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MCQs on Electrical Science

MCQs on DC Machines

Question 1
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 1 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 2
The armature conductor produces the emf across it is
A
sinusoidal.
B
rectangular.
C
triangular.
D
sawtooth.
Question 2 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 3
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 3 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 4
In DC motor mechanical power comes from
A
field system.
B
air gap flux.
C
back emf.
D
electrical input power.
Question 4 Explanation: 
Not only in DC motor in all machines in this universe produces output power from its input power.
Question 5
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 5 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 6
The maximum torque of a DC motor is limited by
A
commutation.
B
heating.
C
speed.
D
armature current.
Question 6 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 7
A DC motor can be thought as a DC generator with the power flow
A
reduced.
B
reversed.
C
increased.
D
modified.
Question 7 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 8
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 8 Explanation: 
We know that the speed of a DC motor is directly proportional to the ration of back emf and field flux.
Question 9
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 9 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.
Question 10
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 10 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 11
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 11 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 12
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 12 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 13
The ratio between back emf and supply voltage indicates
A
efficiency.
B
speed regulation.
C
running torque.
D
starting torque.
Question 13 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 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
There are 14 questions to complete.