Armstrong Retrofits AC Technology for Dragline Mining
A Western Kentucky coal operator is the first to embrace a step change in electrical motors and drives, and quickly demonstrates the benefits
By Steve Fiscor, Editor-in-Chief
Not far away, in Evansville, Indiana, the engineers at Flanders were already drawing up plans for the first AC retrofit for a dragline application. For many years, the company has been working with mines servicing and rebuilding DC systems on draglines and they knew there had to be a better way. Anticipating a need for the technology, Flanders set out to develop a system specifically targeted for the dragline duty cycle.
The Armstrong AC dragline is not the first AC-powered dragline. It is, however, the first AC dragline retrofit operating in the U.S.—the country that has the most draglines in operation. The 23-yd 770 is a small machine with a 215-ft boom, but the technology is scalable. Not long after the machine was commissioned, the mine began to see results. They were also able to compare its performance against the com-pany’s other three machines. Even though the machine is smaller than the others, on a comparative bucket basis, it is moving more than 50% of what the larger 45-cu-yd machines are moving.
The benefits of AC technology, as far as safety and maintenance over DC systems, are well-documented. AC technology is safer. The motor-generator (MG) sets that power most draglines require a lot of main-tenance. The systems are open and guard-ing is a safety concern as well as arc-flash. Electrical maintenance on DC motors, checking carbon brushes and megging motors, centers around the commutator. AC eliminates the MG sets. AC motors have no commutator. AC motors require less maintenance, which keeps people out of harm’s way. So a great deal of the constant electrical maintenance is eliminated. There is less rotating equipment and the associ-ated guarding. The downside is that most of the AC technology of this magnitude was developed for industrial installations and applied to mining. The intense duty cycle for mining applications often brings these industrial systems to their knees.
Flanders wanted to develop a system specifically for dragline mining. Three years ago they recruited an expert from the industrial side of the business and he began teaching them about AC and they began teaching him about mining duty cycles. Together, they developed a proto-type and then they received the call from Armstrong Coal.
Rebuilding the Lewis Creek Dragline
The Lewis Creek surface mine was put into operation in April 2011. The 770 dragline would be used to move overburden. With rolling topography, the overburden depth ranges from 30 to 160 ft. In addition to the dragline, the mine employs a small fleet of drills, front-end loaders, haul trucks and dozers, to expose and extract the 13A and 13B coal seams. The mine currently has a 10-year life.
The dragline was dismantled in Colorado and transported to Western Kentucky. Kenny Allen, vice president-oper-ations, Armstrong Coal, who at one time in his career was a chief electrical engineer working on draglines and shovels, looked at the electrics on the machine and knew he had some decisions to make. “I had some past experience with draglines and I always wanted to build an AC machine and this seemed like the perfect fit,” Allen said. “The rotating DC machinery on this dragline was in pretty bad shape. When it was built new, it wasn’t the best of the best.”
Armstrong Coal approached Flanders, looking at the possibility of retrofitting it with AC drives. They had developed a pro-totype drive. With a short timeline, the engineers began to develop AC motors for “drop-in” applications (same footprint, frame size, shaft height, etc.) for the swing, hoist and drag functions. Flanders performed all of the pre-commissioning test work in advance. “We load tested everything in the shop prior to installa-tion,” said Mike Casson, manager-dragline mining, Flanders. “We also performed sys-tem control testing. We were just waiting to get power on the machine. Once the mine had installed the 7,200-volt line, the dragline was mining in eight days.”
The Armstrong 770 began mining in April 2011. The machine has been run-ning consistently since with very high avail-ability. The machine has been running less than a year and it is already two months ahead of the dig plan. “We have been very, very pleased with the machine,” Allen said. “It has really exceeded our expectations. The run time has been super. We have had very few problems with either the electrical or mechanical side of the drives. This speaks well for the drive, particularly on the mechanical side, because they [Flanders] were very meticulous in their design of the drive to stay within the mechanical parameters of the machine.”
From ballast to drive, the machine is well balanced from a power and speed per-spective, Allen explained. “The operators do not wait for anything,” Allen said. “When they are loading, hoisting and swinging, everything works together and all of the speeds are in synch, which makes for a very productive machine.” E&MJ toured the new Armstrong 770 dragline in late February. Even though the region had suffered from major thunder-storms the night before, including a few tornadoes, the dragline was busy moving overburden. Dragline operator Chris Hayden takes great pride in the machine. “This is the best one we have,” Hayden said. “It’s easier to run. We have no prob-lems. On the uptime log, this one is in the mid to high 90% range. When you want to run it fast, it’s a lot faster.”
When comparing it to the other Armstrong machines, he said the uptime logs for those machines are in the low 90% range. “All of the others are down a day every week or two for maintenance,” he said. “The 770 is down one day a month. We just run. We are running about two months ahead of schedule. A lot of that has to do with not being down. This machine just digs!”
“In my almost 40 years of mining expe-rience, the 770 is one of the most trouble-free machines I have worked on and its performance brings big smiles to manage-ment,” said Gary Durham, chief electri-cian, Midway and Lewis Creek mines, Armstrong Coal.
Power consumption has been below Armstrong Coal’s original estimates. The mining company had to estimate power demands for the power supplier. Allen had never estimated demand for a machine of this type and neither had anyone else for that matter. “We were collaborating with Flanders in providing an estimate for the power company,” Allen said. “We were con-servative and over projected to avoid penal-ties. We probably over-projected a little more than we should have. The power fac-tor has been 99%. It’s at unity almost all of the time and that just doesn’t change.”
When he began discussing the technol-ogy with the engineers at Flanders, Allen had two questions. One dealt with power factor and the other related to harmonics. “They said power factor would not be a problem because they would be operating at unity and I said, ‘Yeah right’ and, well, it is. They have also done a great job of tak-ing care of the harmonics as well.”
As far as cost, Allen agrees with Flanders that this technology is well-suited for retrofit applications. “We have three other draglines and they are doing fine,” Allen said. “I would not shut them down and do the retro-fit. It would not be cost effective. Anyone considering a major rebuild should take a look at this AC system. I would do it again if I had an opportunity.”
Retrofitting an AC Package
At the time Armstrong approached them, Flanders had already developed an AC dragline system and it was 80% complete, but the company had not taken it to market. “Everything on the 770 was obsolete and worn out,” Casson said. “So it needed a completely new electrical package. It was an ideal candidate to go AC rather than DC.” As Allen mentioned, it’s more cost effective when starting from scratch, and the benefits of the AC system quickly payback.
The AC motors were designed to deliver more horsepower. Most frames sizes would see a 30% increase in power. “We designed it as a mechanical drop-in replacement,” Casson said. “It would have the same bolt pattern, footprint, shaft height, and the tapered shafts. It’s still the standard Association Iron & Steel Engineers frame dimensions.” It’s a physi-cal drop-in with the same speed range, so no gearing changes are required.
The AC drive system was designed specifically for a dragline. “This is not an off-the-shelf drive that’s being applied to a mining environment,” said Bob Parke, sen-ior field engineer, Flanders. “It was designed for the machine. We elected to go with a water-cooled arrangement. We felt that was an obvious better choice because of the better efficiency. The system can get the heat out of the drives and the house, but more importantly the cabinets are now sealed.” Without air cooling, there are no filters to change and no dust in the cabinet. Also, everything the technician accesses on the front side of the drive is 24 volts, which provides a zero arc flash rating.
Flanders also opted for a lower voltage application. Typically an AC drive is either 700 volts or 1,400 volts. “We elected to go with the 690 volts to stay below the 1,000-volt threshold,” Casson said. “In many countries, the 1,000-volt threshold triggers a different, more stringent set of safety regulations.”
As far back as 2008, Flanders was already considering the concept. “We want-ed to build an AC dragline system,” Casson said. “We looked at other drives on the mar-ket that we could utilize for this applica-tion. In our opinion, none of them were the right drive for a dragline. So we recruited the people to develop the drive system specifically for a dragline application.”
The AC drive system is designed for 175,000 hours. Typically an MG set lasts about 20 years. “Some would say the drive is over-sized, but we say it’s right-sized for the environment,” Casson said. “A dragline is a long-term investment and we want the drive system to be a long-term piece of equipment.”
The operating environment on an AC dragline is much cleaner than DC. The lack of carbon brushes (no carbon dust) and pedestal bearing oil eliminates the risk for flash over. The drive cabinets have a zero arc flash rating. With an MG set in DC application, there is always a huge poten-tial for arc flash. If the MG sets are down for an extended period of time, they have to be rolled over to oil the bearings.
AC systems run more quietly. The ambi-ent noise for an MG set on a 1570/8200 class dragline runs anywhere from 90 to 96 dBa, according to Parke. “Noise is cumulative and more sources generate louder noise levels,” Parke said. “When the MG sets are eliminated, those sources of noise are removed. The heat exchangers for the water-cooled drives are located on the outside of the house and, with the heat removed from the house, the number of intake fans can be reduced. Or they could be placed on a VFD, which slows them down—energy savings and less noise.”
An AC system is much more efficient. “A typical DC system with an MG set in a Ward Leonard loop is about 80% to 83% effi-cient,” said Casson. “The active front-end drive is about 92% to 93% efficient. So the mine would see significant savings on operat-ing costs as far as electricity consumption.”
Something else that could be consider-able is dragline utilization. When a dragline sits idle, whether it’s for a shift change or a bucket lube, a 1570 size machine is going to consume 1 mw or more of power with the MG sets idling. The AC drive sys-tem consumes virtually no power when the machine is not in operation.
Another side benefit is a decreased diameter on the trailing cable. Ordinarily a dragline trailing cable is sized to start the MG sets. “It’s actually oversized for the power needed to operate the machine,” Parke said. “With the AC static drives, there is no in rush of electricity like there is start-ing an MG set. It’s not a huge difference, but a smaller cable can be used.” It’s easier to handle, lessening the chance for back injury.
Advances in Drive Technology
With all of these benefits, one of the first questions would be: How did the mining business miss the boat? The answer is: It did not. Most of the draglines were built 20 to 30 years ago with a 15- to 20-year life. During that same timeframe, technology advanced and improved. In fact many of the changes have occurred in the last 10 years. Previously AC drive technology could not function properly in a dragline applica-tion such as this before the advent of the active front-end drive. “The field-oriented control with the active front-end can con-trol power factor on the machine just like a synchronous motor,” Casson said. “The harmonics can be reduced to virtually nothing. The drive system operates well below IEEE 519 standards.”
The dragline is such a tough applica-tion because of the cyclic loading. It goes from 200% load to no load or regen every 60 seconds. Previous AC systems could not typically handle the voltage swings and the cyclic loading.
This system can be scaled up to any size dragline, Casson explained. “It’s just a matter of paralleling the modules,” Casson said. “The way we designed it, we used the same module on the active front-end and the inverter. It’s an identical interchange-able module—one spare module. For a typ-ical hoist application, an 824 frame size is replaced with a 1024 AC motor. It has the same shaft height, frame size and footprint. That’s a 1,900-hp induction motor replac-ing a 1,300-hp DC system. It takes 12 modules in parallel for the inverter side to run the 1024. A smaller motor might take nine modules. They are all single phase modules, so there are always three in a batch. So it would run three, six, nine or 12.” Because the modules and drive sys-tems are digital with built-in health moni-toring and diagnostics, there is not a lot of trouble-shooting because the system tells the operator where the problem is located.
Each module has been designed with a maintenance technician in mind. The module has four connections: One AC power connection, the positive and nega-tive DC connections, and one mechanical locking bolt. They each use a ¾-inch bolt. The module slides in on tapered pins and the locking bolt hold it in place. The tech-nician removes four bolts and a small thumb-screw communications cable and the module pulls out. It weighs 50 lb. “One man with a ¾-inch wrench can change the module in less than 3 minutes,” Casson said. “No heavy lifting or cranes required.”
Finding the Solution
To spearhead the AC dragline project, Flanders recruited and hired Stan Mann, technical project manager, Flanders, three years ago. He had been working with AC drives in industrial applications unrelated to mining. He views that experience as an advantage because he was unbiased when he approached the situation. “We spent a lot of time trying to convert the traditional understanding of a DC motor and its rating and what it does to what an AC motor does,” Mann said. “In a way they are the same and they are also very different.”
He sees the Armstrong retrofit as a suc-cessful introduction. “This is first AC dragline retrofit, which amounted to two motors each for hoist and drag, and three for swing,” Mann said. “It’s a multiple-motor-motion application and we were able to validate a lot of our control strategies with parallel motors in the same motions.”
Dragline structures are designed for peak forces and torques. “DC motors have a peak force rating and everyone is sacred about not exceeding that,” Mann said. “There are also gearbox characteristics. As the speed increases, the torque on the gearbox must be reduced to keep it happy. So we have these boundary curves, which are x-y plots of current and speed for DC motors [amps and volts]. It’s the classic analysis of machine performance to take the motor data through a motion and watch how it interacts. It tells them whether a machine is inside or outside its bounds or whether the motor-generator loops are sta-ble or unstable.” This is the way the min-ing business learned how to do this over the last 90 years, Mann explained, and everyone is real comfortable with it.
Now AC has been introduced and it does not work like that anymore. “Instead of amps and volts, we’re talking torque and frequency or speed,” Mann said. “So now we have to translate all of this knowledge into what we really want to have when we are controlling torque and speed instead of amps and volts. That took a year.”
Mann then began the process of deter-mining the AC motor requirements. He vis-ited a few draglines. Parke and Casson taught him about draglines. Then Mann told them this is what AC motors can and cannot do. Then they worked together to figure out the specifications in terms of Newton-meters and RPMs rather than amps and volts. “That took some time— that and understanding the power profiles on the machines,” Mann said.
The use of this technology allows oppor-tunities to do more things differently, Mann explained. “We haven’t done that yet,” Mann said. “We have followed the same pro-files of the classic DC machines. We pro-grammed the drive to deliver the same pro-file because everyone is safe with that. But, we already have people asking about upping the speeds. The curve from max torque to max speed is linear. But the curve for a gear-box is a parabolic function. So why don’t we take the speed up like that? The DC motors used to have commutation problems in the upper limits because of high current and high speed occurring at the same time. The AC motor doesn’t have those problems.” There is opportunity for more performance improvements, but it will place more stress on the machine. Theoretically it’s inside the machine’s performance boundaries.
Within a year Mann had an operational prototype. “Then we initiated the real pro-duction design with the details that we per-ceived the industry needed,” Mann said. “We have to build a good motor. We have to have a drive that will produce the torques and speeds and profiles that we need. But, what’s really important is that the machine runs reliably. Uptime has to be high and it must be easy to maintain. If the dragline is not running, it’s costing the mine a fortune.” That’s what Mann learned from the mines.
One of the biggest changes was the water-cooling, according to Mann. “It took us a long time to convince people that it was O.K. to run water and electricity in the same cabinet,” Mann said. “When they realize that everyone else in the world does this at these power levels, they decided it would be O.K.”
The Armstrong rebuild was a great opportunity. “We built a system to their specifications to fit their application, then proceeded to test and install it,” Mann said. “Within two weeks of commissioning, they were running it full bore. The main power equipment ran fine. We had some mechanical issues on start-up with motors and couplings, and encoders (shaft posi-tion sensors).” The normal bugs encoun-tered in a rebuild.
Cabling was difficult. “The system no longer requires single-conductor cables to the motor,” Mann said. “AC needs a three-phase shielded cable with grounding opti-mized for VFD applications. These kinds of cable are hard to find at these power lev-els especially if the mine wants to have metallic shielding. We used a continuous welded aluminum armored cable, but it could only be purchased with Class B stranding, which is really stiff, making installation difficult. There is a debate in the industry right now about metal armored cable when it comes to VFD applications. Steel armor is the tradition, but manufacturers are not making them that way anymore. Non-metallic jacketed cable is much more flexible and greatly simplifies installation in tight areas.
“As far as maintenance, an AC motor can have a ground fault or a bad bearing. The drive system could have a component failure or ground fault anywhere in the wiring system,” Mann said. “A component failure generates an immediate fault with an obvious response. These are easy to troubleshoot and replace and there’s just not that many parts.”
The machine control is based in the PLC control system. “All they have to do is modify the control algorithm to determine machine performance,” Mann said. “They program the speed and torque reference. They can modify the profile. The drive product is a dumb machine. All it takes from the machine control is a speed com-mand and torque reference. Give me this much torque and don’t exceed this speed. All the intelligence as far as monitoring and interacting with a user comes from the PLCs. All of the variable intelligence is pushed out to the machine master. The drive is the big power amplifier.”
Everyone knew this technology was coming, Mann explained, but Flanders had the vision to get out in front of it. “Everyone agrees its better,” Mann said. “Power factor control on the front-end is so much better.”
Designing the Drop-in
The AC motors were designed to “drop-in” the same configuration as the DC motors. On a DC motor, the big heavy fame is part of the magnetic circuit. Basically, the DC motors slide out and the AC motors slide in, then the brakes and couplings are reat-tached with no need to change the gear-box. When the AC motor is installed, that frame simply becomes a 1 ½-inch steel housing, holding the motor together. In some applications, the AC motor typically uses the stator for structural support, but the deck on a dragline is not solid. That puts stress of the bearings. This concept uses the same bearings and frame, and the frame adds a great deal of rigidity.
Joe Patterson, director of motor manu-facturing, Flanders, was responsible for the design and fabrication of the drop-in AC motors. He had to deal with the constraints around the existing footprint, shaft heights, couplings, bearings, etc. The biggest chal-lenge was to make sure the performance envelope of the AC motors not only mac-thed, but exceeded the existing DC motors. He was able to accomplish this by having a really good understanding of what the hoist, swing, and drag cycles are for the DC motors. “Simply matching the nameplates is not good enough for a dragline applica-tion,” Joe Patterson said. “You have to understand the application and the load cycle to match it appropriately.”
Although the AC dragline retrofit con-cept had been discussed for many years, the design and manufacture took place in really short timeframe from start to finish (five months). On a normal dragline, he would have been allowed 10 to 12 months.
Reinforcing the benefits of AC technol-ogy, Joe Patterson explained that the mine should expect significant efficiency gains. “We would expect about 12% moving from motors and generators to variable frequen-cy drives,” Joe Patterson said. “As far as power consumption, the in rush of power for the synchronous motors is eliminated. That was 600% of starting torque and with a VFD that is not needed so the trailing cable can be reduced.”
The timeline worried him most about the project. “We really had to get it right the first time,” Joe Patterson said. “We did-n’t stray from the path too far, never to the point where we couldn’t course correct.”
No one had attempted to put an AC motor in a DC frame. Airflow around the motor and cooling was a major concern. “We had to make sure we were getting enough air over the right parts,” Joe Patterson said. “AC and DC motors are cooled differently. The armature in DC motor will heat much differently than a rotor, which is the rotating piece on the AC motor. On AC motors, it’s more impor-tant to cool the stator than the rotor, but you still have to direct some air toward the rotor.”
He was very pleased with the commis-sioning process. “When you are doing something that has never been done before, you sort of expect a setback, but it progressed in eight days and that’s about the same commissioning period for a DC retrofit,” Joe Patterson said. There is always room for improvement, he explained, and there are a few accessories that could have been improved upon given more time.
A Commitment to Safety
Flanders committed to developing and building this system before they had a cus-tomer. That’s a gamble that would make many publicly-held companies uncomfort-able, but Flanders is privately held. The Patterson family has been leading the organization for three generations.
Allen Patterson, COO, Flanders (and an electrical engineer), has been with Flanders for 14 years now. “About 18 months ago, we reorganized the company into two busi-ness units, one that focuses on after-market motor repair and the other is the power sys-tems business units, which is engineered products and manufactured systems focus-ing on optimizing assets and improving safety,” Allen Patterson said. “The hope was to develop innovative solutions for min-ing and heavy industry and to be able to grow our people and business in more spe-cific areas.” He is the leader of the power system group and was responsible for the people working on the Armstrong project.
Flanders has been serving the mining business for more than 50 years. They are now seeing more demand from overseas. “We primarily started as wound motor repair services on stripping shovels,” Allen Patterson said. “Listening to the customer has been the key to our success in mining. We also have a good understanding of the application and the duty cycle, and how that affects the mechanical structure of the machine. Rather than having a standard product that we plug into several areas, we have design application-specific solutions.
“As far as the AC dragline, we made the decision to head into that area about three years ago,” Allen Patterson said. “There was not a good drive out there that would withstand the rigors.”
Looking down the road, Patterson sees more possible uses for this technology, such as helper drives and swing motion upgrades. “We can coordinate the drive sys-tem so that we can eliminate some for the mechanical issues, such as backlash,” Allen Patterson said. “There are also more opportunities for autonomous applications.”
The Flanders AC system for dragline mining is step change in technology. DC systems are not going to go away anytime soon. Anytime a mine relocates a major piece of electrically powered machinery that is the ideal time to consider an AC retrofit. Why would a mine invest a signifi-cant amount of money to rebuild aging DC machinery, when it could use that money for AC technology. In addition to improved safety, the AC presents a respectable pay-back with power savings and decreased maintenance costs.