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The Story of Electrical Energy, Pt.15 While there is now considerable interest in battery powered cars, electric and dieselelectric heavy-haul rubber tyred vehicles have been in use for some time and they are huge. Imagine avehicle weighing 85 tonnes and powered by half a megawatt of electricity! By BRYAN MAHER The mining industry is a source of inspiration for electric and mechanical engineers alike. All modern mining concerns think big; big machines, big problems, big costs! This month, we take a look at three vehicles designed for very specific applications in underground and open cut mines. Each uses electrical and mechanical engineering in unique ways to increase material throughput at less cost. Deep underground hard rock mines, such as Mount Isa in western Queensland and Kristine berg in Sweden, produce vast quantities of valuable metals: copper, silver, nickel, lead and so on. The ore must be won, transported and crushed before smelting can extract the pure metal. Such mines invest colossal sums in vertical lift shafts more than 1000 metres deep, together with the cages and hoist equipment. At many levels, networks of horizontal tunnels, up to 6km long, criss-cross the ore body as the miners dig into the rock. Lifts versus ramps Sometimes, it is necessary to work a new ore body below areas presently accessed by lift hoist. For example, the Mount Isa 3000 ore body lies below the existing R62 and U62 vertical shaft systems. In such cases, engineers must assess the comparative costs and viabilities of either deepening the ·lift system or accessing the new ore by ramps from old areas. Where ramps are chosen, large rubber tyred vehicles are used to haul the ore up sloping tunnels to existing underground primary crushers. To maintain production rates, such vehicles must be high-powered so that they can transport large payloads uphill at a reasonable speed. As well, all such vehicles must be of low weight, low profile, highly manoeuvrable and should generate minimum noise, heat and noxious gases. Any heat and/or gases generated by underground vehicles only places an extra burden on the mine ventilation/cooling system. The Kiruna electric truck This old photograph shows one of the GE 100-tonne diesel electric dump trucks used in a South African copper mine. The overhead trolley wires gave an electric boost to the trucks as they drove up the steep access ramp. 86 · SILICON CHIP To meet such a need, the ABB company (ASEA-Brown Boveri), in cooperation with Kiruna Truck AB of Sweden, produced a very special electric dump truck in 1985. It amply fulfilled the above criteria, with the flexibility only a rubber tyred vehicle can provide. Although a large vehicle, its low cross section and small turning radius allow it to easily negotiate underground tunnels. In addition, the Kiruna truck is designed for most types ofloaders. Front loaders, side dumping buckets, Joy loaders and even small power shovels have all been successfully employed. Mount Isa Mines has purchased two Kiruna electric trucks and may obtain more, because of their ad- vantages over other types of rubbertyred load shifters. The Kiruna electric truck model K1050E is a 4-wheel drive vehicle powered by two 230kW DC locomotive traction motors. Electric power at 1000V 3-phase 50Hz is provided by an overhead assembly of three parallel conductors, together with two steel guide rails. As shown in one of the photos accompanying this article, the two outer grounded steel rails support sets of porcelain insulators which hold the three live overhead conductors. The truck makes contact with these conductors using a hydraulically hoisted multiple trolley pole. The driver can automatically raise the boom and accurately connect to the overhead wiring in 3-5 seconds. And that can be done even when the truck is not squarely below the conductors above. Because most deep hard rock mines have high ambient temperatures, the Kiruna truck includes air conditioning for the driver's cab, the computer and control boxes and the battery compartment. - J ,,,,, i!~ ---.... t.if.,,~-•i. fl ' ' .. The overhead system The overhead conductor assembly is of simple lightweight construction. The grounded outer steel members are of square section and are mounted cornerwise. These form the sup-port frame for insulator brackets which support the three live phase conductors, each a copper tube 50mm in diameter. The whole structure is bolted to the tunnel ceiling at 5-metre intervals. The minimum radius of curvature of the overhead trolley line is 15 m,etres. The current requirement is approximately 400-500 amps per truck, allowing for acceleration and uphill running. The trolley line is divided into 1km sections, each supplied at its centre by a 1600kVA transformer. The design provides for any section to power either three trucks running or two trucks accelerating uphill. The supplies to each section are set in the same phase rotation and in phase to allow trucks to run through section junctions at full speed. Control system The motor control uses a microcomputer system, high-powered This photo shows the Kiruna electric truck and the 3-phase electric supply on the mine roof. With electric trucks such as these, there is little noise or heat and no air pollution - all important considerations in an underground mine. thyristor converters and chopper circuits. The microcomputer is also used to accurately align the boom when the driver raises the trolley pole to make contact with the overhead conductors. A thyristor converter produces a variable 0-1200V DC rail from the 1000V 3-phase supply. The two 600V DC traction motors are connected in series, one motor driving each axle through planetary and differential gears. Each motor has its fields connected in series with the armature, giving immense starting torque proportional to the square of the starting current. Once accelerated on level track, the fields can be weakened, allowing the motors to run up to a maximum of 2000 RPM. During acceleration, the control sys- tern allows the motor current to rise to 600 amps. The vehicle is rated at 650kW for 30 minutes, reducing to 460kW continuous. Overhead trolleyline The act of attaching the trolleypole to the overhead lines is a veritable work of art in engineering design. This part of the development program was not at all easy. As the diagram of Fig.1 shows, the square steel bars hang lower than the conductors. With the electric contacts withdrawn (as a cat withdraws its claws), the trolleypole is raised, at first contacting the steel sections. Magnetic sensors establish the degree of misalignment. The computer then actuates small horizontal hydraulic rams to rotate the contact head until it is NOVEMBER1991 87 controllers in the truck in the correct sequence. MINE ROOF PORCELAIN INSULATORS ,.......~-,.--,------r----r7-\;.-~-~~ ooov ..,/ '1 LIVE 50mm COPPER TUBES Battery backup - FIXEO OVERHEAD STEEL VERTICAL HYDRAULIC RAM LIFTS CONTACTS - TROLLEYPOLE 3-PHASE 1000V SOHz TO TRUCK Fig.1: this diagram shows the main components of the 3-phase pickup system used by the Kiruna electric truck. The trolleypole is actuated and aligned by computer before the carbon brushes make contact with the lkV 3-phase supply. aligned with the overhead conductors. Finally, a vertical hydraulic ram raises the set of three separately insulated contacts. The actual connection to each overhead live copper tube is made via double carbon brushes , while multiple springs set the contact pressure. Phase rotation Because the truck may be facing in any direction before attempting overhead contact, once connection is made, the 3-phase supply as picked off by the trolleypole may be in forward or reverse rotation. Therefore, before connecting the current to the thyristor motor controllers , the computer first tests the supply phase rotation direction. It then closes either a forward or reverse electromechanical contactor, thus connecting the 1000V 3-phase supply to the thyristor motor !.E.,,;. ;r..~t.:.11: RCS Radio Pty Ltd is the only company which manufactures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207. Phone (02) 587 3491. 88 SILICON CHIP A large 250 volt 70 amp-hour traction battery of nicad cells mounted in the truck enables it to be driven outside the range of the overhead trolley line. In this mode, it is limited to one quarter full speed and a range of about 200 metres. This is used during loading, unloading and at intersections. The on-board computer also monitors the battery charge condition and regulates charging current when operating on the overhead power. The actual battery power available for offline running depends on the amount of time the truck has just spent running on the overhead power, charging the batteries. The battery is also used for vehicle lighting, powering the computer and the pumps for the hydraulically activated trolleypole, and for truck dumping. The battery circuits operate through a separate chopper isolated from the 1.2kV system. Performance The Kiruna electric truck weighs 35 tonnes empty and can carry a 50tonne payload. Fully loaded at 85 tonnes gross weight, the truck can travel at 18-20km/h up a 12% incline - twice the performance of a diesel truck of similar capacity. Rated top speed is 50km/h. Other important characteristics are that the electric truck is very quiet, is pollution free and generates very little heat. These last attributes are extremely important in deep mines. Down there, the ambient temperature is already quite high, requiring a general mine cooling and ventilating system. Kiruna diesel truck For applications where overhead electric power is impracticable, ABBKiruna provide two diesel truck models of unique design. They both use front wheel drive , thus eliminating problems of bulky differentials, planetary gearing and intermediate shafts. Interestingly, the truck body doesn't just hold the load but is also part of the exhaust system. Firstly, the exhaust gases flow through cooling coils mounted in the body, then pass into the scrubber which contains a large volume of cold water. The poisonous nitrous gases in the exhaust are far more soluble in cold water. The hot gases do heat the water but such is the volume of the water that it does not reach boiling temperature. Therefore , no steam is ejected. Every second shift, the scrubber is flushed and refilled with cold water. Thus, there is no need for continual topping up. This cold water scrubbing process acts as a purifier, removing 95 % of all hydrocarbons and carbon monoxide from the exhaust gases. Also extracted are 95 % of the aldehydes, acroleins and formaldehydes present in diesel exhaust fumes. Unless removed, these toxic gases would be injurious to the eyes, noses and throats of the miners. Electric versus diesel Whether to choose the electric or diesel trucks is a question which must be considered for each project, as circumstances vary. In some mines, the installation of the electric trolley line may be a deterrent. But in others, the high cost of diesel fuel and extra ventilation systems may sway engineers in favour of electrics. Then again, the electric truck carries the greater payload. Variants of the Kiruna truck are also made for use in other materials handling applications, such as in steel works . OVERHEAD TROLLEY WIRES ABOVE RAMP +750V NEG ELECTRIC ASSIST CIRCUIT 4 TRACTION MOTORS STANDARD DIESEL ELECTRIC TRUCK SYSTEM 3-PHASE BRIDGE RECTIFIER Fig.2: this diagram shows the trolley system and 750V DC boost used to assist diesel electric dump trucks operating in an open cut mine with a steep access ramp. Early GE trucks Though open-cut mining may look like a straightforward operation, some giant-sized problems arise. The common practice is to have very large diesel-electric trucks loaded on the floor of the open cut, then driven up the steep ramp to ground level. In one application at the Palabora copper mine in South Africa in the late 1960s, as the mine deepened the slope became too great for the biggest trucks to climb up the ramp and out of the pit. Therefore, the GeneralElectric Company of the USA designed and built an attachment to a standard 100-tonne capacity dump truck. Each wheel is driven by an independent DC electric motor, all powered by an alternator driven by the supercharged diesel engine. High current rectifiers provided a heavy DC supply for the traction motors, using techniques similar to those employed in diesel-electric locomotives. Some mines use the Kiruna diesel truck and these need to be equipped with a complex exhaust gas absorbing system to ensure that pollutants do not enter the mine. They are more expensive to run, are noisier and not as powerful as their electric counterparts. This arrangement provided ample drive for running around the roughly level floor of the open cut pit but extra power was needed to climb the ramp. DC series traction motors can supply enormous power for short times but the limiting factor in this case was the diesel engine. To solve the power problem, twin overhead electric trolley wires were installed above the ramp roadway. These were supplied with 750V DC by a transformer and rectifier system. The truck was loaded and ran across the pit floor under its own power. Upon reaching the ramp, the driver simply raised the trolley poles to contact the overhead DC supply lines; this was done without stopping. This electricity source then provided much more current to drive the traction motors to full power. After reaching the level ground surface, the driver lowered the trolley poles and continued to the dump site self-powered. Acknowledgements .Thanks to ABB Action, ASEA Journal, John Burton and General Electric for information and photos. SC NOVEMBER 1991 89