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.
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| 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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