It is a static device that translats the energy from one circuit to another without changing the frequency.
(OR) "A device that transfers electrical energy from one circuit to another by magnetic coupling".
A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
A changing current in the first circuit (the primary) creates a changing magnetic field; in turn, this magnetic field induces a changing voltage in the second circuit (the secondary). By adding a load to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other.
The secondary induced voltage VS is scaled from the primary VP by a factor ideally equal to the ratio of the number of turns of wire in their respective windings:
Vs/Vp = Ns/Np
By appropriate selection of the numbers of turns, a transformer thus allows an alternating voltage to be stepped up — by making NS more than NP — or stepped down, by making it less.
Basic principles:
The transformer is based on two principles: first, that an electric current can produce a magnetic field (electromagnetism) and, second, that a changing magnetic field within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction). By changing the current in the primary coil, one changes the strength of its magnetic field; since the secondary coil is wrapped around the same magnetic field, a voltage is induced across the secondary.
The primary and secondary coils are wrapped around a core of very high magnetic permeability, such as iron; this ensures that most of the magnetic field lines produced by the primary current are within the iron and pass through the secondary coil as well as the primary coil.
Induction law
The voltage induced across the secondary coil may be calculated from Faraday's law of induction, which states that,
Vs=Ns(dx/dt)
where VS is the instantaneous voltage, NS is the number of turns in the secondary coil and Φ equals the total magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic field strength B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary.
Ideal power equation
If the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and thence to the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power
Pincoming = IsVp = Poutgoing = IsVs
Pincoming = IsVp = Poutgoing = IsVs
Flux leakage
The ideal transformer model assumes that all flux generated by the primary winding links all the turns of every winding, including itself. In practice, some flux traverses pat
hs that take it outside the windings. Such flux is termed leakage flux, and manifests itself as self-inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not itself directly a source of power loss, but results in poorer voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Distribution transformers are therefore normally designed to have very low leakage inductance.
hs that take it outside the windings. Such flux is termed leakage flux, and manifests itself as self-inductance in series with the mutually coupled transformer windings. Leakage results in energy being alternately stored in and discharged from the magnetic fields with each cycle of the power supply. It is not itself directly a source of power loss, but results in poorer voltage regulation, causing the secondary voltage to fail to be directly proportional to the primary, particularly under heavy load. Distribution transformers are therefore normally designed to have very low leakage inductance.Losses in Transformers
An ideal transformer would have no energy losses, and would therefore be 100% efficient. Despite the transformer being amongst the most efficient of electrical machines,
Winding resistance
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
Eddy currents
Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.
Mechanical losses
In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.
Stray losses
Leakage inductance is by itself lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.
Current flowing through the windings causes resistive heating of the conductors. At higher frequencies, skin effect and proximity effect create additional winding resistance and losses.
Hysteresis losses
Each time the magnetic field is reversed, a small amount of energy is lost due to hysteresis within the core. For a given core material, the loss is proportional to the frequency, and is a function of the peak flux density to which it is subjected.
Eddy currents
Ferromagnetic materials are also good conductors, and a solid core made from such a material also constitutes a single short-circuited turn throughout its entire length. Eddy currents therefore circulate within the core in a plane normal to the flux, and are responsible for resistive heating of the core material. The eddy current loss is a complex function of the square of supply frequency and inverse square of the material thickness.
Mechanical losses
In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic forces between the primary and secondary windings. These incite vibrations within nearby metalwork, adding to the buzzing noise, and consuming a small amount of power.
Stray losses
Leakage inductance is by itself lossless, since energy supplied to its magnetic fields is returned to the supply with the next half-cycle. However, any leakage flux that intercepts nearby conductive materials such as the transformer's support structure will give rise to eddy currents and be converted to heat.
What is a split-core current transformer? More specifically how does a split-core current transformer differ from the typical current transformer? Just like the typical current transformer, the split-core current transformer measures alternating current flowing through a conductor. The distinguishing feature of the split core current transformers is that their design permits them to be assembled around a buss bar without disconnecting the buss bar. The typical current transformer is usually a toroidal coil, which is slipped over the end of a buss bar, hence requires disconnecting the buss bar. "C" - cores and "U" core structures are commonly used for split-core current transformers because they are relatively easy to take apart and put back together around the buss bar. Some sort of bracketry or band clamps and holds the assembled pieces of the split-core current transformer together.
Historically, this has not been as practical ( but is possible ) for toroidal coils. The bracketry is more complicated. Typically, the coil(s) must be sector wound on the toroid before cutting the core in half, whereas the “U” and “C” core structure of the typical split-core current transformer permit use of bobbin wound coils which can be wound independently of the core. There are now some flexible toroids, which permit the “split-core” feature of installing it around a buss bar.
The electrical performance of split-core current transformers is not as good as that of the continuous toroidal coil. The “circle” like ( or “ring” like ) shape of the toroid usually offers a shorter magnetic path length than other cores. Since the toroids are continuous, they do not add any air gap to the core structure. Split-core current transformers ( including toroidal split- cores ) add some air gap to the core structure. Consequently, the split-core current transformers will draw more magnetizing ( exciting ) current than a continuous toroidal current transformer made of the same core material ( assuming comparable size and/or weight. ). The toroidal shape provides better magnetic coupling and less leakage inductance than the “C” and “U” core structures commonly used in split-core current transformers.
The electrical performance of split-core current transformers is not as good as that of the continuous toroidal coil. The “circle” like ( or “ring” like ) shape of the toroid usually offers a shorter magnetic path length than other cores. Since the toroids are continuous, they do not add any air gap to the core structure. Split-core current transformers ( including toroidal split- cores ) add some air gap to the core structure. Consequently, the split-core current transformers will draw more magnetizing ( exciting ) current than a continuous toroidal current transformer made of the same core material ( assuming comparable size and/or weight. ). The toroidal shape provides better magnetic coupling and less leakage inductance than the “C” and “U” core structures commonly used in split-core current transformers.
Application Standards
-- Application standards may exclude use of some materials and require use of some materials . Some examples of such standards are minimum temperature ratings ( regardless if actual is less ), flame retardancy, vibration, out-gassing, and required labeling.
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