Electricity, not Diesel. A 3,000 hp diesel engine generates the current. So why doesn’t it just propel the truck too? "The reason is simple. It’s just not worth putting the engine and gears of a car onto the slopes of a mine. A gearbox powerful enough to handle the workload required of these trucks would be enormous, and would also need a lot of maintenance," says Köllner, explaining the drawbacks of purely mechanical propulsion.
Not only do the trucks dispense with gearboxes. Thanks to their electric drive systems, they also do without clutches and brake disks in normal operation. Electrical resistors are used to brake the vehicles, and speed can be steplessÂly adjusted via three-phase current frequency. "Such trucks are essentially driven like a car with an automatic gearbox," says Köllner, who is an engineer and has actually driven one of the behemoths.
For over 30 years now, Siemens has been using three-phase current drives for mining vehicles. "The rotating electric field can be transformed directly into mechanical rotation," says Köllner. Some manufacturers, on the other hand, still prefer DC drive systems. In such motors, however, the current has to be constantly interrupted and re-engaged to generate a rotational movement. This limits the revolutions per minute that a motor of this type can attain. And it requires more parts that need to be maintained regularly. "Our alternating current motors can deliver up to seven percent more performance from the same amount of energy, and downtimes for maintenance and repair work are rare," says Köllner. "Generally, just one technology check a year is all that’s needed."
Giant Trucks, Zero Emissions. AC drives also form the basis for a developÂment from Siemens that can significantly speed up the transport of mining products: trolley trucks. Such vehicles function like streetcarsâ€â€sporting antler-like pantographs that can be raised and lowered at the press of a button. This means that the driver can link the truck to overhead conductors (catenaries), which are generally installed on steep slopes. "This is where conventional trucks, despite their 3,000 plus hp, can only advance at a snail’s pace," says Köllner. The catenaries can provide the drive systems with almost 6,000 hp. This means that the truck’s speed can almost double, and the mine operators can reduce the number of expensive mechanical giants they need to have on site.
The environment benefits from trolley technology too. There are no local emissions, since the diesel engine switches itself off automatically when contact is made with the overhead line. What’s more, the braking energy that is released when a truck rolls downhill is fed back into the network via a second pair of conductors. Thanks to all these benefits, the technology quickly pays for itself, says Köllner. "After no more than three years, a mine operator can recover the costs of buying the trolley trucks and the costs associated with the installation of the overhead lines."