98 ELECTRICAL ENGINEERING PROBLEMS 



would be the e.m.f. of the transformers using two transformers; 

 also using four transformers? (4 min.) 



9. A 3-phase railway converter is giving 57 kilowatts at 550 

 volts, with an efficiency of 93%; what will be the alternating cur- 

 rent e.m.f.? With a 10 to 1 transformer in the circuit what will 

 be the amperes for each line of a 3-wire transmission? (5 min.} 



10. A single-phase converter gives 100 amperes to the direct- 

 current circuit. Plot the currents in three conductors, one midway 

 between the slip-ring connections, one adjacent to one of these 

 connections, and a third halfway between the other two. (20 min.) 



11. Repeat problem 10, with a 3-phase converter. (20 min.) 



12. Repeat problem 10, with a 4-phase converter. (20 min.) 



13. A single-phase converter gives 100 amperes to the direct- 

 current circuit with a lag of 30 degrees in the alternating current. 

 Plot the currents in each of 3 conductors of a segment of the 

 armature lying between two slip rings. Take the first conductor 

 of the segment, the last conductor and one midway between these. 

 (20 min.) 



14. A 3-phase converter gives 100 amperes to the direct- 

 current circuit, with the alternating current leading 30 degrees. 

 Plot the current in each of 3 conductors of a segment of the 

 armature lying between two slip rings. Take the first conductor 

 of the segment, the last conductor and one midway between these. 

 (20 min.) 



15. A certain four-pole, single-phase, synchronous converter for 

 220 volts on the direct-current side has 568 conductors and 2.63 X 

 10 6 maxwells per pole. At what r.p.m. would it run as an inverted 

 converter and what frequency would it give on its alternating- 

 current end? When the machine gives full-load current with a 

 large lag, the back reaction of the armature cuts down the field flux 

 to 4.1 X 10 5 . Neglecting armature drop, at what speed would the 

 converter tend to run under these conditions, and what would be 

 the consequences? (6 min) 



16. A 60-period, 3-phase converter has in series with it a 

 reactance of 20 ohms with negligible resistance; the line e.m.f. 

 is 1100 volts; what will be the generated direct-current e.m.f. on 

 a load of 20 amperes (alternating current), "when the field is excited 

 to give 30 degrees lag, unity power factor and 30 degrees lead? 

 (10 min.) 



17. A converter has a capacity of 125 kw. when used single- 



