Editing Electromagnet (section)

==Side effects==
There are several side effects which occur in electromagnets, which must be considered in their design. These effects generally become more significant in larger electromagnets.

===Ohmic heating===
[[Image:Current carrying busbars at the LNCMI.jpg|thumb|Large aluminum busbars carrying current into the electromagnets at the [[LNCMI]] (Laboratoire National des Champs Magnétiques Intenses) high field laboratory]]

The only power consumed in a [[direct current]] (DC) electromagnet under steady-state conditions is due to the [[Electrical resistance|resistance]] of the windings, and is dissipated as heat. Some large electromagnets require water cooling systems in the windings to carry off the [[waste heat]].

Since the magnetic field is proportional to the product <math>NI</math>, the number of turns in the windings <math>N</math> and the current <math>I</math> can be chosen to minimize heat losses, as long as their product is constant. Since the power dissipation, <math>P=I^2R</math>, increases with the square of the current but only increases approximately linearly with the number of windings, the power lost in the windings can be minimized by reducing <math>I</math> and proportionally increasing the number of turns <math>N</math>, or using thicker wire to reduce the resistance. For example, halving <math>I</math> and doubling <math>N</math> halves the power loss, as does doubling the area of the wire. In either case, increasing the amount of wire reduces the ohmic losses. For this reason, electromagnet windings often have a significant thickness.

However, the limit to increasing <math>N</math> or lowering the resistance is that the windings take up more space between the magnet's core pieces. If the area available for windings is filled up, adding more turns requires a smaller diameter of wire, which has higher resistance, and thus cancels the advantage of using more turns. So, in large magnets there is a minimum amount of heat loss that cannot be reduced. This increases with the square of the [[magnetic flux]], <math>B^2</math>.

===Inductive voltage spikes===
An electromagnet has significant [[inductance]], and resists changes in the current through its windings. Any sudden changes in the winding current cause large voltage spikes across the windings. This is because when the current through the magnet is increased, such as when it is turned on, energy from the circuit must be stored in the magnetic field. When it is turned off, the energy in the field is returned to the circuit.

If an ordinary [[switch]] is used to control the winding current, this can cause sparks at the terminals of the switch. This does not occur when the magnet is switched on, because the limited supply voltage causes the current through the magnet and the field energy to increase slowly. But when it is switched off, the energy in the magnetic field is suddenly returned to the circuit, causing a large voltage spike and an [[Electric arc|arc]] across the switch contacts, which can damage them. With small electromagnets, a [[capacitor]] is sometimes used across the contacts, which reduces arcing by temporarily storing the current. More often, a [[diode]] is used to prevent voltage spikes by providing a path for the current to recirculate through the winding until the energy is dissipated as heat. The diode is connected across the winding, oriented so it is reverse-biased during steady state operation and does not conduct. When the supply voltage is removed, the voltage spike forward-biases the diode and the reactive current continues to flow through the winding, through the diode, and back into the winding. A diode used in this way is called a [[freewheeling diode]] or [[flyback diode]].

Large electromagnets are usually powered by variable current electronic [[power supply|power supplies]], controlled by a [[microprocessor]], which prevent voltage spikes by accomplishing current changes slowly, in gentle ramps. It may take several minutes to energize or deenergize a large magnet.

===Lorentz forces===
In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to the [[Lorentz force]] <math>q\mathbf{v}\times\mathbf{B}</math> acting on the moving charges within the wire. The Lorentz force is perpendicular to both the axis of the wire and the magnetic field. It can be visualized as a pressure between the [[magnetic field lines]], pushing them apart. It has two effects on an electromagnet's windings:
* The field lines within the axis of the coil exert a radial force on each turn of the windings, tending to push them outward in all directions. This causes a [[tensile stress]] in the wire.
* The leakage field lines between each turn of the coil exert an attractive force between adjacent turns, tending to pull them together.{{cn|date=July 2020}}
The Lorentz forces increase with ''<math>B^2</math>''. In large electromagnets the windings must be firmly clamped in place, to prevent motion on power-up and power-down from causing [[metal fatigue]] in the windings. In the [[Bitter electromagnet]] design (Fig. 2), used in very high-field research magnets, the windings are constructed as flat disks to resist the radial forces, and clamped in an axial direction to resist the axial ones.

===Core losses===
In [[alternating current]] (AC) electromagnets, used in [[transformer]]s, [[inductor]]s, and [[AC motor]]s and [[Electric generator|generators]], the magnetic field is constantly changing. This causes energy losses in their magnetic cores, which is dissipated as heat in the core. The losses stem from two processes: eddy currents and hysteresis losses.

''[[Eddy current]]s'': From [[Faraday's law of induction]], a changing magnetic field induces circulating electric currents (eddy currents) inside nearby conductors. 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 [[lamination]]s, 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.

''[[Hysteresis loss]]es'': Reversing the direction of magnetization of the [[magnetic domain]]s in the core material each cycle causes energy loss, because of the [[coercivity]] of the material. These are called hysteresis losses. The energy lost per cycle is proportional to the area of the [[hysteresis loop]] in the ''<math>BH</math>'' 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]]. The energy loss per cycle of the alternating current is constant for each of these processes, so the power loss increases linearly with [[frequency]].