Glossary · Automation fundamentals, platforms and components
Starting current
Also known as: Inrush current, Locked-rotor current
German: Anlaufstrom
In electric drive technology, starting current is the current an electric motor draws while accelerating from standstill. For induction motors started direct on line it is several times the rated current and falls as the motor reaches operating speed.
- Automation components
In one sentence
Starting current is the high current an electric motor draws while accelerating from standstill, several times rated current when started direct on line.
Example
A 45 kW pump motor started direct on line makes the plant lights flicker, so it is changed to start through a soft starter.
How it applies
- Engineering: Starting current determines the selection of circuit breakers, motor protection, cables and supply capacity. Star-delta starters, soft starters and frequency converters reduce it.
- Operation: Frequent starts heat the motor. Motor data sheets and standards limit the number of starts per hour, and Thermal management must allow for it.
- Documentation: State the starting method and current, or the ratio of starting current to rated current, in data sheets and electrical documentation. Installation instructions should tell the operator which supply capacity and protective devices are required.
Starting current vs. inrush current
Motor starting current lasts for the acceleration time, from fractions of a second to several seconds. Inrush current of transformers, power supplies or capacitors is a very short peak when switching on. Both affect the choice of protective devices, but in different ways.