Working Principle of a Transformer:
While the primary is connected to the AC voltage source, an alternating flux is generated in the core with amplitude based on the primary voltage, frequency and number of turns. This mutual flux links the other winding, called the secondary. A voltage is induced in this secondary of the equivalent frequency as the primary voltage but its magnitude depends on the number of secondary turns.
The voltage induced in the primary is
E1 = 4.44 φ f N1 volts ---------- (1)
where E1 = Voltage induced in primary, V
φ = Flux in the core, Wb, and
N1 = Number of turns in primary winding. Similarly, the voltage induced in the secondary is
E2 = 4.44 φ f N2 volts . -------- (2)
where E2 = Voltage induced in secondary, V, and
N2 = Number of turns in secondary winding.
Therefore from the Eqs. (1) and (2), we have
E1/ E2 = N1/ N 2 --------- . (3)
If the secondary voltage is greater than the primary voltage, the transformer is termed a step-up transformer (N2 > N1). If it is less, this is known as a step-down transformer (N1 > N2). If N1 = N2, the transformer is said to have a one to one ratio.
The ratio of transformation can be given by
a = E1 /E2 = N1 /N2 ------- (4)
As in the case of transformers,
Input volt amperes (VA) = Output VA
V1 I1 = V2 I 2 ----------(5)
where V1 and V2 are the terminal voltages of the primary and secondary windings respectively.
Also V1 ≈ E1 and V2 ≈ E2 as winding resistances and reactances are extremely small.
From the Eq. (4), we may get
E1/ E2 = V1 /V2 = N1 /N 2 = a = I2 /I1 ----- (6)