Startstrøm

Startstrøm, or inrush current, is the transient surge of electric current drawn by equipment at the moment it is energised. This initial current is typically much higher than the steady-state running current and lasts for a short time, from a few milliseconds up to several hundred milliseconds, before the device reaches its normal operating state.

Causes and occurrences

Startstrøm occurs because, at start, many electrical devices have low impedance and no back electromotive force (back-EMF) yet. In induction motors, windings present mainly resistance before rotor speed builds up, so current is high until the motor accelerates. Transformers and capacitive loads can also exhibit significant inrush as magnetic cores saturate or capacitors charge to operating voltage.

Typical values

For three-phase induction motors, starting current commonly ranges from about 4 to 7 times the full-load running current. Stall current can be higher and depends on motor design. DC motors and some transformers can show different patterns, but the principle remains: the initial current far exceeds the normal running current.

Impact and mitigation

Inrush can cause voltage dips, nuisance tripping of circuit breakers, and stress on electrical components. Mitigation strategies include soft starters (thyristor-based), variable-frequency drives (VFDs) that ramp voltage and frequency, star–delta or autotransformer starting methods, and pre-charge devices such as NTC thermistors to limit the initial surge. Proper sizing of conductors, protection devices, and power supply infrastructure is also important.

Measurement and standards

Startstrøm is typically assessed with current measurements on energisation, using measuring equipment and alignment with regional electrical standards and safety guidelines. Understanding and controlling startstrøm is important for reliable operation of electrical systems and to minimize disturbances on the power network.