Transformer on On load Condition

When load is connected to the secondary winding of a transformer , I₂ (secondary current) is set up in the secondary winding. The magnitude and phase of I₂ with respect to V₂ (secondary voltage) depends upon the characteristics of the load. Secondary current I₂ is in phase with V₂,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( =N₂I₂ ) and hence it produce magnetic flux Φ ₂ which is in opposition to the main primary flux Φ . The secondary ampere-turns N₂I₂ are known as demagnetizing amp-turns. The opposing secondary flux Φ ₂ weakens the primary flux Φ momentarily, hence the primary winding back e.m.f E₁ tends to be reduced. For a moment V₁ > E₁ and hence more current to flow in the primary winding . Let the additional primary current be I₂’ which is known as load component of primary current. This current (I₂) is anti-phase with I₂.This load component of primary current(I₂’) 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 no-load. 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 I₀ and the other is I₂’ which is anti-phase with I₂ and K times in magnitude. The total primary current is the vector sum of I₀ and I₂’.

In Fig. shown the vector diagram of a transformer when the load is non-inductive 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 non-inductive

I₂= secondary current in phase with E₂= V₂

I₂’= load component of primary current which is anti-phase with I₂ and also equal to it in magnitude( as K=1 )

I₁= Primary current which is vector sum of I₀ and I₂’ and lags behind V₁ by angle φ₁.

In the fig.-2, shown the vector diagram of a transformer when load is inductive,

I₂= secondary current lags E₂ by φ₂.

I₂’= load component of primary current which is anti-phase with I₂ and also equal to it in magnitude( as K=1 )

I₁= Primary current which is vector sum of I₀ and I₂’ and lags behind V₁ by angle φ₁.

It is seen from the fig.-2 that the angle φ₁is slightly greater than φ₂.

In figure , It is seen that I₀ is neglect as compared to I₂’, then φ₁= φ₂. Besides, under this assumption , N₁I₂’= N₂I₁= N₁I₂

It is shown that under the full-load 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 low-range ammeter for measuring currents in circuits.