Working Principle of Transformer

A transformer consists of an iron core (or magnetic core) and a coil, which has two or more windings. The winding connected to the power source is called the primary coil, and the remaining windings are called the secondary coil. It can transform AC voltage, current, and impedance. A simple iron core transformer is composed of an iron core made of soft magnetic material and two coils with unequal turns sheathed on the iron core, as shown in the figure.

The function of an iron core is to strengthen the magnetic coupling between two coils. In order to reduce eddy currents and hysteresis losses within the iron, the iron core is laminated with painted silicon steel sheets; There is no electrical connection between the two coils, and the coils are wound with insulated copper wire (or aluminum wire). One coil connected to an AC power source is called the primary coil (or primary coil), while the other coil connected to an electrical appliance is called the secondary coil (or secondary coil). The actual transformer is very complex, and inevitably there are copper losses (coil resistance heating), iron losses (iron core heating), and magnetic leakage (magnetic induction line closed by air). To simplify the discussion, only the ideal transformer is introduced here. The conditions for the establishment of an ideal transformer are: ignoring the leakage flux, ignoring the resistance of the primary and secondary coils, ignoring the loss of the iron core, and ignoring the no-load current (the current in the original coil when the secondary coil is open). For example, when a power transformer is operating at full load (with the secondary coil output rated power), it is close to the ideal transformer situation.

Transformers are static electrical appliances made using the principle of electromagnetic induction. When the original coil of a transformer is connected to an AC power source, alternating magnetic flux is generated in the iron core, which is universal φ Represent. In the primary and secondary coils φ It"s the same, φ It is also a simple harmonic function, as shown in the table φ=φ MSIN ω T. According to Faraday"s law of electromagnetic induction, the induced electromotive force in the primary and secondary coils is e1=- N1d φ/ Dt, e2=- N2d φ/ Dt. In the formula, N1 and N2 are the turns of the primary and secondary coils. From the figure, it can be seen that U1=- e1, U2=e2 (the PHysical quantity of the original coil is represented by the subscript 1, and the physical quantity of the secondary coil is represented by the subscript 2), and its complex effective value is U1=- E1=jN1 ωΦ、 U2=E2=- jN2 ωΦ， Let k=N1/N2 and measure the transformation ratio of the transformer. From the above equation, it can be obtained that U1/U2=- N1/N2=- k, which is the ratio of the effective values of the primary and secondary coil voltages of the transformer, equal to its turn ratio, and the phase difference between the primary and secondary coil voltages is π.

Furthermore, it can be concluded that: U1/U2=N1/N2

In the case where the no-load current can be ignored, there is I1/I2=- N2/N1, which means that the effective values of the primary and secondary coil currents are inversely proportional to their turns, and the phase difference is π.

Furthermore, it can be concluded that: I1/I2=N2/N1

The power of the primary and secondary coils of an ideal transformer is equal P1=P2. The ideal transformer itself has no power loss. The actual transformer always has losses, and its efficiency is η= P2/P1. The efficiency of power transformers is very high, reaching over 90%.