EXAMPLE OF ALTERNATOR DESIGN 



355 



flux necessary to produce full-load current (700 amp.) in the 

 short-circuited windings, 1,900 ampere-turns per pole are re- 

 quired. This is the excitation which will develop an " apparent" 

 e.m.f . equal to the sum of items (37) and (38) ; the effect of the 

 IR drop being negligible. The armature m.m.f . is almost directly 

 demagnetizing, and the ampere-turns per pole must, therefore, be 

 1,900 + 11,340 = 13,240. The current curve, within the range 

 of the diagram, will be a straight line, and with the full-load ex- 

 citation of 37,000 ampere-turns, the short-circuit current will be 

 1,950 amp., or 2.8 times normal. This is the steady value which 

 the short-circuit current would attain if the field excitation were 

 gradually brought up to full-load value; but at the instant of the 

 occurrence of a short-circuit with full-load excitation, the current 



6 4000 8000 12000 16000 20000 24000 28000 32000 36000 40000 

 Ampere Turns per Pole 



FIG. 146. Curves of open-circuit voltage and short-circuit current 8000 

 x k.v.a. turbo-generator. 



would be limited only by the impedance of the stator windings 

 which must set up a flux of self-induction equal to the total flux. 

 If we neglect the effect of iron saturation and the changes in the 

 paths of the flux leakage lines, the momentary current might be 



700 X -7^5- = 9,150 amp., or 13 times normal-load current. 



It might even be greater than this, depending upon the instan- 

 taneous value of the generated e.m.f. when the short-circuit oc- 

 curs, and the windings should be arranged if possible to with- 

 stand without injury the mechanical forces exerted on the coils 

 under this condition. If this cannot be done, reactance coils 

 external to the machine must be provided; but the tendency 

 to-day is to design machines, even of the largest sizes, with sum- 



