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"Die elektrolytische ReBer., 39, 3291-3299.

ADVANTAGES AND APPLICATIONS OF THE ELECTRIC DRIVE.*

By F. B. CROCKER AND M. ARENDT.

The great economies effected in many manufacturing establishments by the substitution of electric drive for the older methods of counter-shafting and belting have resulted in the rapid extension of this branch of electrical engineering. Many factory managers anxious to decrease running expenses, or increase the plant output, are led, by a general knowledge of what has been accomplished, to believe that this substitution would produce both results. In many instances the desired results may be obtained, and in others, affairs would not be improved; either owing to the fact that the conditions of operation are not suited to the electric drive or because the wrong types of motors or motor control were employed.

It is impossible to give any exact rule which would determine when to adopt the electric drive, or when not to. Each case should be considered according to its own particular features, and the reasons for and against the change must be carefully studied, in order to be able to decide correctly which system or modification would produce the best results in all respects. It is necessary not only to consider the increased output or reduced power bills, but also to take into consideration the items of first cost, interest, depreciation, maintenance and other facts discussed in detail later.

Electric drive may be divided into three general classes, as follows: One motor for the entire plant; group drive, when the machines in each section or bay are driven from one motor common to that section only; individual drive, when each individual tool is equipped with its own motor.

The Common Motor Drive is equivalent to the ordinary engine or other prime mover drive, and employs like them a system of counter-shafts and belts, its advantage being the convenience of electric transmission of power, in comparison with a steam engine

* Portion of Lecture Notes on Electric Power, Electrical Engineering Department Columbia University.

ment.

plant and its attendants, as well as reduced first cost of the equipIt is also employed when the plant is too small to be economically divided into groups or operated by individual motors. The running efficiency under variable loads thus secured is higher than that of the steam or gas-engine, and while this is an advantage, the saving made is too small to consider by itself.

The Group Drive is a compromise between one common motor and individual machine drive, and its employment is frequently advisable. For example, consider a factory having no rush season, but a steady demand for its finished product, so that the work can be turned out according to a certain definite schedule, with the labor so divided that every two or more men operate a certain group of tools, which are used therefore in rotation, and not at the same instant. In a case like this, only one motor is necessary for each group, but it must be large enough to drive the heaviest machine connected to it. The connections are made by counter-shafts, friction-clutches and belts, so arranged that only the particular machine in use is being driven.

It is advisable in such an equipment, to have tools requiring approximately the same power in the same groups, because by so doing, the average size of the motors will be kept down and the first cost reduced.

Group drive is to be recommended, when several similar machines are to be run, each machine requiring only a small amount of power, as for instance in the case of a collection of sewing machines, small saws, grinders, polishers, buffers, etc.

Individual Drive. When the establishment is large and wide ranges of speed are desired, with machines differing greatly in capacity and performing totally different functions, individual drive is to be recommended, on account of the independence of the various machines, reduction in shafting and belting losses, etc. The motor may be connected to the driven apparatus by one of three general means, i. e., direct-connecting, gearing or belting. The first method is employed when the normal speed of the machine is high and corresponds to that of the motor. Very low speed apparatus should not be driven directly, because the cost of the motor would be excessive; low speed motors being larger than high speed ones of the same capacity. When the required reduction of motor speed is small, and does not exceed 3 to 1, belting answers well if the distance between driving and driven shafts can

be made considerable and the power to be transmitted is not great. If the distance between shaft centers is necessarily small (less than four times the diameter of the driving pulley) and the power to be transmitted is considerable, say over 10 h.p., a modified form of belting, i. e., noiseless chain drive is to be recommended. When reduction of driving speed is large and the motor can be placed close to the tool, gearing is to be recommended as the means for transmitting the power. The flexibility of the belt drive is sometimes an advantage since it does not produce the shock upon the motor due to sudden load variations; as for example, those occuring in punching and shearing.

The conditions under which machinery operates in regard to varying speed and power required of the driving motor may be divided into four classes, and a special type of motor is usually best suited to each of these divisions.

(a) Work which requires the motor to operate automatically at a constant speed, regardless of load changes or other conditions. (b) Work requiring frequent starting and stopping of the machine, and wide variations in speed, including sometimes rapid acceleration.

(c) An approximately steady load, or where the work varies as some function of the speed, should it change.

(d) Work in which the power varies regardless of the speed, or where speed variations with constant torque may be desired.

The first case (A), applies to line shaft equipments, with many machines operated by the same motor; and where slight speed variations may be allowed, the alternating current induction motor, the direct current shunt or slightly compounded motor would answer, depending naturally upon the character of electric current available. A special refinement of this problem is encountered in the driving of textile machines where a slight speed variation might influence the appearance of the finished product, in such instances the alternating current motors, polyphase induction or polyphase synchronous are necessary, because the speed of the direct current motors will vary considerably with voltage changes and variation in temperature which occurs after several hours of operation. Whereas the speed of the alternating current motors, unless the voltage varies greatly, is primarily dependent upon the frequency of the supplied current.

The second class (B) is divided into two parts, the first being ap