DISADVANTAGES OF VOLTAGE VARIATION 21 



voltage in the 2, 200- volt circuit which supplies a lighting 

 load, and this is usually done by feeder potential 

 regulators. 



It is allowable to have somewhat larger variations in 

 voltage on motor circuits than on lighting circuits. It 

 is the usual practice to supply motor loads and lighting 

 loads from separate local circuits, thus avoiding any 

 noticeable change in brightness of the lamps when mo- 

 tors are started up, and allowing a smaller size of wire to 

 be used for the motor circuits. But it is distinctly dis- 

 advantageous to have more than a small percentage of 

 voltage variation even in a motor circuit. A motor op- 

 erated at a voltage above normal has a saturated iron 

 circuit, with consequently large magnetizing current and 

 heating. If the voltage is lowered below normal, induc- 

 tion motors and synchronous motors are liable to stop 

 or ''pull out" under heavy loads. There is, also, often 

 difficulty in starting them when the voltage is low, since 

 both the starting torque and the pull-out torque are 

 rapidly reduced with a decrease in voltage. 



Let us now consider the disadvantages of voltage 

 variation on transmission lines. Transmission lines 

 generally carry power from a water-power plant to a 

 city. As stated in the preceding paragraphs, the voltage 

 of a city net- work must be maintained as close as possible 

 to a constant value. But the voltage at the receiver end 

 of a transmission line, that is, the end at the city, may 

 sometimes be allowed to vary 5 or 10 per cent, if suitable 

 feeder potential regulators are installed to correct for 

 this variation. Feeder regulators can scarcely be ex- 

 pected to correct for more than the above amount of vari- 

 ation, since they must also compensate for the load drop 



