They have their differences too. Surprisingly, these two groups rarely talk, even within the same company. Maybe it’s a regional separation combined with different equipment and applications. The amount of material one group moves in a year is the same as the other moves in a month. Surface miners have the experience of working with high-voltage systems and they have seen the evolution firsthand from DC to AC systems. Underground miners have been just as reliant, if not more, on electrical systems, yet they have been confined by space and the environment in which they work.

Today, both areas are again experiencing change. The motivating factors are safety, reduced operating costs, ergonomics and ease of use. As an example, the variable frequency drive (VFD) was introduced to underground coal miners in 1992. Up until about five years ago, they probably had a 5% adoption rate; today it’s 100%. During the last four years, Coal Age has covered the introduction of AC drive systems for electric shovels and draglines. Now, whenever rebuilds are discussed, AC is part of that conversation.

Many of the experts believe the coal business will see even more changes in this space. Those changes include an eventual shift to medium-voltage systems underground, and more use of permanent magnet motors and perhaps gearless systems. They also see the controls for mining equipment being integrated into a larger system where data is used to constantly improve machine interaction.

Underground Acceptance of VFDs
Underground, VFDs have become the default drive for everything, not just fans and pumps, but also for conveyors, shuttle cars and longwall shearers. There is not an application with a motor that is not being touched by the emergence of VFDs in one fashion or another. “VFDs have a lot of strengths and they are pushing the other technologies,” said Steve Shadow, industry business manager for mining for Baldor Electric, which is part of ABB’s Discrete Automation and Motion Division. “Controlled start drives and fluid couplers are still viable tools, but many millions of dollars in research have been invested in medium- and low-voltage drives. Reliability with these systems has improved and the prices have come down. That is all related to that technology investment.”

The way the technology is being presented to the industry allows any mine operator the opportunity to take advantage of it. Shadow cited a small continuous miner operation, which was probably producing 600,000 tons per year. “They were running low-voltage VFDs on their belts,” Shadow said. “The conveyor OEM had a lot to do with that decision. The miners simply adapt to the technology.”

The industry has made substantial improvements with VFD technology over the last four to six years. One of the problems is the noise they generate on the incoming line and the effect it has on other portions of the mine’s operation. “Most miners have learned how to mitigate that with either line reactors or transformers, which is probably the absolute safest way,” Shadow said. “The technology has always been available. People are simply realizing what’s happening more quickly.”

One of the areas that miners are seeing great improvements is with load sharing on multi-drive conveyors. ABB has the direct torque control (DTC) system. Loading conditions on underground conveyors, especially with longwall systems, are constantly changing. “The drives can react to belt tensions and balance the load with the other drives,” Shadow said. 

Low-voltage drives are well-accepted underground, but there is still packaging challenges with medium-voltage drives for underground applications. “Mines have adapted the technology for slope belts by placing the bigger drives on the surface to avoid the packaging challenges,” Shadow said. “When conveyor systems exceed 1,000 hp per drive, low-voltage drives have that capability, but the diameter of the power cables become difficult to work with.”

Miners have learned how to put motors and drives together. Similar to surface and underground miners, however, sometimes electronic engineers developing inverters and motor engineers do not talk until they have a problem. Something along the lines of: The inverter failed the motor; no, the inverter had a problem because the motor was wound wrong.

“Today we are looking at systems as packages more than discrete components,” Shadow said. “With ABB acquiring Baldor, it has broadened that discussion by bringing more resources to the table.”

In the future, Shadow believes underground coal miners may be able to capitalize on permanent magnet motors and the torque density those devices offer. “We may eventually see more gearless drives appear in the coal business,” Shadow said. “The most interesting spot would be in the 1,500- to 3,000-hp range. That technology will go through a maturation process similar to the VFDs and motors and the physical size of the systems will decrease, which would make it compelling for the coal business.”

Deploying Drive Technology Underground
Arun Kesavan, global director for controls and automation for the underground segment, Joy Global, believes that adoption rates for VFDs among coal miners are driven by two forces: the technology itself and miners who are seeing the full benefit on their bottom line. Today, with only a few exceptions, VFDs are a standard offering on most Joy equipment.

The improvements in technology not only support zero harm initiatives at mining operations, but they also give the most reliable, productive solution with no nuisance trips. “As an OEM, we sometimes have to take a broad definition and translate it into an application and get the customer on board, because we do not want to have a negative impact on the day-to-day production activities,” Kesavan said.

As the mines embraced the technology, Joy Global brought more skilled people on board to diagnose problems and support the mines. “We also have regional service centers that are capable of diagnostics and more invasive work to rebuild these systems,” Kesavan said. “That expansion in our service and support model has greatly enabled customer acceptance.”

The benefits of the drive systems and controls are immense. Citing a shuttle car as an example, Kesavan explained that VFDs influence several factors. “The most significant is improved ergonomics for the operator,” Kesavan said. “With the soft start, operator fatigue is greatly reduced. That’s important considering operators are running these machines eight to 10 hours per day. We have heard that some miners are getting to work early so they can operate the new shuttle cars with VFD drives when it was introduced.”

The use of these systems also increases the top speed. Traditionally the top speed for a shuttle car was 4 mph. “VFDs reduce motor slippage and create less heat,” Kesavan said. “Improving motor performance with active controls, we are able to increase speed 50% to 6 mph. Overall it improves the cost per ton situation for the miners.” The heat dissipation factor is applicable across the board.

The Joy Flexible Conveyor Train (FCT) has a traction control system. “We have VFDs distributed across the FCT,” Kesavan said. “We are able to actively manage the speed from one end to the other. It’s all being synchronized through our FaceBoss controls.”

Looking toward the future, Kesavan agrees with Shadow as far as medium voltage making its way underground. “Most of the equipment we are using today is low voltage (1kva and less),” Kesavan said. “It’s gone from an innovative technology to a commodity that is readily available. The power densities have improved. Medium-voltage drives would provide the power and torque levels we need to take the next steps underground.”

Building on Dependability on the Surface
When it comes to P&H electric shovels, Joy Global continues to improve on its Centurion system. “We are continually improving OptiDig and the other systems that can interface with the Centurion system, especially the operator assist type programs that come from our advanced automation group,” said Mark Emerson, program manager for the 2650CX (hybrid shovel) for Joy Global. “That flexibility between the control and the drives is continually improving with the amount of data that can pass between the systems.

“With today’s processors, there are areas within the drive where we can shift some of the code and some of that optimization into the drive itself. That improves response and overall system efficiency. The 4800XPC with Adaptive Controls is a prime example. With the AC systems, we can further improve performance,” Emerson said.

Whether its air- or liquid-cooled drives, the goal is to increase the kilowatts per cubic foot and the switching technology is continually improving the size of the package by removing the heat. “The key is proper dimensioning for the unique duty cycle we encounter in the mining business,” Emerson said. “You have to have the right drive for that duty cycle to maintain reliability and availability.”

As far as maintenance, Emerson explained, if a potential issue develops that could lead to a component failure, such as a fan drive, the diagnostics within these drives can see the temperature differential and alert the operator of the impending failure and the mine can plan accordingly.

“We also have some redundancy in the AC systems,” Emerson said. “We have some extra capability in the active front end. In a matter of 30 minutes or so, miners can move an inverter from the active front end to the motion side and continue to operate at 100% until the PM when they can exchange the module or get the fan replaced. We are always looking to maintain availability.”

Joy Global has a strong partnership with ABB and the two companies are pursuing more developments with diagnostics. “Working with ABB, we are using their tools to understand how to retrieve data and integrate it into our systems,” Emerson said. “We have been able to work closely with them to make some system improvements specifically for our applications.”

One of the aspects that sets them apart from the other is the tight coupling between the control and the drives, Emerson explained. “That allows us to take some extra steps to improve performance and efficiency where possible,” Emerson said.

By knowing where the dipper is located in the dig cycle (Adaptive Controls), in the future, the system will be able to increase bail pull in certain areas; when they are breaking out of the bank or dumping into a haul truck, they are able to reduce the rocking motion on the machine, and other operator assist functions optimize and protect the machine.

With Joy Global’s acquisition of LeTourneau, some cross pollination is taking place. “We are also integrating the switched reluctance (SR) technology into the new 2650CX hybrid shovel,” Emerson said. “LeTourneau developed SR technology for its front-end loader and we will be using it on this shovel to turn the generator connected to the diesel engine into a motor and drive the engine during regeneration in the dig cycle using that energy to feed parasitic and hydraulic loads, which reduces fuel consumption and increases efficiency.”

Justifying Shovel & Dragline Upgrades
With the current market situation, all of the mines are watching their budgets, but they are still spending money. “It’s all about return on investment and, if you can lower that cost per ton, miners are interested in making the investment,” said Mike Casson, director-surface mining for Flanders Electric. “We are seeing the normal rebuild work, which is more about reliability and availability. We also have a lot of interest in rope shovel upgrades.”

Safety is the top priority for the mines and operator comfort ranks high too. “Efficiency improvements, especially semi-automation with adaptive control, is a very hot area for Flanders right now,” Casson said.
Casson said he has seen a lot of interest in AC-powered draglines. “We will have two of them getting ready to walk off the pad in the next year,” Casson said. “On the shovel side, we are supplying a lot of machine health systems that can make real time decisions with position support systems. That’s something we have added into our Freedom Se shovel upgrade platform.”

Flanders recently introduced its M21 crowd motor for harder digging applications such as those found in Appalachia and the Powder River Basin (PRB). In a recent study, the company found that it increased productivity in tons per second by 6% when integrated with existing OEM architecture. Cycle times were also reduced by 4%.  When the system was installed with the company’s Freedom Se platform, productivity improved 10.3%, and cycle times were reduced by 6.93% (or 10.93% over initial baseline) according to Flanders.

“The M21 crowd motor is working well, but it’s more about how well the motor and drive system are working together,” Casson said. “Combined they are getting a productivity increase of more than 17% for rope shovels.”

Flanders is also involved with several DC dragline upgrades overseas increasing the rated suspended load (RSL). “They won’t get all of the benefits of an AC upgrade, but they will get a significant improvement for a relatively low cost.” The improvements include increased rope speed and RSL with existing equipment.