Electromagnets

The duality between electricity and magnetism provides a degree of freedom for magnetic circuit design that is not easily achievable with permanent magnets: field magnitude adjustability. Unlike their permanent magnet counterparts which require mechanical means for field adjustments, electromagnetic field magnitudes can be altered by simply adjusting input power characteristics; more specifically, the input current. This current, multiplied by the number of windings in the circuit, provide the magneto-motive force (MMF) needed to establish a magnetic field in space. By utilizing advanced numerical analysis techniques, we can adjust geometries and materials to optimize electromagnetic designs and minimize MMF requirements.

In general, electromagnets are classified into two categories: DC (direct-current) and AC (alternating-current). The simplest electromagnetic configuration is called an "air-core". These units typically consist of either freestanding copper windings, or copper windings mounted on non-magnetic, non-conductive media. The relatively low inductance of electromagnets constructed in this configuration makes them well suited for applications requiring low magnetic field strengths and high frequency inputs. Consequently, "air-core" electromagnets are rarely used for DC electromagnet applications, and typically find themselves in AC or high field pulsed-current systems.

If higher, continuous, field strengths are required, a high permeability ferrous core (magnetic conductor) may be placed into the electromagnet. Core material selection is based upon input and remanent field requirements of the application. Most DC electromagnet cores can be manufactured from standard cold rolled steels, which are inexpensive and provide significant increases in field magnitude over standard air core configurations. Unfortunately, these materials do exhibit a significant remanent value in their hysterisis curve. Consequently, a non-zero field magnitude may result even after power has been removed from the inputs. Fortunately, various electrical steels are specifically designed to minimize this characteristic, and can easily be implemented into any DC electromagnet. High saturation magnetic steels, that can be utilized to maximize the field strength to current ratios for any given design, are also available. Whatever the case, the selection of core material may be critical to the success of the electromagnet in its final application.

AC electromagnet cores require consideration of a few additional characteristics. Unlike DC units, these cores are prone to eddy-current and hysterisis losses, which are well-documented phenomenon associated with time variant input signals (AC signals) and magnetism. Powdered irons, ferrites, and laminated structures provide a magnetically conductive path, while partially compensating for these losses. Signal frequency and field magnitude are crucial to the proper selection of a core material. Improper core material selection can lead to unexpected losses, resulting in heating that can change the system's performance or lead to self-destruction. Dexter's engineering staff should be consulted to help decide upon the most appropriate core material for your DC and AC electromagnet applications.

In either case, DC or AC, resistive losses in the windings lead to heating. If current densities in the windings are excessive, a forced means of cooling may be required to remove the excess heat and assure consistent performance. Two common methods of cooling electromagnets are forced convection and water-cooling. If such thermal relief is necessary, additional space in the final application should be provided to implement the necessary hardware.

For further inquiries, please e-mail us at info@dextermag.com -or- call us at:  800-345-4082 (In North America) or +44 (0) 1189 602430 (In Europe).