Equipment Focus: Mechanical Magnet Controllers

May/June 2008

Long the workhorses for lifting-magnet work in the scrap industry, mechanical magnet controllers face growing competition from newer technologies.

By Jim Fowler

In a classic Warner Brothers cartoon, Wile E. Coyote ties a big magnet to his waist, puts on roller skates, and follows the Road Runner, who—having consumed "iron bird seed"—attracts the magnet and pulls him down the road, onto the railroad tracks, and into the path of a locomotive engine. Those who use scrap lifting magnets know where Wile E. Coyote went wrong. He needed an electromagnet, a generator to energize the magnet, and a method to manage the power flow to and from the magnet—a controller—that he could have switched off to stop the magnetic pull and move himself out of harm's way.

In all seriousness, there are several ways to operate a lifting magnet on a material handler. You can use a mechanical magnet controller, a solid-state magnet controller along with a separate generator, or a more recent technology, a generator/solid-state magnet controller package. This article further describes mechanical magnet controllers; in the July/August issue, Scrap will examine generator/controller packages.

The Mechanical Approach
Scrap processors have used mechanical magnet controllers for nearly a century, and the equipment has remained largely unchanged in that time. "It's pretty much a mature device," says Roger Moseley, senior applications engineer with Hubbell Industrial Controls (Archdale, N.C.). Processors like them because of their relatively low cost and simplicity of operation. "They can look in a box and see things kicking in and out—they can get a good visual as to whether it's working or not, " says Bob Bedard, manager of National Association Supply Cooperative (New Phil­adelphia, Ohio).

To understand the role of a mechanical controller, you must first know how an electromagnetic circuit works. It typically has three components: the generator (which creates power for the magnet), the controller (which turns the magnet on and off), and the magnet (which performs the work). The generator and controller must match the size of the magnet—the larger the magnet, the greater the power capacity it requires from the generator and controller. Likewise, the lifting magnet must fit the base machine or carrier—it must be neither too small and light nor too large and heavy. Each part of this puzzle must fit properly.

The generator, which is mounted on the base machine, creates the power—direct-current electricity generally flowing at 230 volts—that energizes the magnet. Three possible power sources drive the generator:

• a belt from the shaft of the carrier's engine. Historically, this is the most common and easiest approach.

• a separate diesel or gas engine. This approach is an attractive alternative if, for example, a processor buys a material handler with inadequate power to run a magnet system. One advantage of this approach is that the magnet system is portable. It can be skid-mounted and moved from one carrier to another.

• the carrier's hydraulic system. Material handler manufacturers have recently integrated this capability into equipment they build specifically for the scrap industry. To install hydraulically driven generators on older models, however, you must ensure the proper flow of hydraulic fluid from the carrier's system to the generator. The generator must spin consistently at a specified rpm to maintain the required voltage to the magnet. A generator that runs too slowly will cause the magnet to lose part of its load or not lift anything, notes Ken Richendollar, sales manager for Ohio Magnetics (Maple Heights, Ohio). On the other hand, he adds, a generator that runs too fast will supply excess voltage to the system—it will "burn your magnet," as the saying goes, creating maintenance problems.

The controller's purpose is to control the transmission of the 230-volt DC power from the generator to the magnet for the lift mode, then to stop that current and send a burst of reverse power to the magnet for the drop mode. In practice, the controller works like this: In the base machine's cab, the operator has a control lever, push button, or toggle switch wired to the controller, which has two sets of contactors, one for the lift phase and one for the drop phase. When the switch is turned to "lift," the larger lift contactors inside the controller engage. A set of copper tips on the contactors, one positive and one negative, come together and allow the current from the generator to flow through them to the magnet. The energized magnet lifts its load of scrap and holds it as the base machine moves the load to the desired location. To drop the load, the operator turns the switch to "drop," which disengages the lift contactors and stops sending power to the magnet.

At this point the controller's work is far from done. Even though no power is flowing into the magnet, the electrical conductor inside the magnet—typically a big coil of aluminum or copper—has stored some of the energy it has received during the lift phase, and it must release that energy properly for the magnet to drop its load. When the lift contactors open, the drop contactors engage, sending a reverse current to the magnet to erase the magnetic field—to help it drop material off its face plate—and absorbing any remaining stored energy from the magnet. The stored energy flows into resistors in the controller, which convert it to heat energy and dissipate it into the surrounding area. The controller must send just the right amount of reverse current to clean the magnet's face—too much energy will make the materials continue to stick to the magnet. After this step, the cycle is complete, and the operator is ready to lift the next load.

Because of the heat they generate, the resistors' placement in the controller and the controller's placement on the base machine are both crucial considerations. Resistors often go in a back panel behind the electrical wiring in the controller box to prevent the heat from burning the insulation off the controller's wires. Experts recommend installing the controller in a well-ventilated area on the base machine so that the natural air flow cools the unit. In some controller applications, depending on the size, resistors are mounted in a remote enclosure to enhance cooling.

One manufacturer, Hubbell, uses a varistor—a nonlinear resistor—on some of its magnet controllers to help minimize the magnet's cycle time. The higher the voltage running through the varistor, the lower its ohmic, or resistance, value. "That allows you to dissipate the energy in the magnet much faster" than you could in a resistor-style controller, Moseley says. The voltage induced across the varistor also is limited at 700 volts, reducing the wear-and-tear on components that a higher voltage might cause. A resistor-type discharge path would require voltages to climb much higher—reportedly up to 1,500 volts—to achieve the cycle times the varistor can reach at lower voltages, he says.Randy Creech, engineering manager for The Electric Controller and Manufacturing Co. (St. Matthews, S.C.), previously Square D, notes that his firm's controller has an adjustable timing circuit that's set at the factory, but the operator can change it to either raise or lower the drop current depending on the type of scrap being handled. "A fine material such as turnings might require more drop current than bundles or plate and structural," he notes. Other manufacturers offer similar adjustable-drop rheostats so that operators can vary the "dribble" rate—the amount of time it takes for material to fall free of the magnet.

Know Your Controller
To make sure your controller does the best job and has the longest life, heed the following advice from the experts.

• Choose the correct size controller for your needs. Magnet controllers are sized based on the basic cold amps at which the magnet is designed to operate, Richendollar explains. "Magnets heat up during operation, and the current drops during that period, so you always want to size your controller for the worst-case scenario," he says. "Cold is the worst case because that's when the current is going to be the highest—hot is the lowest."

• Check regularly to make sure the controller is dissipating the stored energy when the magnet is turned off, suggests Jim Butke, sales and marketing manager for Walker Magnetics (Worcester, Mass.). If the controller isn't dissipating the energy, then "the magnet will dissipate that energy—and that's not good," he says. To determine if the controller is doing its job, use a volt meter to check the reverse voltage at the magnet terminal box, Butke recommends.

• Note if the magnet picks up less scrap over time or if material doesn't fall from the magnet bottom as quickly as it should upon release, then begin checking your system for problems. If the magnet picks up less material, then it isn't receiving enough voltage, it's going bad, or both, Richendollar says. To troubleshoot, he notes, "always start with the power supply—the generator—to make sure it's producing the proper voltage from the generator to the controller. Then check the controller to make sure it's lifting and dropping properly. For example, one missing resistor can prevent the magnet from cleaning properly. The last thing you check is the magnet.

• Make sure the mechanical controller is on the operator's preventive maintenance list, Bedard says. Users should have spare parts on hand, such as contact tips, arc shields, and wires.

  Speaking of maintenance, Richendollar recommends keeping a watchful eye on the controller. "It dissipates the energy of the magnet through the series of resistors, and the energy is dissipated as heat," he says. "Anytime you have heat, you have wiring and parts that can fail, so you need to keep a constant eye on them. The controller should be checked on a weekly basis." Specifically, he advises maintaining the gap setting of the lift contacts.

  Bedard suggests removing the cover from the controller regularly to inspect the copper contact tips because they transmit the current. When the contacts are in the process of opening, he explains, the space between the tips increases, and an arc of electricity forms between them until the gap grows so large that the electricity can't bridge it. "This arc of electricity is like a miniature lightning bolt," he says, "and if it is not isolated, it can wreak havoc by burning or melting parts of the controller." The burning material also produces carbon that gets on the controller's panels and components, which could cause a short circuit if it's not cleaned up. "Controllers are designed to minimize the effects of arcing through the use of adjustment screws and arc shields, but regular inspection is essential to avoid arc damage," he says.

  Buying Wisely
  What should you consider when buying a mechanical magnet controller? Vendors suggest these factors.

  Product quality. "The magnet circuit has been around for many years, and there's nothing special about it—two contactors lift and two contactors drop," Creech says. "It's more about the reliability of the product, the quality that goes into it."

  Ease of maintenance. Though maintenance generally is simpler with a mechanical controller, Bedard says, it's still a good idea to ask questions about a controller's maintenance needs and ease of repair. As Richendollar puts it, "You want to know if the controller is easy to work on if you need to."

  Manufacturer support and availability of spare parts. Is help readily available if a particular problem is beyond your maintenance department's abilities? Creech suggests looking for a manufacturer with an established record in the industry that can provide service and parts for its controller.

  Safety features. A charged magnet is dangerous because when the circuit is interrupted, the electricity can jump great distances if there's no discharge path.

  When shopping for a controller, ask if the unit has a permanently connected discharge path. "When power is removed from the magnet, there should be a path for the stored energy to travel," Moseley says. "The stored energy is the collapse of the magnetic field. If you don't have a path, the stored energy will find a path and show up in places you least want it to show up." For instance, the energy can jump to the ground or destroy the insulation in the magnet and on the magnet cables, taking years off the magnet's life.

  Bedard recommends installing a safety disconnect switch with fast-acting fuses between the generator and controller to protect the system from a rush of current. "If a magnet line gets pulled out," he says, "it will flop around until it grounds out. If you don't have a fuse, that surge of current could come back and damage system components." Safety disconnect switches also let maintenance personnel isolate the magnet system using lock-out/tag-out procedures to perform maintenance safely.

  What's New?
  Though mechanical magnet controllers haven't changed much over the years, manufacturers always look for ways to improve the equipment. Ohio Magnetics, for one, just introduced a line of controllers to replace its existing MC-1A duty-cycle models. "Rather than have a lot of controlling parts as we have in the past, we now have one unitized pair of contacts that replace all of the mechanical parts in the controller," Richendollar says.

  Previously, he notes, the controllers had a coil that wound around the core with a mechanical arm attached to the core. When the coil was energized, the arm that held the main set of contacts pulled them together, closing the circuit and allowing energy to flow to the magnet. "We have eliminated the core, coil, and arm that pulled everything together," he says. "We have replaced them with a unitized set of lift contacts that will significantly reduce maintenance on the controller." The new line—which is "basically the same controller with upgrades," Richendollar says—will have the same model numbers and price structure as the firm's previous units.

  Two manufacturers, Magnetech Industrial Services (Boardman, Ohio) and Winkle Industries (Alliance, Ohio), now offer skid-mounted, diesel-powered magnet generator systems. These self-contained systems, designed for use on material handlers, "don't have to be dedicated to any specific machine in the yard," says Larry Woods, inside sales manager, who designed Magnetech's unit, the PowerPro diesel generator. Instead, the unit "can be transferred from one machine to another provided the machine is capable of handling the magnet and the system that it's driving."

  The PowerPro system, which can handle magnets measuring 34 to 48 inches in diameter, includes a Kubota diesel engine, a 100-percent duty cycle Baldor 10-kW DC generator, and a Hubbell 4292P mechanical magnet controller rated from 5 to 50 amps. The system, which sells for about $17,000, weighs 1,200 pounds and measures 56 inches long by 24 inches wide by 36 inches high.

  Winkle's OptiGen comes in models that range from 5 kW to 40 kW, each of which comes with the appropriate size components—Kubota diesel engine, Baldor generator, and Hubbell magnet controller. Prices range from $11,000 for the 5 kW model to $32,000 for the 40 kW model. J. Mark Volansky, Winkle's director of sales, points out that the electric fuel pump that comes standard with OptiGen systems prevents engines from stalling on uneven terrain for improved magnet safety.

  Though mechanical controllers are well established in the scrap industry, they are facing competition from solid-state controllers and generator/solid-state magnet controller packages, both from American and European manufacturers. Will these newer technologies bring about a significant shift in the market in the coming years? We'll explore that question and more in the July/August issue. •

  Jim Fowler is retired publisher and editorial director of Scrap.

Long the workhorses for lifting-magnet work in the scrap industry, mechanical magnet controllers face growing competition from newer technologies.