Cat sets plan of attack on ac electric drive

  • 19-Jun-2008 08:45 EDT
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In the D7E powertrain, the diesel engine drives a generator to produce electricity that ultimately powers two ac-electric-drive motors, which are connected to a differential steering system.

­­“When we gave Caterpillar engineers the opportunity to design a brand new tractor from the ground up, two things happened,” said David Nicoll, Commercial Manager, Track-type Tractors. “First, they were very excited and passionate about it because they don’t get that opportunity very often. Second, they did some amazing things.”

With over 100 patents that have either been applied for or granted, what those engineers ended up developing is an ac-electric-drive system specifically for track-type tractors.

“Often when we talk about electric drives, many people think of locomotives or off-highway trucks, and that’s true, but they use dc technology,” said Amy Moore-McKee, New Product Manager, Track-type Tractors. “A dc-drive system is a rather effective system, but it is hard to maintain the motor pa­ds and brushes.”

Cat has taken advantage of advances of both price and size in powerful semiconductors to provide its newly announced D7E track-type tractor with a brushless ac-electric-drive system. The D7E will ramp up in production and roll out into regulated markets next year, with the DR7 it is replacing to eventually be discontinued in North America. Compared to the D7R Series II, Cat says that even with the same blade, the D7E will deliver 25% more material moved per gallon of fuel, 10% more productivity to the ground, and 10% lower lifetime operating costs.

Using the ac-electric drive enabled engineers to design the system so that the engine operates over a very narrow engine-speed band, reducing the rated speed from the DR7’s 2100 rpm down to about 1700 rpm. “Getting into a narrower engine operating range reduces fuel consumption,” said Moore-McKee. “A narrow range also enhances engine efficiency and durability.”

A 235-hp (175-kW) C9.3 ACERT engine is used in the D7E to drive a generator that produces electricity to power two ac-electric-drive motors, which are connected to a differential steering system. The generator is controlled by a power converter, which is what allows for the brushless design and elimination of solid-state electronics.

“The motors and generator are sealed and liquid-cooled, so this tractor can operate in the muck up to the fan level without any problems,” said Mike Betz, Engineering Manager, Track-type Tractors.

A traditional mechanical transmission is not needed because the electric motors essentially function as a continuously variable transmission. The electric drivetrain has 60% fewer moving parts, and thus fewer parts to maintain, compared to the previous D7’s, with components such as the driveshaft, torque converter, clutch, valves, and belts being eliminated.

Eliminating parts also contributed to design flexibility. “We were able to put components at optimal locations in the machine to increase visibility and increase performance,” said Moore-McKee. “Also, we were able to make use of the electricity on board to provide power to accessories such as the electric drive pump, electric fuel pump, and electric air-conditioning.”

The electric air-conditioning system is a self-contained module that is mounted outside the cab, “much like you see a window unit in a house,” said Betz. “There’s an electric-driven compressor in this unit. It’s all in one box so there are no lines running through the tractor, making it more reliable and reducing operating cost and downtime for the customer.”

Despite so many electric accessories available with the electric drive, the steering system still uses a hydraulic pump.

“The value equation right now for the steering system having electric steering vs. hydraulic steering doesn’t pan out at this point,” said Betz. “We don’t drive each of the tracks with a motor on each side, the two motors act as a single motor, so we have the means to drive the differential via a hydraulic system.”­

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