DC Resistance Tester is a high-precision replacement product for DC single and double arm bridges. The instrument adopts advanced switching power supply technology and displays measurement results on an LCD. It overcomes the disadvantage of other similar products where LED display values are difficult to read in sunlight, and also has an automatic arc suppression function. This instrument has the advantages of fast measurement speed, high accuracy, intuitive display, strong anti-interference capability, small size, low power consumption, stable and reliable test data, and is not affected by human factors. The DC resistance tester can achieve rapid measurement of transformer DC resistance. The produced ZD9202 series DC resistance testers include various specifications such as 10A, 20A, 40A, 50A, and 100A. How to choose the appropriate current size for a DC resistance tester? This article will introduce several calculation formulas to explain how to determine the current value for DC resistance measurement.
Formula One
When the AC no-load current passes through the transformer winding at rated voltage, 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 at this time. When measuring DC resistance, it is necessary to make the magnetic flux density in the iron core greater than Bn, so that the magnetic permeability of the iron core becomes even lower, reducing the circuit time constant and back electromotive force dLi/dt, and shortening the stabilization time. Therefore, the DC current when measuring DC resistance should be at least:
I= k√ 2i0In+ 100
Where k: constant > 1
i0: AC rated frequency, percentage of no-load current at rated voltage (%)
In: rated current of the winding under test (A)
In the formula, the constant √2 is the amplitude of the DC current equivalent to the AC current. When the factor k is greater than 1, the magnetic flux density in the iron core > Bn,
which reduces the magnetic permeability μ of the iron core during DC resistance measurement.
Formula Two
When the transformer winding is star (Y) connected, the line current equals the phase current. From the above formula, the current to be applied when measuring DC resistance is:
IY=1.41 ki0In: 100
Formula Three
When the transformer winding is delta (D) connected, the AC no-load current line current is √3 times the phase current, and the DC current when measuring DC resistance is distributed according to the total current in the proportions of 1/3 and 2/3. Therefore, the current to be applied when measuring DC resistance is:
ID= 1.41x3/2+1/√3 ki0Inx100= 1.22 ki0In÷ 100
When k is taken as 3 to 10, that is, when the excitation ampere-turns during DC resistance measurement are 3 to 10 times the no-load current ampere-turns, the magnetic flux density in the iron core can be made greater than Bn and close to saturation, that is, the DC current when measuring DC resistance is taken as 2% - 10% of the rated current.
If the DC current during measurement is too large and the measurement time is too long, the winding temperature will rise due to heating, causing the resistance to change, thereby increasing the measurement error.
