BEEE Sample Papers MST 2

BEEE Sample Papers MST 2

Syllabus

Operating characteristics of DC motor, working principle, construction and applications of Induction motor, Brushless DC motors, Introduction to special Motors (Geared DC motor, Servo Motors, Stepper motors, Linear DC motor).

Principle of sensing, Basic requirements of transducers, classification of transducers, passive transducers: capacitive, inductive, LVDT, strain gauge, thermistor, Hall-Effect, Active transducers: piezoelectric, photoelectric and thermocouple, Tri-axial Sensors: Gyroscopes, Accelerometers, Magnetometers.

SAMPLE PAPER 1 of 3

Academic Year: 2022- 2023Semester: 1st or 2nd
Time: 1 hourMaximum Marks: 20

Instructions: Attempt all questions

SECTION-A

Each question carries 2 marks.

Question 1: Explain the working principle of a DC motor.

DC motor is an electric motor that converts electrical energy into mechanical energy. It works on the principle of electromagnetic induction. The working principle of a DC motor can be explained as follows:

  • Magnetic field: A DC motor has a stationary magnetic field that is created by either a permanent magnet or an electromagnet.
  • Electric current: The rotor of the motor has a coil of wire wound around it, which is connected to a DC power source.
  • Interaction: When an electric current flows through the coil, it produces a magnetic field that interacts with the stationary magnetic field of the stator,
  • Commutator: The commutator is a mechanical switch that changes the direction of the current in the rotor coil every half rotation, so that the rotor continues to rotate in the same direction.
  • Mechanical power: The interaction between the magnetic fields of the stator and rotor produces a torque, which causes the rotor to rotate and mechanical power is produced.

Question 2: What are the different types of compound DC motors? Discuss their characteristics.

Compound DC motors are of two types: cumulative compound DC motor and differential compound DC motor. Their characteristics are as follows:

  • Cumulative compound DC motor: In this type of motor, the shunt field winding is connected in series with the armature winding, which results in the addition of the two fields. This type of motor has high starting torque but low speed regulation.
  • Differential compound DC motor: In this type of motor, the shunt field winding is connected in parallel with the armature winding, which results in the subtraction of the two fields. This type of motor has low starting torque but good speed regulation.

Question 3: What is a brushless DC motor? List its main components.

A brushless DC motor is a type of motor that uses electronic commutation instead of brushes and commutator to switch the current in the motor windings. Its main components are:

  • Rotor: It is a permanent magnet that rotates and creates the magnetic field.
  • Stator: It contains a set of stationary windings that produce a magnetic field that interacts with the magnetic field of the rotor.
  • Electronic controller: It is a circuit that controls the flow of current to the stator windings based on the position of the rotor, which is detected by sensors.
  • Sensors: They are used to detect the position of the rotor and send signals to the electronic controller.

Question4 : Explain the working principle of a single-phase induction motor.

A single-phase induction motor is a type of motor that works on the principle of electromagnetic induction. Its working principle can be explained as follows:

  • Starting torque: A single-phase AC current is supplied to the stator windings, which creates a magnetic field that interacts with the rotor windings, causing the rotor to start rotating due to the starting torque.
  • Auxiliary winding: A single-phase induction motor has an auxiliary winding that is used to create a rotating magnetic field to provide the starting torque.
  • Run winding: Once the rotor starts rotating, the auxiliary winding is disconnected, and the motor runs on the run winding, which is a single-phase winding that is not self-starting.

Question 5: What are the advantages and disadvantages of a gear motor?

Gear motor is a type of motor that is used to provide high torque at low speed. Its advantages and disadvantages are as follows:

Advantages:

  • High torque: Gear motors are designed to provide high torque at low speed, making them suitable for heavy-duty applications such as lifting and moving heavy objects.
  • Efficiency: Gear motors are more efficient than other types of motors as they provide more mechanical power output for the same electrical power input.
  • Compact: Gear motors are compact and can be used in applications where space is limited.
  • Speed control: Gear motors can be easily controlled to provide a range of speeds using different gear ratios.

Disadvantages:

  • Limited speed range: Gear motors have a limited speed range and cannot be used for high-speed applications.
  • Complexity: Gear motors are more complex than other types of motors as they require additional components such as gears and bearings.
  • Noise and vibration: Gear motors can produce noise and vibration due to the meshing of gears.

SECTION-B

Each question carries 5 marks.

Question 6:

a) Draw and explain the torque-speed curve of a DC shunt motor.

  • The torque-speed curve is a graphical representation of the relationship between the torque and speed of the motor when different loads are applied to it.
  • At no load, the motor runs at its maximum speed, which is also called the no-load speed.
  • As the load on the motor increases, the speed decreases, and the torque increases to compensate for the load.
  • This is known as the pull-out torque, which is the maximum torque that the motor can produce before stalling.
  • The curve also shows the maximum torque that the motor can produce, which is called the rated torque.
  • The rated torque is the torque that the motor can produce continuously without overheating.

b) Design a torque-armature current curve for the same.

  • The torque-armature current curve is a graphical representation of the relationship between the torque and the armature current of the motor.
  • It shows the torque that the motor can produce for different armature currents.
  • The curve is a linear relationship, which means that the torque is directly proportional to the armature current.
  • The slope of the curve is the motor constant, which is a measure of the motor’s performance.
  • The motor constant is given by the ratio of the rated torque to the rated armature current.
  • The torque-armature current curve is used to design the motor for a specific application, and it helps to determine the maximum torque that the motor can produce for a given current.

Question 7: Working Principle of a Piezoelectric Transducer

a) Describe the working principle of a piezoelectric transducer.

  • A piezoelectric transducer converts electrical energy into mechanical energy or vice versa.
  • It works on the principle of the piezoelectric effect, which is the ability of certain materials to generate an electric charge when they are subjected to mechanical stress.
  • The piezoelectric effect is reversible, which means that the same materials can also generate mechanical stress when an electric field is applied to them.
  • Piezoelectric materials are crystals or ceramics that have a special structure, which allows them to generate an electric charge when they are subjected to mechanical stress.
  • When a mechanical force is applied to the material, it causes the crystal structure to change, which creates an electric charge across the material.
  • The amount of charge generated is proportional to the force applied, and the polarity of the charge depends on the direction of the force.

b) Discuss its applications in the field of engineering.

  • Piezoelectric transducers are used in a wide range of engineering applications, including sensors, actuators, and energy harvesting devices.
  • They are commonly used in sensors for measuring pressure, acceleration, and force in industries such as aerospace, automotive, and medical to monitor and control various parameters.
  • Ultrasonic transducers use high-frequency sound waves to image the internal structure of objects or to detect flaws in materials.
  • They are used in medical imaging, industrial non-destructive testing and imaging, and cleaning applications.
  • Piezoelectric actuators are used for precise positioning in various applications such as robotics, optical systems, and nano-manufacturing.
  • Piezoelectric energy harvesting devices are used to convert ambient vibrations into electrical energy and power wireless sensors and other low-power devices.

SAMPLE PAPER 2 of 3

Academic Year: 2022- 2023Semester: 1st or 2nd
Time: 1 hourMaximum Marks: 20

Instructions: Attempt all questions

SECTION-A

Each question carries 2 marks.

Question 1: Explain the working principle of a stepper motor.

Answer:

  • A stepper motor rotates in precise increments, known as steps, when electrical pulses are applied to it.
  • The motor has a stator that contains multiple electromagnets and a rotor with teeth or poles.
  • The rotor moves in small steps when the electromagnetic poles of the stator are energized in a specific sequence.
  • The step angle of a stepper motor is determined by the number of poles in the stator and the teeth or poles in the rotor.

Question 2: What are transducers? Classify them based on their basic requirements.

Answer:

  • Transducers are devices that convert one form of energy into another.
  • They are used to measure physical parameters such as temperature, pressure, force, and displacement.
  • Transducers can be classified into two types based on their basic requirements:
    • Active transducers: These transducers require an external power source to operate.
    • Passive transducers: These transducers do not require an external power source and generate an output signal in response to a stimulus.

Question 3: Describe the construction of a slip ring rotor in a single-phase induction motor. What are its advantages and disadvantages?

Answer:

  • A slip ring rotor in a single-phase induction motor consists of a laminated iron core with slots.
  • The rotor winding is placed in these slots, and the ends of the winding are connected to slip rings.
  • The slip rings allow for the external connection of resistance or reactance, which enables control of the starting and running performance of the motor.
  • The advantages of a slip ring rotor include increased starting torque, better speed regulation, and reduced current surges. The disadvantages include higher cost, complex construction, and maintenance issues.
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Question 4: What is a servo motor? List its advantages and disadvantages.

Answer:

  • A servo motor is an electric motor that is used in a closed-loop control system to precisely control the position, velocity, and acceleration of a load.
  • The advantages of a servo motor include high accuracy, high speed, and high torque-to-inertia ratio. The disadvantages include high cost, complexity, and the need for a feedback system.

Question 5: What are the different types of BLDC motors and what are their applications?

Answer:

  • There are two types of BLDC motors:
    • the trapezoidal motor and the sinusoidal motor.
  • The trapezoidal motor has a simpler design and is suitable for applications such as fans, pumps, and blowers.
    • It is also used in home appliances such as refrigerators, washing machines, and air conditioners.
  • The sinusoidal motor has a more complex design but offers better efficiency and is suitable for high-performance applications such as electric vehicles, robots, and drones.
    • It is also used in industrial automation, aerospace, and medical equipment.
  • The advantages of BLDC motors include high efficiency, low maintenance, and longer lifespan compared to brushed DC motors. They also offer precise speed and torque control, low noise, and reduced electromagnetic interference.

SECTION-B

Each question carries 5 marks.

Question 6: Draw and explain the characteristics curves for a DC motor, i.e., torque-armature current, speed-armature current, and speed-torque curves.

Answer:

  • A DC motor is a device that converts electrical energy into mechanical energy.
  • The characteristics curves for a DC motor are graphical representations that show the relationship between various performance parameters.
  • There are three important curves for a DC motor: the torque-armature current curve, the speed-armature current curve, and the speed-torque curve.

1.1 Torque-armature current curve:

  • The torque-armature current curve is a plot of the torque produced by the motor against the current flowing through its armature.
  • The torque produced is directly proportional to the armature current, and the curve is linear up to a certain point, known as the saturation point.
  • Beyond the saturation point, the torque decreases due to the increased magnetic field saturation in the motor.

1.2 Speed-armature current curve:

  • The speed-armature current curve is a plot of the motor’s speed against the armature current.
  • The curve shows that the speed of the motor decreases with an increase in the armature current, due to the increased armature reaction and the resultant decrease in the magnetic field.

1.3 Speed-torque curve:

  • The speed-torque curve is a plot of the motor’s speed against the torque produced.
  • The curve shows that the speed of the motor decreases with an increase in the torque load.
  • The maximum torque that the motor can produce is known as the stall torque, and the speed of the motor at zero torque is known as the no-load speed.

In conclusion, the characteristics curves for a DC motor are important tools that provide insights into the motor’s performance under different operating conditions. These curves help in selecting and designing a motor for a particular application.

Question 7: Explain the working principle of a Linear Variable Differential Transformer (LVDT). Discuss its applications in the field of engineering.

Answer: 2.1 Working Principle:

  • A Linear Variable Differential Transformer (LVDT) is an electromechanical transducer that converts the linear motion of an object into an electrical signal.
  • It consists of a primary coil, a secondary coil, and a movable core that is connected to the object whose motion is to be measured.
  • The primary coil is excited by an AC voltage, and the secondary coils are connected in series opposition, such that their output voltage is zero when the core is in the center position.
  • When the core moves, it induces a voltage in each of the secondary coils, which are out of phase with each other, resulting in a differential output voltage that is proportional to the core displacement.

2.2 Applications:

  • LVDTs are widely used in the field of engineering for measurement and control applications, where high accuracy, reliability, and durability are required.
  • LVDTs are used in displacement measurement systems, such as in position sensors for machine tools, robotics, and automation systems.
  • LVDTs are used in force measurement systems, such as in load cells for material testing, and in pressure sensors for hydraulic and pneumatic systems.
  • LVDTs are used in vibration measurement systems, such as in accelerometers for monitoring machine vibrations and structural health.
  • LVDTs are used in level measurement systems, such as in fuel and water level sensors for aircraft and automobiles.
  • LVDTs are used in medical devices, such as in ultrasound transducers for imaging, and in heart rate monitors.

2.3 Advantages:

  • LVDTs offer high accuracy, resolution, and linearity, with minimal hysteresis and drift.
  • LVDTs are immune to electromagnetic interference and are intrinsically safe in hazardous environments.
  • LVDTs have a long lifespan and require minimal maintenance, making them highly reliable and cost-effective.

2.4 Limitations:

  • LVDTs require an AC excitation voltage, which may not be suitable for some applications that require DC power.
  • LVDTs are sensitive to temperature variations, and their performance may be affected by environmental factors such as humidity and vibration.
  • LVDTs are not suitable for applications that require high-speed measurements, as their response time is relatively slow.

SAMPLE PAPER 3 of 5

Acadmic Year: 2022- 2023Semester: 1st or 2nd
Time: 1 hourMaximum Marks: 20

Instructions: Attempt all questions

SECTION-A

Each question carries 2 marks.

Question 1: Explain the working principle of a slip ring rotor in a single-phase induction motor. (2 marks)

The working principle of a slip ring rotor in a single-phase induction motor can be explained as follows:

  • A single-phase induction motor consists of a stator and a rotor. The stator is the stationary part of the motor and has a set of windings that create a rotating magnetic field. The rotor is the rotating part of the motor and can either be a squirrel cage or a slip ring rotor.
  • In a slip ring rotor, the rotor windings are connected to slip rings, which are metallic rings mounted on the rotor shaft. The slip rings are connected to brushes that ride on the rings and allow electrical contact with external circuits.
  • During the starting of the motor, the slip rings are short-circuited through a resistor, which limits the current in the rotor windings. The rotating magnetic field in the stator induces a current in the rotor windings, which creates a magnetic field that interacts with the stator field, causing the rotor to rotate.
  • As the rotor speed increases, the slip rings are gradually released from the short-circuiting resistor. This allows the rotor windings to carry the full load current, providing higher torque and smoother acceleration.
  • Once the rotor reaches its rated speed, the slip rings are no longer needed, and the brushes are lifted off the rings, disconnecting the rotor windings from the external circuits.

Question 2: What is a brushed DC motor? Describe its main parts. (2 marks)

A brushed DC motor is a type of DC motor that uses brushes to make electrical contact with the commutator, a segmented cylindrical core mounted on the motor shaft. The commutator allows the motor to change the direction of the current in the rotor windings, causing the rotor to rotate. The main parts of a brushed DC motor are:

  • Stator: The stationary part of the motor that contains the stator windings, which create a magnetic field.
  • Rotor: The rotating part of the motor that contains the rotor windings, which interact with the stator field to produce torque.
  • Commutator: A segmented cylindrical core mounted on the motor shaft that allows the brushes to make electrical contact with the rotor windings.
  • Brushes: Carbon or graphite blocks that press against the commutator and make electrical contact with the rotor windings.
  • Shaft: The central axis of the motor that connects the rotor to the load.

Question 3: Describe the construction and working principle of a photoelectric transducer. (2 marks)

A photoelectric transducer is a type of sensor that converts light energy into electrical signals. The construction and working principle of a photoelectric transducer can be explained as follows:

  • Construction: A photoelectric transducer consists of a light source, a sensing element, and a signal processing circuit. The light source emits light, which is directed towards the sensing element. The sensing element can be a photodiode, phototransistor, or photoresistor, depending on the application.
  • Working principle: When light strikes the sensing element, it generates a photocurrent, which is proportional to the intensity of the light. The signal processing circuit amplifies and filters the photocurrent and converts it into a usable electrical signal. The electrical signal can be used to control a device or as input to a control system.

Question 4: What are the advantages and disadvantages of a thermistor? (2 marks)

A thermistor is a type of resistor that exhibits a change in resistance with temperature. The advantages and disadvantages of a thermistor are:

Advantages:

  • High sensitivity: Thermistors are highly sensitive to temperature changes, allowing for accurate temperature measurements.
  • Low cost: Thermistors are relatively inexpensive compared to other temperature sensors.
  • Small size: Thermistors are compact and can be easily integrated into electronic circuits.
  • Wide temperature range: Thermistors can be designed to operate over a wide temperature range, from -100°C to over 300°C.

Disadvantages:

  • Non-linear response: The resistance-temperature curve of a thermistor is non-linear, which can complicate temperature measurements and calibration.
  • Self-heating: Thermistors generate heat when current flows through them, which can affect the accuracy of temperature measurements.
  • Limited accuracy: The accuracy of thermistors is limited by their non-linear response and self-heating effects.

Question 5: Explain the working principle and applications of a photoconductive cell. (2 marks)

A photoconductive cell is a type of photoelectric transducer that uses a photoconductive material, such as cadmium sulfide or selenium, to convert light energy into electrical signals. The working principle and applications of a photoconductive cell can be explained as follows:

  • Working principle: When light strikes the photoconductive material, it causes a change in the material’s electrical conductivity. The change in conductivity is proportional to the intensity of the light. The photoconductive material is connected to an external circuit, which measures the change in conductivity and generates an electrical signal.
  • Applications: Photoconductive cells are used in a wide range of applications, including:
    • Light sensors: Photoconductive cells can be used to detect light levels in a variety of settings, such as in cameras, light meters, and solar panels.
    • Automatic control systems: Photoconductive cells can be used in automatic control systems to detect changes in ambient light levels and adjust system settings accordingly.
    • Security systems: Photoconductive cells can be used in security systems to detect unauthorized intrusions by sensing changes in light levels.
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SECTION-B

Each question carries 5 marks.

Question 6: Describe the construction, working principle, and applications of a capacitive transducer. (5 marks)

Capacitive transducers are devices that convert physical quantities, such as displacement or pressure, into changes in capacitance. They consist of two conductive plates separated by a dielectric material. The construction, working principle, and applications of a capacitive transducer can be described as follows:

  • Construction: A capacitive transducer consists of two conductive plates, which are separated by a dielectric material. The conductive plates are typically made of metal, while the dielectric material can be air, glass, or a plastic material. The conductive plates are connected to an external circuit, which measures the changes in capacitance.
  • Working principle: When a physical quantity, such as displacement or pressure, is applied to the capacitive transducer, it causes a change in the distance between the conductive plates. This change in distance results in a change in capacitance, which is proportional to the physical quantity being measured. The capacitance is measured by an external circuit, which converts the change in capacitance into an electrical signal.
  • Applications: Capacitive transducers are used in a wide range of applications, including:
    • Displacement measurement: Capacitive transducers can be used to measure small changes in distance or displacement, such as in micrometers or position sensors.
    • Pressure measurement: Capacitive transducers can be used to measure changes in pressure, such as in barometers or pressure sensors.
    • Humidity measurement: Capacitive transducers can be used to measure changes in humidity, by measuring changes in the dielectric constant of a moisture-sensitive material.
    • Level sensing: Capacitive transducers can be used to detect changes in liquid level, by measuring changes in capacitance as the liquid level changes.

Question 7: Explain the working principle and types of a three-phase induction motor. (5 marks)

A three-phase induction motor is a type of electric motor that operates on the principle of electromagnetic induction. It consists of a stator, which contains the stationary windings, and a rotor, which contains the rotating windings. The working principle and types of three-phase induction motors are explained below:

  • Working principle: When a three-phase power supply is connected to the stator windings, a rotating magnetic field is created. The rotating magnetic field induces a current in the rotor windings, which creates a magnetic field that interacts with the stator magnetic field. This interaction causes the rotor to rotate, as the magnetic fields push and pull against each other. The speed of rotation is determined by the frequency of the power supply and the number of poles in the motor.
  • Types of three-phase induction motors:
    1. Squirrel-cage induction motor: In a squirrel-cage induction motor, the rotor windings are made up of conductive bars, which are arranged in a cage-like structure. The bars are short-circuited at both ends by metal rings. When the rotating magnetic field interacts with the short-circuited bars, it induces a current that creates a magnetic field that interacts with the stator magnetic field. This interaction causes the rotor to rotate.
    2. Wound-rotor induction motor: In a wound-rotor induction motor, the rotor windings are made up of separate coils, which are connected to slip rings. The slip rings are connected to external resistors, which can be used to vary the rotor resistance and control the motor speed. When the rotating magnetic field interacts with the rotor windings, the induced current creates a magnetic field that interacts with the stator magnetic field, causing the rotor to rotate.
    3. Double-fed induction motor: In a double-fed induction motor, both the stator and rotor windings are connected to power supplies. The rotor windings are connected to a separate power supply through slip rings, which can be used to vary the frequency and phase of the rotor current. The stator and rotor magnetic fields interact with each other, causing the rotor to rotate.

Three-phase induction motors are widely used in industrial applications, such as pumps, fans, compressors, and conveyor systems, due to their reliability, efficiency, and low maintenance requirements. The type of motor used depends on the specific application requirements, such as the required torque, speed, and control options.

SAMPLE PAPER 4 of 5

Academic Year: 2022- 2023Semester: 1st or 2nd
Time: 1 hourMaximum Marks: 20

Instructions: Attempt all questions

SECTION-A

Each question carries 2 marks.

Question 1: Describe the constructional parts of a DC motor.

A DC motor is an electro-mechanical device that converts electrical energy into mechanical energy. The constructional parts of a DC motor are as follows:

  1. Stator: It is the stationary part of the motor that houses the field winding.
  2. Rotor: It is the rotating part of the motor that houses the armature winding.
  3. Commutator: It is a segmented ring that connects the armature winding to the external circuit through brushes.
  4. Brushes: These are conductive carbon blocks that make contact with the commutator segments to supply electrical power to the armature winding.
  5. Shaft: It connects the rotor to the external load.

Question 2: What is a lap winding and a wave winding in a DC motor? Discuss their differences.

Lap winding and wave winding are two types of armature windings used in DC motors. The differences between them are as follows:

  1. Lap Winding: In lap winding, the armature coils are connected in parallel, and each coil overlaps with the adjacent coil. The number of parallel paths in the armature winding is equal to the number of poles in the motor.
  2. Wave Winding: In wave winding, the armature coils are connected in series, and each coil is wound in one direction only. The number of parallel paths in the armature winding is always two, regardless of the number of poles in the motor.

The main difference between lap winding and wave winding is their output voltage and current characteristics. Lap winding produces a higher voltage but lower current, while wave winding produces a lower voltage but higher current.

Question 3: Explain the working principle of a single-phase induction motor.

A single-phase induction motor is a type of AC motor that operates on a single-phase AC supply. The working principle of a single-phase induction motor is based on the induction of a magnetic field in the rotor due to the interaction of the stator magnetic field and the rotor conductors. The working principle can be explained in the following steps:

  1. When the single-phase AC supply is applied to the stator winding, a magnetic field is produced in the stator core.
  2. The rotating magnetic field produced by the stator induces an emf in the rotor conductors.
  3. The induced emf in the rotor conductors creates a current that produces a magnetic field.
  4. The interaction of the stator magnetic field and the rotor magnetic field produces a torque that causes the rotor to rotate.

Question 4: What are the advantages and disadvantages of a thermistor?

A thermistor is a type of temperature sensor that operates on the principle of the change in resistance with temperature. The advantages and disadvantages of a thermistor are as follows:

Advantages:

  1. High Sensitivity: Thermistors are highly sensitive to changes in temperature, making them suitable for temperature measurement applications.
  2. Fast Response Time: Thermistors respond quickly to changes in temperature, making them ideal for temperature control applications.
  3. Low Cost: Thermistors are relatively inexpensive compared to other types of temperature sensors.

Disadvantages:

  1. Nonlinear Response: The resistance of a thermistor is nonlinear with temperature, which makes temperature calibration and compensation necessary.
  2. Limited Temperature Range: Thermistors have a limited temperature range compared to other types of temperature sensors.
  3. Self-Heating: Thermistors can self-heat, which can affect their accuracy and response time.

Question 5: Discuss the types of photoelectric transducers.

A photoelectric transducer is a type of sensor that converts light energy into electrical energy. The types of photoelectric transducers are as follows:

  1. Photovoltaic Cells: These are devices that produce a voltage when exposed to light. They are commonly used in solar panels to generate electricity.
  2. Photoconductive Cells: These are devices that change their resistance when exposed to light. They are used in light meters, cameras, and other light sensing applications.
  3. Photodiodes: These are semiconductor devices that produce a current when exposed to light. They are used in light sensors, optical communication systems, and other applications.
  4. Phototransistors: These are semiconductor devices that amplify a current when exposed to light. They are used in light sensing applications that require higher sensitivity than photodiodes.
  5. Photoelectric Encoders: These are devices that convert mechanical motion into electrical signals using a combination of a light source and a photoelectric sensor. They are used in motion control systems and other applications that require precise position sensing.

SECTION-B

Each question carries 5 marks.

Question 6: Draw and explain the torque-speed curve for a DC series motor. Discuss its characteristics.

A. Introduction

  • The torque-speed curve of a DC series motor is a graph that shows the relationship between the motor’s speed and torque output.
  • It is a crucial characteristic of a DC series motor and is commonly used in the design and analysis of the motor.
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B. Torque-Speed Curve

  • The torque-speed curve for a DC series motor is a hyperbolic curve that starts at the origin of the graph and extends to the maximum operating speed of the motor.
  • The curve shows that the torque output of the motor decreases as the speed increases.
  • This is due to the fact that the back electromotive force (EMF) of the motor increases with speed, which reduces the net voltage across the armature and hence reduces the torque output.

C. Characteristics

  1. High Starting Torque
  • DC series motors have a very high starting torque due to their high torque output at low speeds.
  • This makes them suitable for applications such as cranes, elevators, and other heavy-duty applications that require high starting torque.
  1. Limited Speed Range
  • DC series motors have a limited speed range because the torque output decreases rapidly as the speed increases.
  • This means that they are not suitable for applications that require high-speed operation.
  1. High Power Density
  • DC series motors have a high power density, which means that they can produce a lot of power in a relatively small package.
  • This makes them suitable for applications where space is limited, such as in electric vehicles and robotics.
  1. Poor Efficiency
  • DC series motors have poor efficiency at high speeds due to their high armature current and high losses in the armature windings.
  • This means that they are not suitable for applications where high efficiency is required.

Question 7: Describe the working principle of a capacitive transducer. Discuss its applications in the field of engineering.

A. Introduction

  • A capacitive transducer is a device that measures changes in capacitance to determine physical quantities such as displacement, pressure, or humidity.
  • It works on the principle of a change in capacitance due to a change in the distance between the capacitor plates or dielectric constant of the medium between them.

B. Working Principle

  • A capacitive transducer consists of two parallel plates with a dielectric medium between them.
  • The capacitance of the transducer varies with the distance between the plates or the dielectric constant of the medium.
  • When a physical quantity such as displacement, pressure or humidity is applied to the transducer, it causes a change in the dielectric constant or distance between the plates, which in turn causes a change in capacitance.
  • This change in capacitance is then converted into an electrical signal, which is measured by an electronic circuit.

C. Applications

  • Capacitive transducers have a wide range of applications in the field of engineering, such as in pressure and humidity sensors, level measurement, displacement sensors, and touch screens.
  • In pressure sensors, the capacitance changes as the pressure is applied, and this change is used to determine the pressure.
  • In humidity sensors, the capacitance changes with the humidity level, and this change is used to determine the humidity.
  • Capacitive transducers are also used in displacement sensors, where the distance between the plates changes due to the displacement of an object, and this change is used to determine the displacement.
  • Capacitive touch screens work on the same principle, where the change in capacitance is detected when a finger touches the screen, and this is used to determine the position of the touch.

D. Advantages

  • Capacitive transducers have high accuracy and resolution, making them suitable for applications where precise measurements are required.
  • They are also highly sensitive to changes in the physical quantity being measured, making them suitable for measuring small changes.
  • Capacitive transducers are also durable and have a long lifespan.

E. Disadvantages

  • Capacitive transducers can be affected by changes in temperature, humidity, and other environmental factors, which can affect their accuracy.
  • They are also sensitive to electromagnetic interference, which can affect their performance.

SAMPLE PAPER 5 of 5

Academic Year: 2022- 2023Semester: 1st or 2nd
Time: 1 hourMaximum Marks: 20

Instructions: Attempt all questions

SECTION-A

Each question carries 2 marks.

Question 1: A DC motor is supplied with a voltage of 220 V and a current of 5 A. If the motor has an efficiency of 80%, what is the output power of the motor?

Given: Input voltage (V) = 220 V Input current (I) = 5 A Efficiency = 80%

To find: Output power of the motor

Formula: Output power = Input power x Efficiency

Input power = V x I

Calculation: Input power = 220 V x 5 A = 1100 W Output power = 1100 W x 0.8 = 880 W

Answer: The output power of the motor is 880 W.

Question 2: A 3-phase induction motor has a slip of 3% and a synchronous speed of 1200 rpm. What is the rotor speed of the motor?

Given: Slip (s) = 3% Synchronous speed (Ns) = 1200 rpm

To find: Rotor speed of the motor

Formula: Rotor speed = Synchronous speed x (1 – Slip)

Calculation: Rotor speed = 1200 rpm x (1 – 0.03) = 1164 rpm

Answer: The rotor speed of the motor is 1164 rpm.

Question 3: A thermistor has a resistance of 5000 Ω at a temperature of 25°C and a resistance of 1000 Ω at a temperature of 100°C. What is the temperature of the thermistor when its resistance is 2500 Ω?

Given: Resistance at 25°C = 5000 Ω Resistance at 100°C = 1000 Ω Resistance at unknown temperature = 2500 Ω

To find: Temperature of the thermistor

Formula: Resistance-temperature relationship for a thermistor can be expressed as: R = R₀ * exp(B*(1/T – 1/T₀)) where R₀ and T₀ are known reference values, B is a constant, T is the temperature in Kelvin, and exp is the exponential function.

Taking natural logarithm of both sides, we get: ln(R/R₀) = B*(1/T – 1/T₀)

Solving for T, we get: T = 1 / (1/T₀ – (1/B) * ln(R/R₀))

Calculation: At 25°C, R₀ = 5000 Ω and T₀ = 298.15 K At 100°C, R₀ = 1000 Ω and T₀ = 373.15 K At unknown temperature, R = 2500 Ω

Substituting the values, we get: T = 1 / (1/298.15 K – (1/3977 K) * ln(2500/5000)) T = 46.84°C

Answer: The temperature of the thermistor when its resistance is 2500 Ω is 46.84°C.

Question 4: A piezoelectric transducer produces a voltage of 50 mV when subjected to a force of 100 N. What is the sensitivity of the transducer?

Formula: Sensitivity = Voltage / Force

Solution:

  • Voltage = 50 mV
  • Force = 100 N
  • Sensitivity = 50 mV / 100 N
  • Sensitivity = 0.5 mV/N

Answer: The sensitivity of the piezoelectric transducer is 0.5 mV/N.

Question 5: A stepper motor has 200 steps per revolution. If the motor is operated at a frequency of 500 Hz, what is the rotational speed of the motor in rpm?

Formula: Revolutions per second = Frequency / Steps per revolution Rotational speed (in rpm) = Revolutions per second x 60

Solution:

  • Steps per revolution = 200
  • Frequency = 500 Hz
  • Revolutions per second = 500 Hz / 200 steps per revolution
  • Revolutions per second = 2.5
  • Rotational speed = 2.5 revolutions per second x 60 seconds per minute
  • Rotational speed = 150 rpm

Answer: The rotational speed of the stepper motor is 150 rpm.

SECTION-B

Each question carries 5 marks.

Question 6: A DC shunt motor has a rated voltage of 220 V and a rated power output of 10 kW. The motor has an armature resistance of 0.2 Ω and a field resistance of 200 Ω. If the motor is operating at full load with an armature current of 40 A, calculate the following:

a. The speed of the motor. b. The torque developed by the motor. c. The efficiency of the motor.

Given:

  • Rated voltage = 220 V
  • Rated power output = 10 kW
  • Armature resistance = 0.2 Ω
  • Field resistance = 200 Ω
  • Armature current at full load = 40 A

Formulae:

  • Back EMF (Eb) = (V – Ia x Ra)
  • Torque (T) = (Eb / Φ) x Kt
  • Power input (Pi) = V x Ia
  • Power output (Po) = Eb x Ia
  • Efficiency (η) = Po / Pi

Solution:

a. Calculation of Motor Speed:

  • Back EMF (Eb) = (V – Ia x Ra)
  • Eb = (220 V – 40 A x 0.2 Ω)
  • Eb = 212 V
  • The back EMF is proportional to the motor speed, so the speed can be calculated using the following formula:
  • Speed = (Eb / Φ) x (60 / 2π)
  • Since this is a DC shunt motor, Φ is constant and given by:
  • Φ = V / Ra
  • Φ = 220 V / 200 Ω
  • Φ = 1.1 A
  • Substituting the values, we get:
  • Speed = (212 V / 1.1 A) x (60 / 2π)
  • Speed ≈ 2000 rpm

Answer: The speed of the motor is approximately 2000 rpm.

b. Calculation of Motor Torque:

  • Torque (T) = (Eb / Φ) x Kt
  • The value of Kt is not given, but we can calculate it using the following formula:
  • Kt = T / (Φ x Ia)
  • Rated power output = 10 kW
  • Therefore, Φ x Ia = 10 kW / V = 45.45 A
  • Substituting the given values, we get:
  • Kt = (10,000 W / 2000 rpm) / (45.45 A)
  • Kt ≈ 0.22 Nm/A
  • Now we can calculate the torque:
  • T = (Eb / Φ) x Kt
  • T = (212 V / 1.1 A) x 0.22 Nm/A
  • T ≈ 42 Nm

Answer: The torque developed by the motor is approximately 42 Nm.

c. Calculation of Motor Efficiency:

  • Power input (Pi) = V x Ia
  • Pi = 220 V x 40 A
  • Pi = 8800 W
  • Power output (Po) = Eb x Ia
  • Po = 212 V x 40 A
  • Po = 8480 W
  • Efficiency (η) = Po / Pi
  • η = 8480 W / 8800 W
  • η ≈ 0.965 or 96.5%

Answer: The efficiency of the motor is approximately 96.5%.

Question 2: A strain gauge with a gauge factor of 2 is used to measure the strain in a cantilever beam. The beam has a length of 1 m, a width of 0.1 m, and a thickness of 0.01 m. If a force of 100 N is applied to the free end of the beam, what is the strain in the beam? Assume that the beam has a Young’s modulus of 200 GPa.

Solution:

Given: Gauge factor = 2 Length of the beam (L) = 1 m Width of the beam (W) = 0.1 m Thickness of the beam (t) = 0.01 m Force applied (F) = 100 N Young’s modulus (E) = 200 GPa = 200 x 10^9 Pa

Formula: Strain = (change in resistance/R) = (GF x F x L)/(E x A)

where, R = initial resistance of the strain gauge GF = gauge factor F = force applied L = length of the beam E = Young’s modulus of the material A = cross-sectional area of the beam

Calculations: The cross-sectional area of the beam can be calculated as: A = W x t = 0.1 m x 0.01 m = 0.001 m^2

Using the formula for strain, we can find the strain in the beam: Strain = (GF x F x L)/(E x A) Strain = (2 x 100 N x 1 m)/(200 x 10^9 Pa x 0.001 m^2) Strain = 1 x 10^-5

Therefore, the strain in the cantilever beam is 1 x 10^-5.

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