DC Motor Armature Control MCQ Quiz - Objective Question with Answer for DC Motor Armature Control - Download Free PDF

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.

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,

E_b = 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

∴ E_b ∝  N

So, Back EMF is proportional to speed.

Important Points

τ = Kϕ I_a

τ ∝  Ia

Torque developed is directly proportional to Armature current.

\({T_2} = {T_1}\)           \({\phi _2} = 0.9{\phi _1}\)            \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)

\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)

R = 1.79 Ω

\({T_2} = {T_1}\)           \({\phi _2} = 0.9{\phi _1}\)            \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)

\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)

R = 1.79 Ω

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.

\({T_2} = {T_1}\)           \({\phi _2} = 0.9{\phi _1}\)            \({\phi _1} \times {I_1} = {\phi _2} \times {I_2}\)

\(\begin{array}{l} {I_2} = 11.11A\\ {E_1} = 220 - 10 \times 1 = 210\ V\\ {E_2} = 220 - 11.1 \times \left( {1 + R} \right)\\ \frac{{{N_2}}}{{{N_1}}} = \frac{{{E_2}}}{{{E_1}}} \times \frac{{{\phi _1}}}{{{\phi _2}}}\\ I = \frac{{220 - 11.1\left( {1 + R} \right)}}{{210}} \times \frac{{{\phi _1}}}{{0.9{\phi _1}}} \end{array}\)

R = 1.79 Ω

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

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.

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,

E_b = 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

∴ E_b ∝  N

So, Back EMF is proportional to speed.

Important Points

τ = Kϕ I_a

τ ∝  Ia

Torque developed is directly proportional to Armature current.

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

The field current corresponds to:

\({I_f} = \frac{{{V_f}}}{{{R_f}}} = \frac{{250}}{{150}} = 1.66\;A\) 

Now:

I_A = I_L - I_f = 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.

E_a = V_S – I_aR_a = 250 – 5.502 = 244.5 V

E_A = 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:

I_a = I_L – I_f = 20 – 1.11 = 18.89 Amps

Thus, the new E_a is:

E_a = V_S – I_aR_a = 250 – 5.667 = 244.33 V

\(\omega = \frac{{244.33}}{{0.0278}} = 8789\;rad/s\)

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.