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Author: Hans Lohninger

  Problem Answer
1 Construct a diagram similar to Fig. 10-21 (a) and (c) for a 4-pole generator.  
2 If the armature of a 50-kw, 230-volt, 4-pole, 1200-rpm lap-wound generator were reconnected to form a wave winding, what would be its new rated voltage at the same speed?  
3 If the lap-wound generator of Problem 2 were 6-pole, at what speed would it have to be driven to generate 230 volts when reconnected to form a wave winding? 400 rpm
4 Would it be necessary to alter the connections of the shunt-field coil of Problem 3 to properly self-excite the generator? 'Explain.  
5 What would be the new kilowatt rating of the generator when reconnected as in Problem 3?  
6 If a self-excited generator failed to build up, and you were required to test for the cause, in what order would you investigate the six causes in the list on page 114?  
7 A certain 4-pole lap-wound generator, rated 50 kw, 125 volts at 1800 rpm, has 89 commutator bars. If each brush covers three bars exactly and a bar is 10 times as wide as a mica strip between bars, what is the average time rate of change of current at full load during its commutating interval?  
8 What must be the per cent of over-compounding of a 220-volt generator to produce 220 volts across a 30-kw load located 500 feet from the generator and supplied through 2/0 conductor?  
9 A certain shunt generator has a no-load voltage of 230 volts, with a field current of 2.75 amperes. In order to maintain that voltage when the load current is 120 amperes, the field current must be increased 20 per cent. How many turns of series-field winding must be added to accomplish the same result by compounding, if each shunt-field coil contains 1200 turns?  
10 At rated speed a generator has a no-load voltage of 230 volts. What will be its no-load voltage, if its field flux is increased 5 per cent and its speed is increased 10 percent?  
11 A separately excited shunt generator has a no-load voltage of 230 volts and a full-load voltage of 215 volts. Its full-load current is 50 amperes. The armature-circuit resistance at full load is 0.2 ohm. How much of the terminal-voltage Joss is caused by armature reaction?  
12 Two 600-volt shunt generators with ratings of 250 kw and 400 kw, respectively, are operating in parallel to carry a 500-kw, 600-volt load divided between them in proportion to their ratings. The voltage regulation of the first generator is 10 per cent, and that of the second is 7.5 per cent. With a declining load, at what ampere load will the two generators deliver equal power? 370 amp, 622 volts
13 What is the full-load rated current of a 1200-kw, 600-volt generator?  
14 What is the load current per conductor at full load in Problem 13, if the armature is lap wound and the generator has 12 poles?  
15 If the shunt-field coils of a lap-wound generator were connected in two or more parallel paths, instead of in series, what would be the effect of unequal resistances of the paths?  
16 A 4-pole, 900-rpm, wave-wound generator has 89 armature coils of 4 turns each. The flux per pole is 107 lines, and 5 per cent of the coils are always under commutation. Calculate the generated emf. 2029 volts
17 If the armature of Problem 16 were connected lap-wound, and all other conditions remained the same, what would be its terminal voltage? 1015 volts
18 If the generator of Problem 16 is rated at 200 kw, what will be the kw rating when the armature winding is changed as suggested in Problem 17?  
19 A 1200-kw generator and a 750-kw generator share a 1500-kw load. How many kilowatts should be carried by each machine?  
20 For a temporary emergency requirement, 25 amperes at 115 volts are needed. A 50-kw, 230-volt, 1200-rpm generator is available. How may this generator be used? Drive at 600 rpm. Reconnect field coils into two parallel circuits, and self-excite as originally.

Last Update: 2011-02-25