NO-LOAD EQUIVALENT CIRCUIT OF A TRANSFORMER

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NO-LOAD EQUIVALENT CIRCUIT OF A TRANSFORMER

WHAT IS MEANT BY NO-LOAD TRANSFORMER?

A transformer is said to be on no-load if its secondary winding is left open circuited i.e, no load is connected to the transformer. In such conditions current in the secondary winding I₂ = 0 and hence I²R losses in the secondary will be zero.

During no-load condition, no-load current (I₀) flows in the primary side. This no-load current is about 3-5% of rated primary current and therefore the I²R losses are so small which can be neglected.

No-load current (I₀) consists of two components Iw and Iu.

Iw is the wattful/active component and it supplies for the core loss in the transformer.

Iu is the wattless/reactive component and it sets up flux in the core (in other words, it magnetizes the transformer).

NO-LOAD EQUIVALENT CIRCUIT

With the help of above information, we can easily draw the no-load equivalent circuit of the transformer. The figure below shows the no-load equivalent circuit of the transformer.  Part inside the dotted box represents the ideal transformer.

no-load equivalent circuit of a transformer
NO-LOAD EQUIVALENT CIRCUIT OF A TRANSFORMER

Impedance of primary winding is not shown in the circuit because no-load current is very small which can be neglected and therefore the impedance and I²R losses of primary winding does not comes into play.

Impedance of secondary winding is not shown in the circuit because secondary winding is left open-circuited and no current flows in the secondary circuit.

R₀ represents the core loss in the transformer and hence the current Iw which supplies for the core loss is shown passing through it.

X₀ called inductive reactance, it takes a reactive current equal to the magnetizing current Iu flowing through X₀.

From the equivalent circuit

V₁ = Iw . R₀ = Iu . X₀

I₀ = Iu +Iw

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