Analysis of 12 pulse three- phase to three - phase cycloconverter drive for induction motor load

Cycloconverters are widely used in speed control of induction motors in industrial applications due to the capability of the drive to start high power motors on full load at very slow speed. A 6-pulse three phase to three phase cycloconverter is a widely analyzed and commonly used circuit. The advantages of 6 pulse cycloconverters are mainly due to its less weight and volume and it proves to be the most cost-effective solution in ship propulsion application. On the other hand, the drive induces high harmonics in input and output currents. Also, it has a low maximum output frequency compared to the input frequency. In addition, the number of harmonics present in step up and step-down operation of the output frequency is very high. These drawbacks limit the application of this high-power converter in induction motor speed control operations. In order to reduce the harmonics and to increase the output frequency with minimum current and torque ripples, the existing 6 pulse converter is modified into a 12-pulse converter and the design of 6 pulse and 12 pulse cycloconverter is discussed in this paper. The performance of the converters using induction motor load is analyzed. The analysis is carried out at a step-up frequency from 25 Hz to 50 Hz and step-down frequency from 25 Hz to 10 Hz. It is observed that 12 pulse three phase to three phase cycloconverter has fastest response time in current and torque control with low harmonic distortion in step up and step-down frequency operation