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MCQ of D.C motor
1.Effect of armature flux on the main flux in a d.c. motor is that
a) It makes distribution of the flux density across both sections unequal
b) It inclines lines of force through the air gap such that they do not remain radial.
c) Both (a) and (b)
d) None of these
Ans: (c)
2. For a series motor, field flux is
a) Proportional to temperature
b) Constant
c) Proportional to armature current
d) Inversely proportional to armature current
Ans:(c)
3. Speed of d.c. series motor at no load is
a) 1500 rpm
b) 3000 rpm
c) Infinity
d) Zero
Ans:(c)
4. Field flux is constant in
a) Compound motor
b) Shunt motor
c) Series motor
d) None of these
Ans:(c)
5. Function of the commutator in a d.c. machine is
a) To change alternating voltage to direct voltage
b) To change a.c. to d.c.
c) To improve commutation
d) For easy speed control
Ans:(a)
6. Dummy coil in a d.c. machine is used to
a) For mechanical balance of armature
b) Eliminate armature reaction
c) Eliminate reactance voltage
d) None of these
Ans:(a)
7. If back e.m.f. suddenly disappears in a d.c. motor
a) Motor will start action as a generator
b) Motor will stop
c) Windings will burn due to high armature current
d) Nothing will happen
Ans:(c)
8. Four point in d.c. motor is used
a) To increase the field current
b) Not to affect the current through hold on coil even if any change in field current takes
place
c) To increase the field current
d) None of these
Ans:(b)
9. Torque-speed characteristic of a series motor is
a) Parabola
b) Rectangular hyperbola
c) Linear
d) None of these
Ans:(b)
10. Direction of rotation of a d.c. series motor can be changed by interchanging
a) Field terminals
b) Voltage terminals
c) Both (a) and (b)
d) None of these
Ans:(c)
11. Shunt motor has
a) Low speed at high loads and high speed at low loads
b) Low speed at low loads and high speed at low loads
c) Widely varying speed
d) Constant speed
Ans: (d)
12. If a d.c. motor is connected across a.c. supply, the motor will
a) Run at lower speed
b) Burn
c) Run at normal speed
d) Run continuously but for sparking at brushes
Ans: (b)
13. If field current of a shunt motor is changed, then
a) Torque remains constant but h.p. will change
b) Horse power remains constant but torque will change
c) Both remains constant
d) Both will change
Ans: (b)
14. In series-parallel control method when two d.c. series motors are connected in series, speed
of the set is
a) Half of the speed of the motors
b) Rated speed of any of the motors
c) One-fourth of the speed of the motors
d) Same as in parallel
Ans: (c)
15. The torque produced by a series combination of two d.c. series motor is
a) Half the torque when they are connected in parallel
b) Twice the torque when they are connected in parallel
c) Four times the torque when they are connected in parallel
d) Equal to the torque when they are connected in parallel
Ans: (c)
16. Synchronous reactance is defined as reactance
a) Due to armature reaction and leakage of flux both
b) Due to armature reaction of the machine
c) Due to leakage flux
d) Of synchronous machine
Ans: (a)
17. Armature winding of a series motor is excited
a) Resistively
b) Conductively
c) Inductively
d) None of these
Ans: (b)
18. Speed of a d.c. motor depends upon
a) Field flux
b) Armature resistance
c) Applied voltage
d) All of these
Ans: (d)
19. For most of the application purpose
a) Depends on requirement
b) Compound motor is used
c) Shunt motor is used
d) Series motor is used
Ans: (a)
20. Use of the starter in d.c. motors is necessary because
a) They are not self-starting
b) To light the high initial current by inserting high resistance
c) To overcome back emf
d) None of these
Ans: (b)
21. In the field flux method of speed control
a) Any speed can be attained
b) Speeds above normal speed can be attained
c) Speeds below normal speed can be attained
d) None of these
Ans:(b)
22. Speed of the d.c. motor can be varied by
a) Varying armature resistance
b) Varying supply voltage
c) Either (a) or (b)
d) Varying field current
Ans:(c)
23. Dynamic braking is generally used for
a) Shunt motors
b) Compound motors
c) Series motors
d) All of these
Ans:(d)
24. Stalling current is the maximum value of
a) Load current for which speed is zero
b) Field current for which speed is maximum
c) Load current for which speed is maximum
d) Diverter current for which speed is maximum
Ans:(c)
25. Torque developed in d.c. motor depends on
a) Armature
b) Speed
c) Magnetic field
d) Both (a) and (c)
Ans:(d)
26. As the load is increased, speed of d.c. shunt motor will
a) Increase slightly
b) Reduce slightly
c) Remains constant
d) None of these
Ans:(b)
27. Which method of speed control has minimum efficiency ?
a) Field control method
b) Voltage control method
c) Armature control method
d) None of these
Ans:(c)
28. Field flux of d.c. motor can be controlled to
a) Steady speed
b) Speeds above rated speed
c) Speeds below rated speed
d) Speeds above and below rated speed
Ans:(b)
29. In case of regenerative braking, the motor
a) Dissipates energy in armature circuit
b) Dissipates energy in field circuit
c) Both (a) and (b)
d) Supply energy to source
Ans: (d)
30. If plugging is applied to series motor for a long time, then
a) It will start revolving in other direction at low speed
b) Motor will stop
c) Motor will burn
d) None of these
Ans: (a)
31. Disadvantage of the field-control method of speed control is
a) That speed above normal speed can be achieved
b) That commutation becomes unsatisfactory
c) Its low efficiency
d) None of these
Ans: (b)
32. Speed regulation of a d.c. motor can be ideally achieved with
a) A.C. excitation to the field of the motor
b) Variable excitation to the field of the motor
c) constant excitation to the field of the motor
d) no excitation to the field of the motor
Ans: (c)
33. Disadvantage of ward-Leonard system is
a) increased maintenance cost
b) its high initial cost
c) its low efficiency at light loads
d) all of them
Ans: (d)
34. Ward-Leonard method is basically a
a) field diverter method
b) field control method
c) armature resistance control method
d) voltage control method
Ans: (d)
35. In ward-Leonard system, minimum number of machines required are
a) two
b) three
c) four
d) five
Ans: (b)
36. Ward-Leonard method of speed control is basically usefully where usually
a) wide range of sensitive speed control is required
b) wide range of speed control is required
c) narrow range of speed control is required
d) none of these
Ans: (a)
