Capital Equipment News June 2017

which it launched in 2014 in southern Africa. Hitachi pioneered the AC-3 technology to its RDT range in 2012 with the launch of the EH5000AC-3. The AC-3 technology was developed using advanced propulsion technology that also powers the OEM’s bullet trains in Japan. Hitachi Construction Machinery Southern Africa introduced the new EH4000AC in the local market in 2014. Its trolley assist is a fuel- saving method which is not available on mechanical drive trucks. It is provided by an overhead catenary power supply supported by poles mounted along main haul roads. Another key benefit of Hitachi’s AC-3 drive system is that it comes with a slip/slide control system. If the system senses slipping or locking of rear wheels when travelling on slippery or frozen roads, it adjusts the torques of the wheel motors accordingly, giving the truck better stable travelling. The AC-3 drive system also incorporates a pitch control system. If this system senses pitching when travelling on bumpy surfaces or an abrupt stop, it adjusts the torques of the wheel motors fittingly to reduce pitching on the truck, ensuring better operating comfort and less load spillage. If the AC-3 Drive Control System’s side skid control function senses oversteer or understeer from the newly equipped sensors on the unit, it adjusts the torques of the wheel motors accordingly to stabilise the side skidding. This gives the gigantic truck smoother movement and more stable steering. While dynamic retarding force is said to be another key benefit for electric trucks, Ingle argues that this is only applied to the rear wheels. “In many cases electric trucks can quote a high retarding power or fast ‘on paper’ retarding speed. In practical field applications, electric drive retarding speeds are mostly reduced to provide protection from exceeding the electric drive retarding capability envelope and because competitive electric drive trucks do not have braking force on their front wheels like Cat mechanical drive trucks (and the Cat 795F AC) which allows a more effective braking force and good machine control – resulting in the ability to better utilise potential retarding speeds,” he says. Ideal conditions According to Lavrenov, BELAZ’s argument that the larger end of the payload scale, starting from around 130 t, needs AC/AC systems is based on the fact that from this payload, trucks are involved in large-scale mining where machines need to get over steep gradients, and electric trucks are preferable. He, however, notes that both drives need good haul roads with gentle sloping, quality maintenance and service, responsible and skilled operators and loading within the rated payload. “However, this is possible in an ideal world only,” says Lavrenov. “At BELAZ we are of the opinion that there are more variants and configurations of operating conditions which suit electric trucks than mechanical. For instance, electric drive is an obvious choice for deep pits where trucks need to haul material from the bottom and get over steep slopes,” says Lavrenov. He adds that from a production point of view, mechanical trucks are notably slower on gradients when moving uphill loaded. Meanwhile, Ingle argues that historically DC drive trucks were used in more flatter, moderate grade or easier applications, while mechanical drive was used in poor underfoot and more severe hauls. “However, AC drive has changed many of the past electric drive limitations. Today, mechanical drive is still recognised as best for many applications, but ultimately, it is more of a function from a mine’s point of view,” says Ingle. Ingle, in conclusion, tells Capital Equipment News that when look- ing to the future of drive technology for large mining trucks, the industry tends to think of some sort of electric hybrid car approach. However, Caterpillar continues to research energy recovery for both mechanical and electric drive systems. Ingle notes that at this time energy recovery for either of the drive solutions is not cost effective to implement. b

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