When an induction motor is thrown across the line by the sudden closing of a switch, transient pulsating torques, which in typical cases may reach values from 3 to 6 times the nominal starting torque, are set up in the shafts, gearing, and couplings of the connected system. The actual magnitude of these torques depends not only upon the electrical characteristics of the motor but also on the mechanical characteristics of the drive, i.e., coupling flexibility, motor, and load inertias. In this paper, which is an elaboration with additional experimental results of a previous paper by the author, a method is developed for calculating such systems, a typical example being the roll-table drive used in continuous-strip mills. Tests on typical drives, carried out in parallel with the theoretical work, indicate that the method of calculation is satisfactory for practical use. A discussion of the transient torques set up during “plugging,” i.e., reversing rotation by interchanging two leads, is also given. A consideration of such torques is particularly important where motors are started and stopped frequently or continuously since, under such conditions, fatigue failure may occur if the mechanical stresses developed exceed the endurance limit of the material.

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