Il8 ELEMENTS OF ELECTRICAL ENGINEERING. 



the kind above specified (where the load may not be always and 

 inseparably connected to the motor) is, that the former motor 

 will not speed up indefinitely (race) when its load is thrown off. 

 The simple shunt motor may be made to operate more or less 

 like a differential compound motor by giving the brushes a back- 

 ward lead (see Art. 51), or like a cumulative compound motor by 

 giving the brushes a forward lead. This is due to the fact that 

 with a backward lead of the brushes of a motor, the armature cur- 

 rent opposes the magnetizing action of the shunt field winding, 

 and that with a forward lead of the brushes, the armature current 

 helps the magnetizing action of the shunt field winding. 



61, The series motor. Series motors are used (with constant 

 or approximately constant voltage supply mains) only where the 

 motor load is always and inseparably connected to the motor, 

 and where constancy of speed with variations of load is not 



especially desired. Thus series motors 



supply main, for driying 



the fan is inseparably attached to the 



starting rheostat $ ^ ^ motor > and for df ivin g automobiles and 



street cars. The necessity of having 

 the load of a series motor attached to it 

 inseparably is due to the fact that if a 

 series motor, connected between con- 

 stant voltage supply mains, were to 

 have its load reduced more and more, 

 its speed would rise indefinitely, and the 

 enormous centrifugal forces would in 

 supply main " a11 probability wreck the armature. 



""" Fig. 87 shows the connections of a 

 series fan motor with its starting rheo- 

 stat. At starting considerable resistance is connected in series 

 with the motor, and as the motor speeds up this resistance is 

 slowly cut out. 



When a street car is operated by a single series motor, the 



