Lifting magnet weak? Five causes and how to check
A lifting magnet is a circuit, not a power rating
When a magnet “gets weak”, the instinct is to blame the magnet. In most cases the magnet is doing exactly what it can do with the conditions it is being given. Holding force is the product of current, coil condition and — above all — how much of the load the magnetic field can actually reach.
Five causes account for almost every case of a magnet that used to lift well and now does not.
The five causes
| Symptom | Most likely cause | First check |
|---|---|---|
| Weak on all loads, gradually | Coil insulation degrading with heat | Coil resistance and insulation test against the original value |
| Weak on one shape of scrap only | Surface contact — the field is reaching air, not steel | Try a flat plate; if that lifts well, it is contact, not the magnet |
| Strong cold, weak after an hour | Duty cycle exceeded; coil overheating | Temperature of the coil housing after a normal working period |
| Intermittent, voltage drops under load | Controller supply or cable voltage drop | Voltage at the magnet terminals while energised |
| Weak only on thin sheet | Not enough material for the flux path | Stack the sheet — if it lifts when stacked, the magnet is fine |
1. The coil degrades with heat, not with age
Every energise/de-energise cycle heats the coil. Insulation degrades slowly, resistance rises, current falls, and holding force falls with it. The change is invisible week to week and obvious over a year.
What to do: measure coil resistance when the machine is new and write it down. That number is your baseline. Without it, “is the coil weak?” is an opinion.
2. Contact area decides more than tonnage
A magnet rated at 2,000 kg is rated on a clean, flat, thick steel plate in contact with the whole face. Loose scrap, tangled rebar and crushed cars present a fraction of that area, and the field has to cross air gaps to reach the rest.
This is why two yards with the same magnet report completely different performance: one lifts plate, the other lifts a bird’s nest.
What to do: test on a flat plate. If the magnet holds its rated load there, the magnet is healthy and the issue is material presentation — how the grab loads it, not the magnet.
3. Duty cycle is a hard limit, not a suggestion
Magnets are rated for a percentage duty cycle — typically 50% to 75% depending on the model. Above that, the coil cannot shed heat and the force falls within the shift.
What to do: check the rated duty cycle for the model, then measure the coil temperature after an hour of normal work. A magnet that is hot to the touch after a shift is being overworked.
4. Voltage at the magnet, not at the panel
Cable length, cable section and connector condition all drop voltage between the controller and the coil. A controller reading 220 V and 195 V at the terminals is a magnet running at 88% of its design current — and force falls faster than current does.
What to do: measure at the magnet terminals while energised, not at the cabinet. Check the connectors for corrosion, which is the usual culprit on a machine working outdoors.
5. Thin material has nowhere for the flux to go
A magnet needs material thickness to complete its magnetic circuit. On thin sheet, most of the field passes through the steel and out the other side into air, where it does nothing.
What to do: stack the sheet and try again. If a stack lifts well and a single sheet does not, the magnet is fine and you need a different handling approach — a grab, or a magnet sized for sheet.
What to measure before calling anyone
- Coil resistance, against the value recorded when new
- Voltage at the magnet terminals while energised
- Coil housing temperature after one hour of normal work
- Holding performance on a flat plate versus on your actual material
- Hours on the machine and whether the duty cycle is respected
Frequently asked questions
Can a magnet be rewound?
Yes, and it is usually cheaper than replacement for a large magnet. The question is whether the coil is the fault — which is why the resistance baseline matters.
Why does it lift steel plate but not rebar?
Contact area. Plate presents a continuous surface; rebar presents edges and gaps. This is normal behaviour, not a fault.
Does rain affect holding force?
Not directly, but water ingress into connectors and the coil housing does. A magnet working outdoors needs its connectors checked far more often than one under a crane indoors.
How do I choose a magnet size for my crane?
From the crane’s rated capacity, the material you handle, and the duty cycle — not from the largest magnet the crane can physically lift. The magnet table lists diameter, weight and rated lifting capacity for each model.
Let’s put the right machine to work.
Tell us your material, required output and destination. We’ll discuss a suitable configuration with you.