Startströme

Startströme, or startup currents, are the transient electrical currents that occur when equipment is energized or switched on. They are typically several times higher than the steady-state running current and decay as the device reaches normal operation. Startströme arise from the need to establish magnetic flux in inductive devices and to charge energy storage components.

In transformers and motors, the dominant cause is magnetization of the core. At startup the core requires a rapid increase in flux, which presents a lower effective impedance and therefore a surge current. In power electronics and power supplies, charging of input capacitors and other energy storage elements generates brief current spikes. Residual magnetization, the phase of the supply, and interaction with network impedance can also influence the magnitude of the inrush.

Quantities and duration vary by device. A typical three-phase induction motor may exhibit inrush currents of 5–10 times the running current; transformers commonly show 2–3 times, though higher values can occur. The duration ranges from a few milliseconds up to several tens of milliseconds, with larger installations potentially causing noticeable voltage dips on the supply.

Impacts and management: Inrush currents can cause voltage dips, nuisance tripping of protective devices, and thermal stress on windings. Mitigation strategies include soft starters, autotransformers, reactors, and variable-frequency drives that ramp voltage or frequency, as well as star-delta starting to reduce initial current.

Measurement and standards: Inrush is characterized by peak current and duration, measured with current probes or clamps. Standards related to electrical safety and utility coordination provide guidance on acceptable levels of inrush and related flicker in power systems.