248 PRINCIPLES OF ELECTRICAL DESIGN 



Output of Y-connected machine = 3(E a I a ) 



= 3^X7 

 V3 



The total output is, of course, the same in both cases. If the 

 power factor is not unity, the output, in watts, of the three- 

 phase generator on a balanced load is: 



W = \/ZEI cos 6. 



where 6 is the angle of lag between terminal e.m.f . of a phase 

 winding and current in the winding. The quantity cos 6 is the 

 power factor when both current and e.m.f. waves are sinusoidal. 



Since the magnetic circuit of an alternating-current generator 

 has to be designed for a certain flux to develop a given voltage, 

 while the copper windings must be of sufficient cross-section to 

 carry a given current, the size of the machine will depend upon 

 the product of volts and amperes, and not upon the actual 

 power output. Alternating-current generators are therefore 

 rated in kilovolt-amperes (k.v.a.), the actual output, in kilowatts, 

 being dependent upon the power factor of the external circuit. 



73. Usual Voltages. Owing to the absence of the commu- 

 tator, A.C. machines can be wound for higher voltages than 

 D.C. machines. Large A.C. generators may be wound to give 

 as high a pressure as 16,000 volts at the terminals, but it is 

 rarely economical to develop much above 13,000 volts in the 

 generator; when higher pressures are required, as for long dis- 

 tance power transmission, step-up transformers are used. In 

 this country, a very common terminal voltage for three-phase 

 generators to be used in connection with step-up transformers, 

 is either 2,200 or 6,600 volts, the higher voltage being adopted 

 for the larger outputs, in order to avoid heavy currents in the 

 machine and between the machine and the primary terminals 

 of the transformer. 



74. Pole Pitch and Pole Arc. Although there can be no 

 sparking at the sliding contacts, as in D.C. designs, with their 

 commutation difficulties, the effects of field distortion and demag- 

 netization are apparent in the voltage regulation of alternating- 

 current generators. A very large pole pitch, involving as it 

 does a large number of ampere-conductors per pole, is objec- 

 tionable, and should be avoided if possible. Where it is un- 

 avoidable, a large air gap must be provided in order to prevent 



