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

Explanation:

Back EMF in a DC Motor

Definition: Back EMF (Electromotive Force), also known as counter EMF, is the voltage that is induced in the armature windings of a DC motor when it rotates. This induced voltage opposes the applied voltage (supply voltage) and is a result of the motor's operation as a generator while it is running. The back EMF is a fundamental characteristic of DC motors and plays a critical role in their operation.

Working Principle:

When a DC motor operates, electrical energy is supplied to the armature windings, creating a magnetic field. This magnetic field interacts with the field produced by the permanent magnets or field windings, resulting in a torque that causes the armature to rotate. As the armature rotates, the conductors within it cut through the magnetic field, inducing an electromotive force (EMF) in accordance with Faraday's Law of Electromagnetic Induction. This induced EMF is known as back EMF because it opposes the current flow that is driving the motor.

The back EMF is given by the formula:

E_b = (P × Φ × Z × N) / (60 × A)

Where:

• E_b = Back EMF (volts)
• P = Number of poles
• Φ = Flux per pole (webers)
• Z = Total number of armature conductors
• N = Speed of the armature (RPM)
• A = Number of parallel paths in the armature winding

Significance of Back EMF:

• Opposition to Current: Back EMF opposes the applied voltage and regulates the current flowing through the armature. This self-regulating mechanism ensures that the motor draws only the necessary current, preventing excessive current flow that could damage the motor.
• Energy Conversion: Back EMF is a direct consequence of energy conversion in the motor. As electrical energy is converted into mechanical energy, the motor also acts as a generator, producing the back EMF.
• Speed Control: The magnitude of the back EMF is proportional to the speed of the armature. As the motor speed increases, the back EMF increases, reducing the net voltage and current in the armature. This relationship helps maintain a stable operating speed under varying load conditions.
• Efficiency: The presence of back EMF minimizes energy losses by limiting the current flow in the armature, improving the efficiency of the motor.

Importance in DC Motors:

Back EMF is an essential feature of DC motors. Without back EMF, the motor would draw an excessive amount of current from the power supply, leading to overheating and potential damage. It provides a natural feedback mechanism that ensures the motor operates safely and efficiently. Additionally, back EMF is used in speed control and monitoring systems to assess the motor's operating conditions.

Correct Option Analysis:

The correct option is:

Option 2: Back EMF

This option correctly identifies the induced EMF in a DC motor that opposes the current flow in the armature conductors as back EMF. The term "back EMF" precisely describes this phenomenon, which is a fundamental characteristic of DC motors and is responsible for regulating current, maintaining efficiency, and ensuring safe operation.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Field EMF

This option is incorrect. Field EMF refers to the electromotive force associated with the field windings of a motor or generator. It is not the induced EMF in the armature conductors that opposes the current flow. Field EMF is related to the creation of the magnetic field, not the regulation of armature current.

Option 3: Induced Voltage

While it is true that back EMF is an induced voltage, this term is too generic and does not specifically describe the opposing EMF in a DC motor. Induced voltage can refer to any voltage generated by electromagnetic induction, including those in transformers, generators, or other electrical devices. Back EMF is a more precise term for the phenomenon in question.

Option 4: Supply EMF

This option is incorrect. Supply EMF refers to the voltage provided by the power source to the motor. It is the applied voltage that drives the current through the armature windings. Back EMF, on the other hand, is the voltage induced within the motor that opposes the supply EMF.

Conclusion:

Understanding the concept of back EMF is crucial for comprehending the operation of DC motors. Back EMF is the induced voltage in the armature windings that opposes the applied voltage, regulating the current and ensuring efficient operation. It is distinct from other types of EMF, such as field EMF or supply EMF, and plays a vital role in the performance and safety of DC motors. The correct answer, option 2, accurately identifies this phenomenon as back EMF.

Explanation:

In a DC Series Motor, at Low or Light Load, Why is the Torque Low?

Definition: A DC series motor is a type of direct current (DC) motor where the field winding is connected in series with the armature winding. This configuration ensures that the same current flows through both the armature and the field winding. The torque produced in a DC series motor is proportional to the product of the armature current and the magnetic flux generated by the field winding. The relationship between the load, current, flux, and torque is a critical factor in understanding the behavior of the motor under different load conditions.

Correct Option Analysis:

The correct option is:

Option 1: Due to the low armature current and low field flux.

This is the correct explanation for the low torque in a DC series motor at light or low load. The torque in a DC series motor is given by the formula:

T ∝ Φ × I_a

Where:

• T is the torque.
• Φ is the magnetic flux produced by the field winding.
• I_a is the armature current.

At light or low load, the motor requires less torque to overcome the load. As a result, the armature current (I_a) is low. Since the field winding in a series motor is directly connected in series with the armature, the field flux (Φ) is also reduced because it depends on the armature current. The reduced flux and low armature current together result in a significantly lower torque output. Thus, the torque decreases due to the combined effect of low armature current and low field flux.

Detailed Explanation:

1. **Torque Equation in a DC Series Motor**:

The torque in a DC motor is generated by the interaction of the magnetic field and the armature current. The relationship can be expressed as:

T = k × Φ × I_a

Here, k is a constant that depends on the design of the motor. In a series motor, since the field winding is connected in series with the armature, the flux (Φ) is directly proportional to the armature current (I_a) up to the point of core saturation.

2. **Effect of Light Load**:

When the load on the motor is light, the required torque is minimal. As a result, the motor draws less current from the supply. The reduced current leads to a decrease in the field flux (Φ), since the field winding is in series with the armature. With both the armature current (I_a) and the field flux (Φ) being low, the torque produced by the motor is also low.

3. **Performance at Low Load**:

At low load, the motor operates at higher speeds because the back EMF (E_b) approaches the supply voltage (V). The reduced current (I_a) in the armature and field winding results in lower magnetic flux (Φ), which, combined with the low armature current, causes the torque to be minimal. This behavior is a characteristic of DC series motors and highlights their dependency on load conditions for efficient operation.

Conclusion:

The low torque at light load in a DC series motor is primarily due to the low armature current and the corresponding low field flux. This explanation aligns with the correct option (Option 1).

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Due to the saturation of the field cores.

This option is incorrect. While core saturation can affect the performance of a DC motor, it is not the primary reason for low torque at light load. Saturation occurs when the magnetic field in the core reaches its maximum level and cannot increase further, even with an increase in current. At light load, the current (and hence the flux) is low, so saturation does not occur.

Option 3: Due to the high field flux.

This option is also incorrect. At light load, the armature current is low, resulting in low field flux, not high field flux. High field flux would require a higher armature current, which is not the case under light load conditions.

Option 4: Due to the high armature current.

This option is incorrect because, at light load, the armature current is low. High armature current occurs only under heavy load conditions, where the motor needs to produce more torque to overcome the load.

Conclusion:

Understanding the behavior of DC series motors under different load conditions is essential for their proper application and operation. At light load, the low armature current and corresponding low field flux result in low torque, as explained in the correct option. The other options misinterpret the underlying principles and do not accurately describe the motor's behavior in this scenario.

Torque in a DC Series Motor at Saturation

Definition: In a DC series motor, torque is initially proportional to the square of the armature current under unsaturated conditions. However, as the motor approaches magnetic saturation, the relationship between torque and armature current changes due to the limitation in further increase of flux.

Working Principle: The torque (T) produced in a DC series motor is generally given by the expression:

T ∝ Φ × I_a

Here:

• T = Torque
• Φ = Magnetic flux produced by the field winding
• I_a = Armature current

Under normal unsaturated conditions, the magnetic flux (Φ) is directly proportional to the armature current (I_a). As a result, the torque becomes proportional to the square of the armature current:

T ∝ Φ × I_a ∝ I_a²

However, as the motor operates at higher currents, the magnetic circuit of the motor approaches saturation. Once saturation occurs, further increases in armature current do not lead to a proportional increase in flux (Φ). Instead, the flux becomes nearly constant, and the torque becomes proportional to the armature current only:

T ∝ I_a

Correct Option Analysis:

The correct option is:

Option 1: Armature current only

At the point of saturation, the magnetic flux (Φ) produced by the field winding becomes constant due to the limitations in further magnetizing the core material. Therefore, the torque is no longer dependent on the flux and becomes directly proportional to the armature current (I_a). This is why, under saturation, the torque in a DC series motor is proportional to the armature current only.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Both armature current and field flux

Under normal (unsaturated) conditions, torque in a DC series motor is proportional to both the armature current and the magnetic flux (Φ × I_a). However, once saturation occurs, the flux becomes constant and no longer varies with armature current. Hence, this option is incorrect under saturation conditions.

Option 3: Field flux only

This option is incorrect because the torque in a DC series motor is never solely dependent on the field flux (Φ). Torque is always a product of flux and armature current (Φ × I_a). Therefore, even under saturation, armature current is essential in determining the torque.

Option 4: The square of the armature current

This option is correct only under unsaturated conditions, where flux is proportional to armature current (Φ ∝ I_a) and torque becomes proportional to the square of the armature current (T ∝ I_a²). However, under saturation conditions, flux becomes constant, and torque is no longer proportional to the square of the armature current.

Conclusion:

At the point of saturation in a DC series motor, the torque becomes proportional to the armature current (I_a) only. This is due to the magnetic saturation of the core material, which prevents further increases in flux despite increases in armature current. Understanding this behavior is crucial for analyzing the performance characteristics of DC series motors in various operating conditions.

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.

Explanation:

4-Point Starter

Definition: A 4-point starter is a type of electrical starter used for starting and protecting DC motors. It is called a 4-point starter because it has four terminals: L (Line), F (Field), A (Armature), and N (No-volt release coil). Unlike the 3-point starter, the 4-point starter's holding coil circuit is made independent of the field circuit, which eliminates the drawback of the 3-point starter where the no-volt release (NVR) coil could potentially disengage during operation due to variations in the field current.

Working Principle: The 4-point starter operates on the principle of electromagnetic force. When the motor is started, the handle of the starter is moved from the OFF position towards the ON position. As the handle moves, the resistance connected in series with the armature is gradually reduced, allowing a controlled increase in armature current. The no-volt release coil ensures that the motor stops in case of a power failure or if the handle is not in the ON position. Since the NVR coil is connected directly across the supply voltage, its operation is independent of the field circuit, which ensures better stability and reliability.

Advantages:

• The holding coil circuit is independent of the field circuit, ensuring stable operation even if the field current fluctuates.
• Provides better protection to the motor against power supply interruptions or faults.
• Prevents the motor from restarting automatically after a power failure, thus ensuring safety.

Disadvantages:

• More complex design compared to a 3-point starter, leading to higher manufacturing costs.
• Requires additional components, such as a separate connection for the no-volt release coil.

Applications: 4-point starters are widely used in DC motors, especially in applications requiring precise control and protection, such as in industrial machinery, elevators, and cranes.

Correct Option Analysis:

The correct option is:

Option 3: 4-point starter

The 4-point starter is specifically designed to address the limitation of the 3-point starter by making the holding coil circuit independent of the field circuit. This design ensures that the no-volt release coil remains energized irrespective of variations in the field current, thereby providing better operational stability and protection for the motor.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: 2-point starter

The 2-point starter is used for smaller DC motors and is rarely employed in practical applications. It does not have a no-volt release coil, which makes it less reliable for protecting the motor during power supply interruptions or faults. This type of starter does not solve the issue of making the holding coil circuit independent of the field circuit, so it is not the correct answer.

Option 2: 3-point starter

The 3-point starter is commonly used for starting DC motors. However, in this design, the no-volt release coil is connected in series with the field winding. This dependency can lead to operational issues if the field current fluctuates, as it may cause the no-volt release coil to disengage and stop the motor. This limitation is addressed in the 4-point starter, making the 3-point starter an incorrect option for the given question.

Option 4: 5-point starter

The concept of a 5-point starter is not commonly used or defined in standard electrical engineering practices. This option is invalid and does not pertain to the question.

Option 5: (No option provided)

This option is left blank, so it is not a valid choice.

Conclusion:

The 4-point starter effectively eliminates the drawback of the 3-point starter by making the holding coil circuit independent of the field circuit. This design ensures stable and reliable operation of the motor, even in the presence of field current fluctuations. The 4-point starter is therefore the correct answer for the given question and is widely used in industrial applications requiring precise motor control and protection.

In a DC motor, T ∝ ϕI_a

In series motor, ϕ ∝ I_a

⇒ T ∝ (I_a)²

In shunt motor, ϕ is constant

⇒ T ∝ I_a

The characteristic of DC series and shunt motor are shown below.

Characteristics of DC series motor:

Characteristics of DC shunt motor:

Regenerative braking: In this type braking back emf Eb is greater than the supply voltage V, which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

Overhauling motor: A motoring motor is converting electrical energy into mechanical energy. An overhauling motor is being driven by the load and is converting mechanical energy into electricity, acting as a generator.

The speed regulation of a DC motor is defined as the change in speed from no load to full load. It is expressed as a fraction or a percentage of the full load speed.

Percentage speed regulation \(= \frac{{{N_{nl}} - {N_{fl}}}}{{{N_{fl}}}} \times 100\)

• The direction of rotation of a DC compound motor may be conveniently reversed by reversing the connection of both series and shunt field winding or may be reversing the armature connection but not both at the same time
• When the direction of power flow reverses, a differentially compounded motor becomes a

           cumulatively compounded generator

• When the direction of power flow reverses, a cumulatively compounded motor becomes a

           differentially compounded generator

Concept:

Torque:

• When armature conductors of a DC motor carry current in the presence of stator field flux, a mechanical torque is developed between the armature and the stator.
•  Torque is given by the product of the force and the radius at which this force acts.

Power:

• Mechanical power developed by the motor is given by P_m = E_b × I_a
• E_b is EMF developed and I_a is armature current
• In terms of torque and speed mechanical power developed is given by P_m = torque × speed.
• Speed is in radians per second (ω).
• \({\rm{\omega }} = 2\;\pi \frac{N}{{60}}\) radians per sec.

Calculations:

\({\rm{\omega }} = \frac{{2\; \times \;{\rm{\pi }}\; \times \;1500}}{{60}}\)

= 157.08 radians/sec.

Power = generated emf × armature current

= 250 × 50

= 12500 watts.

Torque = power/speed.

Torque = \(\frac{{12500}}{{157.08}}\)

Torque = 79.577 Nm.

If back emf of a dc motor vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply.

The voltage equation of dc motor is, E_b = V – I_aR_a

As the back emf vanishes zero, the whole supply voltage appears across armature and heavy current flows.

DC series motor:

It has a very high starting torque. Hence it is used for heavy-duty applications such as traction, driving cranes, hoists, large lifts, air compressors......etc.

DC shunt motor:

It is almost a constant speed motor. Hence it is used for driving constant speed like shafts, lathes, centrifugal pumps, small printing presses, paper making machines, blowers, conveyor belts,.....etc.

DC cumulative compound motor:

It is used for moderately high starting torque applications with intermittent loadings.

Ex:- Rolling mills, escalators, elevators, punching machines, cutting tools, planar machines.....etc.

Concept:

Voltage equation for dc machine working as the motor is

V_t = E_b + I_aR_a

Where,

V_t is the terminal voltage

E_b is induced emf or back emf

I_a is armature current

R_a is armature resistance

Calculation:

Given that,

V_t = 200 V

I_a = 20 A

R_a = 0.5 Ω

Therefore, induced emf in the dc motor is

200 = E_b + (20 × 0.5)

⇒ E_b = 200 – 10 = 190 V

Power flow in the motor

Input power - Armature loss = Developed power

Developed power - (Iron + Mechanical losses) = Shaft power

The relationship between torque and power is:

\(τ= P / ω\)

where, τ = Torque

P = Power

ω = Speed in rad/sec

Calculation

Given, τd = 206 Nm

τshaft = 200 Nm

Iron and mechanical loss = P_d - Pshaft

Iron and mechanical loss = \((\tau_{d}-\tau_{shaft})× \omega_r\)

\(\omega_r={2π N_s\over 60}\)

\(\omega_s={2× π× 1500\over 60}=157\space rad/s=50π \space rad/s\)

Iron and mechanical loss = (206 - 200) × 50 × π  

Iron and mechanical loss = 300 π

• Based on construction, the basic difference between DC & AC motor is the commutator.
• The commutator is only present in DC motor, which performs the commutation phenomenon.
• Commutation can be defined as the reversal of current (AC to DC or DC to AC).
• In DC motor commutator converts DC to AC(Inverter function). 
• In DC generator commutator converts AC to DC (Rectifier function )