MINING SYSTEMS AND TECHNOLOGY –SYNCHRONOUS ELECTRIC DRIVES FOR GRINDING MILLS

摘要

The grinding process in the mining industry has seen an ongoing increase in the size and throughputof AG, SAG, and Ball Mills to accommodate the industry’s needs for greater production and loweroverall production costs.Up to 8 - 9 MW, the mill can be driven by a single electric motor turning the mill through a pinionand ring gear attached to the periphery of the mill. The motor is typically low speed for improvedreliability; although medium speed geared drives are also used. Above the single-pinion drive limit,the dual pinion approach was developed to enable two drive motors to be utilised. This allowed themill input power to be increased to 16 - 18 MW, with corresponding increases in throughput.Recent developments in clutch and ring gear materials and design are allowing greater input powersof 20 - 22 MW to be considered. A joint analysis is currently underway between the mill builder, ringgear, clutch and motor suppliers to determine the allowable increase in motor power rating.To avoid overloading and premature failure of the ring gear or pinion by uneven load sharingbetween the two motors, it is critical that the two machines accurately share load. This is typicallyachieved by one of the following strategies:1. The application of drives capable of controlling load torque between synchronous or inductionmotors. This approach comes at the cost of increased losses and higher running costs, andgenerally higher initial cost.2. For those mills where fixed speed operation is advantageous, GE uses two synchronous motorswith a proprietary control system that accurately controls the load torque from each motor. Thissystem has several unique advantages, including the delivery of leading VARs and bus supportto the power system. Since many mine sites are located at the end of long transmission lines, ordependent upon local generation, these advantages can be significant. Each motor is connectedto the mill through an air-activated clutch to allow smooth motor starts.For mill power ratings beyond the current limits of the dual-pinion system, the industry currentlyuses the ring motor. This design employs a single, shaftless synchronous motor, where the motorstator is located around the periphery of the mill and the rotor poles are fixed to an annular extensionof the mill shell. The mill size requires the motor stator to be built in several ‘segments’. Practicallimitations in motor design necessitate that the motor is supplied by a low frequency, low voltage, VFdrive, which does not allow the motor to deliver bus and power factor support, or to be bypassed tothe power system in the event of a drive failure.There is no standard type of drive suitable for all grinding mills. Tradeoffs exist between the initialcapital costs, operating costs, and process optimisation. A complete cost analysis of the various drivetechnologies available that includes initial cost, net present value of the project operating costs, andmaintenance costs should be made on each project to determine the optimal technology.