28 PROBLEMS IN ALTERNATING CURRENT MACHINERY 



(b) Compare this e.m.f. with what it would have been had the 

 8 inductors per pole been concentrated in one slot. 



26. With normal excitation the resultant air-gap flux in a 60- 

 cycle alternator is 10 6 lines per pole. The equation of the curve 

 which represents the flux density in the air-gap is 



= i sn x-- s sn x 



x is the electrical angle measured from a point midway between 

 the poles. Take B 3 = 0.3#i. 



(a) If the armature winding consists of 3 equally spaced 

 inductors per pole connected so as to give a winding which has a 

 spread of one-third the pole pitch and a coil pitch of 1.0, what is 

 the generated armature voltage per pole? What is the form 

 factor of this e.m.f.? (b) Compare this e.m.f. and form factor 

 with what they would have been had the winding been concen- 

 trated in one slot per pole. 



27. A single-phase turbo-alternator is rated to deliver 1000 

 kv.-a. at 5200 volts when driven at 1500 rev. per min. The 

 armature has 48 slots, 8 of which, or 2 per pole, carry no inductors, 

 so that the spread of the winding is 0.83. In each of the other 

 40 slots there are 4 inductors in series. What is the no-load 

 terminal voltage when the air-gap flux is 40 megalines per pole 

 and the speed is 1510 rev. per min.? 



28. A single-phase alternator is rated to deliver 250 kv.-a. 

 at 2200 volts when driven at 375 rev. per min. The field struc- 

 ture has 24 poles and the armature, 240 slots. The spread of the 

 armature winding is 0.6 and all of the inductors are connected in 

 series. The coil pitch is one. In any belt each of the four cen- 

 tral slots contains 3 inductors while each of the outer slots con- 

 tains 2 inductors. What is the no-load terminal voltage when the 

 air-gap flux is 3.9 megalines per pole and the speed is 370 rev. per 

 min.? 



29. A two-phase water turbine driven generator is rated to 

 deliver 900 kv.-a. at 5000 volts. The field structure has 46 poles 

 and the armature has 2 slots per pole per phase. There are 11 

 inductors in series per slot. The coil pitch is one. What is 

 the no-load terminal voltage when the air-gap flux is 5.4 megalines 

 per pole and the generator is driven at its normal speed of 120 

 rev. per min.? 



30. A 2-phase, 60-cycle, engine driven alternator is rated to 

 deliver 3500 kv.-a. at 11,500 volts. The armature has 576 



