Ratings and Standards

Author: E.E. Kimberly

All motors and generators are guaranteed to perform satisfactorily when operated within certain arbitrary limits. The temperature limits are usually specified on the name plate. The guarantees usually allow a departure in speed or voltage of 10 per cent of the name-plate value in the case of constant-speed machines.

Series motors are used frequently for varying-speed duty in which the power demand changes rapidly through a wide range. Motors for such service have an intermittent rating for perhaps 1/2 hour or 2 hours. Their rating in horsepower indicates their ability to develop the required torque at rated speed without commutating distress, but the time specified indicates the limits of duration of rated load beyond which excessive temperature will occur.