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FACULTY OF APPLIED SCIENCE. 



-. MOTORS 



11 



current when its field is weak, and a leading current 

 when its field is strong. This should be clearly under- 

 stood, because the notation used in this book is not used 

 in all discussions of alternating currents, especially where 

 more emphasis is placed on the action of currents and 

 E. M. F.'s inside the machine than on what takes place 

 in the A. C. line. 



An explanation of the meaning of lagging and leading 

 alternating currents, and of how the relation between 

 voltages, currents, and power-factors may be expressed 

 by vector diagrams when sine wave forms are assumed, 

 will be found in text-books on alternating currents. 



It is to be noted that a synchronous motor with a 

 weak field current has a small pull-out torque and poor 

 synchronizing power, and it will, therefore, more easily 

 drop out of step at changes in its mechanical load or at 

 disturbances in the line voltage than when it has a strong 

 field current, when it is more stable and dependable 



The changes in the armature current and the power- 

 factor of a synchronous motor corresponding to changes 

 in field current are shown by the well-known "V" curves, 

 as in Fig. 2. The minimum value of armature current, 

 between the lagging current and leading current branches 

 of the curve, denotes that the motor is working at that 

 particular stage at 100 per cent power-factor, that is, 

 that the current supplied to the motor is in phase with 

 the voltage and is all employed in doing useful work. It 

 is evident from the "V" curve that a synchronous motor 

 can deliver a certain amount of mechanical power with 

 the least heating when it operates at 100 per cent power- 

 factor; at either lagging or leading power-factor it must 

 carry additional armature current, with correspondingly 



