Why Leakage Reactance in transformer is important for design consideration.

Leakage reactance in power transformer is one of the important parameter for designing a power transformer, choosing a power transformer etc.

In transformer there are primary winding and secondary winding. Both the windings are attached with the core structure. With the current in the primary winding a magnetic flux is produced which passes through the magnetic core structure and this magnetic flux induces voltage in the secondary winding. This is usual operation of a transformer.

Details on leakage reactance in power transformer

What is leakage reactance in power transformer

In usual operation of a transformer the flux is to pass through the magnetic core structure. But some local flux is also generated in the current carrying primary and secondary windings. This flux is called the Leakage flux.

The leakage flux is of opposite direction of the working flux in the magnetic core. Thus it excludes working flux from the core and these effects on the reduction of the secondary voltage. This effect is such like placing a physical impedance in series with the secondary winding; which contribute to the reduction of the secondary voltage. This physical impedance is called the Leakage reactance.

Leakage flux example
Leakage flux example

Measurement of leakage reactance in power transformer

Leakage reactance is measured by short-circuiting one winding of the transformer and increasing the voltage on the other winding until rated current flows in the windings. This voltage divided by the rated winding voltage, then multiplying by 100 results to percent reactance or %X.

So leakage reactance is termed as, %reactance= (reactance voltage drop/winding voltage)/100.

Facts of leakage flux that has effect on leakage reactance

As the magnitude of the leakage flux increases, the magnitude of flux in the core decreases.

  • Leakage flux increases considerably as the currents in the windings increase.
  • Leakage flux travels through empty space instead of through iron, so no saturation, hysteresis and permeability equal to µ0.

Magnitude of leakage reactance

Designer considers the following parameters as, magnitude of this reactance is a function of

  • Number of turns of the primary & secondary winding.
  • The currents in the winding.
  • The geometry of the core & windings.

Controlling the leakage reactance in power transformer

  • Keeping the spaces between the windings small.
  • Placing the windings as close together as possible
  • Varying the winding dimensions.


Effect of leakage reactance in power transformer

  • Reduces the secondary voltage under load.
  • Limits short-circuit currents, this is useful.
  • Consumes reactive power.

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