The role of the core is to strengthen the magnetic coupling between the two coils. In order to reduce the internal eddy current and hysteresis loss, the iron core is formed by the lamination of the coated silicon steel sheet; there is no electrical connection between the two coils, and the coil is wound by an insulated copper wire (or aluminum wire). A coil-connected AC power source is called a primary (or primary), and another coil is connected with an electrical appliance called a secondary (or secondary). The actual transformer is very complex, inevitably there is copper loss (coil resistance fever), iron loss (core heating) and leakage magnetic (through the air closed magnetic induction line), etc., in order to simplify the discussion here only the ideal transformer. Ideal transformer set up conditions are: ignore leakage flux, ignoring the original, auxiliary coil resistance, ignoring the core loss, ignoring no-load current (secondary open primary coil current). For example, the power transformer in full load operation (secondary output rated power) is close to the ideal transformer situation.
Transformers are static electrical appliances made from the principle of electromagnetic induction. When the primary of the transformer is connected to the AC power supply, the alternating magnetic flux is produced in the Iron heart, and the alternating magnetic universal φ is indicated. The original, auxiliary coil φ is the same, φ is also a simple harmonic function, the table is φ=φmsinωt. By Faraday's law of electromagnetic induction, the induction electromotive force in the original and auxiliary coils is E1=-N1DΦ/DT and E2=-N2DΦ/DT. The N1 and N2 in the formula are the turns of the auxiliary coils. According to the figure, U1=-e1, U2=e2 (primary physical volume by the subscript 1, secondary physical quantity with the lower angle of 2), its complex effective value is u1=-e1=jn1ωφ, u2=e2=-jn2ωφ, make k=n1/n2, the transformer's variable ratio. From the upper type can be u1/u2=-n1/n2=-k, that is, the ratio of the voltage valid value of the transformer original and the auxiliary coil is equal to the ratio of the turn number and the position of the original and the secondary coil voltage is π.
In the case of no-load current can be neglected, there is i1/i2=-n2/n1, that is, the primary and secondary coil current valid value is inversely proportional to the turn number, and phase difference π.
and can be
The power of the original and auxiliary coils of the ideal transformer is equal p1=p2. It shows that the ideal transformer has no power loss. The actual transformer total existence loss, its efficiency is Η=P2/P1. Power transformers are highly efficient and can reach over 90%.