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Selection of current value of DC resistance tester

The transformer DC resistance tester can quickly measure the DC resistance of the transformer. The DC resistance tester includes a variety of specifications such as 10A, 20A, 40A, 50A, 100A, etc. How to choose a DC resistance tester with a suitable current size, let me share a few A current calculation formula is used to determine the current value of the DC resistance measurement.

1. When the AC no-load current at the rated voltage passes through the transformer windings, there will be a rated magnetic flux density Bn in the iron core, and the magnetic permeability of the iron core will be relatively low. When measuring DC resistance, it is necessary to make the magnetic density in the core greater than Bn, so that the core conductivity becomes lower, so as to reduce the time constant and back electromotive force dLi/dt of the circuit and shorten the settling time. Therefore, the DC resistance tester is used to measure DC The DC current in resistance should be at least:

I=k√ 2i0In+ 100, where k: constant> 1

i0: AC rated frequency, percentage of no-load current under rated voltage (%)

In: rated current of the winding under test (A)

In the formula, the constant √2 is the magnitude of the DC current equivalent to the AC current. When the factor k is larger than 1, the magnetic density in the iron core> Bn, which reduces the magnetic permeability μ of the iron core during DC resistance measurement.

2. When the transformer winding is connected in star (Y), the line current is equal to the phase current. From the above formula, the current that should be applied when measuring DC resistance is:

IY=1.41 ki0In: 100

3. When the transformer winding is triangular (D) connection, the no-load current line current of the converter is √3 times the phase current, and the DC current when measuring the DC resistance is based on the total current] and 1/3 and 2/3 The proportion of the distribution, so the measurement of DC resistance should add current is:

ID=1.41x3/2+1/√3 ki0Inx100=1.22 ki0In÷ 100

When k is 3~10, that is, when the excitation ampere-turn when measuring the DC resistance is 3-10 times the no-load current ampere-turn, the magnetic density in the core can be greater than Bn and close to saturation, that is, the DC when measuring the DC resistance The current is equal to 2%-10% of the rated current.

The above three formulas can help us accurately choose the appropriate current value of the DC resistance tester. However, we need to pay attention to that if the DC current selected during the measurement is too large, or the measurement time is too long, the heating temperature of the winding will increase , And the resistance value changes, which leads to an increase in the measurement error. Therefore, choose a suitable current value to quickly and effectively complete the DC resistance test.

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