<Li> Eddy currents: From Faraday's law of induction, the changing magnetic field induces circulating electric currents inside nearby conductors, called eddy currents . The energy in these currents is dissipated as heat in the electrical resistance of the conductor, so they are a cause of energy loss . Since the magnet's iron core is conductive, and most of the magnetic field is concentrated there, eddy currents in the core are the major problem . Eddy currents are closed loops of current that flow in planes perpendicular to the magnetic field . The energy dissipated is proportional to the area enclosed by the loop . To prevent them, the cores of AC electromagnets are made of stacks of thin steel sheets, or laminations, oriented parallel to the magnetic field, with an insulating coating on the surface . The insulation layers prevent eddy current from flowing between the sheets . Any remaining eddy currents must flow within the cross-section of each individual lamination, which reduces losses greatly . Another alternative is to use a ferrite core, which is a nonconductor . </Li> <Li> Hysteresis losses: Reversing the direction of magnetization of the magnetic domains in the core material each cycle causes energy loss, because of the coercivity of the material . These losses are called hysteresis . The energy lost per cycle is proportional to the area of the hysteresis loop in the BH graph . To minimize this loss, magnetic cores used in transformers and other AC electromagnets are made of "soft" low coercivity materials, such as silicon steel or soft ferrite . </Li> <P> The energy loss per cycle of the AC current is constant for each of these processes, so the power loss increases linearly with frequency . </P> <P> When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting electromagnets can be used . Instead of using ferromagnetic materials, these use superconducting windings cooled with liquid helium, which conduct current without electrical resistance . These allow enormous currents to flow, which generate intense magnetic fields . Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting . Current designs are limited to 10--20 T, with the current (2009) record of 33.8 T . The necessary refrigeration equipment and cryostat make them much more expensive than ordinary electromagnets . However, in high power applications this can be offset by lower operating costs, since after startup no power is required for the windings, since no energy is lost to ohmic heating . They are used in particle accelerators and MRI machines . </P>

Can you make an electromagnet with ac current