Material Handling
Sunday | 01 January, 2006 | 3:43 am

Powers of attraction

By Rebecca Green

Electropermanent magnets used to be larger and heavier than electromagnets. This difference has been greatly reduced with the development of newer, more powerful rare earth permanent magnetic material. Today, electropermanent magnet systems are much closer in weight and cost to comparable electromagnets.

Electropermanent magnets by Walker Magnetics meet the requirements for handling steel loads in steel service centers, steel processing plants and fabricating shops. These systems can be configured with multiple magnets attached to a beam for handling plate.

A range of standard systems accommodates various plate sizes; spring suspended magnets accommodate bowed plates. Loads are shared equally, and the spring suspension protects against shock and vibration. Adjustable magnet positions for variable size loads are available as an option. A 100 percent duty cycle means that the magnets can be left on indefinitely without overheating.

The strength of the magnetic field generated by the magnet can be controlled by the level that the switchable magnetic pack is magnetized. This is useful when the magnet is used to lift thinner plates off a stack. The magnet control will automatically charge the magnet to full power after the load has cleared the stack of plates.

Controls for electropermanent magnets offer several useful features. For instance, controls can be mounted on the magnet system for smaller plate handling applications or crane mounted for larger slab and coil handling applications. A remote control IR unit allows the operator to stand away from the material being lifted. When the load is ready to be lifted, the operator turns the system on with the push of a button.

Dave Wilber, sales engineer at Walker Magnetics, explains that many of these systems are used at and around cutting table operations, such as burning systems. Traditionally, people climb on the table to get parts, which can be dangerous. "Remote operation allows them to stand off to the side, away from what's being transported," he says.

Holding power stays on
The magnet must be in contact with the load to turn the magnets on, and because the magnets are permanent, electricity is only required to activate the system. Energy is saved, as the magnets require only momentary power to turn them on and off. Once the magnet is on, electricity can be disconnected indefinitely without any interruption to the holding power.

The system also includes an interlock feature to prevent magnet de-energization when a steel plate or part is being lifted. The interlock feature means that the load cannot be released when it's suspended in the air. A non-volatile memory allows the system to remember its on/off status, even after a power interruption.

Another feature of the electropermanent magnet control is a current-sensing circuit. This circuit measures the amount of current flowing to the coil to verify that the magnet is energized to the desired magnetic strength. Electropermanent magnet systems for lifting large loads, such as slabs and coils, also feature a flux sensing circuit. This circuit measures the amount of magnetic flux flowing through the entire magnetic circuit, which includes the magnet and the load to be lifted. Insufficient flux indicates that the air gap in the magnetic circuit is too large and the lift is not allowed.

Not for hot materials
The field generated by an electropermanent magnet is heat sensitive. The material loses strength or the magnetic properties can be changed if they are exposed to a high temperature. Because of this they are not recommended for use on hot material.

Wilber explains that these magnets are air gap sensitive, which is a good term for all permanent magnets, whether they're electropermanent or manually operated permanent magnets. Electropermanent magnets do a good job of lifting solid loads. Applications include plate, slabs and coils. They are not the proper magnet for lifting bundles or scrap. "The cleanliness of the load is important to know about when choosing the right magnet system," says Wilber.

Walker's electropermanent magnets have no internal moving parts. Therefore, the magnets can be sealed so that dust or particles can't enter and create problems over time. MM

By Rebecca Green, from the January 2006 issue of Modern Metals.

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