Cat makes an electric commitment to mining

  • 05-Feb-2009 02:52 EST
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Caterpillar and its team of ac-drive engineers and designers, including strategic partner Mitsubishi Electric, started with a clean sheet of paper when designing its new electric-drive system for mining trucks.

Caterpillar has been committed to mechanical drives for its mining truck lineup for nearly 40 years, after offering dc-drive options for a couple of years in the late 1960s.

While Cat continues to believe mechanical drive is the best solution for the vast majority of mining situations, it announced late last year that it would heed to advances in ac-drive technology and once again offer an electric-drive option on some of its mining trucks.

Cat has placed significant emphasis on its mining equipment in general, beyond the decision by Cat senior management to give the go-ahead to include electric drive in its next generation of mining trucks. It is undertaking its “largest commitment ever to the mining industry,” which entails a $280-million investment in design, engineering, and testing, part of which will fund updated versions of its mechanical-drive trucks, as well as the new ac-drive options.

Mechanical drive offers high efficiency and an “automotive feel,” says Cat, but electric drive complements the mechanical drive by increasing hauling capabilities. Compared to dc-drive systems, ac-drive systems with solid-state controls and brushless motors offer improved drivetrain efficiency, delivering more rimpull over a higher speed range.

The 793 series in the 240- to 250-ton (218- to 227-t) class will now be available in both mechanical (793F) and electric (793F AC) versions. The 793F will feature a larger drive shaft with a fully enclosed drive shaft cover and sport a new transmission with electronic clutch-pressure control that Cat says will contribute to better serviceability.

Compared to the 793D in terms of representative cost/ton, Cat says the 793F is 3% lower in coal (moderate uphill), 4% lower in oil sands (high rolling resistance), and 5% lower in deep pit copper (severe uphill).

Also available in electric drive only will be the 345-ton (313-t) 795F AC, a new size for Cat.

Partner Mitsubishi Electric, considered the world’s largest supplier of traction IGBTs (insulated-gate bipolar transistors) and power inverter devices, is supplying Cat with power semiconductors and inverter components, as well as sharing its core knowledge of ac technology for both the 793F AC and 795F AC models. Complementing the strategic relationship with Mitsubishi, Cat says that it recruited experts with 100 years of heavy-traction ac experience for the project, including seven PhDs in electrical systems and controls and specialists in ECMs and control systems.

Internal Cat experience that went into the ac-drive development included its expertise with CVT and hybrid drive software, ECMs, power generation, and systems integration.

Some innovations of the overall team on the new ac-truck models include four-corner blended braking and retarding (also available on all mechanical-drive trucks), remote-mounted generators with permanently greased bearings, isolated wheel motors, a radial retarding grid, a hydraulically driven cooling system, and proprietary controls and software.

For blended retarding, the operator can select the use of 100% rear dynamic retarding or four-corner retarding with dynamic retarding on rear wheels and proportioned braking on front wheels using Cat oil-cooled wet disc brakes. Cat says four-corner oil-cooled braking provides increased brake capacity and control over dynamic braking alone. It also contributes to improved safety. Incident reports garnered at a mine over four years showed that oil-cooled brakes with four-corner retarding had an incident rate of 0.19 (73 machines, 14 incidents) while the rate for electric rear-axle retarding was 0.58 (19 machines, 11 incidents).

Cat's traction motors are mounted inside the axle, enabling the separate servicing of final drives. Speed sensors are mounted on board for serviceability from the rear-axle housing hatch. The bearings are greased from the center of the axle housing. 

An anti-rollback feature automatically engages brakes to prevent machine rollback when on grade, while the steering assist feature uses the ac-drive system to controls torque transfer to individual rear wheels, enhancing normal steering, reducing tire wear, and providing tighter turning radius.

Twenty-two mechanical- and electric-drive prototypes and pilots were built as of last September, with 15 more scheduled to be built. The trucks will be phased into production over the next two years, with the ac-drive models scheduled to go into production in late 2010.

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