406 



ELEMENTS OF ELECTRICAL ENGINEERING. 



equal. Suppose, for example, that the combined current output 

 of the two machines is 100 amperes, then the points Pand P at 

 which the two machines operate are determined by the two con- 

 ditions : (a) their ordinates must be the same, and (b) the distance 

 PP must represent 100 amperes. In order that each machine 

 may always deliver half of the total current, 0' P must always 

 be equal to 0' F , that is the two characteristics must be exactly 

 alike. Fig. 39 shows the characteristics of two shunt gener- 

 ators of which the terminal voltages are equal at no load. Fig. 

 40 shows the characteristics of two shunt generators of which the 



Volts 



Current output of 



Current output of A 



Fig. 40. 



terminal voltages are unequal at no load. The dotted extension 

 of each characteristic corresponds to motor action, that is to nega- 

 tive current output, or, in other words, to current flowing through 

 the machine in opposition to its induced electromotive force. In 

 this case the two machines operate at the point P" when they are 

 connected in parallel and give a combined current output of zero ; 

 machine B acts as a generator giving out a current equal to 

 P f O and this current flows backwards through machine A 

 which of course acts as a motor. 



