Simulation and Analysis of Medium-Voltage Low-Speed Cyclo-Converter Synchronous Motor Drive and Issues with on-Load Speed Trimming

Medium-voltage cyclo-converter drives are commonly used for low-speed salient pole synchronous motors in mines. Cyclo-converters give very reliable, efficient and simple solution because they use rugged thyristor devices. Generally, the mines are located at remote locations and the availability factor is very important to minimize Business Interruption loss. However, a cyclo-converter drive has the drawback of generation of sub-harmonics and inter-harmonics along with high-order harmonics. These harmonics are harmful to transformers and weak supply networks and may cause saturation and instabilities. The sub-harmonic frequencies are a function of output frequency and change with output frequency. To mitigate these frequencies, huge passive filters are employed. If the output frequency is fixed, the filter designs are relatively simple but with the variable frequency it becomes complicated. For operational requirements in a mill, on-load output frequency trimming is required. This creates additional problems in terms of MVA sizing of equipment and the requirement of additional filter banks. This paper simulates the fixing and trimming of operating speed of medium-voltage cyclo-converter driven, low-speed, salient pole synchronous motor and brings out issues related to equipment sizing, harmonic mitigation, etc.