When a sinusoidal alternating voltage U1 is applied across the primary coil, there is an alternating current I1 in the conductor and an alternating magnetic flux ф1 which forms a closed magnetic circuit along the core through the primary and secondary coils. The mutual inductance potential U2 is induced in the secondary coil, and ф1 also induces a self-induced potential E1 on the primary coil. The direction of E1 is opposite to the applied voltage U1 and the amplitude is similar, thereby limiting the size of I1. In order to maintain the existence of magnetic flux ф1, it is necessary to have a certain power consumption, and the transformer itself 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 generates a current I2, and thus the magnetic flux ф2, the direction of ф2 is opposite to ф1, which acts to cancel each other out, so that the total magnetic flux in the core is reduced, thereby making the primary The self-inductance voltage E1 is reduced, and as a result, I1 is increased, and it can be seen that the primary current is closely related to the secondary load. When the secondary load current increases, I1 increases, ф1 also increases, and the ф1 increase partially complements the portion of the magnetic flux that is offset by ф2 to keep the total magnetic flux in the core constant. If the loss of the transformer is not taken into account, it can be considered that the power consumed by an ideal transformer secondary load is also the primary power drawn from the power supply. 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 allows the load to be consumed.
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