10 CONSTANT- VOLTAGE TRANSMISSION 



motor, and the nature of this current will be quite gen- 

 erally understood when it is stated that a synchronous 

 motor with a weak field current has a similar effect on 

 an A. C. line to the effect of a low power-factor induction 

 motor. It may be mentioned that the lagging reactive 

 current described in this paragraph has the usual effects 

 noticed with low power-factor loads, of increased drop 

 in the A. C. lines, increased current in the transformers 

 and generators supplying the lines, and increased dif- 

 ficulty in holding up the voltage of the generators. 



When a very large field current is used with a syn- 

 chronous motor, a quadrature demagnetizing current 

 flows from the line, so as to keep the magnetic field of 

 the motor at the correct value for the line voltage. This 

 current is, of course, directly opposite to the magnetizing 

 current which flows when the motor is under-excited. It 

 is, therefore, a leading current, and can counteract the 

 bad effects of low power-factor induction motors con- 

 nected to the system. It decreases the voltage drop in 

 the line, and raises the power-factor at the generators. 



When a reactance is connected across an A. C. line, 

 the current which flows is called a lagging current, since 

 it lags behind the line E. M. F. This current is the same 

 as the current which flows when a synchronous motor 

 with a weak field is connected across the line, So, also, 

 when an ordinary condenser, built up of parallel plates, 

 is connected across an A. C. line, the current leads the 

 line E. M. F. and is called a leading current, and this is 

 the same as the current which flows when a synchronous 

 motor with a very strong field is connected across the 

 line. To avoid confusion, in all cases in this book a 

 synchronous motor will be spoken of as having a lagging 



