How To Choose The Type of Dc Resistance Tester？

How to choose a DC resistance tester with appropriate current size? In this article, GDZX will introduce several calculation formulas and explain how to determine the current value of DC resistance measurement.

Formula 1

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

I= k√ 2i0In+ 100

Where K: constant > 1

I0: AC rated frequency, no-load current percentage under rated voltage (%)

In: rated current of tested winding (A)

Where constant √ 2 is the amplitude of DC current equivalent to AC current. When the factor K is larger than 1, the magnetic density in the core is > BN,

The permeability μ of the core is reduced when DC resistance is measured.

Formula 2

When the transformer winding is star (Y) connection, the line current is equal to the phase current. From the above formula, it can be concluded that the current to be applied when measuring the DC resistance is:

IY=1.41 ki0In: 100

Formula 3

When the transformer winding is delta (D) connection, the line current of no-load current is √ 3 times of the phase current, while the DC current when measuring the DC resistance is distributed in the proportion of] and 1 / 3 and 2 / 3 of the total current. Therefore, to measure the DC resistance, add the current as follows:

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

When k is 3-10, i.e. the excitation ampere turn when measuring the DC resistance is 3-10 times of the no-load ampere turn, it can make

When the magnetic density day in the core is greater than BN and close to saturation, that is to say, the DC current when measuring the 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 due to the rise of the heating temperature of the winding.

All the above formulas can accurately select the appropriate current value. Want to learn more about the power test equipment, pay attention to GDZX official website, we will show you more information and technical guidance from time to time!