When we use a bridge to test the leakage inductance of the transformer, we need to short-circuit the secondary side, and the inductance obtained by testing the primary side is the leakage inductance. Have you ever thought about why you need to short-circuit the secondary side? What is the principle of this test?

The picture shows an ideal transformer. The ideal transformer follows the following formula:

V2 = N2/N1*V1

N2: Number of turns on the secondary side

N1: Number of turns on the primary side

But In practice, transformers are always not ideal. There is always a part of the magnetic flux that does not participate in energy transfer and creates trouble on the primary side, causing many adverse effects. The inductance generated by this part of the magnetic flux that does not transfer energy to the secondary side is the leakage inductance. The equivalent diagram of the actual transformer is as follows:

The leakage inductance in the equivalent diagram always has windings in series. In order to measure the inductance of the winding, we use a bridge to apply a certain voltage with a certain frequency for measurement. The measurement principle is as follows:

If a measurement voltage of 1V is applied to the primary side and the turns ratio of the transformer is 2:1, then the voltage on the secondary side is 0.5V. This is the principle of the transformer. The total inductance measured on the primary side includes leakage inductance. In order to measure leakage inductance, try to make the main inductance LP zero, and then what is measured is the leakage inductance. But how to make the main inductance zero?

The method is actually very simple. For example, if you want to measure the leakage inductance of the primary side, short-circuit the secondary side. Then why can you measure the leakage inductance of the primary side after short-circuiting the secondary side? As shown in the picture:

After short-circuiting the secondary side, the voltage of the secondary side is 0V. According to the transformer formula of V2 = N2/N1 * V1, it can be seen that the voltage of the primary side must also be 0V. Since the leakage magnetic flux does not participate in the coupling, there is no effect on the leakage inductance after short-circuiting the secondary side. At this time, look in from the left The measured inductance is the leakage inductance.