480 ALTERNATING CURRENTS 



69. Repeat problem 68 for a three-phase winding. Show the windings 

 of the three phases each in a different color. 



70. An eight-pole alternator has 80 slots. Draw a two-phase, ^-pitoh, 

 lap winding for this machine. 



71. A four-pole alternator has 48 slots. Design a three-phase, full- 

 pitch, lap winding, showing the connections of one phase only. 



72. Repeat problem 71 for a %-pitch winding. 



73. Repeat problem 71 for a full-pitch, wave winding. 



74. A four-pole, 60-cycle, single-phase alternator has a concentrated 

 winding similar to that shown in Fig. 102, page 102. There are eight conduc- 

 tors per slot and 2,800,000 magnetic lines per pole. Assuming that the flux 

 distribution under the pole is practically sinusoidal, determine the electro- 

 motive force of this generator. 



75. A six-pole, 25-cycle, two-phase alternator has one slot per pole per 

 phase and 12 series-connected conductors per slot. Determine the induced 

 volts per phase if there are 3,000,000 lines per pole. Assume that the flux 

 wave is practically sinusoidal. 



76. A single-phase, four-pole, 1,800 r.p.m. alternator has eight slots per 

 pole. Only half of these slots are occupied by the winding so that the 

 breadth factor, 0.907, is the same as that of a two-phase winding having 

 four slots per pole per phase. There are four series-connected conductors 

 in each slot and the winding is full pitch. There are 3,000,000 lines per 

 pole and the flux may be assumed to be distributed sinusoidally. Deter- 

 mine the electromotive force of this alternator. 



77. A three-phase, 12-pole, 600 r.p.m. alternator has 12 slots per pole 

 and a full-pitch winding having six series-connected conductors per slot. 

 There are 3,500,000 lines per pole. What is the open-circuit terminal elec- 

 tromotive force, if the machine is Y-connected? 



78. Repeat problem 77 for a %-pitch winding. 



79. An alternator is rated at 6,000 kw. at 70 per cent, power-factor, 

 (a) What is its kilo volt-ampere rating. (6) How many kilowatts can it 

 safely deliver at unity power-factor? (c) If it. has an efficiency of 95 per 

 cent, at 70 per cent, power-factor, what should be the rating of its prime 

 mover in horsepower? (d) If the prime mover speed is 1,200 r.p.m. what 

 torque does it develop? 



80. A three-phase, 60-cycle, 13,200-volt alternator is rated at 20,000 

 kw. at 70 per cent, power-factor, (a) What is the current per terminal? 

 (6) What would be the safe current per terminal if the power-factor were 

 unity? (c) What would be its kilowatt rating at unity power-factor? (d} 

 At 70 per cent, power-factor, what should be the approximate rating of its 

 prime mover in horsepower? 



QUESTIONS ON CHAPTER VI 



1. Why is the question of the regulation of alternators more important 

 than the regulation of direct-current generators? What factor other than 

 the magnitude of the current determines the regulation of alternators? 

 Why is it usually not desirable to determine the regulation of alternators 

 by actual loading? 



