Electric drive increases efficiency
The Caterpillar D7E is generating a lot of excitement due to its revolutionary drivetrain design. But don?t get confused ? this is not hybrid technology.
?The D7E has electric drive. There is no complex switching back and forth between the systems,? explains Amy Moore-McKee, new product manager, Caterpillar. ?Essentially, the engine drives a generator to continuously supply electricity to power the drive motors and the other machine systems.?
This translates into increased fuel efficiency. ?The engine is optimized to run at a lower rated speed and in a narrower operating range,? says Moore-McKee. ?In addition, the electric drivetrain is also much more efficient than a powershift transmission with a torque divider. There is no gap between gears on the shift curves. When you bring both the engine and the drivetrain together, that?s where we get up to a 20% reduction in fuel usage.?
The continuously variable transmission allows maximum power to the tracks at all speeds, increasing productivity. ?We have demonstrated on average that 10% more material can be moved per hour,? says Moore-McKee. ?When you combine the productivity increase with the fuel efficiency, the result is 25% more material moved per gallon.?
With these types of gains, why wasn?t this technology implemented sooner? ?What enabled this to happen is the recent developments in the semi-conductor power control industry,? Moore-McKee notes. ?You can now get silicon semi-conductors that handle very large power flows with more capacity and at a lower cost. That is really the technology enabler that gives us the ability to use AC systems in large drives like this, compared to the old DC systems. Those used to be very maintenance intensive, hard to control, hard to maintain.?
The electric drive used on the D7E is not exactly the same technology used on mine haul trucks. ?In order to meet the really demanding application needs of a track-type tractor, all of the electric components had to be fully sealed,? says Moore-McKee. This means the electric motors can?t be air cooled, like on the trucks.
?We sealed and liquid cooled the motors, which makes it more robust. Everything is sealed electrically so it can operate in all the same conditions that the D7R can, even partially submerged in water up to the fan blades,? Moore-McKee states. ?We really wanted to design a tractor that could eventually replace the D7R.?
In terms of reliability, the company expects the D7E to perform as good, if not better, than the D7R. ?The D7E has 60% fewer moving parts,? says Moore-McKee. ?So there are fewer components to wear out or potentially break. The components it does have require less service than on any other drive system available.? For example, there are no friction clutches. ?That contributes to a longer drivetrain life and low service requirements.?
Despite a higher initial price, the customer payback is less than two years. ?We are able to do much more in a D7 size class in terms of productivity for the operating cost of a D6. We are doing more with less,? Moore-McKee asserts.
And it is scalable to both larger and smaller tractors. The D7 was purposely designated as the first model. ?It is the perfect mix of heavy-duty dozing and light-duty applications,? says Moore-McKee. ?We wanted to prove this kind of powertrain can work in both applications. The D7E has been able to optimize both. We have superb speed control for fine grading. We have incredible AccuGrade performance.?
Being a totally new design allows designers to address reliability nuisance items that can create downtime and plague some equipment ? for instance, belts, friction clutches, engine-mounted compressors and alternators and long refrigerant lines. ?Those things are not just fixed, they are eliminated,? says Moore-McKee. ?When you design a whole new tractor from the ground up, it gives you an opportunity to look at the whole system anew. The D7E is a quantum leap not only in drivetrain efficiency, but reliability as well.?