Simulink. At very first, single-phase supply converted to two-phase supply through two single-phase Cycloconverters. In these two Cycloconverters, one is directly converted single-phase to single-phase with 0 0 Delay. Second one is converted singlephase to single-phase with 90 0 Delay. These two supplies called as two-phase supply. This two-phase supply directly fed to the Scott-T transformer. Here this transformer converts two-phase to three-phase to drive the three-phase induction load. In the second section the Cycloconverter operation explained with sinusoidal pulse width modulation technique (SPWM). But in this section Cycloconverter operated with manual pulse generator. Because, the output of a Cycloconverter having more dc component by using SPWM technique. To mitigate the dc component with LC-filter. We are not got the filter values exactly. Fig.5a. Cycloconverter & Scott-T Transformer Circuit to drive Three-Phase Inductive Loads in MATLAB Simulink. Fig.5b. Electromagnetic Torque waveform. ISSN: Page 435

6 Fig.5c. Rotor Speed waveform. Fig.5d. Stator Three Phase Current waveform. Fig.5. Performance of Three Phase Induction Motor (i.e., With Two Single-Phase Cycloconverters & one Scott-T Transformer). Fig.5. Shows the characteristics of a Three Phase Induction Motor with the input voltage 220V and frequency 50Hz. The main transformer of a Scott-T having and teaser transformer having as input voltage. These two voltages got by the two single-phase Cycloconverters. Fig.5b. shows the Electromagnetic magnetic torque. Fig.5c. shows the rotor speed. Fig.5d. shows the three-phase stator current. Here input is always equal to output voltage magnitude of the Scott- T transformer, because the transformer ratio is 1:1. The main advantage of this is, to get variable speed by varying the frequency at the input side. This is type of mechanism is very easy and simple compare to other control techniques (i.e., controlling of three-phase supply directly). V. CONCLUSION This paper proposes a new topology for controlling a threephase induction motor with single-phase supply. Here to control of Cycloconverter by the firing pulses. With the help of variable frequencies got the variable speeds of a threephase induction motor. The major role of a Scott-T transformer is used to convert two-phase, output of two Cycloconverters to three-phase. ACKNOWLEDGMENT It is our sincere obligation to thank our well-wishers Dr. M. Venu GopalaRao,Ph.D. EEE HOD, Mr. D. Seshi Reddy,M.Tech. (Ph.D.), Associate Professor & Mrs. S.V.N.L. Lalitha,M.Tech. (Ph.D.), Associate Professor in KL University at Vaddeswaram, Guntur Dist. REFERENCES [1] Mazin, Hooman Erfanian; Gallant, Joey (August 14, 2009, 2010). "A Probabilistic Analysis on the Harmonic Cancellation Characteristics of the Scott Transformer". J. Electromagnetic Analysis & Applications 2: Retrieved 20 December [2] Distribution Transformer Manual, GET-2485T. Hickory, NC: General Electric Company pp. 64. [3] Harold C. Passer, The Electrical Manufacturers, , Harvard, 1953, p [4] Rezgar Mohammed Khalil, Maamoon Al-Kababjie, Modeling and Simulation of multi-pulse Cycloconverter fed AC Induction motor and study of output power factor, Al-Rafidain Engineering, vol.15, no.1, [5] Miyazawa, S. Nakamura, F. and Yamada, N. Effective Approximation Suitable for the Control Algorithm of Microprocessor Based Cycloconverter, IEEE Transaction, August [6] Mohammed, B.A., Microprocessor Based Control of Cycloconverters, M.Sc. Thesis, University of Mosul, Iraq, December ISSN: Page 436