lockrotor

Lock rotor, also known as a locked-rotor condition or stalled rotor, refers to a situation in electric motors where the rotor cannot turn despite the application of supply voltage. At standstill the motor may draw a very high current while delivering little or no torque, causing rapid heating. This condition is most common at startup, when the load torque exceeds the motor’s starting capability or when mechanical binding prevents acceleration.

Causes and contributing factors include mechanical blockage or binding from bearings, seized shafts, misalignment, a jammed driven load, or foreign objects in the rotor path. Excessive load during startup, an undersized motor, or a faulty drive that fails to provide appropriate speed or torque can also lead to lock rotor conditions. Electrical issues such as shorted windings can raise current levels and worsen the thermal stress, but they do not by themselves guarantee a locked rotor.

Operational consequences and protections: Prolonged lock rotor can cause insulation damage, overheating, accelerated wear, and nuisance tripping of circuit breakers or motor protection relays. Protective schemes include overload relays, thermal sensors, and current-based stall detection in motor controllers. Using a soft-start or a variable-frequency drive can reduce inrush current and mitigate stall risk.

Prevention and best practices: Size the motor correctly for the load and required starting torque; implement a proper starting sequence with soft-start or VFD control; maintain bearings, lubrication, and alignment; ensure couplings and mounts are secure and free of obstructions. Employ monitoring of current and temperature and implement stall protection to automatically shut down a motor that remains stalled beyond a safe interval.