Control of a Three Phase Induction Motor using Single Phase Supply

ISSN: 2231-5381

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Andhra Pradesh, INDIA

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 900 phase shift[2] from a three-phase 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 three-phase motors, so a balanced two-three-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].

II. CYCLOCONVERTER

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, Vs is an AC voltage at a frequency, fi 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 αP for

the positive converter and αN 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 Vs, 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).

ISSN: 2231-5381

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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 Vs, 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.

Fig.2a. Positive Gate Pulse for Positive Conversion (for f/2).

Fig.2b. Negative Gate Pulse for Negative Conversion (for f/2).

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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

Fig.3. Scott-T Transformer (2

ø

ø

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 220∟00 and teaser transformer having 220∟900

. 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.

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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.

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single-phase with 0 Delay. Second one is converted single-phase to single-single-phase with 900 Delay. These two supplies called as two-phase supply. This two-phase supply directly

are not got the filter values exactly.

Fig.5a. Cycloconverter & Scott-T Transformer Circuit to drive Three-Phase Inductive Loads in MATLAB – Simulink.

ISSN: 2231-5381

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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 440∟00 and teaser transformer having 440∟900

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 three-phase induction motor with single-three-phase supply. Here to control of Cycloconverter by the firing pulses. With the help of variable frequencies got the variable speeds of a three-phase 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: 18–24. Retrieved 20 December 2011.

[2] Distribution Transformer Manual, GET-2485T. Hickory, NC: General Electric Company. 1996. pp. 64.

[3] Harold C. Passer, The Electrical Manufacturers, 1875-1900, Harvard, 1953, p. 315.

[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, 2007.

[5] Miyazawa, S. Nakamura, F. and Yamada, N. “Effective Approximation Suitable for the Control Algorithm of Microprocessor Based Cycloconverter”, IEEE Transaction, August 1988.

[6] Mohammed, B.A., “Microprocessor Based Control of Cycloconverters”, M.Sc. Thesis, University of Mosul, Iraq, December 1990.