Transformer on load Condition
Transformer on load Condition: When load is connected to the secondary winding of a transformer , I_{2} (secondary current) is set up in the secondary winding. The magnitude and phase of I_{2} with respect to V_{2} (secondary voltage) depends upon the characteristics of the load. Secondary current I_{2} is in phase with V_{2},if load is no inductive , lags if load is inductive and it leads if load is capacitive.
This secondary current sets up m.m.f( =N_{2}I_{2} ) and hence it produce magnetic flux Φ_{ 2} which is in opposition to the main primary flux Φ . The secondary ampereturns N_{2}I_{2 }are known as demagnetizing ampturns. The opposing secondary flux Φ_{ 2 }weakens the primary flux Φ momentarily, hence the primary winding back e.m.f E_{1} tends to be reduced. For a moment V_{1} > E_{1} and hence more current to flow in the primary winding . Let the additional primary current be I_{2}’ which is known as load component of primary current. This current (I_{2}) is antiphase with I_{2}.This load component of primary current(I_{2}’) sets up its own flux Φ_{ 2}’ which is in opposition to secondary flux Φ_{ 2} , but is in the same direction as primary flux Φ. And flux Φ_{ 2}’ is equal to Φ_{ 2}. Hence, the two flux cancel each other out.
So, we can say that whatever the load conditions, the net flux passing through the core is approximately the same as at noload. Due to the constancy of core flux at all loads, the core loss is also practically the same under all load conditions.
As Φ_{ 2}= Φ_{ 2}’
Hence, when transformer is no load, the primary winding has two currents in it; one is I_{0} and the other is I_{2}’ which is antiphase with I_{2} and K times in magnitude. The total primary current is the vector sum of I_{0 }and I_{2}’.
In Fig. shown the vector diagram of a transformer when the load is noninductive and when the load is inductive.
If we assume voltage transformation ratio is unity , ( i.e K= 1 )
In the fig.1, shown the vector diagram of a transformer when load is noninductive
I_{2}= secondary current in phase with E_{2}= V_{2}
I_{2}’= load component of primary current which is antiphase with I_{2} and also equal to it in magnitude( as K=1 )
I_{1}= Primary current which is vector sum of I_{0} and I_{2}’ and lags behind V_{1} by angle φ_{1}.
In the fig.2, shown the vector diagram of a transformer when load is inductive,
I_{2}= secondary current lags E_{2 } by φ_{2}.
I_{2}’= load component of primary current which is antiphase with I_{2} and also equal to it in magnitude( as K=1 )
I_{1}= Primary current which is vector sum of I_{0} and I_{2}’ and lags behind V_{1} by angle φ_{1}.
It is seen from the fig.2 that the angle φ_{1}is slightly greater than φ_{2}.
In figure , It is seen that I_{0} is neglect as compared to I_{2}’, then φ_{1}= φ_{2}.
Besides, under this assumption ,
N_{1}I_{2}’= N_{2}I_{1}= N_{1}I_{2}
_{ }
It is shown that under the fullload condition, the ratio of primary and secondary current is constant. The relationship is made the basis of current transformer, such a transformer which is used with a lowrange ammeter for measuring currents in circuits.
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