Download unit-ii dc generators and armature reaction, commutation

UNIT-II
DC GENERATORS AND ARMATURE REACTION,
COMMUTATION
1. D.C. series generator is used
(a) To supply traction load
(b) to supply industrial load at constant voltage
(c) Voltage at the toad end of the feeder
(d) for none of the above purpose
2. Following D.C. generator will be in a position to build up without any residual
magnetism in the poles
(a) Series generator
(b) shunt generator
(c) compound generator
(d) none of the above
3. Interpole flux should be sufficient to
(a) neutralise the commutating self induced e.m.f.
(b) neutralise the armature reaction flux
(c) neutralise both the armature reaction flux as well as commutating e.m.f. induced in
the coil
(d) Perform none of the above functions
4. D.C. generator generally preferred for charging automobile batteries is
(a) Series generator
(b) shunt generator
(c) long shunt compound generator
(d) any of'the above
5. In a D.C. generator the number of mechanical degrees and electrical degrees will be
the same when
(a) r.p.m. is more than 300
(c) number of poles is 4
(b) r.p.m. is less than 300
(d) number of poles is 2
6. In D.C. generators the polarity of the interpoles
(a) is the same as that of the main pole ahead
(b) is the same as that of the immediately preceding pole
(c) is opposite to that of the main pole ahead
(d) is neutral as these poles do not play part in generating e.m.f.
7. The e.m.f. generated in a D.C. generator is directly proportional to
(a) flux/pole
(b) speed of armature
(c) number of poles
(d) all of the above
8. In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral
axis, when
(a) there is no load on|he generator
(b) the generator runs on full load
(c) the generator runs on overload
(d) the generator runs on designed speed
9. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the
coil is neutralised by all of the following except
(a) interpoles (b) dummy coils (c) compensating winding
(d) shifting of axis of brushes
10. In D.C. generators on no-load, the air gap flux distribution in space is
(a) sinusoidal
(b) triangular
(c) pulsating
(d) flat topped
11. A shunt generator running at 1000 r.p.m. has generated e.m.f. as 200 V. If the speed
increases to 1200 r.p.m., the generated e.m.f. will be nearly.......
(a) 150 V
(b) 175 V
(c) 240 V
(d) 290 V
12. If a shunt generator fails to build up any voltage at no-load, the reason may be
that……
a) there is no residual magnetism
b) field coil may be connected in reverse direction
c) its field resistance is more than the critical resistance
d) all or any one of the above
13. If a D.C. generator fails to build up the probable cause could not be
(a) imperfect brush contact
(b) field resistance less than the critical resistance
(c) no residual magnetism in the generator
(d) faulty shunt connections tending to reduce the residual magnetism
14. Flashing the field of D.C. generator means
(a) neutralising residual magnetism
(b) creating residual magnetism by a D.C. source
(c) making the magnetic losses of forces parallel
(d) increasing flux density by adding extra turns of windings on poles
15. The e.m.f. induced in the armature of a shunt generator is 600 V. The armature
resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be
(a) 640 V
(b) 620 V
(c) 600 V
(d) 580 V
16. In a D.C. generator the critical resistance refers to the resistance of
(a) brushes
(b) field
(c) armature
(d) load
17. Which of the following statement about D.C. generators is false?
(a) Compensating winding in a D.C. machine helps in commutation
(b) In a D. C. generator interpoles winding is connected in series with the armature
winding
(c) Back pitch and front pitch are both odd and approximately equal to the pole pitch
(d) Equilizing bus bars are used with parallel running of D.C. shunt generators
18. A cumulatively compounded long shunt generator when operating as a motor will be
(a) cumulatively compounded long shunt
(b) differentially compounded long shunt
(c) cumulatively compounded short shunt
(d) differentially compounded short shunt
19. In a D.C. generator in case the resistance of the field winding is increased, then output
voltage will
(a) increase
(b) decrease
(c) remain unaffected
(d) fluctuate heavily
20. In case of a flat compounded generator
(a) voltage generated is less than the rated voltage
(b) generated voltage is proportional to the load on the generator
(c) voltage remains constant irrespective of the load
(d) speed varies in proportion to the load on the generator
21. Which of the following generator will have negligible terminal voltage while running
on No load?
(a) Series generator
(b) Shunt generator
(c) Compound generator
(d) Separately excited generator
22. In over compounded generator, full load terminal voltage is
(a) almost zero
(b) less than noload terminal voltage
(c) more than noload terminal voltage
(d) equal to no-load terminal voltage
23. In a level compounded D.C. generator, full load terminal voltage is
(a) Negligibly low
(b) equal to no-load terminal voltage
(c) More than no-load terminal voltage
(d) less than no-load terminal voltage
24. The terminal voltage of a D.C. shunt generator drops on load because of all of the
following reasons except
(a) armature reaction
(b) armature resistance drop
(c) Field weakening due to armature reaction and armature
(d) commutation
25. The voltage regulation of an over compound dc generator is always
(a) Positive
(b) negative
(c) zero
(d) high
26. Which generator has poorest voltage regulation?
(a) Series
(b) shunt
(c) compound
(d) high
27. Compensating windings are used in D.C. generators
(a) Mainly to reduce the eddy currents by providing local short-circuits
(b) To provide path for the circulation of cooling air
(c) To neutralise the cross-magnetising effect of the armature reaction
(d) None of the above
28. The e.m.f. generated by a shunt wound D.C. generator isE. Now while pole flux
remains constant, if the speed of the generator is doubled, the e.m.f. generated will be
(a) E/2
(b) 2E
(c) slightly less than E
(d) E
29. Write the methods for improving the commutation process?
30. Write the formula for cross magnetizing ampere turn per pole?
31. Write the formula for cross magnetizing ampere turn per pole?
32. Write the function of interpoles used in dc machines?
33. Write the function of Compensating wnding used in dc machines?
34. Where is the location of interpoles placed in dc machines?
35. Where is the location of Compensating wnding placed in dc machines?
36. Define Armature reaction?
37. Define the Commutation?
38. Write the formula for reactance voltage?
39. Define the critical speed?
40. Define the critical resistance?
5 Mark questions
1. A short-shunt compound generator delivers a load current of 30A at 220V, and has
a armature, series-field resistance of 0.05Ω, 0.3Ω and 200Ω respectively.
Calculate the induced emf and the armature current. Allow 1V per brush for
contact drop.
2. A 24 KW, 250V, 1600 rpm separately excited d.c. generator has armature circuit
resistance of 0.1Ω. The machine is first run at rated speed and the field current is
adjusted to give an open circuit voltage of 260V. Now, when the generator is
loaded to deliver its rated current, the speed of the driving motor is found to be
1500 r.p.m compute the terminal voltage of the generator under these conditions.
Field flux remains unaltered.
3. A 4-pole generator supplies a current of 143A. It has 492 armature conductors. (i)
Lap –wound (ii) Wave wound. When delivering full load, the brushes are given an
actual lead of 100. Calculate the demagnetizing AT/pole. This field winding is
shunt connected and takes 10 A. Find the number of extra shunt field turns
necessary to neutralize this diamagnetism.
4. Define the armature reaction and write the effects of armature reaction?
5. Explain the compensating winding in briefly?
6. Explain the interpole winding in briefly?
7. Explain briefly about different excitation methods used in the DC generator.
8. Write the types of generators & explain?
9. Explain about the cross magnetizing and De magnetizing effect?
10.Explain the process of self excitation in D.C. shunt generator.
11.What are the conditions for voltage build-up of a shunt generator?
12.Explain the process of self excitation in D.C. series generator.
10 Mark questions
1. The magnetization characteristic for a 4-pole, 110V, 1000 rpm shunt generator is as
follows:
Field
current
0 0.5
1
1.5
2
2.5
3A
O.C
voltage
5 50
85
102 112 116 120V
Armature is lap connected with 144 conductors. Field resistance is 45 Ω.
Determine
i) Voltage the machine will build up at no load.
ii) the critical resistance
iii) the speed at which the machine just fails to excite
iv) Residual flux per pole
2. Define commutation & discuss the methods of improving commutation.
3. (i)A 4-pole, lap wound armature running at 1500 rpm delivers a current of 150A and has
64 commutator segments. The brush spans 1.2 segments and inductance of each
armature coil is 0.05mH. Calculate the value of reactance voltage assuming Linear
commutation.
(ii) What is meant by commutation process in dc machine? How is voltage commutation
achieved in dc machine? Why carbon brushes are used for commutation process?
4. Explain with suitable diagram, the armature reaction in dc machine. How does it affect
the main field flux? Define GNA and MNA.
5. Derive the Expressions for cross magnetizing and De magnetizing ampere turns per
pole?
6. Write the types of commutations and explain them briefly?