This thesis deals with the design and implementation of a three-phase prototype UPS system in order to measure the performance parameters of converters included in the practical UPS systems (rectifier and inverter). The harmonic components present in the output voltage waveform of the six-pulse three-phase bridge rectifier for different firing angles are investigated. It is found that the order of the generated harmonics are the same, but the amplitudes of these harmonics are increased with the increase of firing angle, and the acceptable range of firing angle to give good performance is found to be 30°≤ a<60°. With a<30° the average value of the output D.C is greater than the line to line input voltage and this requires high rating components. Furthermore when a > 60° the amplitudes of the generated harmonic components are increased to a value that gives bad performance. For the practical three-phase bridge inverter controlled by PWM technique, three strategies are used and investigated to control the output voltage of the inverter. The first strategy is the square wave PWM which gave bad performance since its lower order harmonic is found to be greater than 7%, The second strategy used is the sinusoidal PWM which showed some improvements in the performance parameters, and the lower order harmonic is found to be critically acceptable. The third strategy used, which is the modified sinusoidal PWM, showed a good improvement in the output performance parameters of the inverter. Finally, a comparison between the performance parameters of the three strategies is given and discussed. It is found that the modified sinusoidal PWM is the best technique among the three techniques.