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# How to select the DC resistance tester with appropriate current through calculation formula？

When the AC no-load current passes through the rated voltage in the transformer winding, there will be a rated flux density BN in the iron core. At this time, the permeability of the iron core will be relatively low. When measuring DC resistance, it is necessary to make the magnetic density of the magnetic core greater than BN, reduce the conductivity of the magnetic core, so as to reduce the time constant and back EMF of the circuit and shorten the establishment time. Therefore, the DC current when measuring DC resistance shall be at least:

Formula 1
I = k√2i0In + 100
Where K: factor > 1
I0: AC rated frequency, no-load current percentage under rated voltage (%)
Input: rated current of measured winding (A)
Where √ 2 is the amplitude of DC current equal to AC current. When the factor K is greater than 1, the magnetic density in the core is > BN,
Permeability of magnetic core when measuring DC resistance μ Will decrease.

Formula 2
When the transformer winding is of Y connection method, the line current is equal to the phase current, which can be obtained from the above formula. The current to be applied when measuring the DC resistance is:

IY = 1.41 kiloliters: 100

Formula 3
When the transformer winding is delta (D) connected, the line current of the no-load current is √ 3 times of the phase current, while the DC current when measuring the DC resistance is the total current distributed in the proportion of 1/3 and 2/3. Therefore, to measure the DC resistance, add the current as follows:
ID = 1.41x3 / 2 + 1 /√3ki0Inx100 = 1.22 ki0In÷100
When k is 3-10, that is, the excitation ampere turn when measuring DC resistance is 3-10 times of the no-load ampere turn, the magnetic density in the iron core can be greater than BN, which is close to saturation, that is, the DC current when measuring DC resistance is equal to 2%-10% of the rated current. If the DC current is too large and the measurement time is too long, the resistance will change and the measurement error will increase as the winding heating temperature increases.