In an ideal transformer, the secondary winding is loaded, so, but the load on the secondary winding has no current (that is, when it is not working), why is there no current on the primary winding and the AC voltage on the main winding remains unchanged? I understand why the original coil has voltage and path, but no current. An ideal transformer has four ideal conditions: (1) no leakage, that is, the coupling coefficient k= 1 and complete coupling, so there are cases of 1 1= 2 1 and 22= 12. (2) No energy consumption (i.e. no loss) and no energy storage. (3) The inductance of the primary and secondary coils is infinite, that is, l 1→, l2→, but it is a finite value. The proof is as follows: when completely coupled (k= 1), the ratio of the inductance of two coils is equal to the ratio of the square of the turns, that is, the inductance of each coil is proportional to the square of the turns of its own coil. (4) Because there are k= 1, l 1→, l2→, there is m→. The coupling inductance satisfying the above four conditions is called an ideal transformer. Therefore, the ideal transformer can be considered as the limit of coupling inductance. That is to say, in the case of k= 1, l 1→, l2→, m→, it is purely an element that changes the signal to transmit power, but it is essentially different from the coupling inductance. The coupling inductance works according to the principle of electromagnetic induction, and it is a dynamic element, which needs three parameters l 1, l2, m to describe. The ideal transformer has no trace of electromagnetic induction and is a static element. It only needs one parameter n to describe it. The ideal transformer is one of the basic passive components of the circuit. When the precision of core transformer used in engineering is not high, the ideal transformer can be used as its circuit model for analysis and calculation. In a word, the ideal transformer is one without magnetic loss, copper loss and iron loss. Because the inductance of the primary and secondary windings of an ideal transformer is infinite, that is, l 1→, l2→, which is a finite value. Then the inductance is infinite, so there is a voltage path on the primary coil, but no current flows.