AC-Drive systems for mine hoists- The Future
The need for cost cutting in mining puts focus on cost of mine hoisting. High efficiency and availability as well as low maintenance cost become increasingly important issues. Higher network quality calls for drive systems that do not generate harmonic currents and reactive power. Manufacturing and service facilities for low speed high torque DC-motors are becoming scarce. This exposes a risk for the mine production; a motor failure could stop production with unacceptable consequences.
For several years, new large mine hoists have been provided with AC motor and cyclo-convertor. However the generation of harmonics and reactive power has been the major drawback of this solution. Mines are now looking to replace existing DC-drives with AC-drives for their hoists.
The technology that fulfills requirements of the future is now available. AC motors controlled by smart frequency converters are connected to the network without generating reactive power. The harmonics produced are well within the limits of the power utilities without harmonic filtering. Because only active power is consumed, the voltage drop is dramatically lower than when using DC or cyclo-convertor drive. This means that the new drive technology permits weaker power line.
The paper describes the DTC, Direct Torque Control drive as used in mine hoists for direct-coupled synchronous motors and high-speed induction motors connected via gearbox. Practical experience from operating hoists with the new technology is discussed.