1. If residual magnetism of a shunt generator is destroyed accidentally, it may be restored by connecting its shunt field
(a) To earth
(b) to an alternator
(c) in reverse
(d) to a battery
Ans: d
2.The sole purpose of commutator in a dc generator is to
(a) Increase output voltage
(b) reduce sparking at brushes
(c) provide smoother output
(d) convert the induced alternating voltage into unidirectional pulses
Ans: d
3.A conductor is rotating within a magnetic field. At which of the following positions do the zero voltages occur?
(a) along the axis of the magnetic field
(b) at right angles to the axis of the magnetic field
(c) at 45° with the axis of the magnetic field
(d) none of these
Ans: a
4. Which of the following helps in improving the commutation process?
(a) Interpoles
(b) compensating winding
(c) Brush shifting
(d) all of these
Ans: d
5.Commutation in a dc machine may be improved by
(a) reducing to number of turns in the armature and segments of commutator
(b) increasing the resistances of brushes
(c) neutralizing the reactance voltage by producing a reverse emf in the coil undergoing commutation
(d)all of these
Ans: d
6.Brushes are commutators for 220 V dc generator are generally made of
(a) Copper
(b) carbon copper
(c) electrographite
(d) graphited copper
Ans: c
7.Hysteresis losses in a dc generator are basically due to
(a) Current flowing in the armature core
(b) reversal of magnetism of the armature core
(c) initial energy to magnetise the core
(d) none of these
Ans: b
8.A d.c. shunt motor runs at 500 r.p.m. at 220 V. A resistance of 4.5 Ω is added in series with the armature for speed control. The armature resistance is 0.5 Ω. The current to stall the motor is
a)44 A
b)50 A
c)40 A
d)30
Ans: a
9.Core losses in a dc machine occur in
(a) Armature alone
(b) pole faces along
(c) both armature and pole faces
(d) yoke alone
Ans: c
10.The open circuit voltage of a dc compound generator is 200 V. At full load, the terminal voltage
(a) Will always be 200 V
(b) will always be less than 200 V
(c) will always be greater than 200 V
(d) may be greater than or less than 200 V
Ans: d
11.A dc shunt motor is rotating in clockwise direction as viewed from one end. The polarity of connection of the armature and field winding are reversed simultaneously. The motor will
(a) Not rotate at all
(b) rotate in anti-clockwise direction
(c) rotate in clockwise direction
(d) none of above
Ans: c
12.When two D.C. generators are running in parallel an equilizer bar is used
(a) to increase the series flux
(b) to increase the generated e.m.f.
(c) to reduce the combined effect of ar-mature reaction of both the machines
(d) so that the two identical machines will pass approximately equal currents to the load
Ans: d
13.By applying fleming‟s right hand rule to an electric generator, you can find the
(a) direction of magnetic field
(b) direction of induced emf
(c) direction of rotor motion
(d) law of induction
Ans: b
14.Interpoles in dc motors are used for
(a) Increasing the speed of motor
(b) reducing sparking at the commutator
(c) decreasing the counter emf
(d) converting armature current to dc
Ans: b
15.The back emf of a dc motor
(a) Adds to the supply voltage
(b) regulates its armature voltage
(c) helps in energy conversion
(d) usually exceeds the supply voltage
Ans: c
16.DC series motor are best suited for traction because
(a) Torque is proportional to the square of the armature current and speed is proportional to the torque
(b)Torque is proportional to square of the armature current and speed is inversely proportional to the torque
(c)Torque and speed both are proportional to the square of the armature current
(d)None of these
Ans: b
17.The most economical method of electrical braking of a dc motor is
(a)regenerative braking
(b) dynamic braking with self excitation
(c) dynamic braking with separate excitation
(d) plugging
Ans: a
18.A dc motor develops of torque of 100 Nm at 1000 rpm. If the motor now runs at 1200 rpm, the torque will be
(a) 120 Nm
(b) 144 Nm
(c) 100 Nm
(d) 83.3 Nm
Ans: c
19.The nominal power printed on the name plate of any motor signifies the
(a) power drawn in kVA
(b) output power at the shaft
(c) power drawn in kW
(d) the gross power
Ans: b
20.The laws of electromagnetic induction (Faraday‟s and Lenz‟s law) are summarized in the following equation
(a) e =iR
(b) e =Ldi dt
(c) e= – dψ dt
(d) none of the above
Ans: c
21.Sparking at the commutator of a D.C. motor may result in
(a) Damage to commutator segments
(b) damage to commutator insulation
(c) increased power consumption
(d) all of the above
Ans: d
22.Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of
(a)D.C.series motor
(b) D.C.shunt motor
(c) induction motor
(d) all of above motors
Ans: a
23.Which loss in a D.C. generator does not vary with load as well as flux density?
(a) Copper loss
(b) eddy current loss
(c) hysteresis loss
(d) windage loss
Ans: d
24.The condition for maximum efficiency for D.C.generator is
(a) eddy current losses = stray losses
(b) hysteresis losses = eddy current loses
(c) copper losses = 0
(d) variable losses = constant losses
Ans: d
25.During Rheostat braking of D.C.series motors
(a)motor is run as a generator
(b) motor is revered in direction
(c) motor is run at reduced speed
(d) motor runs in toggle mode
Ans: a
26.For which types of D.C.motor, dynamic braking is generally used?
(a)shunt motors
(b) series motors
(c) compound motors
(d) all of the above
Ans: d
27.A 230 V D.C. shunt motor takes 32 A at full load. The back e.m.f. on full load, if the resistance of motor armature and shunt field winding are 0.2 Ω and 115 Ω respectively, will be
(a) 210 V
(b) 215 V
(c) 220 V
(d) 224 V
Ans: d
28.In case of D.C.machines, mechanical losses are primary function of
(a) Current
(b) voltage
(c) Speed
(d) none of above
Ans: c
29.The polarity of a D.C.generator can be reversed by
(a)Reversing the field current
(b) increasing field current
(c) reversing field current as well as direction of rotation
(d) any of the above
Ans: a
30.In D.C generators, the brushes on commutator remain in contact with conductors which
(a) Lie under south pole
(b) lie under north pole
(c) lie under interpolar region
(d) are farthest from the poles
Ans: c
31.The function of pole shoes in the case of D.C machine is
(a) To reduce the reluctance of the magnetic path
(b) To spread out the flux to achieve uniform flux density
(c) To support the field coil
(d) all the above
Ans: d
32.Armature reaction of an unsaturated D.C. machine is
(a) crossmagnetising
(b) demagnetizing
(c) magnetizing
(d) none of above
Ans: a
33.In a D.C generator the ripples in the direct e.m.f. generated are reduced by
(a) using conductor of annealed copper
(b) using commutator with large number of segments
(c) using carbon brushes of superiors quality
(d) using equalizer rings
Ans: b
34.The armature core of a D.C generator is usually made of
(a)silicon steel
(b) copper
(c) non-ferrous material
(d) cast-iron
Ans: a
35.In a D.C. machine stray loss is the sum of
(a) Total copper loss and mechanical loss
(b) armature copper loss and iron loss
(c) shunt field copper loss and mechanical loss
(d) iron loss and mechanical loss
Ans: d
36.A 220 V D.C generator is run at full speed without any excitation. The open circuit voltage will be
(a) Zero
(b) about 2 V
(c) about 50 V
(d) 220 V
Ans: b
37.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
Ans: c
38.Which of the following rule is used to determine the direction of rotation of D.C motor?
a)Coloumb’s Law
b)Lenz’s Law
c)Fleming’s Right-hand Rule
d)Fleming’s Left-hand Rule
Ans: d
39.The ratio of starting torque to full load torque is least in
a)Differential Compound Motor
b)Shunt motor
c)Series Motor
d)Cumulative compound motor
Ans: b
40.A three point starter is suitable for
a)Shunt Motor
b)Series Motor
c)Shunt & Compound Motor
d)Shunt, Series, and compound motor
Ans: c
41.Which DC motor has least percentage increase in Input current for the same percentage increase in torque?
a)Separately excited motor
b)Series motor
c)Shunt motor
d)Compound motor
Ans: c
42.What will happen When the field of a DC shunt motor get opened while the motor is running?
a)Continue to run at same speed
b)Speed of motor will be reduced
c)Motor will attain dangerous high speed
d)Armature current will be reduced
Ans: c
43.According to Fleming’s left-hand rule if the forefinger points in the direction of the field than the middle finger will point in the direction of
a)Current in the conductor
b)Resultant force on the conductor
c)Movement of the conductor
d)None of the above
Ans: a
44.Which DC motor is preferred for Elevator?
a)Differentially compound motor
b)Series motor
c)Shunt Motor
d)Cumulative compound motor
Ans: d
45.What will happen when the D.C motor is connected to A.C supply then?
a)D.C motor will run at rated speed
b)D.C motor will burn
c)D.C motor will run at slow speed
d)Both 2 & 3.
Ans: d
46.The compensating winding in the conductively compensated DC series motor is provided as
a)Parallel with field winding
b)In series with armature winding
c)Parallel with armature winding
d)Series with field winding
Ans: a
47.If a self excited D.C. generator after being installed, fails to build up on its first trial run, the first thing to do is to
(a) reverse the field connections
(b) increase the field resistance
(c) increase the speed of primemover
(d) check armature insulation resistance
Ans: a
48.The function of Dummy coil is to
(a)Reduce eddy current
(b)Reduce hysteresis loss
(c)Increase flux
(d)Mechanical Balancing
Ans: d
49.The flux density under trailing pole tip in DC motor tends to
(a)Decrease
(b)Remain same
(c)Increase
(d)None of the above
Ans: a
50.When the speed of the DC motor is Increases its Armature current
a)Increases
b)Decreases
c)Remain Constant
d)Any of the above
Ans: b
“You cannot change your future, but, you can change your habits, and surely your habits will change your future“
-Dr.Kalam
For detail explanation of each questions you must watch this video.
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