July/August 1991
Operator training and preventive maintenance can extend the life of electromagnets, prevent expensive repairs, and ensure optimum performance.
By Kent Kiser
Kent Kiser is associate editor of Scrap Processing and Recycling.
Let’s begin with a misconception: An electromagnet is a massive, indestructible object that can be used as a drop ball or battering ram and still function effectively.
The truth? Magnets are electric tools that perform work internally and externally with the help of many subcomponents, some of which are as thin and fragile as paper. Any magnet manufacturer or repairman will tell you that magnet misuse is the most common cause of magnet problems--and the fastest route to expensive repairs or magnet failure. “Most magnets are just totally abused,” says Lou Costa, manager of the magnet department for Meade Industrial Services Inc. (Hammond, Ind.).
Considering that scrap processors and recyclers can invest more than $40,000 in a magnet, how can they allow it to be misused? One explanation, says Alan Zelunka, president of Gensco Equipment Co. Ltd. (Toronto), is that “the operator of the magnet is not the guy who paid for it.” But while a few operators may indeed be intentionally careless with a magnet, in many cases lack of training may be responsible--the operator may never have learned proper magnet-handling skills.
This is unfortunate because magnet manufacturers are repair shops agree that the two best ways to reduce magnet repairs, guarantee optimum performance, and extend longevity are to train operators in proper magnet use and implement a preventive-maintenance program.
What Is an Electromagnet?
To make the most of operation training and preventive-maintenance programs, magnet owners and operators should understand the nature of the beast: What are a magnet’s basic components? How does it operate? How is it designed to be used?
The most visible part of a magnet is its case, which contains subcomponents such as the coil, wire connections, waterproof thermoplastic potting, and the bottom plate. The case exterior features a terminal junction box and lugs, or “lifting ears,” that hold the chains that connect the magnet to a crane.
Cases, which can measure 18 to 93 inches in diameter, can be cast or fabricated. A cast case is a single-pour shell that usually has 3/4- to 2-inch walls (depending on the diameter of the magnet), interior extrusions to hold subcomponents, and reinforcing ribs on the outside. Cast cases are the most common type used in scrap plants.
Fabricated cases are made of rolled steel plate with walls also 3/4 to 2 inches thick. These cases can be customized more easily than cast cases, but they take more time to make. “A fabricated case is more labor intensive, whereas a cast case is more material intensive,” observes James A. Butke, sales and marketing manager for O.S. Walker Co. (Worcester, Mass.).
The case’s “official” functions are to protect the interior magnet elements from damage, transfer heat away from the coil, prevent moisture from reaching the coil, and allow magnetic energy to pass through it. Its “unofficial”--and unrecommended--functions are use as a pulverizer, wrecking ball, or drag weight to move railcars.
The coil, or “winding,” is the heart of every magnet. It is a tightly spooled coil of copper or aluminum conductor that has paper-thin insulation between wraps. Its purpose is to conduct electrical energy to produce external magnetic flux-force lines.
A magnet’s bottom plate covers the underside and is welded to the magnet’s inner and outer pole shoes. It is usually made of 3/4- to 1 1/4-inch cast or rolled manganese steel, but--contrary to popular believe--it is not a magnetic surface. It protects the coil by sealing out moisture, dispersing heat, and shielding the coil from external physical damage. Approximately 75 percent of magnet burnout damage is linked to bottom-plate trauma--or, in other words, magnet misuse, Costa says.
A magnet’s “magnetism”--its ability to attract, hold, and deliver payloads--is the work of the pole shoes. Each magnet has a center, cylindrical pole (usually of north polarity) and an outer, circular pole (usually of south polarity). The outer pole shoe of most new magnets is cast as part of the magnet case, whereas most older magnets have bolt-on or weld-on outer pole shoes. The center pole shoe of most magnets is bolted on.
When a magnet is activated on a pile of scrap, the pole shoes create an oval magnetic field that extends downward into the scrap and upward through the magnet case. How deep a magnet's power can extend into the pile of scrap, thus increasing the payload, is its "saturation" level. Standard magnets do not saturate as much as deep-field magnets, which are not as powerful as extra-deep-field magnets.
Common Problems ... And How to Avoid Them
Heat, moisture, and physical trauma are the most common magnet killers. Any of these elements can harm a magnet's components and will eventually impair its overall performance. Even though damage may accumulate gradually, there will come a time when the "cascade effect" occurs and severe breakdowns result, Costa says.
Coil failure, the most common large-scale problem, can often be linked to magnet misuse. Excessive internal temperatures, for instance, can break down the coil insulation, shorting the magnet. Most magnets are designed to run on 230 volts DC, but many operators push the voltage to 250 volts or more to increase the magnet's lifting power. The power formula says, however, that if you increase the voltage, you also increase the wattage, or heat, Costa notes. The increased heat not only can destroy the insulation, but also reduces the magnet's effectiveness after several hours, thus thwarting the operator's original intention.
The coil can also fail if the operator pushes the magnet beyond its recommended “duty cycle,” which normally ranges from 50 to 80 percent. A 50-percent duty cycle means that, in any given minute of operation, a magnet should be on for 30 seconds and off for 30 seconds. A 75-percent duty cycle means that the magnet can be on 45 seconds and off 15 seconds per minute.
In addition, cautions Dave DiCola, magnet product manager for Lectromag Inc. (Massillon, Ohio), a conventional magnet should never be run for more than 5 minutes continuously. To avoid leaving a magnet on too long, manufacturers and repair shops emphasize, operators should turn off the magnet when moving or swinging it to pick up a new load. "You want the magnet to run longer, cooler, and more efficiently," Costa asserts. "Why leave the magnet on if you will lose tonnage throughout the day?"
The coil can also fail if moisture reaches it through minute cracks in the case, through broken welds or loose connections in the terminal box, or through the pores of the case. Overheated magnets can even form condensation inside the case as they cool. One crucial point, DiCola says, is to never place a hot magnet on the ground to cool because it will draw moisture from the earth into the case. Instead, he recommends, hot magnets should be set on wood pallets, railroad ties, used tires, or even a scrap pile. If themagnet won't be used for a while, store it indoors and off the ground, covered with a tarpaulin. DiCola strongly discourages operators from cooling a magnet by submerging it in water, as some do. This practice exposes the magnet's internal and external components to damage due to moisture and physical trauma.
Another common area of concern is the magnet's pole shoes. These components have a rough life, constantly rubbing against all types of scrap, so a certain amount of wear is inevitable. Nevertheless, operators should make sure the shoes do not wear irregularly or excessively. If the pole shoes do not align on the same horizontal plane, the magnet's conductor can be harmed and its lifting power reduced. Check pole-shoe wear at least once a month, recommends David Futa, magnet superintendent for Delta Star Electric Inc. (South Bend, Ind.), keeping in mind that pole shoes should not be worn closer than 1/2 inch to the bottom plate.
Worn pole shoes can be repaired by hard-surface welding, which adds a new layer of metal onto the old surface. Futa suggests taking this step at least once a year. Some manufacturers offer hard-surfacing as an option on new magnets.
To reduce wear and damage to the pole shoes and bottom plate, operators should not energize a magnet until it contacts the material to be lifted. If the magnet is turned on too early, scrap can jump up to meet the magnet and damage it. To achieve the greatest saturation and, thus, move the largest payload, Costa notes, the energized magnet should sit on the scrap for about 10 seconds before the material is lifted.
On-site repairs can also result in significant problems. Most magnet owners and operators are capable of tightening bolts, welding minor cracks, and replacing leads and chains, but, says DiCola, "Anything else and you'll start messing with the mechanical integrity of the magnet." Magnet owners must be vigilant not to "bubble-gum-patch" things together, Futa says, or else they could cause more expensive repairs down the road. For instance, if a magnet owner replaces a lead and does not create a watertight connection where the lead enters the terminal box, moisture could enter the case and destroy the coil.
Implementing a Preventive-Maintenance Program
Industrial magnets are built to last, manufacturers say, but only if they are used within specified parameters and only if they are given the proper care. A magnet can last 8 to 12 years in a scrap plant if maintained well, but only 4 years or less if misused. "We have seen severe abuse destroy a 2-inch bottom plate in 6 months," says Futa.
Magnet problems--not to mention repair bills--can be reduced if the magnet owner implements a preventive-maintenance program that includes regular magnet inspections, recordkeeping, and operator training. Such a program will prevent some problems and catch other problems when they are small, reducing the chance of magnet failure. Identifying problems early also enables the owner to schedule repairs at a convenient time, reducing the loss of productivity due to unexpected downtime.
It's a myth, Costa notes, that preventive maintenance takes too much time. A thorough inspection of one magnet can take as little as 15 minutes, he says, and the program will pay for itself the first time a major problem is averted.
The first step in a preventive-maintenance program is to establish a recordkeeping system. Assign a number to each magnet and weld the number on the magnet case for easy identification during inspections. Next, start an information file for each magnet, including its size, model number, serial number, purchase forms, manufacturer specifications, repair history, and operator comments. Set up an inspection schedule for each magnet, establishing daily checks of leads, connectors, chains, and the terminal box, with monthly checks of the case, pole shoes, and other major components. Make one person or small group of people in charge of recording the maintenance information, and make sure they are diligent in executing the program. Preprinted magnet maintenance logsheets are available from many manufacturers and repair shops.
Most magnet manufacturers agree that a physical inspection should center on the following parts and considerations:
Coil: Magnet owners should purchase both a volt-ohm meter and a 500-volt insulation tester (megger) to test coil resistance and coil insulation, respectively. The volt-ohm meter is hooked up to the coil leads, while the megger is hooked up to one lead and a clean spot on the magnet case. To guarantee the most reliable results, all readings should be taken under the same conditions--that is, at the beginning of the workday, after four hours of operation, or at the end of the workday--during every inspection.
Magnet temperature can affect both tests. Seasonal and interior temperature changes, in fact, can alter coil resistance readings up to 30 percent. Therefore, when checking coil resistance, operators should record the outside, or ambient, temperature and note whether the magnet is cold or hot. If the reading falls within 10 percent of manufacturer specifications, then the coil should be functioning properly. Butke recommends taking the readings when the magnet is cold because manufacturers base their specifications on a cold magnet and a cold magnet eliminates concerns about temperature variations. If the coil resistance reading varies significantly from specification, the operator should contact the manufacturer or a repair shop.
For insulation tests, a cold magnet (68 to 77 degrees) should have a minimum reading of 10 meg ohms, while a hot magnet (320 degrees or more) should not read less than 0.1 meg ohm. If the readings fall below those levels, the magnet may need to be "baked out" to eliminate moisture and also may need to be rewound with new insulation. A total coil replacement with a new bottom plate is the most expensive magnet repair and can take the magnet out of operation for 10 days to a month, DiCola says.
Terminal Box: This area is critical not only because it is the electrical junction of the magnet, but because it can be easily damaged, thus exposing the coil to moisture. Check for broken welds and lead clamps, wire insulation stress at clamps, arc tracking (burns) under leads, and leakage of potting compound. Make sure all seals, plugs, bolts, and gaskets are snug. If there is any dirt on the box or in its seams, clean it off as it can conceal damage, hold moisture, or fall into the box during repairs.
Leads and Connectors: These electrical lines are a magnet's most vulnerable parts because they can be cut, cracked, scraped, and strained during operation. In fact, Futa says, lead damage is the most common day-to-day magnet problem. Make sure that all leads and connectors fit snugly together or in their sockets and that the insulation around them is not cut, cracked, worn, or strained. Sometimes protective hoses or "cable channels" are wrapped around leads for extra protection; if so, make sure they aren't bent, crimped, or crushed, which could indicate lead damage. Gland nuts that plug into the terminal box must be tightly threaded to create a watertight seal; or if male/female plugs are used, the protective metal shield should not be damaged and should be bolted tightly to the box. Many magnet users install strain-relief clamps made of chain, springs, or cable to prevent strain on the leads. These clamps should be checked for overtightness and wear.
Pole Shoes: In addition to checking Pole shoe wear, operators should inspect them for scrapes, gouges, broken or missing bolts, and broken welds between the shoes and the bottom plate. Remember, these are the essential magnetic elements, so their health will directly affect the lifting power of the magnet.
Bottom Plate: Damage to the bottom plate is usually a sign that the operator needs training. Check for dents or warps by using a straight edge, while also noting any broken welds and moisture and/or potting leakage at the seams. Serious damage to the bottom plate could mean damage to the coil, which rests on the plate. If the bottom plate crushes the internal conductors, Futa says, repairs can total up to 98 percent of a new magnet price. In such cases, the magnet must usually be scrapped.
Case: The magnet's "protective shell" should be scrutinized for cracks, dents, broken welds, severe wear, and lug wear. Most case cracks will appear in the pole shoe area and where the side of the case joins the top.
Chains: Worn chains and pins are a safety hazard. Chains in which any link is worn more than 20 percent should be replaced, DiCola says, and operators should never try to weld-repair a chain link or pin.
Fill Hole Plug: Make sure the set plug is not damaged, missing, or leaking potting or water at its threads.
One final note: Emphasize safety during all physical inspections. Employees should never stand under a suspended magnet; instead, lay the magnet on its back or side, or set it on sturdy supports. Also, Futa notes, make sure all power to the magnet is turned off before disconnecting any leads; otherwise, up to 10,000 volts of direct current could be unleashed.
Manufacturers Do Their Part
Magnet manufacturers and repair shops are becoming more active in educating their customers and promoting preventive maintenance programs. Ohio Magnetics Inc. (Maple Heights, Ohio), for example, offers magnet maintenance seminars as well as a poster that operators can put in their crane cabs and consult for proper operating procedures. Meade has a magnet care brochure available for the asking. Delta Star Electric offers on-site equipment surveys, diagnostic services, operator training, and preventive-maintenance workshops. O.S. Walker provides operating and maintenance instructions with all its magnets and offers training services on request. And Lectromag distributes a preventive-maintenance/troubleshooting service manual for magnets and magnet systems, promotes its "Safewatch" preventive-maintenance program, and offers magnet inspection services anywhere in the United States. •
Operator training and preventive maintenance can extend the life of electromagnets, prevent expensive repairs, and ensure optimum performance.