Computer modelling techniques for industrial and marine cyclo-converter drives
Large cyclo-converter drives using either synchronous or induction machines are found in a number of low speed high torque applications: these include steel mills, gearless cement mills, mine winders and ice breakers. The cyclo-converter output frequency is synthesised from portions of the high voltage supply system. At low frequencies this produces an output current waveform which is reasonably sinusoidal, as the frequency increases the waveform become more distorted. There is concern for the level of current harmonics produced by the cyclo-converter in both the motor windings and the supply system. The waveform distortion is mainly due to the switching action of the cyclo-converter, this is modified by the effects of interbridge changeover delay, commutation overlap, the supply and decoupling reactances. Industrial drives are required to conform to the Electricity Council's regulations on input current harmonics and voltage waveform distortion. In marine drives the main generators feed the ship propulsion system and the ship's auxiliary supplies. The level of interference produced by the converter input line current on the supply network is of particular concern. High levels of voltage distortion may cause malfunction of the auxiliary equipment and produce large torque pulsations in the supply alternator. The cyclo-converter generates unwanted harmonics in the output current waveforms, these are responsible for torque pulsations on the motor drive. With naval marine drives the torque pulsation is of particular interest, since this level must be minimised to increase the sensitivity of detecting instruments such as passive sonar. To date, mathematical models and computer based models have not detailed all the aspects which are responsible for the distortion to the converter input and output waveforms. This thesis seeks to address this and identify the effects on the converter performance of, the converter interbridqe delay, overlap and the supply parameters supply and decoupling reactance. Various motor models will also be examined. The author has used the mathematical work by Pelly as a basis, and has identified the percentage errors in the level of harmonic currents which are likely to be introduced if his results are used in designing new systems. Comparisons will be made with experimentally produced results obtained from a mine winder at the Wearmouth Colliery. Results from a lower powered cyclo-converter produced experimentally and computationally at Newcastle University will also be examined.