Basic principles of transformers

When a sinusoidal AC voltage U1 is applied to both ends of the primary coil, there is an alternating current I1 in...

When a sinusoidal AC voltage U1 is applied to both ends of the primary coil, there is an alternating current I1 in the wire and an alternating magnetic flux ф1 is generated. It passes through the primary coil and the secondary coil along the core to form a closed magnetic circuit. A mutual induction potential U2 is induced in the secondary coil, and ф1 will also induce a self-induction potential E1 on the primary coil. The direction of E1 is opposite to the direction of the applied voltage U1 and the amplitude is similar, thus limiting the size of I1.

In order to maintain the existence of the magnetic flux ф1, a certain amount of power consumption is required, and the transformer itself also has a certain loss. Although the secondary is not connected to the load at this time, there is still a certain current in the primary coil. This current is called "no-load current" .

If the secondary is connected to the load, the secondary coil will generate a current I2, and therefore generate a magnetic flux ф2, the direction of ф2 is opposite to ф1, which has the effect of canceling each other, reducing the total magnetic flux in the iron core, thus making the primary The self-inductance voltage E1 decreases, and as a result I1 increases. It can be seen that the primary current is closely related to the secondary load.

When the secondary load current increases, I1 increases and ф1 also increases, and the increased part of ф1 just supplements the part of the magnetic flux that is offset by ф2 to keep the total magnetic flux in the core unchanged. If the loss of the transformer is not considered, it can be considered that the power consumed by the secondary load of an ideal transformer is the electric power obtained by the primary from the power supply.

The transformer can change the secondary voltage by changing the number of turns of the secondary coil as needed, but it cannot change the power allowed by the load.