Electro vs. Permanent Overhead Magnets: How to Choose

Both magnet types have the same function — pull ferrous metal up and out of a moving burden of material — and the choice between them is governed by one variable above all: how far the magnetic field has to reach. An electromagnet generates its field with a powered coil, reaches deeper, and can be adjusted; a permanent magnet generates its field with magnetic material, costs nothing to run, and never needs more than mechanical maintenance. Get the suspension height and burden depth right, and the rest of the separation usually makes itself.

How each one works

An electro overhead magnet is a DC coil wound around a steel core. Field strength scales with the current through the coil, which means two things: the field can be made very deep for a given magnet size, and it is adjustable — it can be tuned to the duty, or switched off entirely to release the circuit for maintenance. The cost of that capability is electrical: the coil consumes power continuously, generates heat that must be managed (oil-filled or air-cooled designs), and depends on a rectifier or controller that becomes one more component to maintain. Duty matters — a coil running hot loses field strength as resistance rises, so continuous heavy duty needs to be in the specification, not discovered in service.

A permanent overhead magnet is a block of magnetic material — ceramic ferrite in most heavy-duty separators, rare-earth where intensity at short range matters — in a steel housing. No power, no rectifier, no cooling, no controller. Field strength is fixed at manufacture and degrades only negligibly over decades of normal service. Its limitation is the same fact in reverse: what you buy is what you get, and the practical field depth for a given physical size is shallower than an electro of comparable footprint.

Suspension height and burden depth: the governing logic

Magnetic field strength falls off steeply with distance — small increases in gap cost large fractions of pulling force. The working distance the magnet must cover is the suspension height above the belt plus the depth of material the steel may be buried under. A piece of rebar at the bottom of a deep burden must feel enough force, through the full burden, to climb out through the material above it.

That arithmetic drives the selection. Wide belts, deep burdens, high belt speeds, and large suspension heights (often forced by lump size — the magnet must clear the biggest piece that will pass under it) push you toward electro. Shallow, well-spread burdens on narrower belts at moderate suspension heights are well within permanent range. There is no universal crossover number — it depends on burden, belt width, and the ferrous you must catch — but as a rough industry pattern, modest suspension heights favour permanent and tall ones favour electro.

One upstream lever is cheaper than any magnet upgrade: spread the burden. A thinner, evener bed at the magnet position reduces the depth the field must penetrate and improves recovery with either technology.

Self-cleaning configurations: cross-belt and inline

On a recycling line the magnet must discharge continuously, so the standard arrangement is self-cleaning: a short belt with cleated flights runs around the magnet housing, sweeping captured ferrous off the face and out of the field, where it drops to a collection conveyor or bunker.

Two mounting orientations exist. A cross-belt magnet hangs perpendicular to the conveyor and discharges to the side — it can be positioned anywhere along the conveyor run. An inline (axial) magnet hangs parallel to the flow, over the head pulley discharge, and throws ferrous forward. Inline placement gets help from physics: at the discharge, material is in the air, the burden opens up, and steel is easier to extract from a loose trajectory than from a packed bed. Where layout allows it, inline over the discharge is usually the higher-recovery position; cross-belt wins where the discharge zone is already spoken for.

The drum magnet alternative

Where headroom is missing or the burden is too deep and harsh for an overhead unit, a drum magnet does the same job in a different geometry: material flows over a rotating drum with a stationary magnetic arc inside; ferrous holds to the shell through the arc and discharges underneath, separated from the non-magnetic trajectory. On primary unders in a C&D line — the heaviest ferrous duty on the plant — the answer is an overhead self-cleaning magnet or a drum magnet, never a magnetic head pulley, which is quickly overloaded by that burden.

Cost of ownership, not purchase price

The permanent magnet wins the operating-cost column outright: zero energy, no rectifier, maintenance limited to the cleaning belt and its drive. The electro carries a higher purchase price too — the coil, rectifier, and cooling are real capital — on top of continuous power consumption, cooling, and controller upkeep. But the comparison is incomplete without the recovery column: a permanent magnet that cannot reach the bottom of your burden leaves steel in the stream — steel that damages shredders, de-stoners, and eddy current rotors downstream, and steel that was sellable product. Missed ferrous costs more than electricity. In streams where the ferrous fraction is high enough — C&D among them — that extra recovery pays back the electro's higher capital and running cost; in lean or shallow-burden streams it may not, and the permanent magnet is the better buy. The cheapest magnet is the one that actually pulls your steel from your burden at your suspension height.

Selection checklist

Belt width, belt speed, and design burden depth at the magnet position
Largest lump that must pass under the magnet — this sets minimum suspension height
Ferrous profile: rebar, strapping, sheet, fasteners — size and shape change extraction difficulty
Mounting position available: inline over the discharge (preferred where possible) or cross-belt
Headroom — if there is none, evaluate a drum magnet instead
Duty cycle and ambient temperature if electro; coil cooling type
Downstream machines being protected (eddy current, de-stoner) — their tolerance sets your required recovery, not the magnet brochure

Sherbrooke OEM sizes and integrates ferrous separation into every line it builds — overhead self-cleaning, drum, and head-pulley configurations placed per duty — as part of the magnets and sorters range. For the related question of keeping the cleaning belt alive in C&D service, see protecting overhead magnet belts. To size a magnet against your actual burden rather than a catalogue page, talk to engineering.