Distribution.— In arranging such a system of power distribution the average power re quired to drive is of as much importance as the maximum, for in a properly arranged group system the motor capacity need not be the equivalent of the total maximum power re quired to operate the several machines in the group, but may be taken at some value less than the total, depending upon the number of the machines and the average period of operation. On the other hand, as already shown, the motor capacity of independently driven machines must not only equal the maximum power required to drive the machine at full load, but it must be capable of exerting a greatly increased momen tary torque. In any case large units should be avoided, for the multiplication of machines driven from one motor entails additional shaft ing, counter-shafts and belting which may read ily cause the transmission losses to be greater than those obtained with engines and shafting alone, besides frustrating some of the principal objects of this method of transmission.
As far as the efficiency of transmission is concerned, it is doubtful whether,• in a large number of cases, motor-driving per se is any more efficient than well-arranged engines and shafting. The principal thing to be kept in mind is a desired increase in efficiency of the shop plant in turning out product, with a reduc tion in the time and labor items, without espe cial reference to the fuel items involved in the power •production. On account of the subdi vision of power which results from the use of many motors, there is less liability of interrup tion to manufacture, and in case of overtime it is not necessary to operate the whole works, with its usual heavy load of transmitting machinery. Another advantage is the adapta bility to changes and extensions; new motors may always be added without affecting any already in operation, and the ease with which this system lends itself to varying the speed of different unit groups is a very potent factor in its favor. When power is delivered to the machine by direct connected motors, a serious difficulty is often experienced in obtaining speed variation, which is so necessary with a large proportion of the machines in common use. A certain amount of variation can be obtained by rheostatic control — a wasteful method; or by using a single voltage system with shunt field regulation; but the variation in either case is very limited. This, however, may be increased by using a double commutator if space will permit. The three-wire, 110-220-volt system, offers many advantages for both power and lighting systems, and is very frequently em ployed. Variations of speed may be obtained with this system by using a combination of field regulation with either voltage, and, in rarer cases, by the use of a double commutator motor.
A method which has been used with consider able satisfaction involves the use of a wire generator, with collector rings connected to armature winding similar to that of a two phase rotary converter. Balancing coils are used, and the middle points of these are con nected to the third wire, which is thus main tained at a voltage half way between the outer Wires. This system is simple and economical, and possesses all the advantages of the ordinary three-wire method; it permits similar variations in speed by field regulation with either voltage; and if still wider ranges are desired a double commutator motor may also be used.
In other installations the four-wire mutiple voltage system is used, which permits of very wide variations of speed in the operation of the tool. This system gives excellent results and removes one of the objections urged against direct-connected motor-driven tools, namely, that such machines are not sufficiently flexible in regard to speed variation, and that such vari ation can only be obtained by throwing in resist ances which cut down the efficiency of the motor or-by varying the strength of field which reduces the torque. The multiple voltage sys tem, however, has some serious disadvantages. It cannot usually be operated from an outside source of power without rotary transformer• the generating sets and switchboard are com plicated, and the total cost of installation is .expensive; yet with these drawbacks the system is growing in favor, as it has manifest advan tages which outweigh the objections. Storage battery power has been used to some extent to obtain multiple control and is suggestive of interesting possibilities, but in its present form it is not altogether desirable for this purpose. In many of the larger sizes of certain metal cutting machines marked changes in design have taken place, and these m2ehines are now built with direct-connected motors incorporated in them, possessing wide variations of speed and power.
Improvements in the manufacture of tool steel have shown indisputably that the older designs of machine tools are not sufficiently heavy to stand up to the work in order to obtain the economy of operation which results from the use of such steels. Higher speeds, heavier cuts and greater feeds may be obtained if the machines will stand the strain, but in most cases the capacity of the machine is not commensurate witdt the ability of the tool to remove metal. With cutting speeds of 100 to 200 feet per minute, it is evident that the power requirements will be much greater than for those machines which use ordinary carbon steel with cutting speeds of from 10 to 30 feet per minute. Here then an interesting field for the direct-connected motor with ample power and speed variation for any work which it may be called upon to perform.