UNIT-6: D.C.MOTORS
1. In a DC machine, the number of commutator
segments is equal to [c
]
(a)number of conductors (b)number of poles (c)number of
coils (d)both a and b
2. Main advantage of differentially compound
motor is that [c ]
(a)stable under heavy loads (b)speed increases to very high
values on no load
(c)speed remains constant if load is not excessive (d)start
under heavy load
3. Which of the following motor should never be
started on no load [ d ]
(a)shunt (b)compound cumulative (c)compound differentially
(d) series
4. The yoke of a dc machine is made up of [ d ]
(a)silicon steel (b)soft iron (c)aluminium (d)cast steel
5. The value of back emf in a dc motor is maximum
at _______ [ a ]
(a)no load (b)full load (c)half full load (d)one fourth
full load
6. The brushes that carry the current
to the load are made up of Carbon
7. Power lost as heat in the armature and field windings of a
dc machine is called copper loss
8. Series dc motor should never be started on
no load.
9. The relation between speed, flux and
backemf in a dc motor is NαEb/Ф
10. The field winding of a d.c shunt machine
usually carries ______ of the rated current of the machine [ a]
(a) 2% to 5% (b) 15% to 20% (c) more than 20% (d) less than
0.5%
11. The speed of a d.c. motor can be controlled by
changing [ d ]
(a) its flux (b) armature circuit resistance
(c) applied voltage (d) all of the above
12. By putting controller resistance in series with the
armature of a d.c motor, we can obtain speeds
[ b]
(a) above the normal speed only (b) below the normal speed
only
(c) above as well as below the normal speed (d) none of the
above
13. The only
disadvantage of field control method for controlling the speed of a d.c shunt
motor is that it [ c]
(a) give speeds lower than the normal speed (b) is wasteful
(c) needs a large rheostat (d) adversely affects
commutation
14. The motor terminal voltage equation
is given by V = Eb+ IaRa
15. The most economical method of
finding no-load losses of a large d.c. machine is swinburne’s test.
16. The Commutator in a dc machine can be convert
[ c]
(a) ac to
dc (b) dc to ac (c) both a and b (d) None of the above
17. Fleming's left hand rule is applicable for: [b ]
(a) dc
generator (b) dc motor (c) alternator (d) Transformer
19.. The armature torque of the dc shunt motor is
proportional to [a ]
(a) armature current only (b) field flux only (c) armature
current and flux both (d) None
20.. Motor which should not run at no-load is [a ]
(a) Series motor (b) shunt motor (c) compound motor (d)
induction motor
21. Yoke is made of which material Cast iron or Cast steel
22 Losses in DC machine are Iron losses,Cu losses,
Mechanical losses
23. Types of DC motors , D.C series motor,
d.c shunt motor
24. In DC shunt motor the Torque is directly proportional
to Ia
25. High permeability magnetic materials helps [a ]
A) To confine the flux within the magnetic circuit. B) To
allow the flux to leak
C) In producing more current. D) None of the
26. A Commutator in a DC machine can convert ____
to ______ [ c]
(a) AC, DC (b) DC, AC (c) either ‘A’ or ‘B’ (d) None
27. The current flowing through armature
conductors of a dc motor is _____________ [c ]
a) Pulsating b) Dc c) Ac d) none of these
28.. In Swinburne’s test of a Dc machine [ b]
a) No load losses are calculated and
copper losses are measured
b) No load losses are measured and
copper losses are calculated
c) Both the losses are measured
d) Both the
losses are calculated
29. The supply terminals for a dc shunt motor are
reversed. What will be effect on the motor [b ]
a) It will run as Dc generator b) runs in reverse direction
c)it stops d)its burns
30. The starting resistance for a dc
shunt motor Low
31. A 3 point starter is considered
suitable for dc Shunt motor
32. The magnitude of force acting on a current
carrying conductor placed in a magnetic field is
independent of [ c ]
A) Flux density B) length of conductor
C) cross-sectional area of conductor D) current flowing
Through the conductor
35. When a dc machine is connected in the dc
supply main it will produce [ d]
A) Emf in opposition to applied voltage. B) Unidirectional
torque
C) Copper losses in the windings D) all of the above
36. Emf induced in each conductor of a DC Machine
is [a ]
A) An alternating emf B) A direct emf
C) A pulsating emf D) Emf of random waveshape
37. The starting torque of a dc shunt motor is [b ]
A) Zero B) low C) high D) very high.
38. The armature of a dc machine is a
laminated to reduce Eddy current losses
39. If the flux of a dc motor approaches
zero its speed will Infinite
40. The direction of mechanical force
experienced on a current carrying conductor placed in a magnetic Field is
determined by Fleming’s left hand rule
41. The function of interpoles in a dc
machine is to improve commutation
42 The speed of a DC motor is directly
proportional to its Back emf and
inversely proportional
To Flux
43. The current drawn by a dc motor at
the starting instant will be High
44. The eddy current losses is directly
proportional to [ b ]
a) f b)f^2 c)f^1.6 d)f^3
45. The iron losses [ d]
a) core losses b) eddycurrent losses c) hystersis losses d)
all the above
46.. The speed of a d.c motor can be controlled by
varying [d ]
a)its flux per pole b)resistance of armature circuit c)applied
voltage d)all the above
47.. The collection of current from
armature conductors to the external load is the function of Commutator and brushes
48. The iron or core losses are also
known as Constant losses
49. In d.c motor unidirectional torque
is produced with the help of-Brushes-and
commutator.
50 Under constant load conditions, the
speed of a d.c motor is affected by V, Ra, Ø
51. The Eb/V ratio of a d.c motor known
as Efficiency
52. An invertor is employed to convert DC voltage to AC voltage
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