THE SYNCHRONOUS MOTOR 323 



85. Use of a Synchronous Motor as a Synchronous 

 Condenser. The carrying capacity of any electrical cir- 

 cuit, in power, depends upon the power factor of the receiv- 

 ing circuit. A certain transmission line, for example, is 

 rated to carry 100 amperes at 60,000 volts and hence, 

 when the current and voltage are in phase with each other, 

 the carrying capacity of this line is 6000 kw. But if the 

 receiving circuit is made up mostly of induction motors 

 and lightly loaded transformers, the current in the trans- 

 mission line may lag as much as 45 behind the voltage. 

 Under these conditions the carrying capacity of the line 

 is equal to 60,000 X 100 X cos 45 = 4240 kw. Now the earn- 

 ing capacity of the transmission line in this case is only 



FIG. 210. A Circuit, Electrically Equivalent to the Armature of an 

 Overexcited Synchronous Motor. 



70.7% of what it is when the carrying capacity is 6000 kw. 

 For we must bear in mind this fact, the customer pays 

 only for the actual power he uses, i.e., El cos <j> and not 

 for the apparent power, EL 



Improvement of the Line Power Factor. It is therefore 

 advantageous for the transmission line owners to contract 

 with customers in such a way as to bring about a high 

 power factor in the line. This can be done if a customer, 

 using a synchronous motor, is persuaded to run the motor 

 with an overexcited field. When such an arrangement is 

 not possible, it generally pays, on long, expensive, trans- 

 mission systems, for the owners to install at the end of the 

 line a synchronous motor. In this case it is likely that the 

 motor will not supply any mechanical power, but that it 



