252 PRINCIPLES OF ELECTRICAL DESIGN 



machines. Higher densities are used in some steam-turbine- 

 driven machines, with a view to reducing the size of the rotor. 

 A good system of forced ventilation is then imperative. 



77. Length of Air Gap. Inherent Regulation. In A.C. ma- 

 chines, just as in D.C. machines, the length of air gap should 

 depend upon the armature m.m.f and therefore on the pole 

 pitch, r, and the specific loading, q. In salient-pole machines, 

 the air gap will not be of constant length, but will increase from 

 the center outward, in order to produce the required distribution 

 of flux. A practical method of shaping the pole face will be 

 explained later. The clearance to be allowed between pole 

 face and armature surface at the center of the pole may be de- 

 termined approximately by making it of such a length that the 

 open-circuit field ampere-turns shall be not less than 1.25 to 1.75 

 times the full-load armature ampere-turns. In large turbo- 

 alternators this ratio may be as low as 1 to 1.5, in order to reduce 

 the weight of copper on the rotor, and keep the short-circuit 

 current within reasonable limits. The distribution of armature 

 m.m.f. will be discussed later; but, for the purpose of estimating 



the air gap, the ampere-turns per pole may be taken as ~- 



i 



In no case should the air gap be less than one-third to one-half 

 the slot opening. 



A large air gap has the effect of improving the regulation of 

 the machine; but otherwise it is objectionable, seeing that it 

 leads to increased magnetic leakage and higher cost, due mainly 

 to the greater weight of copper in the field coils. 



The inherent regulation of a generator, at any given load, may 

 be defined as the percentage increase in terminal voltage when 

 the load is thrown off; the speed and field excitation remaining 

 constant. Owing to the low power factors resulting from the 

 connection of induction motors on alternating-current circuits, 

 it is practically impossible to design a generator of which the 

 inherent regulation is so good that auxiliary regulating devices 

 are unnecessary. It is, therefore, uneconomical to aim at very 

 good inherent regulation, especially as efficient automatic field 

 regulators are now available. The inherent regulation of com- 

 mercial machines usually lies between 5 and 9 per cent, at full 

 load on unity power factor, while it may easily be 20 per cent., 

 or higher, on 85 per cent, power factor, with normal full-load 

 current taken from the machine. This very marked effect of 



