Transformer-หม้อแปลงไฟฟ้
Induction law
A varying electrical current passing through the primary coil creates a varying magnetic field around the coil which induces a voltage in the secondary winding. The primary and secondary windings are wrapped around a core of very high magnetic permeability, usually iron,[d] so that most of the magnetic flux passes through both the primary and secondary coils. The current through a load connected to the secondary winding and the voltage across it flow in the directions indicated in the figure at left. (See also Polarity.)
The voltage induced across the secondary coil may be calculated from Faraday’s law of induction, which states that:
where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil, and dΦ/dt is the derivative[e] of the magnetic flux Φ through one turn of the coil. If the turns of the coil are oriented perpendicularly to the magnetic field lines, the flux is the product of the magnetic flux density 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.