Current inrush phenomenon in transformer is the flow of high magnetizing current for short time in primary winding when the transformer is switched ON at the instant when applied voltage is zero.
Figure 1.
As the primary winding of transformer is purely inductive hence during steady state operation the flux (ϕ) lags the supply voltage by 90º as shown in figure (1). From figure (1), it is seen that when t = 0 the supply voltage is zero and the flux is maximum negative.
Now consider the transient condition when the transformer is switched ON at the instant when applied voltage is zero.
Figure 2.
Ideally, the flux at this moment of switching should be maximum negative. But practically, it is impossible to develop maximum flux within zero time. Hence, the flux will start from zero and attains a value equal to twice the value of steady state flux (ϕm) as shown in figure (2).Such effect is known as doubling effect. Due to high amount of flux, core of transformer is saturated and high magnetizing current is drawn by primary from the source. This current is called inrush current.
Nature of Transformer Inrush Current
Figure 3.
The nature of inrush current is that it decays slowly. The magnitude of inrush current is very high i.e., of the order of 8 to 30 times the full load current of transformer. Inrush current has peaky non-sinusoidal waveform as shown in figure (3).
Problems Associated with Transformer Inrush Current
Figure 4.
The inrush current causes many problems like interruptions in fuse or circuit breaker, injection of noise and harmonics into the mains. It also leads to mechanical stress on transformer thereby affecting the magnetic property of the core of transformer. Also, high inrush current requires over-sizing of fuses or circuit breaker-s.
To prevent inrush current phenomenon the primary of transformer should be switched ON at the instant when applied voltage is maximum positive. When applied voltage is maximum positive, the flux which lags applied voltage by 90º will start from zero and attains maximum value equal to steady state flux as shown in figure (4). Thus, inrush current is absent in this situation.