DC machine MCQ #3

1) In a D.C. generator the ripples in the direct e.m.f. generated are reduced by using

              A) filters.

              B) carbon brushes of superior quality.

              C) commutator with large number of segments.

              D) using equaliser rings.

2) A 6 pole wave wound d.c. generator has 460 armature conductors carrying armature current of 180 A. The armature reaction demagnetizing and cross magnetizing ampere turns per pole if brushes are shifted by 6 degree electrical from G.N.A are

              A) 3220, 230

              B) 3220, 260

              C) 260, 3220

              D) 230, 3220

3) A 120 V shunt generator running at 850 rpm has its armature and shunt field resistance of 0.15 ohm and 50 ohm respectively. It supplies 200 lamps each rated at 60 W, 100 V. The friction and windage  and core loss of the machine is 400 W. its armature copper loss on full load and shunt field loss is

              A) 2156.7 W, 200 W

              B) 2232.6 W, 200 W

              C) 2156.7 W, 240 W

              D) 2232.6 W, 240 W

4) A machine operating as motor may go into regenerative braking mode if its speed becomes

              A) Sufficiently high to make back emf greater than supply voltage

              B) Sufficiently low to make back emf greater than supply voltage

              C) Sufficiently high to make back emf half of the supply voltage

              D) Sufficiently low to make back emf half of the supply voltage

5) The torque constant (Km), of a permanent magnet d.c. motor is 0.25 V sec / radian and armature resistance is 2 ohm. If the applied armature voltage is 90 V, then its zero speed torque is 

              A) 10.45 N-m

              B) 11.25 N-m

              C) 13.65 N-m

              D) 15.56 N-m

6) In electric traction, which type of motor is generally used?

              A) Shunt motor

              B) Series motor

              C) Cumulative compound motor

              D) Differential compound motor

7) A 4 pole 240 V, d.c. series motor has a wave wound armature with 180 conductors. The armature resistance and the field winding resistance are 0.10 ohm and 0.2 ohm respectively. If the motor is drawing 40 A current at 0.015 Wb  flux per pole, then the speed under this condition is

              A) 2533.3 rpm

              B) 2644.4 rpm

              C) 2756.9 rpm

              D) 2987.6 rpm

8) For d.c. series motor, if Ia is the armature current then the torque developed is

              A) α Ia

              B) α Ia^2

              C) α (1 / Ia^2)

              D) α (1 / Ia)

9) A shunt generator can self-excite

              A) Only if the resistance of the field circuit is less than critical value

              B) Only if the resistance of the field circuit is greater than critical value

              C) Irrespective of the value of the resistance in the field circuit

              D) None of the above

10) 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

| (1)-C | (2)-D | (3)-B | (4)-A | (5)-B | (6)-B | (7)-A | (8)-B | (9)-A | (10)-C |

11) If the no load speed of d.c. motor is 1350 rpm and full load speed is 1150 rpm, then its voltage regulation is

              A) 11.56 %

              B) 15.36 %

              C) 17.39 %

              D) 19.39 %

12) A 220 V, d.c. motor draws an armature current of 20 A. Its armature resistance is 0.6 ohm. Then the induced emf in the motor will be

              A) 195 V

              B) 202 V

              C) 208 V

              D) 215 V

13) In d.c. machines, equalizer connections are provided in

              A) Only wave winding

              B) Only lap winding

              C) Both wave winding and lap winding

              D) None of these

14) The terminal voltage of a series generator is 150 V when the load current is 5 A If the load current is increased to 10 A, the terminal voltage will be

              A) 150 V

              B) Less than 150 V

              C) Greater than 150 V

              D) None of the above

15) For neutralising the demagnetising effect of armature reaction, an extra number of winding turns are provided on each pole. To calculate number of extra turns per pole the equation used is

              A) (ATd / I) * leakage coefficient

              B) (ATd / I) / leakage coefficient

              C) Leakage coefficient / (ATd / I)

              D) (ATd / I)

16) If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by

              A) Blv

              B) Blv2

              C) Bl2v

              D) Bl2v2

17) A 240 V, 17 kW d.c. shunt motor draws an armature current of 80 A at full load. The armature and shunt field resistances are 0.2 ohm and 195 ohm respectively. The rotational losses and efficiency of motor at full load is

              A) 87.20 %

              B) 89.32 %

              C) 89.67 %

              D) 90.03 %

18)  A 4 pole d.c. shunt generator having a wave winding supplies 45 lamps, each of 50 W at 100 V. The armature and field resistance are 0.15 ohm and 50 ohm respectively. The current in armature conductor is

              A) 11.25 A

              B) 11.50 A

              C) 12.25 A

              D) 13.50 A

19) The current flowing through the armature of a d.c. shunt machine at maximum efficiency is given by

              A) √(Pi / Ra)

              B) √(Ra / Pi)

              C) √(Pi / R2a)

              D) √(Ra / P2i)

20) For the parallel operation of two or more D.C. compound generators, we should ensure that

              A) Voltage of the incoming generator should be same as that of bus bar

              B) Polarity of incoming generator should be same as that of bus bar

              C) All the series fields should be run in parallel by means of                                        equaliser connection

              D) Series fields of all generators should be either on positive side or                            negative side of the armature

| (11)-C | (12)-C | (13)-B | (14)-C | (15)-A | (16)-A | (17)-A | (18)-C | (19)-A | (20)-D |

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