Blockedrotor

Blocked rotor, also known as locked-rotor, refers to a condition in electric motors where the rotor cannot turn despite the application of voltage to the stator windings. This situation can arise from mechanical jam, excessive load, bearing failure, or a fault causing the rotor to seize. In this state the motor operates with maximum slip (approximately unity for an induction motor), which produces a momentary surge of current determined by the standstill impedance.

Electrical characteristics of a blocked rotor include a high inrush or starting current, typically several times the motor’s full-load current. The exact ratio depends on motor design, voltage, and frequency. The starting torque at standstill is governed by the motor’s design and the rotor resistance; it can be substantial in some motors, but if the load remains engaged, overheating and damage may occur due to prolonged high current. Blocked-rotor conditions are short-lived in normal starting sequences but can cause nuisance tripping of protection devices if not properly managed.

Blocked-rotor test is a common method in motor engineering to determine standstill parameters. By applying rated voltage with the rotor mechanically blocked, engineers measure standstill impedance, resistance, and torque characteristics to derive equivalent circuit parameters used for performance modeling and protection settings.

Protection and control strategies include proper overcurrent protection, thermal protection, and the use of soft-start or VFD drives to limit inrush. In some contexts, “blocked rotor” is used interchangeably with “locked rotor” to describe the same condition.