Surge current

Inrush current, input surge current or switch-on surge is the maximum, instantaneous input current drawn by an electrical device when first turned on. Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady state currents, due to the charging current of the input capacitance. The selection of overcurrent protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The overcurrent protection must react quickly to overload or short circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.

When a transformer is first energized, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles. Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage, (which for a pure inductance would be the current maximum in the AC cycle) and if the polarity of the voltage half cycle has the same polarity as the remanence in the iron core has. (The magnetic remanence was left high from a preceding half cycle). Unless the windings and core are sized to normally never exceed 50% of saturation, (and in an efficient transformer they never are, such a construction would be overly heavy and inefficient) then during such a start up the core will be saturated. This can also be expressed as the remnant magnetism in normal operation is nearly as high as the saturation magnetism at the "knee" of the hysteresis loop. Once the core saturates however, the winding inductance appears greatly reduced, and only the resistance of the primary side windings and the impedance of the power line are limiting the current. As saturation occurs for part half cycles only, harmonic rich waveforms can be generated, and can cause problems to other equipment.

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