DESIGN OF ALTERNATORS 



253 



a low power factor will be explained later in detail; but it may 

 be stated here that the effect of a lagging armature current 

 is very similar to that of a change of brush position in a con- 

 tinuous-current dynamo, causing the armature ampere-turns 

 which on unity power factor have merely a distorting effect 

 to become partly demagnetizing. 



Not only must the armature be weak relatively to the field, 

 but the inductance of the armature windings should be small 

 if the inherent regulation is to be good. Thus the regulating 

 qualities of an alternating-current generator depend on both 

 armature reaction and armature reactance; but since these cannot 

 be made so small as to dispense entirely with external regulating 

 devices, the designer rarely aims at getting very good inherent 

 regulation. 



With steam-turbine-driven machines, in which the pole pitch 

 is always very large, the air gap frequently exceeds 1 in. in 

 length. The writer knows of a machine, designed for an output 

 of 5,500 k.v.a. at a speed of 1,000 revolutions per minute, and 

 a frequency of 33 J^, with the single air gap 3J4 in. long. Whether 

 or not the designer was justified in trying to obtain satisfactory 

 regulation by this costly and somewhat crude expedient is at 

 least questionable. Machines of three times this output are 

 now built with air gaps from 1 in. to 1J4 in. long. 



Good inherent regulation means that the current on short- 

 circuit may be very large, and this is sometimes objectionable. 

 With the exception of high-speed, steam-turbine-driven units, 

 the short-circuit current in modern A.C. generator (with full- 

 field excitation) is about three to five times the normal full-load 

 current; but in connection with the larger units, and on systems 

 dealing with large amounts of energy, power-limiting reactances, 

 external to the generator, are usually installed to prevent the 

 current attaining a dangerous value before the automatic circuit- 

 breakers have had time to operate. Many of the largest units, 

 driven at very high speeds by steam turbines, are now purposely 

 designed with large armature reaction and highly inductive 

 windings, in order that they may be able to withstand momen- 

 tary short-circuits without mechanical injury; but notwith- 

 standing these features of recent introduction, the momentary 

 short-circuit current in some of the 20,000 to 30,000 k.v.a. units, 

 may be of the order of 15 to 20 times the normal full-load current. 



For certain electro-metallurgical work, or electric smelting, 



