DEPT.
OF ELECTRICAL & ELECTRONICS ENGINEERING
SRM UNIVERSITY, Kattankulathur – 603 203
COMPREHENSION - I
Class : IV Sem Date: 25-02-12
Coverage : Electrical Machines – I, II Time: 90min
1. A magnetic core which is ac – excited should be made of silicon steel because it
a) reduces hysteresis loss b) reduces eddy current loss
c) reduces both hysteresis and eddy current loss d) has high permeability
2. The magnetic field is employed as the medium for electromechanical energy conversion rather
than electric field because
a) specific dielectric loss is more than specific magnetic loss
b) stored energy density for practicable field strength is low in electric field
c) stored energy density for practicable flux density is low in magnetic field
d) electric field presents insulation difficulties
3. The no load current in a transformer with respect to the primary voltage
a) leads by 90o b) lags by 90 o
o
c) leads by slightly less than 90 d) lags by slightly less than 90 o
4. Why is it that in a dc m/c, field polar cannot form the rotor and armature a stator?
a) As field poles are projecting, they will be subjected to strong centrifugal forces on the rotor
b) Such a construction will require higher field current for a certain air gap field
c) Such a construction will make the air gap flux sinusoidal, which is not desirable for a dc
machine.
d) The commutator which is connected to the armature winding is a rotating mechanical rectifier
and the armature must be a rotor.
5. According to Fleming’s left hand rule, when the fore finger points in the direction of the field or
flux, the middle finger will point in the direction of
a) current in the conductor b) movement of the conductor
c) resultant force on the conductor d) none of the above
6. What will happen if the back e.m.f of a DC motor vanishes suddenly?
a) the motor will stop b) the motor will continue to run
c) the armature will burn d) the motor will run noisy
7. A dc series motor develops a torque of 30 Nm at 4A of load current. If the load current is
increased to 8A, the developed torque will be
a) 15Nm b) 30Nm c) 60Nm d) 120Nm
8. The methods of improving commutation can be obtained by
a) resistance commutation b) torque commutation
c) speed commutation d) none of the above
9. A synchronous motor develops maximum power when load angle is
a) 45o b) 60 o c) 90 o d) 120 o
10. An unexcited single phase synchronous motor is
a) reluctance motor b) universal motor
c) repulsion motor d) AC series motor
11. In a synchronous motor, the armature current has the highest value when excitaton is
a) zero b) low c) high d) high and low
12. If air gap of an induction motor is increased, its
a) power factor will increase b) magnetising current will decrease
c) magnetising current will increase d) power factor will decrease
13. In a 3 induction motor, input power on no load is W o = 800 W and windage and friction losses
are Wf =180 W. The stator copper loss will be
a) 180 W b) 600 W c) 780 W d) 440 W
14. Minimum value of insulation resistance of induction motor is
a) 0.5 M/KV b) 1.0M/KV c) 0.75M / KV d) 2.00M / KV
15. For domestic sewing machine, the size of motor required is
a) 10-30 watts b) 30-60 watts
c) 100-150 watts d) 250-500 watts
16. 1 motors are commercially manufactured for maximum rating of
a) 1HP b) 2HP c) 4HP d) 10HP
17. When a 400Hz transformer is operated at 50Hz, its KVA rating is
a) reduced to ¼ b) reduced to 1/8
c) reduced to ½ d) reduced to 1/16
18. Natural oil cooling is used for transformers up to a rating of
a) 3000KVA b) 1000KVA c) 500KVA d) 250KVA
19. If stator voltage of a squirrel cage induction motor is reduced to one-half of its normal value, its
starting current is reduced to ------------- percent of its full voltage value.
a) 75 b) 60 c) 50 d) 15
20. A centrifugal switch is used to dis-connect the starting winding when motor has
a) run for about 1 minute b) run for about 5 minutes
c) picked up about 50 to 70 percent of rated speed
d) picked up about 10 to 25 percent of rated speed
21. In air crafts, alternators have operating frequency of
a) 800Hz b) 400Hz c) 60Hz d) 50Hz
22. Four point starter in the DC motor is used
a) to decrease the field current b) to increase the field current
c) not to affect the current passing through ‘Hold on’ coil even if any change in the field current
takes place
d) all of the above
23. In an induction motor, the rotor reactance per phase is proportional to
a) s b) 1/s c) 1/s2 d) s2
24. EMF of a dc generator depends on
a) number of poles b) flux per pole
c) number of conductors in the armature core d) all the above
25 To reduce the eddy current losses within the conductor, the thickness of the rectangular
conductor selected for ℓ.v and h.v windings should not be greater than
a) 3.5mm b) 2 mm c) 2.5mm d) 3 mm
26. Which of the following statements is false?
a) current density in l.v. winding should be taken lesser than that in the h.v winding.
b) circular conductors are used for h.v winding of transformers
c) cooling conductors are used for h.v winding compared to l.v windings.
d) it is essential to provide proper insulation between core to l.v and l.v to h.v windings
27. In a dc machine, the interpole winding is connected
a) in series with the field winding
b) in parallel with the field winding
c) in series with the armature winding
d) in parallel with the armature winding
28. A 3-phase induction motor designed with comparatively lesser air gap leads to
a) improved cooling b) improved power factor
c) reduced power factor d) increased over load capacity.
29. A 3-phase, 6 pole induction motor has 54 stator slots and 48 rotor slots. The motor will have a
tendency of
a) cogging b) crawling c) noisy operation
d) developing cusps in torque-speed characteristics.
30. Which of the following statements is correct?
a) slip ring induction motor is more robust compared to double cage induction
b) inner cage of double cage induction motor is of high reactance
c) cogging phenomenon in squirrel cage induction motor can be avoided by choosing the
number of rotor slots equal to number of stator slots.
d) Double cage induction motor provides much higher starting torque compared to slip ring
induction motor.
31 The maximum flux in an ac excited core is determined by
a) impressed voltage only b) frequency of impressed voltage only
c) both impressed voltage and its frequency d) reluctance of the core only
32 In an electromagnetic relay the force acts in a direction
a) to maximize reluctance and maximize coil inductance
b) to maximize reluctance and minimize coil inductance
c) to minimize reluctance and minimize coil inductance
d) to minimize reluctance and maximize coil inductance
33 The test(s) needed to be performed to determine the leakage reactance of a transformer are
a) OC b) SC c) OC and SC d) Test by an impedance bridge
34 The phase relationship between flux phasor and induced emf phasor E in a synchronous
machine is
a) and E are in phase b) lags E by 90o
c) leads E by 90 o
d) phase relationship is dependent on the pf of the load
35. A face plate starter is employed for starting
a) induction motor b) universal motor
c) synchronous motor d) dc series motor
36. A brake test on DC motors is usually restricted to
a) small horse power motors b) variable speed motor
c) high speed motors d) open frame type motor
37. A dc motor develops a torque of 100 Nm at 1000 rpm. If the motor now runs at 1200 rpm, the
torque will be
a) 120Nm b) 144 Nm c) 100Nm d) 83.3Nm
38 In a dc motor iron losses occur in
a) the field b) the armature c) the yoke d) none of these
39. The minimum armature current of the synchronous motor corresponds to operation at
a) zero power factor leading b) unity power factor
c) 0.707 power factor lagging d) 0.707 power factor leading
40. Hunting in a synchronous motor can not be due to
a) variable supply voltage b) variable frequency
c) variable load d) windage friction
41. For a P-pole machine, the relation between electrical () and mechanical (m) degrees is given
by
2 p. m
a) C b) C = p.m c) C = m d) C =
p. m 2
42. For a 3, 50Hz, squirrel cage induction motor, rotor leakage reactance at stand still is twice of it
resistance. The frequency of the supply at which maximum torque is obtained at starting is
a) 50 Hz b) 105Hz c) 100Hz d) 75Hz
43. Two induction motors, having 6 poles & 4 poles respectively when runs in cascade combination
from 60Hz 3 supply can give the following synchronous speeds in rpm.
a) 720, 1200, 1800 and 3600 b) 1520, 1200, 1800
c) 600, 1000, 1500 d) 1520 and 3000
44. Simplest method of eliminating the harmonic induction torque is by
a) skewing b) chording c)using integral slot winding d) none of the above
45. Single phase motors generally get over heating due to
a) over loading b) short circuited windings c) bearing troubles d) any of above
46. Which of the following applications makes use of a universal motor?
a) floor polishing machine b) oil expeller
c) portable tools d) lathe machines
47. Scott connection of a transformer permits conversion of
a) three phase to two phase b) three phase to single phase
c) single phase to three phase d) none of the above
48. The auto – starters can be used to start cage induction motor of the following type
a) star connected only b) delta connected only
c) both (a) & (b) d) none of the above
49. What could be the size of a universal motor for use in vacuum cleaners?
a) 2H.P b) 1H.P c) ¾ H.P d) ¼ H.P
50. The speed of a salient pole machine is nearly
a) 500 rpm b) 1000 rpm c) 1500 rpm d) 3000 pm
51. In case of a uniformly distributed winding, the value of distribution factor is
a) 0.65 b) 0.76 c) 0.85 d) 0.995
52. Compensating winding in a DC machine is placed
a) on yoke in the pole faces b) on yoke in the interpolar gap
c) on armature d) none of the above
53. Generated emf and current are in the same direction in case of
a) series motors b) shunt motors
c) dc generators d) all of these
54 The power drawn by the primary winding of a transformer on no-load is predominantly absorbed
by
a) copper loss b) core loss
c) copper loss + core loss d) eddy current
55. During the design of magnetic frame of a transformer, average current density was assumed 2.4A
/ sq.mm, then the proper choice of current density for designing the l.v winding is
a) 2.4A / mm2 b) 2.6A / mm2 c) 2.2A/mm2 d) 2.8A/mm2
56. Which of the following statements is true?
a) In large transformers, oil ducts are provided in core and yoke
b) Tappings are provided in the low voltage winding
c) Class H Insulation is used between l.v and h.v windings.
d) On-load tap changers are used for small transformers.
57. During the design of armature of a 4 pole dc machine, the product D 2L = 0.00837 m3 . Most
suitable values of D and L are
a) 0.35m, 0.07m b) 0.2m, 0.21m
c) 0.18m, 0.26m d) 0.25m, 0.13m
58. In a dc machine, designing the armature with lesser slots leads to
a) reduced output b) improved commutation
c) noisy operation d) reduced torque
59. Designing the induction motor at higher gap flux density leads to
a) increased cost b) decreased overload capacity
c) lower temperature rise d) reduced size
60. Designing an induction motor at higher current density would mean
a) increased weight of copper b) lower temperature size
c) reduced resistance of the winding d) higher copper losses.
DEPT. OF ELECTRICAL & ELECTRONICS ENGINEERING
SRM ENGINEERING COLLEGE, Kattankulathur – 603 203
KEYS
COMPREHENSION - 1
Class : IV Year Date :
Coverage : Electrical Machines – I, II & Electrical Machine Design
Contributors : NKK, MSP, VKS
Q.No Ans Q.No. Ans Q.No. Ans
1 B 26 A 51 A
2 D 27 C 52 B
3 D 28 D 53 C
4 B 29 B 54 B
5 A 30 D 55 C
6 C 31 C 56 D
7 D 32 D 57 D
8 A 33 B 58 C
9 C 34 C 59 A
10 A 35 D 60 D
11 D 36 A
12 C 37 C
13 A 38 D
14 B 39 B
15 C 40 C
16 B 41 D
17 A 42 B
18 A 43 A
19 C 44 B
20 A 45 D
21 B 46 C
22 C 47 A
23 C 48 D
24 D 49 C
25 A 50 C
