Latest DC Machines MCQs ( Electrical Machines ) MCQs – Competitive Electrical Machines MCQs
Latest Electrical Machines MCQs
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Electrical Machines MCQs – DC Machines MCQs ( Electrical Machines ) MCQs
The most occurred mcqs of DC Machines MCQs ( Electrical Machines ) in past papers. Past papers of DC Machines MCQs ( Electrical Machines ) Mcqs. Past papers of DC Machines MCQs ( Electrical Machines ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related DC Machines MCQs ( Electrical Machines ) Mcqs. The Important series of DC Machines MCQs ( Electrical Machines ) Mcqs are given below:
Circuit Model of DC Machines
1. The circuit depicting the equation V=Ea + I*Ra.
a) 
b) 
c) 
d) 
Answer: a
Explanation: This is a motor performance equation.
2. The circuit depicting the equation V=Ea – I*Ra.
a)
b)
c)
d) None of the mentioned
Answer: a
Explanation: This is a generator performance equation.
3. If the terminal voltage of 220-V dc generator having armature resistance of 1 ohms. The induced emf produced is 200-V. The armature current for the above machine is?
a) 20 A
b) -20 A
c) -10 A
d) 10 A
Answer: a
Explanation: I = V-E/r
= 220-200/1
= 20 A.
4. The voltage drop at brush-commutator contact is variable (1-2V) and dependent of armature current.
a) True
b) False
Answer: b
Explanation: The voltage drop at brush-commutator contact is variable (1-2V) and independent of armature current.
5. What is the shaft power at the DC Generator?
a) sum of mechanical power and rotational losses
b) only mechanical losses
c) no-load losses and mechanical power
d) any of the mentioned
Answer: a
Explanation: The shaft power is sum of mechanical power and rotational losses.
6. If the electromagnetic torque in a DC shunt-generator is opposite, what can be further concluded?
a) Mechanical power is absorbed by the machine
b) Mechanical power is delivered by the machine
c) Electromagnetic torque is in same direction of prime mover
d) None of the mentioned
Answer: a
Explanation: When the electromagnetic torque is in opposite direction, it is of motoring nature.
7. The conductor EMF and current are in _____ direction and developed torque is in _____ for generating mode.
a) same, opposite
b) same, same
c) opposite, same
d) opposite, opposite
Answer: a
Explanation: The conductor emf and current will be in same direction and the developed torque is in opposite direction for a generator.
8. If the armature terminal voltage is more that its induced EMF, the DC machine given is ________
a) motoring mode
b) generating mode
c) regenerative mode
d) none of the mentioned
answer: a
Explanation: As the terminal voltage is lesser than armature voltage, the supply is fed to the machine and so it will be acting like a motor.
9. Consider a 200V, 25kW, 30A DC machine lap connected with armature resistance of 0.4 ohms. If the machine is later wave wound, then the developed power is?
a) 25 kW
b) 12.5 kW
c) 20 kW
d) 50 kW
Answer: a
Explanation: The power of the machine remains unaltered by the type of connections.
10. If the DC machine is held constant at 3000 rpm. The DC voltage is 250V. If the field is held constant with 250V. Is this machine generator or motor?
a) Motor
b) Generator
c) None of the mentioned
d) Any of the mentioned
Answer: a
Explanation: From the speed and emf relation, E = 250*2950/3000
= 245.8 V
This is less than the terminal voltage. Hence it is a motor.
11. A shunt generator has an induced voltage on open circuit of 127 V. When the machine is on load the terminal voltage is 120 V. The load current if the field resistance be 15 ohm and armature resistance be 15 ohm.
a) 342 A
b) 350 A
c) 358 A
d) 8 A
Answer: a
Explanation: Ia*Ra = E-V
= 127 – 120
= 7 V
Ia = 350 A
Ish = 120/15
= 8 A
I = 350 – 8
= 342 A
Methods of Excitation
1. Consider a DC generator running at the rated speed of 2000 rpm, suddenly there is an insulator falls on the field circuit and breaks it. Then __________
a) the motor stops in a few rounds
b) it continues to run but as dc motor
c) it continues to run as a motor
d) none of the mentioned
Answer: a
Explanation: As the field circuit gets broken there is interruption in the flux to the dc machine, and as a result the motor stops in few seconds.
2. If a self excited DC generator is failed to run, this refers to ________________
a) zero residual voltage
b) field MMF, that it is not cumulative
c) resistance is greater than critical resistance
d) any of the mentioned
Answer: a
Explanation: Any of the reasons are valid for no build up of an emf.
3. The self-excited dc generator with gradual build up of residual voltage and EMF corresponds to __________
a) positive feedback
b) negative feedback
c) saturation
d) any of the mentioned
Answer: a
Explanation: It responds to a positive feedback because the gain is always more than one in the linear region.
4. The critical resistance refers to __________
a) the resistance above which machine does not excite
b) the resistance below which machine does not excite
c) the resistance at which machine does not excite
d) any of the mentioned
Answer: a
Explanation: After taking a glance at the magnetization curve, it can be concluded that the emf will not build up if the resistance is greater than critical resistance.
5. If a DC shunt generator fails to start, the most primitive action would be __________
a) reversing field connection to armature
b) reversing direction of rotation
c) any of the mentioned applicable ways
d) none of the mentioned
Answer: a
Explanation: The first measure is to check if the field winding as properly connected with additive polarity so that the flux is aiding in nature.
6. Flashing is used to __________
a) temporarily excited field from a battery source for large DC generator
b) temporarily excited field from a battery source for small DC generator
c) temporarily excited field from a battery source for large DC motor
d) temporarily excited field from a battery source for large DC motor
Answer: a
Explanation: It is a method to give a kick start to dc machine when its residual magnetism is vanished.
7. A series generator, having external characteristics which is a straight line through 0 to 50 V at 200 A is connected as a booster, between a station bus-bar and feeders of 0.5 ohm resistors. The voltage at the far end of feeder at current of 50 A is?
a) 25 V
b) 17 V
c) 8 V
d) 50 V
Answer: a
Explanation: V = voltage drop-voltage added up by boosters
= (0.3*50 – 50*50/200)
= 15-12.50
= 2.5 V.
8. The number of parallel paths in the armature is increased by __________
a) increasing number of magnetic poles
b) decreasing number of magnetic poles
c) lap number of magnetic poles
d) using more brushes
Answer: a
Explanation: It is increased by increasing magnetic poles which give parallel path to the magnetic circuit.
9. A dc series generator as armature and field connections are reversed, the generator __________
a) stops
b) opposite direction
c) same direction
d) none of the mentioned
Answer: a
Explanation: It happens due to non build up voltage.
10. _____________ is responsible for mechanical power output of a DC motor.
a) Electrical input power
b) Any of the mentioned
c) Air-gap flux
d) Armature emf
Answer: a
Explanation: Primarily it is the electrical input power that starts the motoring action.
MMF and Flux Density Waveforms in DC Machines
1. The armature mmf affects __________
a) commutation
b) generated voltage
c) torque
d) any of the mentioned
Answer: a
Explanation: Armature reaction thus armature mmf affects all of the mentioned above.
2. The space distribution of flux produced in a DC machine is?
a) 
b) 
c) 
d) None of the mentioned
Answer: d
Explanation: Distorted triangular waveform shape is that of flux distribution under poles, which is due to the armature reaction.
3. Armature reaction in a dc machine is __________
a) cross magnetizing in nature
b) magnetizing in nature
c) demagnetizing in nature
d) none of the mentioned
Answer: a
Explanation: It is cross magnetizing in nature which happens due to interaction between stator and field mmf.
4. The air-gap flux density waveform has decreased flux under one pole tip and measured under the other is due to __________
a) cross magnetization
b) magnetization
c) demagnetization
d) saturation of iron magnetic circuit
Answer: a
Explanation: The flux density increases under one pole tip while it decreases under the other due to the saturation of iron magnetic circuit.
5. The flux distortion caused by cross magnetizing armature reaction is more pronounced in a __________
a) dc shunt motor
b) dc series motor
c) any of the mentioned
d) differential Compound Motors
Answer: a
Explanation: Since field excitation remains substantially constant, while aarmature mmf reaches higher values at loading.
6. Which method needed to limit cross-magnetizing effect?
a) by increasing reluctance of cross flux path
b) by chamfering the pole faces
c) by compensating winding
d) any of the mentioned
Answer: a
Explanation: By ultimately increasing the reluctance of the magnetic circuit at the pole tips so that the armature reaction effect is neutralized.
7. The brushes of a dc motor, are shifted by 5′, from the main field axis, then __________
a) demagnetizing or magnetizing effect may occur depending on machine
b) cross-magnetization will be pronounced
c) both cross-magnetization as well as magnetization and demagnetization will occur
d) none of the mentioned
Answer: a
Explanation: When the brushes are shifted from GNA, there will either be magnetizing or demagnetizing effect will be observed.
8. A 100KW, 250 V, 400 A, a long shunt compound generator has an armature resistance of 0.025 ohms. There are 1000 shunt fields turns per pole and 3 series field turns per pole. The series field is connected in a such a fashion that positive armature current produces direct-axis MMF which adds to that of the shunt field. Compute the gross MMF at the rated terminal current when shunt field current is 4.7A and speed is 1150 rpm.
a) 5.9AT
b) 3.5AT
c) 4.7AT
d) 1.2AT
Answer: a
Explanation: Series field current = Is = I(l)+I(f)
= 400+4.7
= 405 A(approx)
Main field mmf = I(f)+(Ns/Nf)*I(s)
= 4.7+(3/1000)*405
= 5.9 AT.
9. Compensating windings are embedded in pole faces and having the same polarity as that of adjoining armature winding.
a) True
b) False
Answer: b
Explanation: Compensating windings are embedded in pole faces and having an opposite polarity as that of adjoining armature winding.
10. Compensating winding has an advantage of __________
a) eliminating flux distortion
b) importing speed of response
c) protecting overloading
d) any of the mentioned
Answer: a
Explanation: Any of the corrective measures will be applicable.
Electromechanical Energy Conversions MCQs
11. The correct positioning of the windings in the below diagrams is?
a) 
b) 
c) 
d) Any of the mentioned
Answer: a
Explanation: Commutating and compensating fields act along armature axis. Shunt and series fields act along main field axis.
12. A dc shunt motor is connected to the source through 3-point starter. If the field id kept open and starter handle is moved from off to on position, then __________
a) motor will not start
b) armature will draw large current from source
c) no sparking would occur
d) all of the mentioned
Answer: d
Explanation: Leaving the field winding open, the machine will not start as the exciting flux is zero.
13. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the filed winding terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected
Answer: a
Explanation: Reversing the field will change the direction of rotation but will not affect the commutation.
14. A dc shunt motor is running at 1000 rpm at the rated load torque. If we reverse the supply terminals, then the __________
a) direction of rotation reverses and commutation will be unaffected
b) direction of rotation will be same and commutation will be unaffected
c) direction of ration reverses and commutation will be affected
d) direction of ration will be same and commutation will be affected
Answer: b
Explanation: Reversing the supply in dc machine will not change the direction of rotation and will not affect the commutation too.
15. A dc shunt motor is running at 1000 rpm at the rated load torque. If few of the field windings get shot circuited then the __________
a) motor speed will increase and more armature current is drawn
b) motor speed will decrease and more armature current is drawn
c) motor speed will increase and lesser armature current is drawn
d) motor speed will decrease and lesser armature current is drawn
Answer: a
Explanation: If field winding get short circuited, field excitation reduces causing rise in speed and more armature current.
Effect of Brush Shift and Compensation Windings
1. The brushes are placed __________
a) along geometrical neutral axis
b) perpendicular to geometrical neutral axis
c) along magnetic neutral axis
d) perpendicular to magnetic neutral axis
Answer: a
Explanation: The brushes are placed along geometrical neutral axis so that in ideal conditions commutations take place without sparking. Also placing brushes along the magnetic neutral axis requires consistent variations as load varies which is not desirable.
2. The brushes should be shifted in _______ direction in generator for the satisfactory operation of commutation.
a) forward
b) reverse to rotation
c) all of the mentioned
d) none of the mentioned
Answer: a
Explanation: For the generator, the electromagnetic torque produced is in the opposite direction and so the induced current. So to eliminate the sparking the brushes should be shifted in the forward direction of rotation.
3. With no commutating poles used the brushes are given backward lead in the dc motor.
a) True
b) False
Answer: a
Explanation: The armature reaction flux at the trailing edge weakens the flux and at the leading edge is strengthens the flux in the dc motor.
4. If the critical commutation time is 2 ms but it was observed that the practical commutation took 2.5 ms. This is the case of __________
a) under commutation
b) over commutation
c) critical commutation
d) none of the mentioned
Answer: a
Explanation: Since the time taken is more than the prescribed time limit for the commutation, it is under commutation.
5. The coil under going commutation lies along _____ while the coil getting short circuited lies along ____ for a practical dc machine.
a) gna, mna
b) mna, gna
c) gna, gna
d) mna, mna
Answer: a
Explanation: In a practical dc machine, the coil undergoing commutation lies along GNA so as the brushes and the coils which actually get short circuited lies along mna. This lead to sparking as a voltage difference is created at the coils.
6. The principle contributors to magneto motive force is/are __________
a) field current and armature current
b) field excitation
c) electromagnetic torque
d) all of the mentioned
Answer: a
Explanation: The interaction between the field excitation and armature supply create the mmf and the interconnected behaviour creates the emf.
7. The absence of compensating winding lead to __________
a) statically induced emf in armature
b) fluctuations in supply
c) reduction in the flux
d) any of the mentioned
Answer: a
Explanation: The static emf induced inside gets reduced due to the armature reaction in large machines when there is no compensating winding present.
8. The number of compensating conductors/pole faces is _______ (where Z is the number of active armature conductors/pole; A is number of parallel paths; Ia is the armature current).
a) Z/A*Ia
b) 2*A*Ia/Z
c) Z*Ia/A
d) Z/2A*Ia
Answer: a
Explanation: Compensating conductors per pole face is Z/A*Ia.
9. A 400-V, 1000-A, lap wound dc machine has 10 poles, 860 armature conductors. The number of conductors in the pole face to give full compensation if pole face covers 70% of pole span is?
a) 3010
b) 4300
c) 2400
d) 2800
Answer: a
Explanation: AT/pole for the compensating winding = 0.7*(Z*I/2P)
= 0.7*(860*100/2*10)
= 3010 AT.
10. Each of the following is valid for interpoles except __________
a) they are connected in parallel with the armature so that they carry part of armature current
b) they are small yoke fixed poles spaced in between main poles
c) their polarity, in case of generator is same as that of main poles ahead
d) they automatically neutralize not only reactance voltage but cross-magnetization also
Answer: a
Explanation: All the options are valid for the interpoles except a because they are connected in series not in parallel.
11. Mark the most incorrect. In dc generator, commutation can be improved by __________
a) using interpoles
b) using carbon brushes
c) shifting brush axis in direction of armature rotation
d) none of the mentioned
Answer: a
Explanation: All the given measures are used to eliminate the effect of armature reaction and hence improves commutation also.
12. The commutation process involves basically reversal of current in armature coil as it crosses MNA.
a) True
b) False
Answer: a
Explanation: By reversing the current direction in the MNA the current can be limited in the coils to avoid sparking.
13. The armature reaction is produced mainly by __________
a) load current in armature
b) load current in field
c) all of the mentioned
d) none of the mentioned
Answer: a
Explanation: The armature reaction is initiated by the loaded conditions of the machine because an unloaded dc machine does not armature flux to get distorted.
14. The compensating winding in dc generator is provided to __________
a) neutralize cross-magnetizing flux
b) neutralize demagnetizing flux
c) neutralize armature mmf
d) maintain magnetizing mmf
Answer: a
Explanation: The compensating winding functions such that to nullify the cross magnetization effects in the armature.
15. If the magnetic circuit of the dc machine is in the saturation region, the armature reaction __________
a) does not affect flux/pole
b) increases the flux/pole
c) decreases flux/pole
d) affects the flux/pole only when armature current is small
Answer: a
Explanation: In the saturation region the flux will be constant.
Efficiency and Testing of DC Machines
1. A dc motor is connected in the short-shunt configuration, but the series and the shunt windings get interchanged by mistake and the motor is connected to the rated voltage?
a) neither of the windings get short-circuit
b) series winding gets overloaded
c) shunt winding gets overloaded
d) both of the winding get overloaded
Answer: a
Explanation: In the short shunt configuration, the winding interchanging does not change the configuration of the set up so none of the winding get affected.
2. What is the main concern before doing hopkinson’s test for finding efficiency?
a) needs one motor and one generator is that it
b) ignores iron and stray losses
c) needs one motor
d) requires identical shunt machines
Answer: d
Explanation: It is must that both the machines are identical as required by the calculations.
3. Retardation test on DC shunt motor is used for finding _______
a) stray losses
b) copper losses
c) friction losses
d) iron losses
Answer: a
Explanation: Retardation test is used for finding the stray losses.
4. The quantities needed to complete retardation test are/is _________
a) dw/dt and moment of inertia
b) dw/dt
c) current
d) any of the mentioned
Answer: a
Explanation: We need the angular acceleration and the moment of inertia of the machine.
5. ______ test is used for determining the efficiency of a traction motor.
a) Field
b) Retardation
c) Hopkinson
d) Swineburne’s
Answer: a
Explanation: Field test is used for finding efficiency of a traction motor.
6. If the field current and armature current are reversed, then the __________
a) direction of rotation remains same
b) direction of rotation reverses
c) stops
d) none of the mentioned
Answer: a
Explanation: When both field current and armature current are reversed then the direction will not change.
7. Mark the possible causes of overheating of commutator in a DC machine.
a) Overload
b) Restricted ventilation
c) Shorted winding
d) Any of the mentioned
Answer: a
Explanation: All the factors can cause the overheating of commutator.
8. In the hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as motor?
a) B
b) A
c) A, B
d) None of the mentioned
Answer: a
Explanation: Ea/Eb = 1.4/1.3
Ea = 1.077 Eb > Eb
So, the machine will act as motor.
9. In the Hopkinson’s test on two DC machines. Machine A has field current of 1.4A and B has field current of 1.3A. Which machine acts as generator?
a) B
b) A
c) A,B
d) None of the mentioned
Answer: b
Explanation: Ea/Eb = 1.4/1.3
Ea = 1.077 Eb > Eb
So, the machine will act as generator.
10. Hopkinson’s test is also called regenerative test due to ____________
a) energy of one machine is used to drive other
b) feedback
c) losses are least
d) extra motor is used
Answer: a
Explanation: Regenerative phenomena utilizes the energy of the running machine to tap the dynamically produced energy and use it to charge the battery.
11. Which is the most appropriate relation to find efficiency of the generator?
a) output/(output + losses)
b) (output – losses)/input
c) (output – losses)/(output + losses)
d) any of the mentioned
Answer: a
Explanation: For a generator the output can be measured and so this expression is used.
12. The efficiency in the swineburne’s test can be found.
a) True
b) False
Answer: a
Explanation: Because the constant losses are known.
13. The possible assumption is/are made while doing swineburne’s test.
(i) mechanical loss constant
(ii) Armature reaction neglected
(iii) Increases in flux
a) (i) and (ii)
b) (iii) and (i)
c) (i), (ii), and (iii)
d) (i)
Answer: a
Explanation: There is decrease in flux due to positive temperature coefficient of resistance in shunt machine.
14. Swineburne’s test is applicable to those machines in which flux is practically _______
a) constant
b) linear
c) non-linear
d) none of the mentioned
Answer: a
Explanation: Because the constant losses are known in advance of the test is conducted.
15. Similarity between Hopkinonson’s and field’s test is that _______
a) both need similar mechanically coupled motor
b) both need similar electrically coupled motor
c) regenerative power
d) negligible power
Answer: a
Explanation: The machines have to be mechanically coupled together.
DC Machine Applications
1. Inspite of heavy initial investments, dc motors are used due to _____________
a) flexibility and ease of control
b) lower losses
c) improved power factor of the system
d) all of the mentioned
Answer: a
Explanation: The control of dc machines is very much simplified when compared to other machines. So this makes it very useful when compared.
2. Separately excited dc generators are used in __________
a) Ward leonard system of speed control
b) Hopkinson’s testing
c) Voltage control
d) None of the mentioned
Answer: a
Explanation: Separately excited dc generators are used in Ward leonard system of speed control.
3. Maximum torque in dc series motor is limited by __________
a) commutation
b) heating
c) field control
d) all of the mentioned
Answer: a
Explanation: Commutation is the process which reduces the induced emf and so the torque.
4. Hoists, cranes and battery powered vehicles use _________ motors in the locomotive.
a) dc series
b) dc shunt
c) induction
d) reluctance
Answer: a
Explanation: Hoists, cranes require large starting torque which can be provided by dc series motor.
5. There is an application which required pulsating loads, punch presses. The most preferred machines would be __________
a) compound dc machine
b) series dc machine
c) shunt dc machine
d) all of the mentioned
Answer: a
Explanation: It is compound machine whose flux can varied higher as well as lower in order to have variable torques.
6. The manufacturer has mentioned a medium starting torque and 15% speed regulation. Which is the most appropriate motor for this requirement?
a) dc shunt motor
b) induction motor
c) differential motor
d) none of the mentioned
Answer: a
Explanation: The dc shunt motor provides a medium values of the torque.
7. Centrifugal pumps, fans-blowers use __________
a) shunt as well as induction motor
b) only shunt motors
c) only induction motor
d) none of the mentioned
Answer: a
Explanation: Centrifugal pumps, fans-blowers use shunt as well as induction motor.
8. Most commercial compound dc generator are normally supplied by manufacturers as over compound machines because __________
a) degree of compounding can be adjusted by diverters acrosss series field
b) they have ideally best for HVDC
c) cost effective than shunt
d) zero percent regulation
Answer: a
Explanation: It is usually over compounded so that degree of compounding can be adjusted by diverters across series field.
9. Which of the following are applied with differential compound motor?
a) Hoist
b) Cranes
c) Drilling machines
d) None of the mentioned
Answer: a
Explanation: Practically it is never used as the speed can rise very high.
10. Most commercial compound dc generator are normally supplied by manufacturers as ______ compounded machines.
a) over
b) under
c) level
d) none of the mentioned
Answer: a
Explanation: Most commercial compound dc generator are normally supplied by manufacturers as over compounded.
11. Long back ago, magnetic casette were in use, having permanent magnet dc motors which had?
a) magnets on stator and armature on rotor
b) magnet on rotor and armature on stator
c) electronic commutation
d) all of the mentioned
Answer: a
Explanation: PMDC motors have field fixed on the stator and rotor has coils.
12. If a dc shunt motor is stopped by forcing starter handle back to OFF position, then the __________
a) dangerous sparking occurs at last stud
b) dangerous sparking occurs at all stud
c) it stops normally
d) heavy sparking
Answer: a
Explanation: The sparking will occur at the last stud not all the studs of the starter.
13. Interpoles are used to start the dc motor above the base speed.
a) True
b) False
Answer: b
Explanation: Interpoles do not reduce the flux across the windings, hence the speed does not rise above base speed.
14. A 200 V dc motor has external resistances of Ra and Rf in armature and field circuits respectively. The starting current is reduced when?
a) Ra is maximum and Rf minimum
b) Rf is maximum and Ra minimum
c) Ra is minimum and Rf minimum
d) Ra is maximum and Rf maximum
Answer: a
Explanation: When Ra is maximum and Rf minimum then the current to the armature is reduced. And hence the sparkings are reduced while starting.
15. A d.c. generator has been provided tappings on the armature winding at intervals of 120 degrees from the side of commutator. The connection will be __________
a) delta connected alternator
b) star connected alternator
c) star connected induction motor
d) any of the mentioned
Answer: a
Explanation: Dc generator if has displaced by 120 degrees physically then it will act as Ac machine.
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