108 ELEMENTS OF ELECTRICAL ENGINEERING. 



The withdrawing of the plungers diverts the larger part of the 

 reduced flux into the pole tips where it is most effective in the 

 prevention of sparking, and at the same time interposes a greatly 

 increased reluctance in the path of the cross-flux, CC t Fig. 108. 

 (4) Multivoltage speed control. From equation (26) it is evi- 

 dent that the zero load speed (and consequently the full load speed 

 to nearly the same extent) may be made high or low by supplying 

 current to the armature at high or low voltage E x , the field flux 

 4> being kept constant. The range of speed control of a shunt 

 motor by this method is limited only by the speed the motor 

 can stand mechanically, and by the safe limit of impressed 



supply main A 



supply main 



supply main C 



T, 6oVo/(S 

 supply main JJ ; 



120\VOltS 



Fig. 82. 



voltage E x . In carrying out this method of speed control, cur- 

 rent is supplied over a number of mains, for example, A, B, C 

 and D y Fig. 82, between which constant electromotive forces of 

 any chosen values are maintained by a set of generators. The 

 field winding of the shunt motor is permanently connected to 

 one pair of these supply mains, and arrangements are made, by 

 means of a suitable " controller," for easily and quickly connect- 

 ing the motor armature to any pair of mains at will. 



This multivoltage method of speed control gives a series of 

 distinct and widely separate speeds. For example, with the vol- 

 tages indicated in Fig. 82 the available speeds would be n, 2n 

 and 4#, where n is the speed corresponding to E x = 60 volts. 

 The multivoltage method is usually combined with the field 

 rheostat method, so that by the latter method the intermediate 

 speeds may be obtained. 



(5) The Ward Leonard method of speed control. A wide range 

 of speed control of a shunt motor by means of a field rheostat can- 



