All questions of Electromechanical Energy Conversion Principles for Electrical Engineering (EE) Exam

Understanding Electromagnetic Torque
In the context of electromagnetic energy conversion devices, torque is a critical factor. The equation provided outlines different components of torque, but only one specifically represents the electromagnetic torque.
Breaking Down the Torque Equation
The equation given is:
Te = 1/2is2dLs/dθr + 1/2ir2dLr/dθr + isirdMsr/dθr
Each term represents a different contribution to the total torque (Te):
- 1/2is2dLs/dθr: This term relates to the stator current and the derivative of the stator inductance with respect to rotor position.
- 1/2ir2dLr/dθr: This term corresponds to the rotor current and the derivative of the rotor inductance concerning rotor position.
- isirdMsr/dθr: This is the term of interest, representing the interaction between the stator and rotor magnetic fields.
Why Option C is Correct
The third term, isirdMsr/dθr, specifically represents the electromagnetic torque:
- Interaction Between Currents: It accounts for the coupling between the stator and rotor currents (is and ir) and their mutual inductance (Ms).
- Energy Conversion: This term showcases the conversion of electrical energy into mechanical energy, which is the fundamental purpose of electromagnetic devices.
Conclusion
Thus, the electromagnetic torque in the equation is effectively captured by the term isirdMsr/dθr, which signifies the energy conversion process. This is why option C is the correct answer, as it encapsulates the essence of electromagnetic torque in such devices.

Doubly excited magnetic systems are commonly used in machines such as generators and motors. In these systems, both the stator and rotor have separate windings, allowing for independent control of the magnetic fields. In the given system, the stator is of the salient pole type, which means it has protruding poles that concentrate the magnetic flux.

The equation for magnetic field stored energy in establishing the currents from zero to is and ir can be derived as follows:

1. Magnetic Field Energy in Stator Winding (Wfld_stator):
The energy stored in the stator winding can be calculated using the formula:
Wfld_stator = 1/2 * is^2 * Ls,
where is is the stator current and Ls is the stator winding inductance.

2. Magnetic Field Energy in Rotor Winding (Wfld_rotor):
The energy stored in the rotor winding can be calculated using the formula:
Wfld_rotor = 1/2 * ir^2 * Lr,
where ir is the rotor current and Lr is the rotor winding inductance.

3. Magnetic Field Energy due to Mutual Inductance (Wfld_mutual):
The energy stored in the magnetic field due to the mutual inductance between the stator and rotor windings can be calculated using the formula:
Wfld_mutual = Mrs * is * ir,
where Mrs is the mutual inductance between the stator and rotor windings.

4. Total Magnetic Field Energy (Wfld_total):
The total magnetic field energy in the system is the sum of the energy in the stator winding, rotor winding, and mutual inductance:
Wfld_total = Wfld_stator + Wfld_rotor + Wfld_mutual
= 1/2 * is^2 * Ls + 1/2 * ir^2 * Lr + Mrs * is * ir.

Therefore, the correct equation for magnetic field stored energy in establishing the currents from zero to is and ir is:
Wfld = 1/2 * is^2 * Ls + 1/2 * ir^2 * Lr + Mrs * is * ir. (Option C)

Given data:
Current (I) = 5A
Flux linkages (Φ) = 2.4 wb-turns

To calculate the energy stored in the magnetic field of the coil, we can use the formula:

Energy (W) = 0.5 * L * I^2

Where,
L = Inductance of the coil
I = Current flowing through the coil

To find the inductance (L), we can use the formula:

L = Φ / I

Substituting the given values, we have:

L = 2.4 wb-turns / 5A
L = 0.48 H

Now, substituting the values of L and I in the energy formula:

W = 0.5 * 0.48 H * (5A)^2
W = 0.5 * 0.48 H * 25A^2
W = 0.5 * 0.48 H * 625
W = 7.5 Joules

Therefore, the energy stored in the magnetic field of the coil is 7.5 Joules.

Since none of the given options match the calculated value, it seems there might be an error in the question or the options provided. However, the closest option to the calculated value is option 'A' with a value of 6 Joules.

Explanation:

The electromagnetic force and torque are developed in any physical system due to the interaction between magnetic fields and electric currents. These forces and torques can act in different directions depending on the specific configuration of the system. However, in this particular scenario, we are considering the effect of these forces and torques on the magnetic stored energy.

Understanding magnetic stored energy:

Magnetic stored energy refers to the energy stored in a magnetic field. It is proportional to the square of the magnetic flux and inversely proportional to the permeability of the medium. When current flows through a conductor, it creates a magnetic field around it, and this magnetic field stores energy.

The effect of electromagnetic force and torque on magnetic stored energy:

The electromagnetic force and torque developed in a physical system tend to decrease the magnetic stored energy at constant flux. This can be explained as follows:

When a force or torque is applied to a magnetic system, it tends to change the magnetic field configuration. If the force or torque causes the magnetic field to change in such a way that the magnetic flux decreases, then the magnetic stored energy also decreases. This is because the energy stored in the magnetic field is directly proportional to the square of the magnetic flux.

Example:

Let's consider the example of an electromagnetic relay. An electromagnetic relay consists of a coil of wire wound around a soft iron core. When current flows through the coil, it generates a magnetic field that attracts an armature, causing mechanical movement. In this case, if an external force is applied to the armature, it will tend to move in a direction opposite to the magnetic force developed by the coil. This will result in a change in the magnetic field configuration, leading to a decrease in the magnetic flux and consequently, a decrease in the magnetic stored energy.

Conclusion:

In summary, the electromagnetic force and torque developed in any physical system tend to decrease the magnetic stored energy at constant flux. This is because the force or torque causes a change in the magnetic field configuration, resulting in a decrease in the magnetic flux and thus, a decrease in the magnetic stored energy.

Flux Linkages in a Toroid

To extract energy from the supply system, the flux linkages of the magnetic field in a toroid must be changing or varying. This is because the generation of an electromotive force (EMF) in a coil is directly proportional to the rate of change of magnetic flux passing through the coil.

Explanation:

- In a toroid, the magnetic field is confined within the core due to its closed-loop structure, which helps in increasing the flux linkage with the coils wound around it.
- When the magnetic flux passing through the toroid changes, it induces an EMF in the coils due to Faraday's law of electromagnetic induction.
- This EMF can then be used to extract energy from the supply system for various applications.
- If the magnetic flux linkages are constant, there will be no change in flux, and hence no induced EMF will be generated in the coils.
- Therefore, for energy extraction to occur, the flux linkages must be changing or varying, allowing for the generation of EMF in the coils.

Conclusion:

In conclusion, in order for a toroid to extract energy from the supply system, the flux linkages of the magnetic field must be changing or varying to induce an electromotive force in the coils. This change in flux is essential for the conversion of magnetic energy into electrical energy.

Magnetic Field Energy Storage Capacity

The energy storing capacity of magnetic field is given by the formula:

U = (1/2) L I^2

where U is the energy stored in the magnetic field, L is the inductance of the coil, and I is the current flowing through it.

Similarly, the energy storing capacity of electric field is given by the formula:

U = (1/2) C V^2

where U is the energy stored in the electric field, C is the capacitance of the capacitor, and V is the voltage across it.

Comparison of Magnetic Field and Electric Field Energy Storage Capacity

The energy storing capacity of magnetic field is about 25,000 times greater than that of electric field. This can be explained by the following reasons:

1. Inductance vs Capacitance

Inductance is a property of a coil that opposes any change in current flowing through it by inducing a voltage in the opposite direction. It is proportional to the square of the number of turns in the coil and the area enclosed by them. As a result, inductance can be increased by increasing the number of turns or the area of the coil.

Capacitance, on the other hand, is a property of a capacitor that opposes any change in voltage across it by storing charge on its plates. It is proportional to the area of the plates and inversely proportional to the distance between them. As a result, capacitance can be increased by increasing the area of the plates or decreasing the distance between them.

Since inductance can be increased more easily than capacitance, the energy storing capacity of magnetic field is higher than that of electric field.

2. Current vs Voltage

The energy stored in a magnetic field depends on the square of the current flowing through the coil, while the energy stored in an electric field depends on the square of the voltage across the capacitor. Since the current flowing through a coil can be much higher than the voltage across a capacitor, the energy storing capacity of magnetic field is higher than that of electric field.

Applications of Magnetic Field Energy Storage

The high energy storing capacity of magnetic field makes it useful in various applications, such as:

1. Inductors in electronic circuits to store energy and filter out unwanted frequencies.

2. Transformers to transfer energy from one circuit to another.

3. Electric motors and generators to convert electrical energy into mechanical energy and vice versa.

4. Magnetic storage devices such as hard disk drives to store digital data.

Conclusion

In summary, the energy storing capacity of magnetic field is about 25,000 times greater than that of electric field due to the higher inductance of coils and the higher current flowing through them. This makes magnetic field useful in various applications where high energy storage is required.

And (iii) are correct.

The correct answer is option 'C': (i), (iii), (iv).

Explanation:
(i) Electromagnetic torque can exist only if both windings carry current:
- Electromagnetic torque is generated in an electric motor when there is a magnetic field interaction between the stator and rotor windings. This magnetic field is created by the current flowing through both the stator and rotor windings. Therefore, electromagnetic torque can exist only if both windings carry current.

(iii) Reluctance torque doesn't depend on the direction of current in stator or rotor windings:
- Reluctance torque is a type of torque that arises due to the variation in the magnetic reluctance of the magnetic path between the stator and rotor of an electric motor. It is independent of the direction of current in the stator or rotor windings. Reluctance torque is generated when the stator and rotor teeth are not aligned properly, causing a variation in the magnetic reluctance and resulting in torque production.

(iv) Electromagnetic torque depends on the direction of currents in the stator and rotor windings:
- The direction of current in the stator and rotor windings plays a crucial role in determining the direction of the magnetic field produced by these windings. The interaction between the magnetic fields of the stator and rotor windings results in the generation of electromagnetic torque. Therefore, the direction of the currents in the stator and rotor windings directly affects the direction of the electromagnetic torque produced.

In summary, electromagnetic torque requires both windings to carry current, and its direction depends on the direction of currents in the windings. On the other hand, reluctance torque is independent of the direction of current in the windings.

Explanation:

Electromagnetic Force Direction:
The electromagnetic force developed in any physical system acts in such a direction as to tend to increase the co-energy at constant mmf.

Co-energy:
Co-energy is a measure of the energy stored in a system due to the presence of magnetic fields. It is defined as the energy required to establish a certain magnetic field in the system.

Constant mmf:
When the magnetomotive force (mmf) in a system remains constant, the electromagnetic force tends to increase the co-energy. This means that the direction of the electromagnetic force is such that it works to store more energy in the form of magnetic fields.

Implications:
- This behavior of the electromagnetic force is crucial in understanding the energy dynamics in electromagnetic systems.
- By increasing the co-energy at constant mmf, the system can store more energy for future use or for performing work.
In conclusion, the electromagnetic force in a physical system is directed in such a way that it tends to increase the co-energy at constant mmf. This principle is essential in the design and analysis of electromagnetic systems.

Doubly-excited magnetic systems refer to systems that have two independent sources of excitation, resulting in a magnetic field that is produced by two separate windings. These windings are often referred to as the field winding and the armature winding. In this case, all of the mentioned options, namely synchronous machines, loudspeakers and tachometers, and D.C shunt machines, are examples of doubly-excited magnetic systems.

Synchronous Machines:
Synchronous machines are AC machines that can operate as generators or motors. They have a field winding placed on the rotor and an armature winding on the stator. The field winding is excited by a DC source, while the armature winding carries the AC current. This dual excitation creates a magnetic field that interacts with the armature winding, resulting in the generation or conversion of electrical power.

Loudspeakers and Tachometers:
Loudspeakers and tachometers are examples of electromagnetic devices that utilize doubly-excited magnetic systems. In a loudspeaker, the field winding is represented by a permanent magnet, while the armature winding is created by a coil of wire attached to the diaphragm. The interaction between the magnetic field produced by the permanent magnet and the current flowing through the coil generates sound.

Similarly, in a tachometer, the field winding is represented by a permanent magnet, while the armature winding is created by a coil of wire. The rotation of the armature winding generates a voltage that is proportional to the speed of rotation, allowing the tachometer to measure the rotational speed of a machine.

D.C Shunt Machines:
D.C shunt machines are another example of doubly-excited magnetic systems. They have a field winding connected in parallel with the armature winding. The field winding is excited by a separate DC source, while the armature winding carries the load current. The interaction between the magnetic field produced by the field winding and the current flowing through the armature winding enables the machine to generate or convert electrical power.

In conclusion, all of the mentioned options, namely synchronous machines, loudspeakers and tachometers, and D.C shunt machines, are examples of doubly-excited magnetic systems. They all utilize two separate windings, namely the field winding and the armature winding, which interact to produce the desired electromagnetic effects.

Explanation:

Energy Conversion in a Magnetic Relay:
- In a magnetic relay, electrical energy is converted into magnetic field energy when the relay is energized.
- This magnetic field then acts on the armature, causing it to move and mechanically switch the relay contacts.

Energy Input during Armature Movement:
- When the armature moves slowly from the open position to the closed position, both mechanical and magnetic energy are involved in the process.
- The electrical energy input to the relay is divided between the mechanical work done in moving the armature and the magnetic field energy.

Calculation of Electrical Energy Input:
- The total electrical energy input (Welec) to the relay during this process can be divided into two parts: the mechanical work done (Wmech) and the magnetic field energy (Wfld).
- Since the armature moves slowly from the open to the closed position, the electrical energy input can be approximated as the average of the mechanical and magnetic energies: Welec = Wmech/2 + Wfld/2.

Conclusion:
- Therefore, the correct answer is option 'C', where the electrical energy input to the relay during the slow movement of the armature from open to closed position is divided equally between the mechanical work done and the magnetic field energy.

Energy stored in the mechanical system of linear motion type

The correct formula for calculating the energy stored in a mechanical system of linear motion type is option B, which is given as 1/2 mv^2.

Explanation:

1. Understanding mechanical energy:
Mechanical energy refers to the energy possessed by an object due to its motion or position. In the case of linear motion, mechanical energy is associated with the movement of an object along a straight line.

2. Types of mechanical energy:
There are two forms of mechanical energy: kinetic energy and potential energy.
- Kinetic energy (KE) is the energy possessed by an object due to its motion. It depends on the mass (m) and velocity (v) of the object.
- Potential energy (PE) is the energy possessed by an object due to its position or height. It depends on the mass (m) of the object and the height (h) at which it is located.

3. Energy stored in the mechanical system of linear motion:
In the case of linear motion, the energy stored in the mechanical system is solely kinetic energy since there is no change in height or position.
The formula to calculate the kinetic energy is given by KE = 1/2 mv^2, where:
- KE is the kinetic energy of the object.
- m is the mass of the object.
- v is the velocity of the object.

4. Derivation of the formula:
To understand why the formula for energy stored in the mechanical system of linear motion is 1/2 mv^2, let's consider the following steps:

- The work done (W) on an object is given by the product of the force applied (F) and the displacement (d) along the direction of the force, i.e., W = Fd.
- According to Newton's second law of motion, force (F) is given by the product of mass (m) and acceleration (a), i.e., F = ma.
- Substituting the value of force (F) in the work equation, we get W = mad.
- The equation for acceleration (a) is given by the change in velocity (Δv) divided by the change in time (Δt), i.e., a = Δv/Δt.
- Substituting the value of acceleration (a) in the work equation, we get W = m(Δv/Δt)d.
- Rearranging the equation, we get W = mΔv(d/Δt).
- Since velocity (v) is the change in displacement (Δd) divided by the change in time (Δt), i.e., v = Δd/Δt, we can substitute the value of velocity (v) in the work equation, giving W = m(v)(d/Δt).
- The term (d/Δt) represents the average velocity (v_avg) of the object.
- Therefore, the work done (W) can be written as W = m(v)(v_avg).
- The work done (W) is equal to the energy stored in the mechanical system.
- Thus, the energy stored in the mechanical system is given by E = m(v)(v_avg).
- Since v_avg is half of the final velocity (v