Corriente de arranque (inrush)

Inrush current is the brief, large current a load draws in the first instant after it is switched on — often many times what it draws once it is running. It lasts anywhere from a fraction of a cycle to a few seconds, then settles to the normal running current.

Where the surge comes from

A motor at rest has no spin to push back against the supply, so for the first moments its windings look like little more than their resistance and the current is limited by almost nothing. As the rotor comes up to speed it generates a counter-voltage that pushes back, and the current falls to its running value. That starting current is the one the nameplate calls locked-rotor current.

A transformer switched on at the wrong point in the voltage wave can drive its core into saturation for a cycle or two and gulp current while the magnetism settles. An LED driver or electronic power supply has capacitors at its input that are empty at switch-on and fill almost instantly — a very tall, very short spike. A cold incandescent or heating element has a lower resistance than when it is hot, so it too draws more for the first moment.

Why it matters on the job

Inrush is why the lights dim when a big compressor kicks on: the extra current drops extra voltage across every bit of conductor it passes through. It is why a circuit of many LED fixtures can trip its breaker at the wall switch even though the running load is small. And it is why breakers and fuses are built to be time dependent: they let a large current through for a short time and only open if it lasts. Which device and curve suits a given inrush is a manufacturer-data and code question.

One worked, so the idea sticks

A motor starting on a long run

1. A motor runs at 10 A. Starting, it draws about six times that for a couple of seconds: 60 A.
2. The conductors out and back have 0.2 Ω between them. Running: 10 A × 0.2 Ω = 2 V dropped in the wire.
3. Starting: 60 A × 0.2 Ω = 12 V dropped — six times as much, for as long as the start lasts. Every light on the same run sees the dip.

Practice numbers. The real starting current is on the motor’s nameplate or data sheet, and the multiple varies a lot from motor to motor.

Preguntas frecuentes

Ver también

• Glossary: inrush
• Wye-delta starting — cutting the starting current
• Motor starting methods
• Back to the glossary