112 



ELEMENTS OF ELECTRICAL ENGINEERING. 



smaller motor would run at a higher efficiency. The use of 

 variable speed motors is justified, however, in many cases because 

 of the great ease and quickness with which speed changes can be 

 accomplished. 



59. The shunt motor starting rheostat. From equation (2$c] it 

 is evident that an excessively large current, / a [= (-. &Z'n)jR^ 

 will flow through the motor armature at starting if the armature 

 terminals are connected directly to the supply mains, inasmuch as 

 QZ'n is zero at starting, since n is then zero, and the resistance 

 R a of the armature is small. It is necessary to prevent this exces- 

 sive flow of current at starting 

 by connecting an auxiliary 

 resistance in series with the 

 motor armature (not in series 

 with the field). While the 

 motor is speeding up, the value 

 of QZ'n increases, and the 

 current tending to decrease, 

 it is permissible to gradually 

 cut out the auxiliary resistance. 

 When the motor has reached 

 nearly full speed, the auxiliary 



Fig. 83. 



resistance is wholly cut out. 



This auxiliary resistance, when conveniently arranged for being 

 put in series with the armature and slowly cut out as the motor 

 speeds up, is called a starting rheostat. 



Fig. 83 shows the simplest arrangement of a shunt motor with 

 a starting rheostat, the motor being connected to the supply mains 

 through a double-pole single-throw switch. The closing of this 

 switch connects the field circuit of the motor to the mains, the 

 first movement of the rheostat arm a connects the armature to the 

 supply mains through the whole of the starting resistance R, and 

 the continued movement of the rheostat arm slowly cuts out 

 the starting resistance. 



