Core losses
In alternating current In alternating current the movement (or flow) of electric charge periodically reverses direction. An electric charge would for instance move forward, then backward, then forward, then backward, over and over again. In direct current (DC), the movement (or flow) of electric charge is only in one direction (AC) electromagnets, used in transformers A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils. A varying current in the first or primary winding creates a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force or ", inductors Inductance (measured in henries) is an effect resulting from the magnetic field that forms around a current-carrying conductor that tends to resist changes in the current. Electric current through the conductor creates a magnetic flux proportional to the current. A change in this current creates a change in magnetic flux that, in turn, by Faraday', and AC motors and generators, the magnetic field is constantly changing. This causes energy losses in their magnetic cores that are dissipated as heat in the core. The losses stem from from two processes:
- Eddy currents An eddy current is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body: From Faraday's law of induction Faraday's law of induction describes a basic law of electromagnetism, which is involved in the working of transformers, inductors, and many forms of electrical generators. The law states:, the changing magnetic field induces circulating electric currents Electric current can mean, depending on the context, a flow of electric charge or the rate of flow of electric charge (a quantity). The electric charge that flows is carried by, for example, mobile electrons in a conductor, ions in an electrolyte or both in a plasma inside nearby conductors, called eddy currents An eddy current is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body. The energy in these currents is dissipated as heat in the electrical resistance The electrical resistance of an object is a measure of its opposition to the passage of a steady electric current. An object of uniform cross section will have a resistance proportional to its length and inversely proportional to its cross-sectional area, and proportional to the resistivity of the material 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 An eddy current is an electrical phenomenon discovered by French physicist Léon Foucault in 1851. It is caused when a conductor is exposed to a changing magnetic field due to relative motion of the field source and conductor; or due to variations of the field with time. This can cause a circulating flow of electrons, or a current, within the body 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.
- Hysteresis losses A system with hysteresis can be summarized as a system that may be in any number of states, independent of the inputs to the system. To be exact, a system with hysteresis exhibits path-dependence, or "rate-independent memory" . By contrast, consider a deterministic system with classical dynamics but no hysteresis. In that case, one can: 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 In a deterministic system with no dynamics or hysteresis, it is possible to predict the system's output at an instant in time, given only its input at that instant in time. In a system with hysteresis, this is not possible; there is no way to predict the output without knowing the system's current state, and there is no way to know the system's. The energy lost per cycle is proportional to the area of the hysteresis loop A system with hysteresis has memory. This means that the system may be in more than one state, and that the system's current state is independent of its input at the current instant in time. A system with hysteresis is said to exhibit path-dependence, or "rate-independent memory". 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 Electrical steel, also called lamination steel, silicon electrical steel, silicon steel or transformer steel, is specialty steel tailored to produce certain magnetic properties, such as a small hysteresis area and high permeability or soft ferrite.
The energy loss per cycle of the AC current is constant for each of these processes, so the power loss increases linearly with frequency Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency. The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency.
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