July/August 2009
The right drive powers a shredder's rotor and hammers to optimize its productivity and minimize its operating costs. Shredder size and type and the yard’s access to electricity can help determine which drive is right for you.
By Kim Fernandez
When asked the age of his shredder motor, Tom Barnett of Sadoff Iron & Metal Co. (Fond du Lac, Wis.) has to think for a minute. "We bought that in 1972—it's our original one," he recalls. For most yard managers, the shredder drive is out of sight, out of mind—they selected it when they bought and installed the shredder and, with regular maintenance, it hums along for years or even decades. And that's a good thing, considering the six-figure cost of the equipment and its installation.
But when it's time to get a new drive—typically with the purchase of a new shredder—these managers have several important decisions to make, and the implications of their choices are huge. The cost and logistical challenges of buying and installing a new shredder are substantial. Scrapyards undertake months of research and work with vendors and local power suppliers to pick the best equipment for their needs and capabilities. When it comes to shredder drives, things can get complicated quickly, and a decision based on incorrect information—or a lack of information—can have a huge impact on the shredder's efficiency, operating cost, and overall usefulness.
The shredder drive is the motor and controls that turn the shredder's rotor and hammers. Drives must be compatible with the shredder model and size, leaving the most important variable the type of drive: alternating-current or direct-current electric motor or natural gas or diesel engine. Each has its own advantages and disadvantages.
Although the drive might seem like just one of many shredder variables, selecting the right one is critical to preventing downtime and lost productivity. "Your choice of drive will affect initial cost, operating costs, power usage, and productivity," says one Midwestern drive manufacturer. "It's going to have a dramatic effect on the bottom line of this huge investment in a shredder."
Electricity: AC or DC?
Alternating-current motors were once the most common shredder drives. The original AC wound-rotor drives with liquid rheostats were developed in the 1960s to offer optimum power to big shredding machines. There's nothing wrong with the AC motor, this drive vendor says, but the liquid rheostat is "a very old and crude control. You're going to get power spikes, which are going to make your demand charge high and probably guarantee that
you'll need a dedicated [electricity] substation and dedicated power line for your shredder."
Developed in the 1950s, liquid rheostats are essentially tanks of salt water with electrodes that are moved together or apart to reduce or add resistance to the rotor circuit. Adding resistance changes the characteristics of the motor: It allows it to keep its torque at a lower rpm, which helps start the motor, helps prevent the motor from stalling under a heavy load, and somewhat controls "flicker," or power surges. But adding resistance also causes the motor speed to fall, which results in lower productivity, and rheostat controls still allow power spikes of 200 percent to 300 percent, creating problems for the local power grid. Though equipment is available to further control flicker, it's quite expensive, the drive manufacturer says.
Recently, drive manufacturers have come up with new controls for AC wound-rotor motors that avoid some of these problems and allow the scrap company to hook up to local power sources directly. One vendor now offers a slip power recovery drive instead of a liquid rheostat on its AC shredder drives. This solid-state technology maintains the motor speed despite heavy loads, the company says. When the current hits a pre-set limit, the motor speed drops, but the speed recovers more quickly than it would with a liquid rheostat. The SPR drive also gives the scrap company the option of regenerating power the liquid rheostat would have wasted and sending it back onto the power grid. SPR drives require less maintenance than liquid rheostats and don't require a cooling tower or power-factor-correction capacitors, the company says.
Direct-current drives gained popularity in the 1990s, when people discovered that shredders could reuse motors from the main generators of locomotives. "They are very reliable and very sturdy, [and] also very cost-effective," the drive vendor says. "Because the motors are so reasonably priced, people can afford to put sophisticated controls in front of [them]." These controls can limit the electric current to an amount the scrap company and power company set. This controls current spikes and limits demand charges. With these controls, DC drives don't require the dedicated electrical substation that an AC drive might, making their installation possible where that's not an option. They're also less expensive at the outset: They can be 10 percent to 15 percent cheaper than their AC counterparts, the vendor says.
Though people used to believe that DC motors don't have as much power as comparable AC systems, they reportedly have the same or better productivity as AC systems with the same rating. Further, the manufacturer says, DC systems are better at working through the occasional jam. With AC systems, the torque drops off at lower rpm, making them more prone to jams at slower speeds, he explains. With DC systems, the torque remains constant even when the motor speed drops. Also, when the shredder jams, the operator can toggle the DC system back and forth to clear the jam, while AC systems have to be cleared manually.
DC drives have certain limitations, however. Because of their original purpose, they come in only a few sizes, such as 1,000, 1,500, 2,000, and 2,300 hp. "We can put two together on a skid and make 4,600 horsepower, but we can't go back in time and ask locomotive manufacturers to put bigger generators on [their trains] so we can harvest them for shredders," the drive vendor says. Yards typically install them for applications requiring 4,600 hp or less, to the point that "they've become the industry standard for small and midsized shredders"—those 80 inches or smaller, he says.
One company, American Railroad Equipment (Roanoke, Va.), sells a shredder drive that's a little bit different: It uses a diesel—or biodiesel—fueled locomotive engine to generate AC power, which it then converts to DC, which powers two 2,500-hp electric motors for a shredder. For more on this drive, read "Power Play" in the July/August 2006 issue of Scrap or online at www.scrap.org.
The Flexibility of Fuel
Depending on the shredder yard's location and the surrounding infrastructure, drives that run on natural gas or diesel fuel might be a good choice. Because they don't run on the main power grid, they don't require a dedicated electrical substation—which isn't feasible at every location—or negotiations with the local power company to ensure a sufficient flow of electricity.
Before selecting a natural gas—or diesel—fueled drive, shredder owners would be well advised to study how the cost of that energy source compares with electricity. The prices of all three have fluctuated over the years, explains an Ohio-based drive vendor. Though in the past natural gas often had a cost advantage, more recently "these advantages have [swung] back and forth between electricity and natural gas, and today it would take a site evaluation to determine which would be more economical."
Even if the costs are favorable, natural gas and diesel-fueled engines have some drawbacks. A scrapyard interested in a natural gas-fueled drive must work with local natural gas distributors to ensure they can provide adequate volume and pressure for the application. Diesel engines require on-site fuel storage, and their exhaust emissions and smoke can be problematic, the Ohio manufacturer says. "The frequent load/no load cycles that are inherent in a shredder's operation exacerbate these issues, particularly with today's environmental concerns with exhaust pollutants and particulate emissions," he says.
Further, diesel and natural gas engines have no overload capacity, the Midwestern vendor points out. An electric motor that usually runs at 4,000 hp can run briefly at 6,000 hp or even 8,000 hp to work through a heavy load, whereas a diesel or natural gas motor cannot, he explains. Thus, these motors need more horsepower to do the same job. A 4,000-hp diesel engine is the equivalent of a 2,500-hp electric engine, the vendor says.
And keep in mind that these shredders are not completely off the electrical grid. The fuel-powered engine drives the rotor of the shredder, the Ohio manufacturer says, "but the customer will still need utility power to drive the electric motors for other accessories." Natural gas and diesel drives come in myriad sizes and horsepowers, so shredder owners should be able to find the right size for their particular shredding equipment.
Maintenance and Repair
Shredder owners can give any drive greater reliability and longer life by maintaining it regularly. AC and DC drives require maintenance about twice a year. That includes replacing the brushes—DC motors have more brushes than their AC counterparts—and changing the filters to ensure the air moving through the motor is clean. Additional maintenance includes checking the rheostat's chemistry and adjusting it, if necessary, and keeping the cabinets vacuumed and clean. "Owners can expect to get 15 to 20 years from their drives with small amounts of maintenance," another vendor states.
Engine drives, whether natural gas or diesel, need more regular maintenance than their electric counterparts, the Ohio manufacturer says. "You want to check the lubricating oil and the coolant daily," he says. "And then you want to do the normal checks that you'd do on a car or truck about every three months." These drives also need valve adjustments and new spark plugs periodically. But those intervals depend largely on how often and how long the shredder runs. "Shredders are typically not 24/7 operations,"
he says. "They typically run between six and 12 hours a day, five or six days a week. A lot of maintenance will depend on how much the shredder is running." Though electric drives can be sent out for service, he says reciprocating engines normally are serviced on site.
Sadoff Iron's Barnett, whose motor is more than 37 years old, says he sends it out for major maintenance every four or five years. "Every time we pull the rotor, we pull the motor, too, and send it to be reworked," he says. The manufacturer does "a major cleaning, changes the bearings, and makes sure everything is up to the original specifications.
"We do a quarterly vibration analysis, and take oil samples twice a year for the bearings," he adds. "But we've had no issues. We monitor the motor closely. It's been very reliable." •
Kim Fernandez is a writer based in Bethesda, Md.
The right drive powers a shredder's rotor and hammers to optimize its productivity and minimize its operating costs. Shredder size and type and the yard’s access to electricity can help determine which drive is right for you.