Motor Locked-Rotor Current (LRC) and Short-Circuit Contribution

The current drawn by an asynchronous motor during direct-on-line starting (without a soft starter or variable-frequency drive) for a fraction of a second up to a few seconds, typically five to eight times the rated current (Locked Rotor Current, LRC), which must be accounted for when sizing cables, protective devices and voltage-drop calculations to avoid nuisance tripping of overcurrent protection. Besides this design aspect, running motors briefly feed an additional short-circuit current contribution back into the network during a fault elsewhere in the installation, because the still-rotating rotor continues to act as a generator until the magnetic energy has decayed; in installations with many large motors (such as in process industry), this motor feedback must be included in short-circuit current calculations per NEN-EN 60909, otherwise the actual making capacity a switch must handle is underestimated.