Lifting Electromagnet

Overview

A lifting electromagnet grips ferrous loads with magnetic force instead of hooks, slings or clamps. Suspended from a crane, the circular magnet is lowered onto scrap, slabs, billets, plates or coils; energising the coil at 220 V DC pulls the load against the pole faces and the crane lifts magnet and load together. No rigger touches the load, attachment takes seconds, and oddly shaped scrap that could never be slung is handled in bites of hundreds of kilograms. Scrap yards, steel mills and foundries run these magnets through thousands of lift cycles a day.

The machine comprises the Magnet Body itself, the Magnet Controller that rectifies and sequences the coil current, the Battery Backup System that holds the load through a power failure, and the Suspension Gear and Power Cabling connecting it to the crane.

How it works

The magnet is a short, fat solenoid wrapped in iron. The annular Exciting Coil, strip-wound anodised aluminium or copper, class H insulated, sits in the groove between the central Centre Core Pole pole and the surrounding Outer Pole Shell, which serves as both housing and outer pole. When the coil carries current, flux travels down the core, crosses into the load, passes radially through it, and returns up the shell rim: the load itself completes the magnetic circuit. Holding force follows from the Maxwell stress, proportional to flux density squared times pole area, so force collapses rapidly with any air gap. A magnet rated 20,000 kg on a machined slab may hold only 600 kg of loose scrap, because scrap touches the poles at a few points and most of the circuit is air. Manufacturers therefore quote two ratings: dead lift on a thick, flat, low-carbon plate, and a practical scrap rating roughly 3–5% of it. ASME B30.20 requires the working load limit be no more than half the breakaway force in the actual application.

A non-magnetic manganese-steel Bottom Wear Plate between the poles takes the impact of being dropped onto scrap thousands of times, protecting the coil and its Encapsulation Compound encapsulation. Coil heating is the operating constraint: at 75% duty the winding runs near its 180 °C class H limit, and as copper or aluminium resistance rises with temperature the current, and the lift force, falls, which is why a hot magnet noticeably "weakens" during a shift.

Control and release

The Thyristor Rectifier Bridge in the controller converts plant 380–480 V AC to controlled DC. Switch-off is the hard part: the coil stores tens of kilojoules in its inductance, and interrupting the current abruptly would generate a destructive voltage spike. The controller instead commutates the energy into the Discharge Resistor and then drives a brief reverse-current pulse through the coil. The reverse pulse cancels residual magnetism in the load and pole faces, so plates fall away cleanly rather than dribbling off as remanence decays, the difference between a 1-second and a 10-second drop cycle. The Power Level Selector allows reduced coil voltage for picking a single plate off a stack; at partial flux the field does not penetrate to the second sheet.

Battery backup and safety

Because the load falls if current fails, a suspended-load power failure is the defining hazard. The Battery Backup System addresses it: a float-charged 12 V Battery bank monitored by a BMS Board sits behind a Changeover Contactor contactor that transfers the coil to battery within milliseconds of mains loss, holding the load for 10–20 minutes while the Backup Alarm Unit sounds. The operator's standing instruction is to set the load down immediately and not re-lift until mains returns. EN 13155 mandates the backup supply and alarm for magnets used over areas where people may be present; site rules still prohibit carrying loads over personnel.

The Release Switch is shrouded or two-action so the load cannot be released by a brushed elbow. Mechanically the magnet hangs on a three-leg Grade 80 Suspension Chain Leg from welded shell lugs, proof-tested with the magnet; power arrives through an EPR Trailing Cable on a spring-rewind Cable Reel that follows hoist motion.

Variants

Rectangular (bipolar) magnets suit billets, rails and beams, with the two poles bridged lengthwise by the load. Slab-handling magnets gang three to five circulars on a spreader beam. High-temperature designs with extra coil clearance and heat shields work loads to 600 °C in rolling mills. Electro-permanent magnets use NdFeB or AlNiCo material magnetised and demagnetised by a current pulse, drawing power only during switching, they hold indefinitely through power loss without batteries, and have displaced electromagnets in machine-shop plate handling, though pure electromagnets still dominate scrap because their deep field reaches through irregular, rusty material.