As shown in the figure, the primary winding N1 of the iron core passes the direct current I1, and the secondary winding N2 is connected to the alternating voltage U2. When the current I1 is changed, the total impedance of the winding N2 changes due to the change of the magnetic permeability in the iron core, thereby changing the current I2 taken from the AC network. This is the basic principle of the DC transformer. 
However, such a structure is also different from that of a DC transformer because the AC current I2 is proportional to the measured DC current I1 in only one half-wave period.
The DC transformer usually uses two identical cores (A, B) for the DC transformer. The primary and secondary windings on the core are constructed such that the DC flux in a core is within one half-wave period of the AC voltage D2. The AC flux direction is the same, and the AC and DC fluxes in the other core are reversed. In the next half-wave cycle, the fluxes in the two cores are opposite. The two iron cores work in rotation, and the DC transformers work continuously throughout the entire cycle. Because of the opposite connection of the AC windings, the AC potentials of the two primary DC windings are opposite and cancel each other out. The secondary current I2 of the DC transformer can be directly connected to Electromagnetic ammeter measurement of the secondary circuit. However, the secondary current is often rectified by a bridge-type solid-state rectifier, and it can be measured by a galvanic current meter with equidistant calibration, which has high sensitivity and low loss.
DC transformers have series and parallel two basic lines. The series circuit can also be added on the secondary side of the intermediate AC transformer, and then rectified, so that it has a secondary current ratio, which facilitates the measurement of large currents. Anti-magnetic compensation type DC transformer is a series reverse connection, in the increase of iron core, plus anti-magnetic compensation winding.
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