A variable-speed drive for local manufacture

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Rajamani, Haile-Selassie
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Recent developments in the design of variable-speed drives have been aimed at the control of a standard squirrel cage induction motor because of its many attractive features including robustness, reliability and low maintenance requirements. This dissertation demonstrates the design of a drive package that meets specified requirements, while being sufficiently simple and economic to be attractive to local industry. The drive is designed for applications that do not require very precise speed control and fast dynamic response, such as water pumping and food processing.In Chapter 1 an overview of variable speed drives is given.Chapter 2 describes the open loop control strategy used.Constant torque capability and acceleration of the motor are studied, illustrating the advantage of variable speed drive. A review of Pulse Width Modulation (PWM) techniques and their implementation is given in Chapter 3. Chapter 4 presents a study of asynchronous PWM technique using a high carrier frequency with particular reference to voltage utilization and harmonic distortion. An optimum waveform is suggested that ideally provides a pure sinusoidal output current. The effect of dead-time and harmonic losses are also considered. Chapter 5 describes the inverter circuit built with MOSFET's. The following chapter describes the implementation of the control technique using a specialized integrated circuit, the MAS 18, and a computer. In Chapter 7 the implementation and testing of the drive system is detailed and the results presented.The optimum waveform suggested is shown to give good utilization of rail voltage. The current distortion is also shown to be low. In Chapter 8, the practicability of the simple design is discussed.The final drive unit is shown to be very simple consisting of the inverter circuit, the MAS 18, a micro-controller chip and a low voltage power supply. The inverter circuit is also a simple design that avoids the use of transformers and opto-couplers.Possible modifications and suggestions for future work are given.
Variable Speed Drives , Electric Driving, Variable Speed