Motor Locked Rotor Current

Locked rotor current (LRA — locked rotor amperes) represents the worst-case sustained current demand of an induction motor, occurring when the rotor cannot turn due to a mechanical jam, bearing seizure, or excessive load. Unlike normal starting current, which decreases as the motor accelerates, locked rotor current persists indefinitely until the protective device operates or the motor overheats and fails. IEC 60034-12 classifies motors by code letter indicating the ratio of locked rotor current to rated current — Design A motors have higher ratios (6.3–7.1:1) while Design B motors have moderate ratios (5.0–5.6:1) and higher-efficiency Design C motors have lower ratios. The NEC uses locked-rotor indicating code letters (Table 430.7(B)) that express LRA in kVA/hp. Motor protection must distinguish between acceptable starting current (high but transient) and locked rotor conditions (high and sustained): thermal overload relays are set to trip after a time that exceeds the motor's starting time but is short enough to prevent winding damage during a stall. Instantaneous settings on motor circuit breakers must be set high enough to ride through starting inrush without nuisance tripping.