The design and optimization of a system using an induction motor driven pump, powered by solar panels

Master Thesis

1992

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University of Cape Town

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This thesis describes the design and evaluation of an induction motor driven water pumping system which is powered by solar panels. The system consists of a positive displacement pump, solar panels and an induction motor with a microprocessor controlled inverter. The reason that an induction motor has been chosen for the project is that these motors are cheaper and more robust than the more conventional DC motors. It is expected that by using an induction motor, the system performance will improve significantly for the same investment. The motor has a power rating of 0.75kW and it has been specially designed for a solar application. The system has been designed to operate from between five and seven solar panels, which yields a system capacity of 350W. The capacity could be extended to operate up to the full rating of the motor. A variable frequency drive has been designed to control the motor speed. The drive consists of a power MOSFET inverter bridge which is controlled by an 8031 microcontroller. Software has been written for the controller to generate the required pulse-width modulated signals to the inverter. Also included in the system design is an array tracker which optimizes the power output of the solar panels. The efficiency of the motor has been optimized for the torque requirements of the pump. This has been achieved by implementing an optimized voltage frequency curve and by providing for operation above the rated frequency of the motor. The motor has been operated in the frequency range of 5 - 80Hz. The inverter efficiency was high at 87% and this is expected to increase at higher power ratings. The combined motor and inverter efficiency was found to be 67% over a frequency range of 45 - 80Hz. This is only marginally less than the efficiency found in DC systems where a DC-DC converter is required to drive the motor. The control method for the system incorporated a method of maximizing the water delivery. This was achieved by optimizing the motor speed while monitoring the panel voltage. The voltage was monitored because of the high inertia of the pump, which made pure speed control difficult to implement. A field test was conducted to compare the developed AC system with a Mono DC system. The gearing of the DC system was not optimal and hence a higher flow rate was achieved with the AC system. However, the efficiency of the DC motor and converter combination proved to be slightly higher than that of the AC system. This comparison neglected the effect of poor maximum power point tracking of the DC system. In conclusion, the implementation of an AC induction motor system offers significant advantages over a DC system in terms of cost and reliability, while similar efficiencies are expected from the two systems. The cost reduction with a seven panel system will more than cover the cost of another panel, which represents a 15% increase in the system input power. The speed control of the system ensures that the water delivery is maximized at all operating irradiance levels and hence the panel output is fully utilized. The system performance is further enhanced with the use of an array tracker, which will improve the panel output by approximately 20%.
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