In the general power transformer, the winding resistance voltage drop is very small and can be ignored, so the voltage U1 = E1 can be considered in the primary winding. Due to the open circuit of the secondary winding and the current I2 = 0, its terminal voltage U2 is equal to the induced electromotive force E2, that is, U2 = E2. Therefore, from the above induced electromotive force formula of primary side and secondary side, it is obtained that:
Ratio transformer
In the formula, K is the ratio of primary side voltage U1 to secondary side voltage U2, and the value of K is called transformer transformation ratio.
The above shows that the voltage ratio of the primary and secondary windings of the transformer is equal to the turn ratio of the primary and secondary windings, so if the primary and secondary windings have different voltages, just change their turns. When N1 > N2, k > 1, transformer step-down; when N1 < N2, K < 1, transformer step-up.
For the supported transformer, K is the fixed value, so the secondary side voltage is proportional to the primary side voltage, that is to say, the secondary side voltage increases with the increase of the primary side voltage, and decreases with the decrease of the primary side voltage. However, the voltage at both ends of the primary winding must be rated. Because when the applied voltage slightly exceeds the rated voltage, the current passing through the primary winding will increase greatly. If the transformer with rated voltage of 220 V is wrongly connected to the 380 V line, the current of the primary winding will increase sharply, causing the transformer to burn down.
After the load of the secondary winding of the transformer is connected, there is current I2 passing through the secondary circuit. At this time, it is called transformer load operation. Because the current I2 in the secondary winding will also produce magnetic flux (i.e. self induction phenomenon) in the iron core, this kind of magnetic flux plays a demagnetizing role for the magnetic flux generated by the primary winding, that is, the magnetic flux in the iron core should be the combination of the magnetic flux generated by the current in the primary winding and the secondary winding. However, under the condition that the applied voltage U1 and power frequency f remain unchanged, the approximate formula is as follows:
Primary voltage
It can be seen from the above formula that the resultant magnetic flux Φ should remain basically unchanged. Therefore, with the appearance of I2, the current I1 passing through the primary winding will increase, so that the magnetic flux in the primary winding will not be corrected by the magnetic flux in the secondary winding, and the synthetic magnetic flux in the iron core will remain unchanged on the other side. Therefore, the primary current I1 of transformer is determined by the secondary current I2.
From the energy point of view, the power P1 drawn by the primary coil of the transformer from the power supply should be equal to the output power P2 of the secondary coil (ignoring the coil resistance and flux transfer loss of the transformer)
P1 = P2 or i1u1 = i2u2
Therefore, the transformation ratio:
Transformer transformation ratio
It can be seen that the current ratio of primary side and secondary side of transformer is inversely proportional to their turns ratio or voltage ratio. For example, if the number of turns of a transformer N1 ＜ N2 is a step-up transformer, the current I1 ＞ I2; if the number of turns of winding N1 ＞ N2 is a step-down transformer, the current I2 ＞ I1. In other words, the current on the high side is small, while the current on the low side is large.