Safe Locked Rotor Time: How Safe Is It?

Stalling of an induction-motor, or its failure to accelerate upon start-up, produces both thermal and mechanical stress within the stator and rotor which can be damaging. Whether the stator winding or rotor cage reaches unsafe stress limits first depends upon individual motor design. "Safe locked time" is considered the maximum period a motor can be locked on the line without significant loss of motor life. The nature of these stresses imposed on motor components, how they vary with design, and why the nature of acceleration heating differs from that of locked rotor heating is explained. Equally important, the effect of the motor\´s "safe time-current characteristic" (which expresses internal stress limits in terns of line current) on the problem of protective relaying is described. Solution of this problem inust depend upon the system designer\´s understanding of this characteristic.