Stall time, acceleration time, frequency of starting: the myths and the facts [electric motors]

The safe stall time of an electric machine is often compared with the time required to accelerate the motor and the load from standstill to rated speed. Most users of electric machines feel that the safe stall time must be longer than the acceleration time in order to safely protect the unit against thermal damage. It is also felt that the longer stall time with respect to the acceleration time, the more frequently a large motor may be started. These ideas embody several of the many myths surrounding these two machine parameters since, in reality, greater thermal damage often results more from the frequency of starting than from the acceleration time being greater than the stall time. A prime objective is to generate a broader understanding of the definitions, the methods of calculation, and the thermal effects of stalling, accelerating, and repeated starting of medium and large fabricated squirrel-cage induction machines