52 



SCIENCE. 



[N. S. Vol. XVII. No. 419. 



thing to be kept in mind is a desired in- 

 crease in efficiency of the shop plant in 

 turning out product, with a reduction in 

 the time and labor items, without especial 

 reference to the fuel items involved in the 

 power production. 



On account of the subdivision of power 

 which results from the use of many motors, 

 there is less liability of interruption 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 adaptability 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. 



One serious obstacle to the use of con- 

 nected motors with machine tools is the dif- 

 ficulty of 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, 220-volt system offers 

 many advantages for both power and 

 lighting systems, and is very frequently 

 employed. Variations of speed may be 

 obtained with this system by using a com- 

 bination of field regulation with either 

 voltage, and, in rarer cases, the use of a 

 double commutator motor. 



A method which has been used recently 

 with considerable satisfaction involves the 

 use of a three-wire generator, with collector 

 rings connected to armature winding sim- 

 ilar to that of a two-phase rotary con- 



verter. Balancing coils are used, and the 

 middle points of these are connected to 

 the third wire, which is thus maintained 

 at a voltage half-way between the outer 

 wires. This system is simple and eco- 

 nomical, and possesses all the advantages 

 of the ordinary three-wire method; it per- 

 mits similar variations in speed by field 

 regulation with either voltage; and if still 

 wider ranges are desired a double commu- 

 tator motor may also be used. 



In other recent installations the four- 

 wire multiple 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-con- 

 nected motor-driven tools, namely, that 

 such machines are not sufficiently flexible 

 in regard to speed variation, and that such 

 variation can only be obtained by throwing 

 in resistances which cut down the efficiency 

 of the motor, or by varying the strength 

 of field which reduces the torque. 



The multiple voltage system, however, 

 has some serious disadvantages. It can 

 not usuallj^ be operated from an outside 

 source of power without rotary transform- 

 ers; the generating sets and switch-board 

 are complicated and the total cost of 

 installation is expensive; yet with these 

 drawbacks the system is growing in favor, 

 as it has manifest advantages which out- 

 weigh the objections. 



The storage battery 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 de- 

 sirable for machine tools. 



In many of the larger sizes of certain 

 metal-cutting machines it is probable that 

 marked changes will be produced in the 

 immediate future, and the indications are 

 that direct-connected motors with wide 

 variations of speed and power will be in- 

 corporated in the new designs. 



