Machinery's Handbook, 31st Edition
Electric Motor Applications 2653 loaded to full capacity. Where operation is to be at several speeds, the horsepower require ment for each speed should be considered. Torque: Starting torque requirements may vary from 10 percent of full load to 250 per- cent of full load torque depending upon the type of machine being driven. Starting torque may vary for a given machine because of frequency of start, temperature, type and amount of lubricant, etc., and such variables should be taken into account. The motor torque supplied to the machine must be well above that required by the driven machine at all points up to full speed. The greater the excess torque, the more rapid the acceleration. The approximate time required for acceleration from rest to full speed is given by the formula: Time T 308 N WR seconds a 2 # # = where N = full load speed in rpm T a = torque = average foot-pounds available for acceleration. WR 2 = inertia of rotating part in pounds feet squared ( W = weight and R = radius of gyration of rotating part). 308 = combined constant converting minutes into seconds, weight into mass and radius into circumference. If the time required for acceleration is greater than 20 seconds, special motors or starters may be required to avoid overheating. The running torque T r is found by the formula: T N 5250 HP foot pounds r # = where HP = horsepower being supplied to the driven machine N = running speed in rpm 5250 = combined constant converting horsepower to foot-pounds per minute and work per revolution into torque. The peak horsepower determines the maximum torque required by the driven machine and the motor must have a maximum running torque in excess of this value. Inertia: The inertia or flywheel effect of the rotating parts of a driven machine will, if large, appreciably affect the accelerating time and, hence, the amount of heating in the motor. If synchronous motors are used, the inertia ( WR 2 ) of both the motor rotor and the rotating parts of the machine must be known since the pull-in torque (torque required to bring the driven machine up to synchronous speed) varies approximately as the square root of the total inertia of motor and load. Space Limitations in Motor Selection.— If the motor is to become an integral part of the machine which it drives and space is at a premium, a partial motor may be called for. A complete motor is one made up of a stator, a rotor, a shaft, and two end shields with bearings. A partial motor is without one or more of these elements. One common type is furnished without drive-end end shield and bearing and is directly connected to the end or side of the machine which it drives, such as the headstock of a lathe. A so-called shaftless type of motor is supplied without shaft, end shields or bearings and is intended for built- in application in such units as multiple drilling machines, precision grinders, deep well pumps, compressors and hoists where the rotor is actually made a part of the driven ma- chine. Where a partial motor is used, however, proper ventilation, mounting, alignment and bearings must be arranged for by the designer of the machine to which it is applied. Sometimes it is possible to use a motor having a smaller frame size and wound with Class B insulation, permitting it to be subjected to a higher temperature rise than the larger-frame Class A insulated motor having the same horsepower rating.
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