Fundamentals of Transformers

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, which forms a closed magnetic circuit through the primary coil and the secondary coil along the iron core. A mutual inductance potential U2 is induced in the secondary coil, and ф1 also induces a self-inductance potential E1 on the primary coil. The direction of E1 is opposite to 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 amount of 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 thus generate a magnetic flux ф2. The direction of ф2 is opposite to that of ф1, which cancels each other out, so that the total magnetic flux in the iron core is reduced, so that 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 complements the part of the magnetic flux that is offset by ф2, so as to keep the total magnetic flux in the iron 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 source from the power source. The transformer can change the secondary voltage by changing the number of turns of the secondary coil as needed, but cannot change the power that the load is allowed to dissipate.