The leakage reactance of the transformer is caused by flux leakage from the magnetic core. The entire flux does not link to both the primary & secondary transformer winding and thus some part of the** flux leaks** from the magnetic path.

On application of AC voltage to primary of the transformer, the flux generates in the primary winding which travels through the core and links to the secondary.

The flux generated in the primary must link to both the **primary and secondary winding. **Does it is practically possible that all the flux generated will link to both of the winding?

In an ideal transformer, all the flux links with the both primary and secondary winding. However, In reality, all the flux do not link with both winding in a practical transformer, all the flux may link either winding but not both.

**Leakage Flux in Transformer**

The part of the flux diverts its path from the core, and it passes through winding insulation to transformer oil. The leakage of flux from the main core is the** leakage flux. **The magnetic leakage takes place in every transformer which causes **leakage reactance.**

The leakage flux begets leakage reactance in the transformer. The transformer draws more current to produce the same amount of flux to nullify the effect of leakage flux. The current drawn by the transformer causes an additional voltage drop in the primary and secondary winding. Thus, the voltage regulation of the transformer deteriorates with an increase in leakage flux or leakage reactance. Moreover, the percentage impedance of the transformer also increases with an increase in the leakage flux.

We can calculate the total voltage drop in the primary and secondary winding by measuring the voltage drop caused by the resistance and reactance of the transformer. The primary and secondary winding of the transformer has copper as a winding material that has a very low resistance. The combined resistance of the transformer is known as the resistance of the transformer.

**How do leakage reactance and resistance cause voltage drop?**

The combination of the resistance and reactance is called the** impedance of the **transformer.If R1 and R2 and X1 and X2 are primary and secondary resistance and leakage reactance respectively, then the impedance of primary(Z_{1}) and secondary(Z_{2}) windings are as follows;

If we apply AC voltage V_{1} across the primary of the transformer, the voltage drop in the primary will take place due to a voltage drop in the leakage reactance and resistance. Let the primary current be I_{1}. The total voltage drop in primary on account of leakage reactance and resistance is ;

The voltage equation of the transformer on the primary side is as given below.

Similarly, the transformer secondary supplies current to load. Let the secondary current be I_{2}. The secondary resistance and reactance are R_{2} and X_{2} respectively. EMF induced in the secondary is E_{2}.

The total voltage drop in secondary on account of resistance and reactance is ;

The voltage equation of the transformer on the secondary side is as given below.

The secondary voltage is ;

**Effect of Leakage Flux on Transformer Performance**

- The leakage flux flow opposite in the main flux which reduce the net flux in the core. The reduction in the net flux cause
**reduction in secondary voltage of the transformer.** - The transformer primary current increase with an increase in the leakage flux, the increased current is inductive in nature and therefore the power factor gets poor with increased reactance.
- The leakage reactance causes voltage drop in primary and secondary of the transformer, hence it deteriorates the transformer voltage regulation and increases leakage reactance.
- Increased transformer current with high leakage reactance cause more copper losses in the transformer. The efficiency of transformer lowers with increased leakage reactance.
- The short circuit capacity improves with increase in the leakage reactance. This is positive point with increased leakage reactance.

**How to Control Leakage Flux of Transformer?**

- Placing the primary and secondary winding as close as possible
- Keeping space between primary and secondary winding small
- By taking the proper dimensions of winding

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