The design and development of a solid state, single phase, variable-frequency motor speed control as a working laboratory test and demonstration unit

Includes bibliographical references.Includes illustrations.Last page labeled 63.Statement of the Problem. Direct current machines have dominated the variable motor speed control industry as a result of the relative ease of affecting speed regulation through direct current voltage regulation. However, due to (1) the recognized superior characteristics of a.c. squirrel-cage induction motors and (2) the development of a high current, high speed switching semiconductor, known as the silicon controlled rectificer (thyristor), variable-speed control of a.c. machines has been accomplished with the use of variable-frequency inverters. The object of this study was (1) to present an account of contemporary motor speed control systems, (2) to design a rectifier inverter variable-frequency speed control for use with a single phase fractional horsepower a.c. squirrel-cage induction motor, (3) to construct the variable-speed control as a working laboratory experiments to disclose the characteristics of the unit, and (5) to analyze the results of the experiments. Sources of Data. Research design information and an account of contemporary motor speed control was obtained primarily from a review of published materials such as textbooks, technical publications, and periodical journals. Technical data was obtained as a result of rests and experimentation with motor speed control equipment designed and built as a part of the experiment. Method of Research. This study was an experiment involving (1) the design and construction of a parallel capacitor commutated inverter drive capable of delivering single phase variable-frequency power to a fractional horsepower a.c. squirrel-cage induction motor, (2) the selection and construction of a variable-frequency oscillator controlled bi-stable multivibrator to produce square wave pulses for triggering the inverter thyristors, (3) the design and construction of pulse amplifiers for the multivibrator outputs, (4) the selection and construction of a direct current power supply for the inverter, (5) the design and construction of a demonstration unit chasis, and (6) the design and performance of a set of experimental tests revealing the characteristics of the variable-frequency adjustable-speed drive. Findings. The study indicated (1) that variable-speed control of single phase squirrel-cage induction motors is possible through the use of parallel capacitor commutated inverters, (2) that speed control is smooth and free from hunting through a speed range ratio of 6 to 1, (3) that the power conversion is most efficient at the resonating frequency of the commutating elements, (4) that the motor acts as a constant horsepower unit with high starting and low speed torque characteristics, and (5) that transformer saturation characteristics have a substantial effect on output voltage waveform. Conclusions. The results indicate that the parallel capacitor commutated inverter can be used successfully to drive single phace a.c. motors in industrial applications where low level constant horsepower variable-speed control is needed. Although the analysis of operation is complex, the circuitry of the drive is quite simple and could be almost maintenance-free. The drive makes full use of solid state components, insuring reliable operation and long life. Considering the advantages and limitations on the basis of the system's prime function as a small motor drive, this approach to variable-speed motor control appears to be sound.M.S. (Master of Science