Leakage Reactance of Transformer

Updated: Mar 12


When the AC voltage is applied to the primary of the transformer the flux is generated in the primary winding which travels through the core and gets linked 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 the both of the winding?


In an ideal transformer , all the flux gets linked with both primary and secondary winding. However, In reality, all the flux do not link with the both winding in a practical transformer, all the flux may link either winding but not both. The part of the flux diverts its path from the core and pass through winding insulation and transformer oil rather than passing through the core.The leaked flux from the main core is called the leakage flux. The magnetic leakage takes place in every transformer which cause 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 cause additional voltage drop in the primary and secondary winding. Thus, the voltage regulation of the transformer deteriorates with increase in leakage flux or leakage reactance.Moreover, the percentage impedance of the transformer also increase with increase in the leakage flux.


The total voltage drop in the primary and secondary winding can be calculated by measuring the voltage drop caused by resistance and reactance of the transformer. The primary and secondary winding of the transformer is made of copper which has very low resistance. The combined resistance of the transformer is known as resistance of transformer.


How do leakage reactance and resistance cause voltage drop?


The combination of the resistance and reactance is called the impedance of transformer.If R1 and R2 and X1 and X2 are primary and secondary resistance and leakage reactance respectively, then impedance of primary(Z1) and secondary(Z2) windings are as follows;






If we apply AC voltage V1 across primary of transformer, the voltage drop in the primary will take place due to voltage drop in the leakage reactance and resistance. Let the primary current is I1. The total voltage drop in primary on account of leakage reactance and resistance is ;



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

Similarly, the transformer secondary supplies current to load. Let the secondary current is I2. The secondary resistance and reactance is R2 and X2 respectively.EMF induced in the secondary is E2.


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



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

The secondarily voltage is given as ;






Effect of Leakage Flux on Transformer Performance

  1. 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.

  2. 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.

  3. The leakage reactance cause voltage drop in primary and secondary of the transformer, hence transformer voltage regulation gets deteriorated with increased leakage reactance.

  4. Increased transformer current with high leakage reactance cause more copper losses in the transformer. The efficiency of transformer lowers with increased leakage reactance.

  5. 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

  1. Placing the primary and secondary winding as close as possible

  2. Keeping space between primary and secondary winding small

  3. By taking the proper dimensions of winding

Related Posts:

  1. Working principle of transformer, turn ratio & transformation ratio

  2. Percentage impedance of transformer and its calculation

  3. Transformer winding Resistance

  4. Difference between power and distribution transformer

  5. Cooling classes of transformer

  6. Difference between an ideal and practical transformer

  7. What is transformer, EMF, Types of transformers and efficiency

  8. What is a step-up and step down transformer?





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