Inrush Current — Transformer Glossary

The transient surge of magnetizing current that flows when a transformer is first energized. Can reach 8–12 times the rated full-load current.

Backelectrical

Inrush Current

The transient surge of magnetizing current that flows when a transformer is first energized. Can reach 8–12 times the rated full-load current.

Inrush current occurs because the core may retain residual flux from the previous energization. If the voltage waveform is at a point that adds to the residual flux, the core saturates and draws excessive current. Inrush current decays exponentially over 10–40 cycles as the core establishes steady-state flux. Protection relays must be set to distinguish between inrush current (which is harmless) and fault current. Modern relays use second harmonic restraint to prevent false tripping during energization.

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Transformer CoreThe laminated magnetic circuit that provides the path for magnetic flux between the primary and secondary windings.

Core SaturationThe condition where the core's magnetic flux density reaches a level where further increases in magnetizing current produce diminishing increases in flux.