- Transformer Inrush Current Definition: Transformer inrush current is defined as the high transient current drawn by a transformer when it is energized.
- Initial Flux and Voltage: At startup, there is no initial flux, and the flux wave starts from zero, following the voltage waveform.
- Peak Flux and Core Saturation: The flux can peak at double the steady-state maximum value, leading to core saturation and high inrush current.
- Transient Nature of Inrush Current: Inrush current is transient, lasting only a few milliseconds, but can be up to 10 times the normal rated current.
- Impact on Circuit Operation: High inrush current can cause fuse or breaker interruptions, component failures, and introduce noise and distortion into the electrical system.
When a transformer is first switched on with the secondary circuit open, it behaves like an inductance. Normally, the flux in the transformer core is in quadrature with the applied voltage.
The flux wave will reach its maximum value, 1/4 cycle or π/2 angle later, reaching the maximum value of the voltage wave. As per the waves shown in the figure below, at the instant when the voltage is zero, the corresponding steady state value of flux should be the negative maximum (i.e. minimum value).
However, it’s not possible to have flux immediately when the transformer is switched on. There is no flux in the core before the power is applied.
The steady state value of flux will not be reached instantly. Although it’s very fast from our perspective – it takes a non-zero amount of time. The speed of this process depends on how fast the circuit can take energy.
This is because the rate of energy transfer to a circuit cannot be infinity. So the flux in the core also will start from its zero value at the time of switching on the transformer. According to Faraday’s law of electromagnetic induction the voltage induced across the winding is given as e = dφ/dt. Where φ is the flux in the core. Hence the flux will be integral of the voltage wave, which can be calculated using the formula below:
If the transformer is switched on when the voltage is zero, the flux wave starts from the same point as the voltage wave. The flux value at the end of the first half-cycle of the voltage wave can be calculated using:
Where φm is the maximum value of the steady-state flux. The transformer core is generally saturated just above the maximum steady state value of flux. But in our example, when switching on the transformer the maximum value of flux will jump to double its steady state maximum value.
After reaching the maximum steady-state flux, the core becomes saturated, and producing more flux requires a lot of current. This high peak current drawn by the transformer primary is called transformer inrush current or magnetizing inrush current.
Magnetizing inrush current in transformer is the current which is drown by a transformer at the time of energizing the transformer. This current is transient in nature and exists for few milliseconds. The inrush current may be up to 10 times higher than normal rated current of transformer.
Although the magnitude of inrush current is so high but it generally does not create any permanent fault in transformer as it exists for very small time. But still inrush current in power transformer is a problem, because it interferes with the operation of circuits as they have been designed to function.
Some effects of high inrush include nuisance fuse or breaker interruptions, as well as arcing and failure of primary circuit components, such as switches. High magnetizing inrush current in transformer also necessitate over-sizing of fuses or breakers. Another side effect of high inrush is the injection of noise and distortion back into the mains.
