132 



ALTERNATING CURRENTS 



when, owing to an extremely heavy overload, the motor is caused to 

 drop out of step. The values of the armature current corresponding 

 to unstable conditions of running are also shown dotted in the current 

 curves of Fig. 98. 



The power curves of Fig. 99 show very clearly that corresponding 

 to each excitation (or each value of the motor e.m.f.) there is a certain 

 maximum load represented by the top of the curve, where it becomes 



1000 



MOTOR. . M. F. 



FIG. 100. Connection between Excitation and Overload Capacity. 



horizontal, and where the stable descending branch passes into the 

 unstable ascending one beyond which the motor will refuse to run. 

 This maximum load is seen to increase with the excitation, and the 

 relation connecting the maximum load with the motor e.m.f. is repre- 

 sented graphically in Fig. 100. From this it will be seen that if a 

 large overload capacity is desired, the motor e.m.f. should have a 

 relatively large value.* 



* If the excitation is increased indefinitely, then beyond a certain value the over- 

 load capacity decreases with further increase of excitation. The value of the excitation 

 corresponding to this is, however, so great as to be practically unrealizable under 

 ordinary conditions of working. 



