184 



ELECTRICAL MACHINERY 



in contact with the metal to be punched or formed. The 

 rolling mill is another instance of a tool calling for an irregu- 

 lar power supply. 



If the mass of the moving parts is not very great in 

 such machines the power consumption of the driving motor 

 will be very irregular. Fig. 112 illustrates this point; the 

 full line curve shows the current consumption of a motor 

 driving a shears for cutting large iron bars. 



FIG. 111.- Some of the Contact of a Railway Controller. A magnetic 

 " blow-out " scheme is used to prevent serious arcing at the 

 contacts. General Electric Company. 



Now the motor in which such a current is flowing must 

 be made sufficiently large to commutate successfully the 

 greatest value of current, as at A. If the motor driving the 

 shears were a compound-wound motor with a comparatively 

 large number of turns' in its series field (say 30%-40% 

 compounding), and a heavy flywheel is put on the same 

 shaft with the armature, the current to the motor, while 



