Hey, today we are going to know why the resistance of winding or coil always measured with DC not AC and what will be the problem if we measure winding resistance with the help AC instead of DC. The winding resistance test is performed on transformers, motors, etc. Many important tests are performed on the transformers, motors before releasing to the market, winding resistance test one of them.

Tell me, how we can measure the resistance of a conductor? The answer is very simple.

We know that the basic resistance formula is,

Most of the resistance measuring instruments follow this equation to measure the resistance. The above equation is valid if the voltage and current are DC because we know that in the AC circuit we measure impedance, not resistance.

The impedance formula is,

So from the above explanation, you may understand why we use DC for the measurement of winding resistance. The simple concept is, in the AC circuit we cannot measure resistance(R), we can measure only impedance(Z = R + XL).

Now let's find the Practical Answer

When we apply AC supply to a coil or winding, the alternating current flowing through it. In this case, the winding opposes the flow of current in two way. First, the actual internal resistance of the conductor of the winding opposes the flow of current and the second is the inductive reactance of the winding also oppose the flow of current.
So, when we apply AC supply to a coil the current opposed by the actual resistance and the inductive reactance.

But when we apply DC supply to a coil the current opposed by only the internal resistance. There is no inductive reactance occurs.

Here the formula of inductive reactance,

You can see in the above equation, the frequency is included. So as DC has zero frequency, the inductive reactance also is zero.

So the conclusion is if we try to measure winding resistance using AC supply, we will not get the value of actual resistance due to the inductive reactance(which occurs due to the self-induction of the winding or coil).

Other Reasons

If we measure winding resistance using AC supply, the Skin effect and proximity effect occurs as they depend upon the frequency. But in case of DC supply, as there is no frequency, so the proximity and skin effect do not occur.