Electrical Machines(DC Generator) Part-01

Electrical Machines(DC Generator) Part-01:

1. Iron losses are considered to be the constant losses. It depends on

a. Voltage
b. Frequency
c. Both (a) and (b)
d. None of these

2. Copper losses in a rotating machine are

a. Variable losses
b. Constant losses
c. Both (a) or (b)
d. None of these

3. Stray losses are the losses which vary with the load but their relationship with load current cannot be identified. Stay losses is maximum in

a. Synchronous machines
b. D.C. machines
c. Induction machines
d. Equal in all types of machines

4. The brush contact losses in a d.c. Machine is

a. Inversely proportional to the square of current
b. Directly proportional to the square of current
c. Inversely proportional to the current
d. Directly proportional to the current

5. Electrical power output in a d.c. generator is equal to

a. Electrical power developed in armature – copper losses
b. Mechanical power input – iron and friction losses
c. Electrical power developed in armature – iron and copper losses
d. Mechanical power input – iron and friction losses – copper losses

6. Mechanical power developed in armature of a d.c. motor is given by

a. Electrical power input to motor – copper losses
b. Motor output + iron and friction losses
c. Both (a) & (b)
d. None of these

7. Out of electrical, mechanical and magnetic losses, the losses which is minimum is

a. All are equal
b. Electrical losses
c. Magnetic losses
d. Mechanical losses

8. The hysteresis motor are mainly used in tape recorders because

a. Of its extremely steady torque
b. Constant speed
c. Reduced initial current
d. None of these

9. If % η_m is the mechanical efficiency of a DC machine and % η_e is the electrical efficiency. Then the overall efficiency of DC machine is given by

a. % η _m + % η _e
b. % η _m X % η _e
c. (% η _m + % η _e) / % η _m
d. (% η _m + % η _e) / % η _e

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

a. √(P_i / R_a)
b. √(R_a / P_i)
c. √(P_i / R²_a)
d. √(R_a / P²_i)

11. 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

12. A d.c. shunt generator delivers 190 A at a terminal voltage of 220 V. The copper losses and stray losses are 2000 W and 1000 W respectively. The efficiency of the generator is

a. 91.35 %
b. 92.60 %
c. 93.30 %
d. 94.23 %

13. 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

14. A 120 V d.c. shunt motor runs at speed of 1200 rpm. When the motor is operated unloaded but an additional resistance of 4 ohm is connected in series with the shunt field, then the speed increases and reaches to 1370 rpm with the same terminal voltage. The value of series resistance is

a. 28.23 ohm
b. 32.48 ohm
c. 35.72 ohm
d. 36.82 ohm

15. 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 %

16. Out of different methods available for testing of d. c. motors, Swinburne’s test and Hopkinson’s test are commonly used in practice on

a. Shunt generators
b. Series motors
c. Shunt motors
d. All of these

17. Swinburne’s test and brake tests

a. Both are direct method of testing
b. Direct method of testing, indirect method of testing
c. Indirect method of testing, direct method of testing
d. Both are indirect method of testing

18. Swinburne’s test can be performed at

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a. Any load
b. Only no load
c. Only full load
d. Only half load

19. While performing Swinburne’s test, the iron losses are assumed to be

a. Constant
b. Absent
c. Variable
d. None of these

20. The speed in d.c. machine can be measured by using

a. Anemometer
b. Tachometer
c. Voltmeter
d. Ammeter

21. While carrying out brake tests if the belt snaps, then the motor will

a. Rotate at reduced speed but in forward direction
b. Rotate at increased speed but in forward direction
c. Rotate at reduced speed but in backward direction
d. Rotate at increased speed but in backward direction

22. While performing retardation tests, the machine whose test is to be taken is run at a speed which is

a. Slightly less than its rated speed
b. Equal to its rated speed
c. Slightly greater than its rated speed
d. All of these

23. The rotational or stray losses includes

a. Iron losses only
b. Iron losses, friction and windage losses
c. Iron losses, copper losses, friction and windage losses
d. None of these

24. The rotational losses in d.c. machines is equal to the

a. Kinetic energy of armature
b. Half of the kinetic energy of armature
c. Square of the kinetic energy of armature
d. Rate of change of kinetic energy

25. In regenerative test, the supply is given to the motor