Multiple Choice Questions (MCQs) on DC Machines

mcq on dc machines

Multiple Choice Questions are very important for different competitive examinations. Also, we call these as MCQs in short. Again, some of us call these as objective type questions. So, we have provided such questions for betterment of preparation for such examinations.  The subject of electrical machines is the backbone of electrical engineering. We can categories electrical machines as ac machines and dc machines. Again dc machines have two groups. These are dc motors and dc generators. Hence, we can understand that the Multiple Choice Questions or MCQs on DC Machines(MCQs) are very essential to study.

Importance of Multiple Choice Questions (MCQs) on DC Machines

For clearing different engineering examinations we need to practice such questions in deep. We have provided explanations for each answer. No book will provide you such readymade explanations of the answers in such a compact form. We have not segregated the MCQs as per motor and generator. There are mixtures of such questions but all are related to DC machines only. We continually, research for finding out new conceptual questions and try to provide here for your purposes.

Why online Multiple Choice Questions (MCQs) on DC Machines are essential?

Nowadays, most of the competitive examinations are in online mode. So practicing such MCQs online is the best process for preparing such examinations.

The Chapters of Multiple Choice Questions (MCQs) on DC Machines

This list is mainly about Multiple Choice Questions (MCQs) on DC Machines. So, we have covered all the possible sections of dc machines. For example, we have covered, the working principle of dc motors and working principle of dc generators. Then we have covered types of dc motor and also the types of dc generators. Also, we have included characteristics curves of dc motor and generator. In addition to these, we have included different multiple questions from testing of dc machines and their different applications. The speed control of dc motors and voltage regulation of dc generators are also provided. Many more have been provided along with their explanations.

MCQs on DC Machines

Question 1
A DC machine has back pitch of 6 and front pitch of 4. What is the pitch of the winding, if the machine has wave winding?
A
10
B
6
C
4
D
2
Question 1 Explanation: 
The distance between the two coil sides measured from back of the armature is back pitch. dc machine back pitch The distance between the coil sides connected together at the front of the armature before get connected to the same commutator segments, is the front pitch. The distance between the same coil side of two adjacent interconnected coils is the resultant pitch. The resultant pitch is generally known as the pitch of the winding.
Question 2
What is the value of pole pitch in electrical degree
A
90°
B
45°
C
180°
D
360°
Question 2 Explanation: 
The pole pitch is the distance between two adjacent poles in the machine. It is measured in terms of armature slots available between the axis of two adjacent poles. The poles are opposite in to two adjacent positions in the DC machine. That if a certain pole is north pole then the adjacent pole will be south pole. Hence, the distance between them is always 180° (electrical).
Question 3
The maximum torque of a DC motor is limited by
A
commutation.
B
heating.
C
speed.
D
armature current.
Question 3 Explanation: 
In DC shunt motor the torque is directly proportional to the armature current. In DC series motor the torque is directly proportional to the square of armature current. So here in both cases when torque is increased armature current is increased. The increased armature current causes more armature reaction in the machine which ultimately results disturbances in the commutation. And this is why the maximum torque in a DC motor is determined considering its proper commutation.
Question 4
The expression of mechanical power developed by the armature of a DC motor is
A
product of armature current and back emf.
B
input power – losses.
C
product of efficiency and output power.
D
power output + iron losses.
Question 4 Explanation: 
The back emf of a DC motor is nothing but the voltage generated across the armature during its operation. And armature current is the current flowing through the armature winding so the product of this voltage that is back emf and armature current is nothing but the electrical power consumed in the armature. And this electrical power then gets converted to the mechanical power developed in the armature.
Question 5
Some through holes are provided in the laminated armature core in a DC machine to
A
reduce the weight of the core.
B
safe the steal for lamination.
C
balance the weight of the core for balanced rotational motion.
D
ventilate air during rotation.
Question 5 Explanation: 
The purpose of the holes provided in the armature core in DC machine is only for air ventilation during rotation of the machine. The through holes provide the flow of air through the machine during natural and forced cooling. These holes are called air ducts.
Question 6
Under constant load condition the speed of a DC motor is effected by
A
field flux.
B
armature current.
C
back emf.
D
both field flux and back emf.
Question 6 Explanation: 
We know that the speed of a DC motor is directly proportional to the ration of back emf and field flux.
Question 7
What is the commutator pitch in duplex lap winding DC machine?
A
2
B
1
C
4
D
6
Question 7 Explanation: 
The commutator pitch is measured by the number of commutator segments between the leads of a coil. In general in the case of lap winding, it equals the 'plex' of the winding. That means commutator pitch is 1 for simplex winding. The commutator pitch is 2 for duplex winding. The commutator pitch is 3 for triplex winding and it is 4 for quadruplex winding and so on.
Question 8
What is the commutator pitch in a quadruplex lap winding DC machine?
A
2
B
1
C
4
D
6
Question 8 Explanation: 
The commutator pitch is measured by the number of commutator segments between the leads of a coil. In general in the case of lap winding, it equals the 'plex' of the winding. That means commutator pitch is 1 for simplex winding. The commutator pitch is 2 for duplex winding. The commutator pitch is 3 for triplex winding and it is 4 for quadruplex winding and so on.
Question 9
The material by which the yoke of a DC generator is made of must have
A
low permeability.
B
high permeability.
C
zero permeability.
D
moderate permeability.
Question 9 Explanation: 
The yoke has to carry the field magnetic flux of the machine. Therefore it must allow the magnetic flux to pass through low reluctance paths in it. So the material of yoke must have high permeability.
Question 10
The purpose of commutator a DC generator is to
A
collect the armature current.
B
rectify the armature current.
C
amplify the output current.
D
collect and rectify the armature current.
Question 10 Explanation: 
The current induced in the armature of a DC generation is always alternating in nature. So there must be some means in the DC generator to rectify it to DC before the current being supplied to the load. The commutator of DC machine does both.
Question 11
The back emf of a DC motor
A
is more than supply voltage.
B
is added to the supply voltage.
C
helps in energy conversation.
D
regulates its armature voltage.
Question 11 Explanation: 
The back emf in a DC motor is nothing but the armature generated voltage of the machine. When the armature rotates in the magnetic field due to motoring action in a DC motor, like a generator the armature conductors also cut the magnetic field and thereby an emf is produced in the armature itself. This emf is in the opposite direction of the applied voltage to the armature and thereby we call this emf as back emf of the armature. The mechanical power produced in the armature, comes from the electrical power of the armature and the electrical power of the armature is nothing but the product of armature current and armature voltage. And the armature voltage is nothing but the back emf of the motor.
Question 12
The yoke of a DC Generator serves for providing  
A
mechanical support to the poles.
B
cover to the whole machine.
C
path for the field magnetic flux.
D
all of above.
Question 12 Explanation: 
Yoke is the outer cover of a DC generator. It is made of steel or iron. It holds the field magnetic poles. Also it provide path to the magnetic flux created by the field magnetic poles. Yoke of a DC generator
Question 13
The purposes of pole shoes in a DC generator are  
A
to spread out the flux in air gap.
B
to provide larger cross section to flux path for reducing reluctance.
C
to hold the field coil in place.
D
all of above.
Question 13 Explanation: 
Pole shoe of a DC generator From the above figure it is clear that the pole shoe holds the coil in its position. It prevents the coil to slide down from the pole core during vibration of the machine. It also spreads the flux and provides higher cross section of machine path in the air gap.
Question 14
Load saturation characteristic of a dc generator gives the relation between
A
output voltage and armature current.
B
armature emf and armature current.
C
armature emf and field current.
D
output voltage and field current.
Question 14 Explanation: 
The load saturation curve of a dc generator is also known as open circuit characteristic of the machine. It gives the open circuit terminal voltage various field currents. Also, it shows the saturation level of the field core. open circuit characteristics of dc generator
Question 15
Which of the following quantity maintain the same direction whenever a DC machine runs as a generator or a motor?
A
Induced emf in the armature.
B
Armature current.
C
Field current.
D
Supply current.
Question 15 Explanation: 
When the machine rotates as generator the direction of motion and magnetic field and current in the armature conductor are determined by Fleming’s right hand rule. On the other hand when the machine runs as a motor these directions are determined by Fleming’s left hand rule. If the direction of magnetic field and direction of rotation are same in the both cases then obviously induced emf, armature current will be opposite in both cases. Again in case of motor the current get supplied from the mains. In case of generator the current get supplied from the machine to mains. So sub-direction of supply current also alters when a DC motor behaves as DC generator. Only field system of the machine remains same in both cases. That is why field current is the correct answer.
Question 16
The relation between the number of commutator segments and the number of slots is given as
A
Number of commutator segments = number of slots
B
Number of commutator segments > number of slots
C
Number of commutator segments < number of slots
D
Number of commutator segments = ½ of number of slots
Question 16 Explanation: 
The number of commutator segments is equal to the number of slots or coil sides became the front ends of conductors are joined to the segments pairs.
Question 17
The armature conductor produces the emf across it is
A
sinusoidal.
B
rectangular.
C
triangular.
D
sawtooth.
Question 17 Explanation: 
When the conductor goes from north pole to south pole in a DC motor. The rate of flux linkage changes with the angel of rotation. This rate of change of flux linkage can be represented with a sinusoidal function because it is a circular rotation. And induced emf in the conductor is directly proportional to the rate of change of flux linkage and thereby we can say the pattern of induced emf in the armature conductor is purely sinusoidal.
Question 18
If the armature of a 4 pole DC machine, has 48 slots, the full pitched coil span would be
A
12
B
24
C
32
D
4
Question 18 Explanation: 
In full pitch coil. the coil span is exactly equal to the pole pitch. The pole pitch of a 4 pole DC machine having 48 armature slots, is (48÷4)=12
Question 19
In DC motor mechanical power comes from
A
field system.
B
air gap flux.
C
back emf.
D
electrical input power.
Question 19 Explanation: 
Not only in DC motor in all machines in this universe produces output power from its input power.
Question 20
The ratio between back emf and supply voltage indicates
A
efficiency.
B
speed regulation.
C
running torque.
D
starting torque.
Question 20 Explanation: 
The product of armature current and armature voltage that is back emf is the expression of mechanical power produce in the armature and the product of input voltage and armature current is the expression of input power. Here the shunt field current of the motor is so small compare to the armature current we have neglected it. And the efficiency of a motor is nothing but the ration of output voltage and input voltage.
Question 21
A DC motor can be thought as a DC generator with the power flow
A
reduced.
B
reversed.
C
increased.
D
modified.
Question 21 Explanation: 
When we supply power to a DC machine it rotates as a DC motor. But if we mechanically rotate the armature of the machine it produces power at the terminals of the DC machine. And then this DC machine behaves as DC generator. So we can say the same machine can be used as DC motor as well as DC generator but the power flow is reversed.
Question 22
The normal value of armature resistance of a DC motor is generally
A
10 Ohm.
B
100 Ohm.
C
0.05 Ohm.
D
0.5 Ohm.
Question 22 Explanation: 
The armature resistance in a DC motor should be low enough to limit the armature resistance drop of the machine. Generally we keep 0.5 Ohm as its value.
Question 23
If an armature of a 4 pole DC machine has 72 slots. Find out the pole pitch
A
4
B
8
C
16
D
18
Question 23 Explanation: 
The number of poles in the machine is 4. The number of armature slots in the machine is 72. So the number of armature slots between two adjacent pole is (72÷4)=18
Question 24
If we suddenly disconnect the field circuit of a dc shunt motor
A
the speed of the motor becomes excessively high.
B
the motor gets stopped instantly.
C
the motor gets tripped.
D
the motor will continue to rotate but with reduced speed.
Question 24 Explanation: 
The torque of a dc shunt motor is directly proportional to the product of field and armature current. Now when we disconnect the shunt field the flux becomes almost zero. Hence for maintaining the torque the motor draws excessively high armature. This ecessive high current causes tripping of the switchgear associated with motor.
Question 25
A 4 pole DC machine, having 35 armature slots, has coil span  
A
9
B
7
C
8
D
5
Question 25 Explanation: 
The number of slots is 35. The number of poles is 4. Then coil span or coil pitch Since, it is customary to drop fractions.
Question 26
If we make the field flux of DC motor 0 then its speed
A
approaches to 0.
B
approaches to infinity.
C
does not change.
D
becomes unstable.
Question 26 Explanation: 
From the equation of speed of a dc motor, we get This indicates that the speed of a dc motor is inversely proportional to its field flux.
Question 27
A DC motor produces unidirectional torque with the help of
A
brushes.
B
commutator segments.
C
inter poles.
D
both brushes and commutator segments.
Question 27 Explanation: 
During one rotation each conductor of the armature winding comes under all poles one by one. Here two adjusten poles are opposite to each other. So the field direction in front of each pole face alters. Again the direction of force on the conductor depends on the relative direction of current and field. So to produce unidirectional torque the direction of current in the conductor has also to alter its direction when it comes to one pole from other. Hence for unidirectional torque the direction of current in the armature has to alter. This alteration of supply direct current in the armature is done by brush and commutator. For more details go to the page commutator of DC machine.
There are 27 questions to complete.

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