Inrush current

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.

The selection of over-current-protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated.

The over-current protection must react quickly to overload or short-circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.

Safeguarding against the filter capacitor’s charging period’s initial current inrush flow is crucial for the performance of the device.

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).

As saturation occurs for part half-cycles only, harmonic-rich waveforms can be generated and can cause problems to other equipment.

For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away (decay time proportional to XL/R) and the regular AC equilibrium is established.

To avoid magnetic inrush, only for transformers with an air gap in the core, the inductive load needs to be synchronously connected near a supply voltage peak, in contrast with the zero-voltage switching, which is desirable to minimize sharp-edged current transients with resistive loads such as high-power heaters.

For high-power motors, the winding configuration may be changed (wye at start and then delta) during start-up to reduce the current drawn.

Negative-temperature-coefficient (NTC) thermistors are commonly used in switching power supplies, motor drives and audio equipment to prevent damage caused by inrush current.

When a transformer is switched off on its primary side, inductive kick produces a voltage spike on the secondary that can damage insulation and connected loads.