Giant Terex Trucks Prove Hybrid-electric Drive Practical
By Tom Jackson
If the term "electric drive" conjures up images of tiny cars straining downhill in a strong tailwind while hauling a trunkful of batteries, you may be surprised to find out what's powering four new 150-ton trucks up and down the ramps and roads of the Imerys calcium carbonate quarry in Sylacauga, Alabama.
Earlier this year the quarry took delivery of four new Terex/Unit Rig MT 3300 AC mining trucks-the smallest AC electric drive mining trucks built to date-but trucks that can nonetheless haul almost a third of a million pounds of rock and dirt at speeds of up to 40 mph.
The power for these two-story tall trucks starts with a Detroit Diesel/MTU 12V-4000 engine. But instead of spinning a mechanical driveshaft and differential, this engine pushes its 1,570 horsepower into a General Electric GE150AC drive system that fires 1,171 kilowatts of electric juice into a pair of motorized wheels.
That's enough electricity to keep the lights burning in 100 homes, but when used to turn the tires on these giant trucks, it may be the most cost efficient way to move mountains.
A partnership in technology
"The trucks were put together by a fairly significant partnership with General Electric, to provide the AC drive systems; Detroit Diesel, the engine; and Terex and Unit Rig to put that package together," said Brian King, president of Terex Mining at a luncheon at the Imerys mine in May to celebrate the formal introduction of the trucks.
"We also spent a lot of time on operator safety," said King. "We went to every serious accident in North America and looked at the causes and tried to engineer fixes for those causes into the truck. It has tailgating protection, a sophisticated roll-over protection structure and a ride quality I think you'll find different from anything else."
GE first introduced the electric drive concept for mining trucks more than 40 years ago. But the first trucks were DC rather than AC. The DC-drive trucks proved powerful enough to haul up to 320 tons, but according to Matthew Dickey, manager of off-highway vehicle marketing and sales for GE Transportation Systems, the DC motors limited trucks to continuous grades no steeper than 8 percent of incline. And DC motors use brushes and commutators which eventually wear out and have to be replaced.
"AC drive is inherently a better drive and motor combination," said Dickey. The problem in the past, however, was that AC power electronics were too big and heavy to put into mining and quarry trucks. But in the early '90s advances in the technology of the inverter drives brought the size and the costs down. "We first introduced AC drive to locomotives, then brought it over to 320-ton mining trucks in 1996," he said.
Since then, the components have been increasingly miniaturized to the point where GE was able to develop AC drives for smaller truck classes: the Terex/Unit Rig 150-ton trucks and a soon-to-be introduced 240-ton AC drive, said Dickey.
New trucks to meet new challenges
Although 150 tons may be small for the mining business, the four Terex trucks now in operation at the Imerys site are the largest used there to date.
Located in the rolling hills of central Alabama, the Imerys quarry, once famous for its Sylacauga marble, now produces about two-million tons of calcium carbonate a year. The Imerys facility is the largest producer in the world of this important industrial mineral that finds its way into thousands of products -- everything from paper and plastics to toothpaste, floor tiles and landscaping chips.
Getting to the white rock, however, was becoming more of a challenge as the depth of the deposits angled further into the ground, increasing the amount of overburden -- rock, clay and dirt -- that had to be removed.
Faced with that challenge, and an aging fleet of mechanical-drive trucks, Mike Butts, Imerys North American mining manager, decided it was time to re-examine his costs per ton. "We were looking for the lowest unit cost to move material from point A to point B," he said.
The choices were AC drive, mechanical drive, or DC drive. After weighing the options he decided on the Terex/Unit Rig AC drive trucks.
"We replaced six smaller trucks with the four Terex/Unit Rig trucks, and we replaced two 8 yard shovels with one 18.5 yard shovel," said Butts. "We went from 350 tons of truck capacity, to 600 tons. By going with fewer pieces of equipment and going that much bigger I replaced the need to supplement overburden removal with a subcontractor."
Speed, inclines and cost efficiency
But payload size was not the only factor in the decision to buy the trucks. The quarry has several steep ramps, 10 percent or more, as well as long flat sections.
"The AC trucks can handle ramps and the flat haul, whereas the DC trucks have to be geared for one or the other," said Butts. "These trucks (Terex/Unit Rigs) will climb a steeper grade than the mechanical drive trucks, and they'll do 40 mph on the flats...but we tune them back for safety."
Thanks to the trucks' climbing power, the only changes made at Imerys were to widen some of the haul roads to accommodate the width of the new trucks.
Energy efficiency also plays a part in the equation. "The AC truck had the highest energy efficiency, then the mechanical drive. If the AC had not been available we would have stayed with the mechanical drive trucks," he said. "DC-drive trucks were third in terms of cost per ton."
GE's Dickey said most mines rate trucks in cost per ton hauled, and AC drives with their better top-end speeds, low maintenance and climbing torque are gaining in popularity. He cites as evidence two of the largest mining companies in the world that recently converted to all electric fleets.
Despite the sophistication of these trucks, most of the components are well known and well proven, said Butts. "We were comfortable with GE. They had been putting AC technology in locomotives for years. The armature on the wheels of the truck is the same as what's on an Amtrak locomotive. The I-beam frame of the truck came from Unit Rig and there are probably hundreds of those out."
Seven trucks to deliver one
Delivery and assembly of these 150-ton trucks took considerable effort and organization. Each truck required seven tractor-trailer loads. The dump beds were fabricated in two parts, each half requiring its own tractor-trailer. Once the components were delivered to the quarry, it took workers there about 30 days to assemble all four trucks.
"We had a few bugs in the first few months, but they've fixed all those," said Butts. Since then they've really settled down and we're getting around 90 percent availability." Having spent a lot of time studying and comparing the various types of trucks, Butts says he'll have a more accurate idea of the final cost per ton when the trucks approach the 20,000-hour mark and the service and maintenance costs are well established.
For the operators, the electric drive trucks offer several improvements over the mechanical drive models they had been using. "Drivers have to get used to the fact that there is no shifting," said Butts, "and they really like the brake retarding. When you hit the brakes, the retarder will get you down to less than one mile per hour without ever using the friction brake, so you're not burning up your brakes. On a DC truck, the retarding will only get you down to three to four miles per hour."
The MT3300's dynamic retarding system can slow the truck from full speed to 90 percent stopped in less than two seconds.
Butts said his drivers also speak well of the trucks' smooth ride, which he attributes to the Unit Rig front end I-beam suspension. The biggest change for the drivers, he said, was getting comfortable with the size of the trucks. "Getting out of a 50 or 70 ton truck onto these 150 ton trucks takes some getting used to, especially when you're backing up to the shovel," he said.
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