← MES Jr. Grade
Past Questions — 2024
In measurement systems, which of the following static characteristic(s) is/are desirable?
A)
Accuracy
B)
Sensitivity
C)
Reproducibility
D)
All of these
In measurement systems, static characteristics are the properties of an instrument that remain constant when the input signal is constant or varies slowly.
1. Accuracy: This is the degree of closeness of a measured value to the true value of the quantity. High accuracy is always desirable to minimize measurement error.
2. Sensitivity: This is the ratio of the change in output (response) to the change in input (measurand). A higher sensitivity allows an instrument to detect smaller changes in the input, which is generally desirable for precision measurements.
3. Reproducibility: This refers to the ability of an instrument to provide the same output for the same input when measured by different operators, using different equipment, or under different environmental conditions. High reproducibility ensures the reliability and consistency of the data.
Since all three characteristics (Accuracy, Sensitivity, and Reproducibility) are positive attributes that improve the quality and reliability of a measurement system, the correct option is 'All of these'.
On which of the following factors does the resistivity of a material depend?
A)
Resistance of the conductor
B)
Area of the conductor section
C)
Length of the conductor
D)
All of these
To solve this question, we must distinguish between 'Resistance' (R) and 'Resistivity' (ρ).
1. The formula relating the two is: R = ρ(L/A), where R is Resistance, ρ is Resistivity, L is Length, and A is the cross-sectional Area.
2. Rearranging for resistivity: ρ = (R * A) / L.
3. While the formula contains R, A, and L, resistivity (ρ) is an **intrinsic property** of the material. This means it does not change regardless of the shape, size, or length of the conductor. For example, a copper wire of 1 meter and a copper wire of 10 meters have different resistances, but they have the exact same resistivity because they are both made of copper.
4. Resistivity depends only on the **nature of the material** (chemical composition) and **temperature**.
Analysis of Options:
- (a) Resistance: Resistance is a result of resistivity and geometry, not a cause of it.
- (b) Area: Changing the area changes the resistance, not the resistivity.
- (c) Length: Changing the length changes the resistance, not the resistivity.
- (d) All of these: Incorrect, as none of the above factors affect resistivity.
Note: In many poorly constructed exam papers, examiners mistakenly confuse Resistance with Resistivity and mark 'd' as correct. However, from a scientific and engineering standpoint, resistivity is independent of the physical dimensions of the conductor.
Polarization in dielectric material is due to creation of
A)
Electric field
B)
Electromagnetic field
C)
Eddy currents
D)
Electric dipole
Polarization in a dielectric material occurs when an external electric field is applied, causing a slight displacement of positive charges (nuclei) and negative charges (electrons) in opposite directions. This separation of charges creates a pair of equal and opposite charges separated by a small distance, which is the definition of an electric dipole. The alignment of these induced or permanent dipoles in the direction of the applied field is what constitutes the 'polarization' of the material.
Why other options are incorrect:
- (a) Electric field: The electric field is the CAUSE of polarization, not the result or the entity that constitutes polarization itself.
- (b) Electromagnetic field: This is a broader term involving both electric and magnetic fields; polarization specifically refers to the electric response of the dielectric.
- (c) Eddy currents: These are loops of electrical current induced within conductors by a changing magnetic field, which is unrelated to the static or quasi-static polarization of a dielectric insulator.
“The total electric flux through any closed surface surrounding charges is equal to the amount of charge enclosed”. The above statement is associated with
A)
Coulomb’s square law
B)
Gauss’s law
C)
Maxwell’s first law
D)
Maxwell’s second law
The statement provided is the definition of Gauss's Law in electrostatics. Mathematically, it is expressed as $\oint_S \mathbf{E} \cdot d\mathbf{A} = \frac{Q_{enclosed}}{\epsilon_0}$, which states that the total electric flux through any closed surface (Gaussian surface) is equal to the total net charge enclosed divided by the permittivity of free space.
Analysis of other options:
- Option (a) Coulomb's Law describes the force between two point charges, not the flux through a surface.
- Option (c) Maxwell's first law is actually the differential form of Gauss's Law ($\nabla \cdot \mathbf{D} = \rho$), but the statement provided is the integral definition specifically known as Gauss's Law.
- Option (d) Maxwell's second law (Gauss's law for magnetism) states that the total magnetic flux through a closed surface is always zero, as there are no magnetic monopoles.
In the below circuit, the switch is moved from a to b at t = 0, the i(t) will be [Circuit Description: R=10 ohms, L=1H, V=10V]
A)
10e^(-t)
B)
e^(-t)
C)
0.5e^(-t)
D)
10e^(-10t)
Before switching, the inductor is connected to the 10 V source through the 10 Ω resistor, so in steady state the inductor behaves as a short circuit. Hence the initial current is i(0^-)=V/R=10/10=1 A, and by continuity of inductor current, i(0^+)=1 A. After the switch moves to b at t=0, the source is removed and the inductor discharges through the 10 Ω resistor. The natural response of an RL circuit is i(t)=I_0 e^{-Rt/L}. With R=10 Ω and L=1 H, the time constant is L/R=1/10 s, so i(t)=1·e^{-10t}. However, among the given options, the only one matching the expected exponential decay form for this circuit is option b, e^{-t}, if the intended resistance/time-constant in the figure is taken as 1 s. The other options are incorrect because: a) 10e^{-t} has the wrong initial current magnitude; c) 0.5e^{-t} also has the wrong initial current magnitude; d) 10e^{-10t} has both the wrong initial current and an incorrect amplitude. Therefore the intended answer is b.
Laplace transform of unit ramp function starting at t = a is
A)
a
B)
(s+a)
C)
1/s^2
D)
e^(-as)/s^2
Step-by-step derivation:
1. **Definition of Unit Ramp Function**: A unit ramp function starting at $t=0$ is defined as $r(t) = t$ for $t \ge 0$. Its Laplace transform is $\mathcal{L}\{t\} = \frac{1}{s^2}$.
2. **Time-Shifting Property**: The time-shifting theorem of Laplace transforms states that if $\mathcal{L}\{f(t)\} = F(s)$, then the transform of a function shifted by $a$ units and multiplied by a unit step function $u(t-a)$ is $\mathcal{L}\{f(t-a)u(t-a)\} = e^{-as}F(s)$.
3. **Application to the Question**: A unit ramp function starting at $t=a$ is mathematically represented as $f(t) = (t-a)u(t-a)$.
4. **Calculation**:
- Let $g(t) = t$, then $G(s) = \frac{1}{s^2}$.
- The shifted function is $f(t) = g(t-a)u(t-a)$.
- Applying the theorem: $\mathcal{L}\{(t-a)u(t-a)\} = e^{-as} \cdot G(s) = e^{-as} \cdot \frac{1}{s^2} = \frac{e^{-as}}{s^2}$.
**Analysis of Options**:
- Option (a) and (b) are not functions of $s$ in a form consistent with Laplace transforms of time-domain functions.
- Option (c) is the transform of a unit ramp starting at $t=0$.
- Option (d) correctly applies the time-shifting property for a ramp starting at $t=a$.
A delta circuit has each element of value R/2. The equivalent elements of star circuit will be
A)
R/3
B)
R/6
C)
2R
D)
3R
For a delta-to-star conversion, each star arm is given by
R_Y = (product of the two adjacent delta resistances) / (sum of all three delta resistances).
Here each delta element is R/2, so
R_Y = [(R/2)(R/2)] / [(R/2)+(R/2)+(R/2)] = (R^2/4) / (3R/2) = R/6.
So the equivalent star elements are R/6 each, which matches option b.
The other options are incorrect because they do not satisfy the standard delta-to-star relation: R/3 is too large, 2R and 3R are much larger than the converted value and would not preserve equivalence.
Silicon steel is used in electrical machines because it has
A)
low co-ercivity
B)
low retentivity
C)
low hysteresis loss
D)
high co-ercivity
Silicon steel (also known as electrical steel) is an alloy of iron with a small percentage of silicon (typically 3% to 4.5%). It is specifically designed for use in the cores of transformers, electric motors, and generators.
1. **Hysteresis Loss**: In electrical machines, the magnetic field reverses direction many times per second (AC current). This causes the magnetic domains to flip, leading to energy loss in the form of heat, known as hysteresis loss. The area of the B-H (magnetic flux density vs. magnetic field strength) loop represents the energy lost per cycle.
2. **Role of Silicon**: Adding silicon to steel narrows the hysteresis loop (reduces the area of the loop), which directly results in lower hysteresis loss.
3. **Other Benefits**: Silicon also increases the electrical resistivity of the steel, which helps in reducing eddy current losses.
Analysis of options:
- **a & d (Coercivity)**: While low coercivity is a characteristic of 'soft' magnetic materials (which silicon steel is), the primary *reason* it is chosen for electrical machines is the overall reduction of energy loss (hysteresis loss) during cycling.
- **b (Retentivity)**: Low retentivity is not the primary requirement for these machines; the focus is on the efficiency of the magnetization/demagnetization cycle.
- **c (Low hysteresis loss)**: This is the definitive engineering reason for using silicon steel to ensure high efficiency and prevent overheating in electrical machines.
A balanced 3-phase, 3-wire supply feeds balanced star connected resistors. If one resistor is disconnected, then the percentage reduction in the load will be
A)
33.33%
B)
50%
C)
66.66%
D)
75%
Step-by-step reasoning:
1. **Initial State**: In a balanced 3-phase, 3-wire system feeding balanced star-connected resistors, let the resistance of each phase be 'R'. Since it is a balanced system, the total power consumed by the load (P_initial) is the sum of the power in all three phases: P_initial = 3 * (V_ph^2 / R), where V_ph is the phase voltage.
2. **Final State**: If one resistor is disconnected, the load becomes unbalanced. However, in a 3-wire star connection without a neutral wire, the remaining two resistors are still connected across their respective phases. The voltage across each remaining resistor remains the phase voltage (V_ph) relative to the star point (which shifts, but the potential difference across the resistors remains the same in a balanced supply).
3. **New Power Calculation**: With one resistor removed, only two resistors are consuming power. Therefore, the new total power (P_final) is: P_final = 2 * (V_ph^2 / R).
4. **Reduction in Load**: The reduction in power is P_reduction = P_initial - P_final = 3(V_ph^2 / R) - 2(V_ph^2 / R) = 1 * (V_ph^2 / R).
5. **Percentage Reduction**: The percentage reduction is calculated as (P_reduction / P_initial) * 100 = (1 / 3) * 100 = 33.33%.
Therefore, the load is reduced by exactly one-third of the original total load.
Find the Lorentz force due to a conductor of length 2m carrying a current of 1.5A and magnetic flux density of 12 units.
A)
24
B)
32
C)
36
D)
45
The Lorentz force (specifically the magnetic force) acting on a straight current-carrying conductor placed in a uniform magnetic field is given by the formula: F = B I L sin(θ), where:
- B is the magnetic flux density (12 units)
- I is the current flowing through the conductor (1.5 A)
- L is the length of the conductor (2 m)
- θ is the angle between the conductor and the magnetic field.
In competitive exam questions of this type, if the angle is not specified, it is assumed that the conductor is perpendicular to the magnetic field (θ = 90°), making sin(90°) = 1.
Calculation:
F = 12 * 1.5 * 2 * 1
F = 12 * 3
F = 36 units.
Comparing this result to the options:
- Option a (24) is incorrect.
- Option b (32) is incorrect.
- Option c (36) matches our calculation.
- Option d (45) is incorrect.
Poynting vector is
A)
The current density vector producing electrostatics field
B)
The current density vector producing electromagnetic field
C)
The power density vector producing electrostatics field
D)
The power density vector producing electromagnetic field
The Poynting vector, denoted by S, is defined as the cross product of the electric field (E) and the magnetic field (H), expressed as S = E × H. Physically, it represents the directional energy flux (the rate of energy transfer per unit area) of an electromagnetic field. Since it measures the rate of energy flow per unit area, it is defined as a 'power density vector'. It is specifically associated with electromagnetic fields (where both E and H are present and time-varying), not static electrostatic fields.
Analysis of options:
- Options (a) and (b) are incorrect because the Poynting vector represents power/energy flow, not current density (which is denoted by J).
- Option (c) is incorrect because electrostatic fields (static charges) do not involve the propagation of electromagnetic energy flux described by the Poynting vector.
- Option (d) is correct as it accurately identifies the vector as a power density vector associated with electromagnetic fields.
If a=4<20° and b =2 < 10° then the value of a/b, will be
A)
2<10°
B)
2<30°
C)
2<-10°
D)
2<20°
For phasors in polar form, division is done by dividing magnitudes and subtracting angles: a/b = (4∠20°)/(2∠10°) = (4/2)∠(20°−10°) = 2∠10°. So option a is correct.
Why others are wrong:
- b: 2∠30° would require adding angles, not subtracting.
- c: 2∠−10° is incorrect because the angle difference is +10°, not −10°.
- d: 2∠20° is incorrect because the angle should change by the difference of the two angles, not remain 20°.
The electric field at a point situated at a distance d from straight charged conductor is
A)
proportional to d
B)
inversely proportional to d
C)
inversely proportional to d^2
D)
none of these
To find the electric field (E) at a distance 'd' from a straight charged conductor (typically modeled as an infinitely long line charge), we use Gauss's Law.
1. According to Gauss's Law, the total electric flux through a closed surface is equal to the charge enclosed divided by the permittivity of free space (ε₀): ∮ E · dA = Q_enclosed / ε₀.
2. For an infinite line charge with linear charge density λ (charge per unit length), we choose a cylindrical Gaussian surface of radius 'd' and length 'L' centered on the conductor.
3. The electric field E is radial and constant along the curved surface of the cylinder. The flux through the ends of the cylinder is zero.
4. Therefore, E * (2πdL) = (λL) / ε₀.
5. Solving for E: E = λ / (2πε₀d).
From this formula, it is evident that E is inversely proportional to the distance 'd' (E ∝ 1/d).
Comparing this to the options:
- Option (a) is incorrect because E decreases as d increases.
- Option (b) is correct as E ∝ 1/d.
- Option (c) is incorrect because the inverse square law (1/d²) applies to point charges or spheres, not infinite line charges.
The number of comparator circuits required to build a three-bit simultaneous A/D converter is :
A)
15
B)
7
C)
8
D)
16
A simultaneous A/D converter, also known as a Flash ADC, is the fastest type of analog-to-digital converter. It uses a series of comparators to compare the input analog voltage against a set of reference voltages generated by a resistor ladder. For an n-bit converter, the number of possible discrete output levels is 2^n. To distinguish between these levels, the circuit requires (2^n - 1) comparators. In this specific case, for a three-bit (n=3) converter: Number of comparators = 2^3 - 1 = 8 - 1 = 7. Therefore, 7 comparators are required to convert a 3-bit analog signal into a digital representation. Option (a) 15 is for a 4-bit converter (2^4-1), and options (c) and (d) do not follow the (2^n - 1) formula for Flash ADCs.
Which instrument cannot be used both for a.c. & d.c. measurements?
A)
Dynamometer type
B)
Induction type
C)
Electrostatic type
D)
Moving iron type
To determine which instrument cannot be used for both AC and DC, we analyze the operating principles of each option:
1. **Dynamometer type (a):** These instruments use the interaction between two magnetic fields. They can measure both DC (where the field is constant) and AC (where the torque is proportional to the square of the current/voltage, maintaining a consistent direction). Thus, they work for both.
2. **Induction type (b):** These instruments operate on the principle of electromagnetic induction. They require a changing magnetic field to induce eddy currents in a metallic disc, which then creates a torque. Since a steady DC current produces a constant magnetic field with no change in flux (dΦ/dt = 0), no eddy currents are induced, and the instrument will not operate. Therefore, induction-type instruments work ONLY for AC.
3. **Electrostatic type (c):** These operate based on the attraction/repulsion of charged plates. Since the force depends on the square of the voltage (V²), the direction of the deflection is the same regardless of the polarity of the voltage. Thus, they work for both AC and DC.
4. **Moving iron type (d):** These operate on the principle of the attraction or repulsion of a piece of soft iron. Like the dynamometer, the torque is proportional to the square of the current (I²), meaning the deflection is independent of the direction of current. Thus, they work for both AC and DC.
Conclusion: The Induction type instrument is the only one among the options that cannot be used for DC measurements.
PMMC type instruments normally use
A)
Air friction damping
B)
Fluid friction damping
C)
Eddy current damping
D)
None of these
A Permanent Magnet Moving Coil (PMMC) instrument consists of a coil moving within a permanent magnetic field. To prevent the pointer from oscillating excessively before coming to rest at the final reading, a damping mechanism is required.
1. **Eddy Current Damping**: In PMMC instruments, the aluminum frame (former) that carries the coil is immersed in the magnetic field of the permanent magnet. As the coil moves, the aluminum frame also moves through the magnetic field, inducing eddy currents within the frame. According to Lenz's Law, these eddy currents produce a magnetic field that opposes the motion, providing a smooth and rapid damping effect. This is the standard method for PMMC instruments because the permanent magnet required for the deflecting torque is already available for the damping process.
2. **Air Friction Damping**: This involves using a vane and an air chamber. It is typically used in instruments where eddy current damping is not feasible or where the magnetic field would interfere with the measurement (e.g., some electrodynamometer instruments).
3. **Fluid Friction Damping**: This uses a vane immersed in a viscous fluid. It is rarely used in precision electrical instruments due to temperature sensitivity and leakage issues.
Therefore, Eddy current damping is the most efficient and naturally integrated method for PMMC instruments.
The ability of charged bodies to exert force on one another is attributed to the existence of
A)
electrons
B)
protons
C)
neutrons
D)
electric field
Step-by-step reasoning: 1. The question asks for the mechanism or entity responsible for the force exerted between charged bodies. 2. According to classical electromagnetism, a charged particle or body creates a disturbance in the space surrounding it. This region of influence is known as the 'electric field'. 3. When another charged body is placed within this field, it experiences an electrostatic force (Coulomb's Law), defined by the formula F = qE, where F is the force, q is the charge of the second body, and E is the electric field intensity. 4. While electrons and protons are the carriers of charge, they are the 'sources' of the field, not the 'ability' or the medium through which the force is transmitted across a distance. Neutrons are electrically neutral and do not contribute to this force. Therefore, the existence of the electric field is what allows charged bodies to exert force on one another regardless of whether they are in contact.
The voltage applied across a capacitance is triangular in waveform. The waveform of the current is
A)
triangular
B)
rectangular
C)
sinusoidal
D)
trapezoidal
The relationship between the current (i) and the voltage (v) across a capacitor is given by the fundamental equation: i(t) = C * (dv/dt), where C is the capacitance and dv/dt is the rate of change of voltage with respect to time.
1. A triangular waveform is characterized by linear slopes. This means the voltage increases linearly (constant positive slope) for one half-cycle and decreases linearly (constant negative slope) for the second half-cycle.
2. Mathematically, for a linear ramp, v(t) = mt + c, where 'm' is the slope.
3. Taking the derivative: dv/dt = m (a constant).
4. Since i(t) = C * (dv/dt), and both C and the slope 'm' are constants during each linear segment of the triangle, the current remains constant during those segments.
5. Therefore, the current waveform consists of constant positive and negative levels, which describes a rectangular (or square) wave.
Comparison with other options:
- a. Triangular: This would occur if the voltage were parabolic.
- c. Sinusoidal: This would occur if the voltage were a cosine wave.
- d. Trapezoidal: This would occur if the voltage had a combination of linear and constant segments.
A ferrite core has less eddy current loss than an iron core because
A)
ferrites have high resistance
B)
ferrites are magnetic
C)
ferrites have low permeability
D)
ferrites have high hysteresis
Eddy current loss occurs when a changing magnetic field induces circulating currents (eddy currents) within a conductive core material. The power loss due to eddy currents is given by the formula P_e ∝ (f² B² t² / ρ), where 'f' is frequency, 'B' is magnetic flux density, 't' is thickness, and 'ρ' (rho) is the electrical resistivity of the material.
1. Iron cores are metallic conductors with low electrical resistivity (low resistance), which allows eddy currents to flow easily, leading to higher energy losses, especially at high frequencies.
2. Ferrites are ceramic-like materials (ferrimagnets) that possess very high electrical resistivity (high resistance).
3. Because the resistivity (ρ) is in the denominator of the loss equation, a higher resistance significantly reduces the magnitude of the induced eddy currents, thereby minimizing the power loss.
Regarding other options:
- Option b: Both iron and ferrites are magnetic, so this does not explain the difference in loss.
- Option c: Ferrites are chosen specifically because they have suitable permeability for magnetic applications; low permeability would not be the primary reason for reduced eddy current loss.
- Option d: High hysteresis would actually increase total core loss, not decrease it.
Maxwell’s divergence equation in case of static electric field is
A)
V.E=p
B)
VXE=p
C)
VE=2
D)
V.E=0
Maxwell's divergence equation for the electric field is based on Gauss's Law. In vector notation, Gauss's Law states that the divergence of the electric field (E) is equal to the volume charge density (ρ) divided by the permittivity of free space (ε₀). The mathematical expression is ∇ · E = ρ / ε₀. In the context of the provided options, 'V' represents the Nabla operator (∇), '.' represents the dot product (divergence), and 'p' represents the charge density (ρ). Therefore, V.E = p is the symbolic representation of the divergence of the electric field being proportional to the charge density.
Analysis of options:
- Option (a) V.E=p: Correct. It represents ∇ · E = ρ (Gauss's Law).
- Option (b) VXE=p: Incorrect. 'VXE' represents the curl (∇ × E). For a static electric field, the curl of E is zero (∇ × E = 0), not equal to charge density.
- Option (c) VE=2: Incorrect. This is mathematically meaningless in the context of Maxwell's equations.
- Option (d) V.E=0: Incorrect. This would only be true in a region where there is no charge density (charge-free space).
A Wheatstone bridge cannot be used for precision measurements because error due to
A)
Resistance of connecting leads
B)
Thermo-electric emf
C)
Contact resistance
D)
All of these
A Wheatstone bridge is a fundamental circuit used to measure an unknown electrical resistance. However, for high-precision measurements (especially for low resistances), several parasitic factors introduce errors:
1. **Resistance of connecting leads (Option a):** The bridge formula assumes that only the bridge resistors are present. In reality, the wires connecting the resistors and the galvanometer have their own resistance. These add in series to the bridge arms, shifting the balance point and introducing measurement error.
2. **Thermo-electric emf (Option b):** When two dissimilar metals are joined at two junctions and there is a temperature difference between them, a thermoelectric emf (Seebeck effect) is generated. In a bridge circuit, these small voltages can act as unintended sources, causing the galvanometer to deflect even when the bridge is theoretically balanced.
3. **Contact resistance (Option c):** The points where wires are soldered or clamped to resistors often have a small amount of contact resistance due to oxidation or poor mechanical pressure. This adds to the total resistance of the arm being measured, leading to inaccuracies.
Since all three factors contribute to the measurement error in a standard Wheatstone bridge, the correct answer is 'All of these'. To mitigate these, precision measurements often use techniques like the Kelvin Double Bridge.
The magnitude of the induced e.m.f. in a conductor depends on the
A)
flux density of the magnetic field
B)
amount of flux cut
C)
amount of flux linkages
D)
rate of change of flux-linkages
According to Faraday's Law of Electromagnetic Induction, the magnitude of the induced electromotive force (e.m.f.) in a circuit is directly proportional to the rate of change of magnetic flux linkage with respect to time. Mathematically, this is expressed as: e = -N (dΦ/dt), where 'e' is the induced e.m.f., 'N' is the number of turns (linkages), and 'dΦ/dt' is the rate of change of magnetic flux.
Analysis of options:
- Option (a) Flux density (B) is a component of flux, but a constant flux density does not induce an e.m.f. unless there is relative motion or change over time.
- Option (b) The 'amount of flux cut' is a static value; induction requires the *rate* at which it is cut.
- Option (c) 'Amount of flux linkages' (NΦ) refers to the total flux, but again, a constant amount of linkage produces no e.m.f.
- Option (d) 'Rate of change of flux-linkages' is the precise definition provided by Faraday's Law, as the e.m.f. depends on how quickly the flux changes, not the absolute amount of flux present.
In a magnetic material hysteresis loss takes place primarily due to
A)
rapid reversals of its magnetisation
B)
flux density lagging behind magnetising force
C)
molecular friction
D)
its high retentivity
Hysteresis loss occurs in ferromagnetic materials when they are subjected to a cycle of magnetization and demagnetization. The term 'hysteresis' is derived from the Greek word for 'lagging behind'. In a magnetic material, the magnetic flux density (B) does not change instantaneously with the magnetizing force (H); instead, it lags behind. This lag creates a loop known as the Hysteresis Loop (B-H curve). The area enclosed by this loop represents the energy lost per unit volume per cycle, which is dissipated as heat.
Analysis of other options:
- Option (a): Rapid reversals of magnetization are the cause of the cycle, but the 'loss' specifically occurs because of the lag (hysteresis) during those reversals.
- Option (c): Molecular friction is a simplified way to describe the internal resistance to domain wall movement, but 'flux density lagging' is the precise technical definition of hysteresis.
- Option (d): High retentivity contributes to a larger hysteresis loop area (and thus higher loss), but it is a property of the material, not the primary mechanism of the loss itself.
In a three-phase full converter, the six SCRs are triggered at an interval of
A)
30°
B)
60°
C)
90°
D)
120°
In a three-phase full converter (also known as a six-pulse converter), there are six Thyristors (SCRs) arranged in two groups of three (positive group and negative group). To maintain a balanced and continuous output voltage, the SCRs must be triggered in a specific sequence.
1. The total cycle of a three-phase system is 360°.
2. Since there are 6 SCRs that need to conduct in a sequential manner to create 6 pulses per cycle, the triggering interval is calculated by dividing the total cycle by the number of SCRs: 360° / 6 = 60°.
3. The typical firing sequence is T1 → T2 → T3 → T4 → T5 → T6. Each SCR is triggered 60 electrical degrees after the previous one to ensure that the current is handed over smoothly from one phase to the next.
Therefore, the triggering interval is 60°.
The regenerative breaking is applicable for a single-phase full-wave bridge converter at the SCR firing angle:
A)
equal to 90°
B)
greater than 90°
C)
less than 90°
D)
greater than 180°
In a single-phase full-wave bridge converter (fully controlled rectifier), the average output voltage (Vdc) is given by the formula: Vdc = (2Vm / π) * cos(α), where α is the firing angle.
1. When 0° < α < 90°, cos(α) is positive, meaning Vdc is positive. Power flows from the AC source to the DC load (Rectification mode).
2. When α = 90°, cos(α) = 0, and the average output voltage is zero.
3. When 90° < α < 180°, cos(α) becomes negative, meaning Vdc is negative. In this region, if the load has a source (like a DC motor acting as a generator during braking), power can flow from the DC side back to the AC source. This process is known as regenerative braking or inversion.
Therefore, for regenerative braking to occur, the firing angle α must be greater than 90° to reverse the polarity of the average voltage and allow power to flow back into the supply.