Lockedrotor

Locked-rotor refers to a condition in electric motors where the rotor is prevented from turning while electrical power is applied. In this state the motor draws a very high current because there is little or no back-EMF (back electromotive force) to oppose the applied voltage, leading to maximum torque potential but also excessive heating risk.

This situation can occur when a motor is unable to start due to an overload, a mechanical jam, binding bearings, or an improperly sized drive. If the load torque exceeds the motor’s starting torque, the rotor remains stationary and the current remains near the locked-rotor level, rather than settling to running current.

In many motors, the locked-rotor current is several times higher than the running current. This inrush is a critical design and protection consideration, influencing wiring size, circuit breakers, and cooling requirements. Some motors specify a locked-rotor or starting current as part of their datasheet, which helps in selecting appropriate starters and protection schemes.

Protection and control approaches to manage locked-rotor conditions include soft starters, reduced-voltage starting, star-delta configurations, and variable-frequency drives that limit inrush and control acceleration. Proper mechanical design to avoid jams, regular maintenance, and appropriately rated protections are essential to prevent overheating or electrical faults during starting.

See also: starting current, stall current, inrush current.