What is the principle of transformer leakage inductance? Why do we need to short-circuit the secondary coil of the transformer when testing leakage inductance?

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What is the principle of transformer leakage inductance? Why do we need to short-circuit the secondary coil of the transformer when testing leakage inductance? [Copy link]

When we use a bridge to test the leakage inductance of a 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 wondered why we need to short-circuit the secondary side, and what is the principle of this test? The figure shows an ideal transformer, which 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 reality, transformers are always not ideal. There is always a part of the magnetic flux that does not participate in energy transfer, causing trouble on the primary side, resulting in many adverse effects. The inductance generated by 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:

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

If a 1V measurement voltage is applied to the primary side and the turns ratio of the transformer is 2:1, the voltage on the secondary side is 0.5V. This is the transformer principle. The total inductance measured on the primary side includes the leakage inductance. In order to measure the leakage inductance, we need to find a way to make the main inductance LP zero, and then the leakage inductance is measured. 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, you 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 figure:

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 known that the voltage of the primary side must also be 0V. Since the leakage flux does not participate in the coupling, short-circuiting the secondary side has no effect on the leakage inductance. At this time, the inductance measured from the left is the leakage inductance.