110 ELEMENTS OF ELECTRICAL ENGINEERING. 



of a speed control ranging from full speed to zero speed, pro- 

 vided the motor load is fairly constant. 



The control of speed by a field rheostat does not involve 

 much waste of power, inasmuch as the field current is very 

 small in comparison with the armature current of a shunt motor. 

 Additional advantages of this method are : simplicity and cheap- 

 ness, the absence of great fluctuations of speed with load, and 

 the possibility of fine gradations of speed control when a finely 

 subdivided rheostat is used. The disadvantage of this method is 

 that it has a limited range, as already explained. 



The control of speed by the change of reluctance method pos- 

 sesses all of the advantages of the field rheostat method using a 

 very finely subdivided rheostat, and it saves the cost of such a 

 rheostat and gives a wider range of speed control because of the 

 partial elimination of sparking as before explained. On the other 

 hand the change of reluctance method requires a motor of a 

 special and rather expensive design. 



The multivoltage method of speed control has the great disad- 

 vantage that it is expensive, requiring as it does, several genera- 

 tors,* an elaborate system of wiring, and a complex controller for 

 each motor. The disadvantage that it does not provide for a finely 

 graded speed control is entirely overcome by combining with it 

 the field rheostat method for giving intermediate speeds, as al- 

 ready explained. The combined multivoltage and field rheostat 

 system of speed control is extensively used in machine shops 

 for driving machine tools where it is desired to make arrange- 

 ments quickly to drive the tool at that speed which will accom- 

 plish a particular kind of work in the shortest time. 



The advantage of the multivoltage method over the armature 

 rheostat method of speed control in securing a greater constancy 

 of speed under variations of load, is well illustrated by the ex- 

 perimental data in the following table. These observations were 



* In practice a single main generator is used, and this main generator operates one 

 or more motor-generator sets, technically called balancers, for subdividing the voltage 

 of the main generator, as explained in the chapter on electrical distribution. 



