478 ALTERNATING CURRENTS 



61. A two-phase generator rated at 1,500 kv-a., 2,300 volts, 60 cycles, 

 has two coils. What is the current rating per coil? If this machine 

 supplies a five- wire system show how the wattmeters would be connected 

 in order to measure its output. 



62. Each of the coils of the alternator of problem 61 consists of two sepa- 

 rate sections connected in series. The machine may be then connected 

 in mesh. Determine its voltage and current per terminal under these 

 conditions. 



63. A two-phase four-wire system has a potential difference of 115 volts 

 between adjacent wires giving 163 volts, across diametrically opposite 

 wires. Four resistances of 10 ohms each are connected between adjacent 

 wires, as shown in Fig. 63(A). Determine the total power and the current 

 flowing in each line. 



64. How much current and how much power is supplied by the two alter- 

 nator coils to the load in problem 63? 



QUESTIONS ON CHAPTER V 



1. Why can a rotating field and a stationary armature be used for alter- 

 nators where they cannot be conveniently used for direct-current machines? 

 Give two reasons why it is advantageous for alternators to be of the ro- 

 tating-field type. 



2. What two conditions must a coil of an alternator armature winding 

 fulfill? Compare the wave and the lap winding of alternators with these 

 same types of winding in direct-current machines. 



3. Illustrate by a simple sketch the difference between a half-coil and a 

 whole-coil winding. What is the difference between a single-and a two- 

 layer winding? What are the objections to using one slot per pole? 



4. In what way does the spiral winding differ from the lap and wave 

 windings (barrel type) as regards mechanical disposition of the coils? 

 Why is the interior coil usually omitted? What is meant by "single 

 range"? 



6. Show that a two-phase winding is an extended application of the single- 

 phase winding. How may the chain winding be adapted to two-phase? 

 Why is the two-range feature necessary? State the advantages and the 

 disadvantages of this type of winding. 



6. State the advantages of the lap winding. In the full-pitch lap winding 

 what relation exists between the coil sides in any one slot? 



7. Show that a three-phase winding consists of three single-phase windings 

 properly spaced. 



8 Under what conditions are coils of special shape required in three- 

 phase, two-range, chain windings? 



9. State the advantages and disadvantages of fractional-pitch windings. 



10. Compare the types of stampings required for machines of large and 

 machines of small diameter. Why are there perforations back of the slots 

 in the stampings used for the larger machines? 



11. How are the armature laminations held in position in engine-driven 

 generators? Why is the frame usually of the hollow-box type? 



