CHAP. V] EXCITING AMPERE-TURNS 91 



and the field current of the machine. From the known specifica- 

 tions of the machine this curve can be easily converted into one 

 which gives the useful flux per pole against the ampere-turns per 

 pole as abscissae. The lower part of such a curve is always a 

 straight line, there being then practically no saturation in the iron. 

 ( )n t his part of the curve, practically the whole m.m.f. is consumed 

 in the air-gap, so that the actual permeance of the air-gap is found 

 1)\ dividing one of the ordinates by the corresponding abscissa. 

 The permeance of the simplified air-gap is calculated from eq. 

 and the ratio of the two gives the value of the coefficient k a . This 

 value is then used in the design and calculation of the performance 

 of new machines with similar proportions. Engineering judg- 

 ment and practical experience are factors of considerable impor- 

 tance in estimating the values of k a for new machines. 



The same method of calculating the air-gap ampere-turns is 

 applicable to induction machines (Fig. 23). The ampere-turns 

 are computed, assuming both the rotor and the stator to have 

 smooth iron surfaces, without slots; the result is then multiplied 

 by a factor k a larger than unity, determined from tests upon 

 machines of similar proportions. 



A more accurate, though more elaborate, method for calcula- 

 ting the air-gap ampere-turns is explained in the next article. 



Prob. 14. Calculate the air-gap ampere-turns per pole for a 6600 v. f 

 25-cycle, 375-r.p.m. alternator to be built according to the following 

 specifications: The bore 2.4 m.; the gross axial length of the armature 

 core 55 cm.; seven air ducts 9 mm. each; the minimum air-gap is 15 

 mm.; the maximum air-gap is 30 mm. The poles cover 66 per cent of 

 the periphery; the axial length of the pole shoes is 53 cm. The useful 

 flux per pole at no-load and at the rated voltage is 19.1 megalines. The 

 air-frap factor is estimated to be about 1.15. Ans. 10,600. 



Prob. 15. The no-load characteristic obtained from the test upon 

 tli- machine specified in the preceding problem has a straight 

 that at a field current of 52 amp. the line voltage is 4000 v. Kadi tirKl 

 oil has 120 turns. What is the true value of the air-gap fa 



1.12. 



Prob. 16. A pole shoe is so shaped that the minimum air gap is a, 



an<! the maximum air-gap is a, -o+Ja, the increase in the length being 



proportional to the square of the distance from the renter of the pole. 



What is the length a tq of the equivalent uniform air-gap such that its 



rmeance is the same as that of the given air-gap? Assume a 



smooth lol v armature, ami neglect tin- fringing at the pole-tips. S.hi- 

 I.t the ; ! width <>f the pole be 2u>; then the length of 



