A Mathematical Model To Choose Optimal Subsynchronous Cascade Elements In Electric-Motor Drives For Belt Conveyors

Belt conveyors are a dominant mean of transport in modern open-pit mines in Bosnia and Herzegovina. They are usually driven by three-phase induction slip-ring motors of a nominal voltage of 6 kV and nominal power from 200 to 1000 kW. Efficiency of the transport system is the crucial parameter of the level of economical exploitation of an open-pit mine as a whole. The main problem which prevent optimal transport-system efficiency is insufficient synchronization between the production and transport capacities. Since the conveyors in the open-pit mines in Bosnia and Herzegovina are mainly driven by three-phase induction slip-ring motors, the most appropriate solution to control the conveyor speed (the speed of the drive motors) is to use a subsynchronous constant torque cascade with a static voltage and frequency converter. The basic elements of the subsynchronous cascade are a diode bridge rectifier and a thyristor inverter bridge, as well as a power transformer whose nominal power depends on the control range and which sends electricity back to the power grid. The paper describes a mathematical model to calculate the subsynchronous cascade parameters used in controlling the speed of electric-motor drives for belt conveyors.