DC Motor Armature Control MCQ Quiz - Objective Question with Answer for DC Motor Armature Control - Download Free PDF
Last updated on Jun 13, 2025
Latest DC Motor Armature Control MCQ Objective Questions
DC Motor Armature Control Question 1:
When the armature and field of a DC motor are supplied with current, what does the armature experience?
Answer (Detailed Solution Below)
DC Motor Armature Control Question 1 Detailed Solution
Explanation:
Correct Option: A force that tries to rotate it in an appropriate direction
Definition: When the armature and field of a DC motor are supplied with current, the armature experiences a force due to the interaction of the magnetic field generated by the field windings and the current flowing through the armature conductors. This force causes the armature to rotate, driving the motor in the desired direction.
Working Principle:
The operation of a DC motor is governed by the principle of electromagnetic induction. When the current flows through the armature conductors, positioned within the magnetic field created by the field windings, a force is exerted on the conductors according to Lorentz's force law. The direction of this force is determined by Fleming's Left-Hand Rule:
- The thumb represents the direction of the force (motion).
- The index finger represents the direction of the magnetic field.
- The middle finger represents the direction of the current.
As a result, the armature experiences a torque that causes it to rotate. This rotation is the fundamental working mechanism of DC motors, enabling them to convert electrical energy into mechanical energy.
Key Components Involved:
- Armature: The rotating part of the motor where the current flows through the conductors.
- Field Windings: The stationary coils that produce a magnetic field when energized.
- Commutator: A device that ensures the current direction in the armature windings changes periodically, maintaining consistent torque in a single direction.
- Brushes: Conductive components that transfer current to the rotating armature through the commutator.
Advantages of DC Motors:
- Precise control of speed and torque, making them suitable for applications requiring variable speeds.
- High starting torque, which is ideal for applications such as electric trains, cranes, and elevators.
- Reliable performance and simple construction.
Applications:
- Industrial machinery requiring adjustable speed and torque.
- Transportation systems such as electric vehicles and trains.
- Home appliances like fans and washing machines.
Correct Option Analysis:
The correct option is:
Option 4: A force that tries to rotate it in an appropriate direction.
This option accurately reflects the fundamental working principle of DC motors. When the armature and field are supplied with current, the armature experiences a force due to the electromagnetic interaction, resulting in rotation. This rotation is the desired outcome for converting electrical energy into mechanical energy.
Additional Information
To further understand the analysis, let’s evaluate the other options:
Option 1: An unstable force that tries to halt rotation.
This option is incorrect as the force generated in the armature does not halt rotation. Instead, it produces torque to initiate and sustain rotation. In a properly functioning DC motor, the electromagnetic force is stable and contributes to smooth operation.
Option 2: No force at all.
This option is incorrect because, when the armature and field are supplied with current, an electromagnetic force is always generated due to the interaction of current and magnetic fields. Without this force, the armature would not rotate, and the motor would be non-functional.
Option 3: A force that resists rotation.
This option is incorrect because the force exerted on the armature due to electromagnetic induction aids rotation rather than resisting it. A resisting force would oppose the intended operation of the motor, which is not the case in a properly functioning DC motor.
Conclusion:
The operation of a DC motor is a direct application of electromagnetic principles. When the armature and field windings are supplied with current, the interaction of the magnetic field and current generates a torque that causes the armature to rotate. This rotation is harnessed to perform mechanical work, making DC motors indispensable in various industrial and domestic applications. Understanding the forces at play within the motor clarifies why Option 4 is the correct answer and highlights the inaccuracies in the other options.
DC Motor Armature Control Question 2:
Which of the following assumptions is true for armature controlled DC motor?
Answer (Detailed Solution Below)
DC Motor Armature Control Question 2 Detailed Solution
Armature Controlled DC Motor:
This method is used when speeds below the base speed are required.
As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or resistance in series with the armature circuit as shown in Figure.
As controller resistance is increased the voltage across the armature is decreased, thereby decreasing the armature speed.
For a load constant torque, speed is approximately proportional to the voltage across the armature or Back emf.
\({E_b} = \frac{{NPϕ Z}}{{60A}}\)
Where,
Eb = Induced or Generated EMF Or Voltage in a parallel path.
P = Number of poles.
ϕ = Flux per poles in Weber.
A = Parallel paths.
N = Speed in RPM
∴ Eb ∝ N
So, Back EMF is proportional to speed.
Important Points
τ = Kϕ Ia
τ ∝ Ia
Torque developed is directly proportional to Armature current.
DC Motor Armature Control Question 3:
A 220V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1 and armature current is 10A. If the excitation of the machine is reduced by 10% the extra resistance to be put in the armature circuit to maintain the same speed and torque will be
Answer (Detailed Solution Below)
DC Motor Armature Control Question 3 Detailed Solution
\({T_2} = {T_1}\) \({\phi _2} = 0.9{\phi _1}\) \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)
\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)
R = 1.79 Ω
Top DC Motor Armature Control MCQ Objective Questions
A 220V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1 and armature current is 10A. If the excitation of the machine is reduced by 10% the extra resistance to be put in the armature circuit to maintain the same speed and torque will be
Answer (Detailed Solution Below)
DC Motor Armature Control Question 4 Detailed Solution
Download Solution PDF\({T_2} = {T_1}\) \({\phi _2} = 0.9{\phi _1}\) \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)
\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)
R = 1.79 Ω
When the armature and field of a DC motor are supplied with current, what does the armature experience?
Answer (Detailed Solution Below)
DC Motor Armature Control Question 5 Detailed Solution
Download Solution PDFExplanation:
Correct Option: A force that tries to rotate it in an appropriate direction
Definition: When the armature and field of a DC motor are supplied with current, the armature experiences a force due to the interaction of the magnetic field generated by the field windings and the current flowing through the armature conductors. This force causes the armature to rotate, driving the motor in the desired direction.
Working Principle:
The operation of a DC motor is governed by the principle of electromagnetic induction. When the current flows through the armature conductors, positioned within the magnetic field created by the field windings, a force is exerted on the conductors according to Lorentz's force law. The direction of this force is determined by Fleming's Left-Hand Rule:
- The thumb represents the direction of the force (motion).
- The index finger represents the direction of the magnetic field.
- The middle finger represents the direction of the current.
As a result, the armature experiences a torque that causes it to rotate. This rotation is the fundamental working mechanism of DC motors, enabling them to convert electrical energy into mechanical energy.
Key Components Involved:
- Armature: The rotating part of the motor where the current flows through the conductors.
- Field Windings: The stationary coils that produce a magnetic field when energized.
- Commutator: A device that ensures the current direction in the armature windings changes periodically, maintaining consistent torque in a single direction.
- Brushes: Conductive components that transfer current to the rotating armature through the commutator.
Advantages of DC Motors:
- Precise control of speed and torque, making them suitable for applications requiring variable speeds.
- High starting torque, which is ideal for applications such as electric trains, cranes, and elevators.
- Reliable performance and simple construction.
Applications:
- Industrial machinery requiring adjustable speed and torque.
- Transportation systems such as electric vehicles and trains.
- Home appliances like fans and washing machines.
Correct Option Analysis:
The correct option is:
Option 4: A force that tries to rotate it in an appropriate direction.
This option accurately reflects the fundamental working principle of DC motors. When the armature and field are supplied with current, the armature experiences a force due to the electromagnetic interaction, resulting in rotation. This rotation is the desired outcome for converting electrical energy into mechanical energy.
Additional Information
To further understand the analysis, let’s evaluate the other options:
Option 1: An unstable force that tries to halt rotation.
This option is incorrect as the force generated in the armature does not halt rotation. Instead, it produces torque to initiate and sustain rotation. In a properly functioning DC motor, the electromagnetic force is stable and contributes to smooth operation.
Option 2: No force at all.
This option is incorrect because, when the armature and field are supplied with current, an electromagnetic force is always generated due to the interaction of current and magnetic fields. Without this force, the armature would not rotate, and the motor would be non-functional.
Option 3: A force that resists rotation.
This option is incorrect because the force exerted on the armature due to electromagnetic induction aids rotation rather than resisting it. A resisting force would oppose the intended operation of the motor, which is not the case in a properly functioning DC motor.
Conclusion:
The operation of a DC motor is a direct application of electromagnetic principles. When the armature and field windings are supplied with current, the interaction of the magnetic field and current generates a torque that causes the armature to rotate. This rotation is harnessed to perform mechanical work, making DC motors indispensable in various industrial and domestic applications. Understanding the forces at play within the motor clarifies why Option 4 is the correct answer and highlights the inaccuracies in the other options.
DC Motor Armature Control Question 6:
A 220V, DC shunt motor is operating at a speed of 1440 rpm. The armature resistance is 1 and armature current is 10A. If the excitation of the machine is reduced by 10% the extra resistance to be put in the armature circuit to maintain the same speed and torque will be
Answer (Detailed Solution Below)
DC Motor Armature Control Question 6 Detailed Solution
\({T_2} = {T_1}\) \({\phi _2} = 0.9{\phi _1}\) \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)
\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)
R = 1.79 Ω
DC Motor Armature Control Question 7:
A 50KW dc shunt motor is loaded to draw rated armature current at any given speed. When driven
- At half the rated speed of armature voltage control and
- At 1.5 times the rated speed by field control the respective output power delivered by the motor are approximately
Answer (Detailed Solution Below)
25KW in (i) and 50 KW in (ii)
DC Motor Armature Control Question 7 Detailed Solution
Below rated speed the machine behaves as a constant torque drive hence
\(P \propto N\) \(\frac{{{P_1}}}{{{P_2}}} = \frac{{{N_1}}}{{{N_2}}}\)
\(\frac{{{P_2}}}{{50KW}} = \frac{{0.5{N_1}}}{{{N_1}}}\)
\({P_2} = 25\ KW\)
Above base speed machine acts as a constant power drive hence the power is nothing but the rated power 50KW
DC Motor Armature Control Question 8:
In an adjustable speed dc drive operated by an ac-dc converter with armature voltage control upto base speed,
Answer (Detailed Solution Below)
DC Motor Armature Control Question 8 Detailed Solution
Speed control methods:
Basic speed control methods in DC drives/motors are
- Field weakening speed control
- Armature resistance control
- Voltage control
Armature resistance control :
- This method is used to vary the speed from rated to below the rated speed.
- By connecting an external resistance in series with the armature, the voltage across the armature is varied as well as speed varied.
- The resistance varies from maximum position to the minimum position.
- This method is also known as the Constant Torque-variable speed control method.
- Here power is varied linearly, but Torque remains constant.
The characteristics of Power, Torque vs Speed is shown below.
Points to remember:
- The field weakening control method is used to vary the speed beyond the rated speed.
- In the field weakening control, Power remains constant and Torque varies.
DC Motor Armature Control Question 9:
Which of the following assumptions is true for armature controlled DC motor?
Answer (Detailed Solution Below)
DC Motor Armature Control Question 9 Detailed Solution
Armature Controlled DC Motor:
This method is used when speeds below the base speed are required.
As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat or resistance in series with the armature circuit as shown in Figure.
As controller resistance is increased the voltage across the armature is decreased, thereby decreasing the armature speed.
For a load constant torque, speed is approximately proportional to the voltage across the armature or Back emf.
\({E_b} = \frac{{NPϕ Z}}{{60A}}\)
Where,
Eb = Induced or Generated EMF Or Voltage in a parallel path.
P = Number of poles.
ϕ = Flux per poles in Weber.
A = Parallel paths.
N = Speed in RPM
∴ Eb ∝ N
So, Back EMF is proportional to speed.
Important Points
τ = Kϕ Ia
τ ∝ Ia
Torque developed is directly proportional to Armature current.
DC Motor Armature Control Question 10:
A dc series motor driving an electric train faces a constant power load. It is running at rated speed and rated voltage. If the speed has to be brought down to 0.25p.u the supply voltage has to be approximately brought down to
Answer (Detailed Solution Below)
0.5 p.u
DC Motor Armature Control Question 10 Detailed Solution
For constant power drive NT is constant and also for series motor \(T \propto I_a^2\)
\(\frac{{{N_1}}}{{{N_2}}} = \frac{{I_2^2}}{{I_1^2}}\) from the relation we can find that \({I_2} = 2{I_1}\)
\({V_1}{I_1} = {V_2}{I_2}\) \({V_2} = \frac{{{V_1}{I_1}}}{{2{I_1}}} = 0.5{V_1}\)
If V1 is 1pu then V2 is 0.5pu
DC Motor Armature Control Question 11:
A 250-V DC shunt motor has a shunt field resistance of 150 Ω and an armature resistance of 0.3 Ω. For a given load. The motor runs at 1400 rpm, and drawing 20 A current. If a resistance of 75 Ω is added in series with the field, find the new speed (rad/sec). Assume load torque and flux and constant.
Answer (Detailed Solution Below) 8785 - 8795
DC Motor Armature Control Question 11 Detailed Solution
The field current corresponds to:
\({I_f} = \frac{{{V_f}}}{{{R_f}}} = \frac{{250}}{{150}} = 1.66\;A\)
Now:
IA = IL - If = 20 – 1.66 = 18.34 Amps
Observe that the current of the field circuit is much smaller than the armature current. Thus, adding the speed control resistance leads to minor power dissipation.
Ea = VS – IaRa = 250 – 5.502 = 244.5 V
EA = K ϕ ω
\(K\phi = \frac{{{E_A}}}{\omega } = \frac{{244.5}}{{2 \times \pi \times 1400}} = 0.0278\)
\({I_f} = \frac{{{V_s}}}{{{R_f} + {R_{adj}}}} = \frac{{250}}{{225}} = 1.11\;Amps\)
The new armature current corresponds to:
Ia = IL – If = 20 – 1.11 = 18.89 Amps
Thus, the new Ea is:
Ea = VS – IaRa = 250 – 5.667 = 244.33 V
\(\omega = \frac{{244.33}}{{0.0278}} = 8789\;rad/s\)
DC Motor Armature Control Question 12:
When the armature and field of a DC motor are supplied with current, what does the armature experience?
Answer (Detailed Solution Below)
DC Motor Armature Control Question 12 Detailed Solution
Explanation:
Correct Option: A force that tries to rotate it in an appropriate direction
Definition: When the armature and field of a DC motor are supplied with current, the armature experiences a force due to the interaction of the magnetic field generated by the field windings and the current flowing through the armature conductors. This force causes the armature to rotate, driving the motor in the desired direction.
Working Principle:
The operation of a DC motor is governed by the principle of electromagnetic induction. When the current flows through the armature conductors, positioned within the magnetic field created by the field windings, a force is exerted on the conductors according to Lorentz's force law. The direction of this force is determined by Fleming's Left-Hand Rule:
- The thumb represents the direction of the force (motion).
- The index finger represents the direction of the magnetic field.
- The middle finger represents the direction of the current.
As a result, the armature experiences a torque that causes it to rotate. This rotation is the fundamental working mechanism of DC motors, enabling them to convert electrical energy into mechanical energy.
Key Components Involved:
- Armature: The rotating part of the motor where the current flows through the conductors.
- Field Windings: The stationary coils that produce a magnetic field when energized.
- Commutator: A device that ensures the current direction in the armature windings changes periodically, maintaining consistent torque in a single direction.
- Brushes: Conductive components that transfer current to the rotating armature through the commutator.
Advantages of DC Motors:
- Precise control of speed and torque, making them suitable for applications requiring variable speeds.
- High starting torque, which is ideal for applications such as electric trains, cranes, and elevators.
- Reliable performance and simple construction.
Applications:
- Industrial machinery requiring adjustable speed and torque.
- Transportation systems such as electric vehicles and trains.
- Home appliances like fans and washing machines.
Correct Option Analysis:
The correct option is:
Option 4: A force that tries to rotate it in an appropriate direction.
This option accurately reflects the fundamental working principle of DC motors. When the armature and field are supplied with current, the armature experiences a force due to the electromagnetic interaction, resulting in rotation. This rotation is the desired outcome for converting electrical energy into mechanical energy.
Additional Information
To further understand the analysis, let’s evaluate the other options:
Option 1: An unstable force that tries to halt rotation.
This option is incorrect as the force generated in the armature does not halt rotation. Instead, it produces torque to initiate and sustain rotation. In a properly functioning DC motor, the electromagnetic force is stable and contributes to smooth operation.
Option 2: No force at all.
This option is incorrect because, when the armature and field are supplied with current, an electromagnetic force is always generated due to the interaction of current and magnetic fields. Without this force, the armature would not rotate, and the motor would be non-functional.
Option 3: A force that resists rotation.
This option is incorrect because the force exerted on the armature due to electromagnetic induction aids rotation rather than resisting it. A resisting force would oppose the intended operation of the motor, which is not the case in a properly functioning DC motor.
Conclusion:
The operation of a DC motor is a direct application of electromagnetic principles. When the armature and field windings are supplied with current, the interaction of the magnetic field and current generates a torque that causes the armature to rotate. This rotation is harnessed to perform mechanical work, making DC motors indispensable in various industrial and domestic applications. Understanding the forces at play within the motor clarifies why Option 4 is the correct answer and highlights the inaccuracies in the other options.