CHAPTER III 



SYNCHRONOUS MOTORS 



THE constant-voltage system consists essentially in 

 counteracting directly by means of synchronous motors 

 the variation in voltage which is the most troublesome 

 feature encountered in designing lines for the transmis- 

 sion of electric power. In order to show how the voltage 

 variation may be overcome, a short description of the 

 properties of a synchronous motor is necessary. 



A synchronous motor is in itself practically a duplicate 

 of an alternating-current generator. It, therefore, has 

 definite poles and must rotate at the exact speed deter- 

 mined by the frequency of the power circuit to which it 

 is connected. It also has its field excited by direct cur- 

 rent, and the amount of the field current can be con- 

 trolled by a field rheostat. 



A reduction in the field current of a generator weakens 

 the magnetism of the poles and lowers the voltage of the 

 generator. However, weakening the field current of a 

 synchronous motor cannot produce a proportionate de- 

 crease in voltage, because the motor is connected to an 

 A. C. source of power of more or less constant voltage. 

 It is found that when the field current of a synchronous 

 motor is reduced, a lagging quadrature current flows 

 from the A. C. line, which takes the place of the defi- 

 ciency in field current in magnetizing the machine. This 

 current is quite similar to the quadrature or reactive 

 current drawn from the line to magnetize an induction 



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