← Labour Officer
Past Questions — 2026
Pulse Width Modulation (PWM) is used to:
A)
Control output voltage of inverters
B)
Rectify AC
C)
Convert AC to DC
D)
Reduce ripple in DC
Pulse Width Modulation (PWM) is a technique used in power electronics to control the average voltage delivered to a load. In the context of inverters (which convert DC to AC), PWM is used to vary the width of the pulses of the DC voltage applied to the load. By adjusting the duty cycle (the ratio of 'on' time to the total period), the fundamental component of the output AC voltage can be precisely controlled, and the harmonic content can be reduced.
Analysis of other options:
- Option b (Rectify AC): This is the function of a rectifier (using diodes or thyristors), not PWM.
- Option c (Convert AC to DC): This is the definition of rectification, which is the opposite of what an inverter does.
- Option d (Reduce ripple in DC): Ripple reduction is achieved using filters (capacitors or inductors), not PWM. While PWM is used in DC-DC converters (Buck/Boost), its primary purpose there is voltage regulation, and the ripple is actually a byproduct of the switching process that must be filtered out.
A half-controlled bridge rectifier consists of:
A)
Four SCRs
B)
Two SCRs and two diodes
C)
Four diodes
D)
One SCR and one diode
A bridge rectifier is a circuit that converts AC to DC. Depending on the components used, it can be categorized as follows:
1. Fully Uncontrolled Rectifier: Uses four diodes. The output voltage cannot be controlled.
2. Fully Controlled Rectifier: Uses four SCRs (Thyristors). The output voltage can be precisely controlled by adjusting the firing angle.
3. Half-Controlled (Semi-Controlled) Rectifier: This is a hybrid configuration. It replaces two of the diodes in a full bridge with two SCRs. Typically, the top two components are SCRs and the bottom two are diodes (or vice versa). This allows for partial control of the output voltage while reducing the cost and complexity of the triggering circuitry compared to a fully controlled bridge.
Therefore, a half-controlled bridge rectifier consists of two SCRs and two diodes.
The opcode of an instruction specifies:
A)
Operation to be performed
B)
Memory location
C)
Input data
D)
Output data
In the context of the 8085 Microprocessor and computer architecture, an instruction consists of two main parts: the Opcode (Operation Code) and the Operand.
1. **Opcode (Operation Code):** This is the part of the instruction that tells the microprocessor exactly what task to perform. For example, in the instruction 'MOV A, B', the 'MOV' part is the opcode, which tells the CPU to move data from one register to another.
2. **Operand:** This is the data on which the operation is performed or the address of the memory location where the data is stored. In 'MOV A, B', 'A' and 'B' are the operands.
Evaluating the options:
- **Option (a) is correct** because the opcode specifically defines the operation (e.g., ADD, SUB, MOV, JMP).
- **Option (b) is incorrect** because the memory location is specified by the operand (addressing mode).
- **Option (c) is incorrect** because input data is the operand.
- **Option (d) is incorrect** because output data is the result of the operation, not the specification of the instruction itself.
A microprocessor can be interfaced with which type of memory?
A)
ROM
B)
RAM
C)
Both ROM and RAM
D)
None
A microprocessor requires two primary types of memory to function effectively:
1. **ROM (Read-Only Memory):** This is non-volatile memory used to store the 'firmware' or the boot-up program (BIOS/Monitor program). Since a microprocessor cannot perform any action without instructions, the ROM provides the initial set of instructions that the CPU executes upon reset.
2. **RAM (Random Access Memory):** This is volatile memory used to store temporary data, variables, and the stack during program execution. It allows the microprocessor to read and write data quickly while the system is running.
In the context of the 8085 Microprocessor, the address bus and data bus are designed to interface with both types of memory chips. The control signals (RD' and WR') allow the processor to read from both ROM and RAM, but write only to RAM. Therefore, a complete microprocessor system must be interfaced with both to be functional.
For a half wave bridge inverter, the output voltage:
A)
Vo = – Vs/2 for 0 < t < T/2
B)
Vo = – Vs/2 for T/2< t
C)
Vo = – Vs for 0 < t < T/2
D)
Vo = Vs/2 for T/2< t < T
In a half-bridge inverter, the DC source (Vs) is split into two equal capacitors, each providing a voltage of Vs/2. The output voltage (Vo) is measured across the load connected between the midpoint of the capacitor bank and the switching nodes.
1. During the first half-cycle (0 < t < T/2), one switch (e.g., S1) is turned ON. This connects the load to the negative terminal of the capacitor bank, resulting in an output voltage of Vo = -Vs/2.
2. During the second half-cycle (T/2 < t < T), the other switch (e.g., S2) is turned ON. This connects the load to the positive terminal of the capacitor bank, resulting in an output voltage of Vo = +Vs/2.
Evaluating the options:
- Option (a) correctly describes the first half-cycle: Vo = -Vs/2 for 0 < t < T/2.
- Option (b) is incorrect because for T/2 < t < T, the voltage should be +Vs/2.
- Option (c) is incorrect because the magnitude in a half-bridge inverter is Vs/2, not Vs (Vs is the magnitude for a full-bridge inverter).
- Option (d) is partially correct in magnitude, but the standard convention for the first half-cycle is negative and the second is positive; however, compared to the standard definition of a half-bridge operation, option (a) is the definitive characteristic of the initial state.
If a BJT has: IC = 4mA, IB = 0.1mA, Emitter current IE is:
A)
4.1 mA
B)
3.9 mA
C)
4.5 mA
D)
5 mA
Option A is correct: 4.1 mA.
FM is preferred over AM because:
A)
Requires less bandwidth
B)
Less affected by noise
C)
Simpler circuits
D)
Lower power consumption
Frequency Modulation (FM) is preferred over Amplitude Modulation (AM) primarily due to its superior noise immunity. Most electrical noise (such as atmospheric interference or industrial sparks) manifests as amplitude spikes. In AM, the information is carried in the amplitude of the carrier wave, meaning the receiver cannot distinguish between the signal and the noise. In FM, the information is carried in the frequency variations, and the amplitude remains constant. FM receivers use 'limiters' to clip off amplitude variations (noise) without losing the frequency-encoded information, resulting in a much clearer signal.
Analysis of other options:
- Option (a) is incorrect: FM actually requires significantly more bandwidth than AM (e.g., an FM channel is typically 200 kHz, whereas an AM channel is about 10 kHz).
- Option (c) is incorrect: FM circuits (modulators and demodulators/PLLs) are generally more complex than the simple envelope detectors used in AM.
- Option (d) is incorrect: While FM transmitters can be more power-efficient in terms of the carrier, the primary reason for its preference in communication quality is noise rejection, not power consumption.
Explain working of DC chopper.
An amplifier has: Output voltage = 5 V, Input voltage = 50 mV. Voltage gain is:
A)
10
B)
50
C)
100
D)
200
Option C is correct: 100.
A circuit used to remove part of a waveform above or below a reference level is called:
A)
Clipper
B)
Clamper
C)
Oscillator
D)
Amplifier
A clipper circuit (also known as a limiter) is specifically designed to 'clip' or remove a portion of an input signal that exceeds a predefined voltage level (reference level) without distorting the remaining part of the waveform. This is typically achieved using diodes and resistors.
Why other options are incorrect:
- b. Clamper: A clamper circuit shifts the entire DC level of a waveform up or down to a different reference level without changing the shape of the signal; it does not remove any part of the waveform.
- c. Oscillator: An oscillator is a circuit that generates a periodic AC signal (like a sine or square wave) from a DC source; it does not modify an existing waveform by removing parts of it.
- d. Amplifier: An amplifier increases the amplitude (power, voltage, or current) of a signal; while it can cause 'clipping' if it reaches saturation, its primary purpose is amplification, not the intentional removal of waveform parts based on a reference level.
Digital transmission is preferred over analog because:
A)
Less affected by noise
B)
Easier to encrypt
C)
Supports error detection
D)
All of the above
Digital transmission is superior to analog transmission for several fundamental reasons:
1. **Noise Immunity (Option a):** In analog signals, noise is additive and becomes an inseparable part of the signal. In digital transmission, signals are represented by discrete levels (e.g., 0s and 1s). As long as the noise does not push a '0' above the threshold to be read as a '1' (or vice versa), the original data can be perfectly recovered using regenerative repeaters. This makes digital signals significantly less affected by noise.
2. **Encryption (Option b):** Digital data consists of binary sequences, which can be mathematically manipulated using algorithms (like AES or RSA) to encrypt information. Analog encryption (like frequency inversion) is far less secure and more limited in complexity compared to digital encryption.
3. **Error Detection and Correction (Option c):** Digital systems allow for the addition of redundant bits (such as parity bits, Checksums, or Cyclic Redundancy Checks - CRC). These allow the receiver to detect if bits were flipped during transmission and, in some cases, correct them without needing a retransmission.
Since all three statements are technically accurate and represent the primary advantages of digital over analog communication, the correct choice is 'All of the above'.
Classify instruction set of 8085 microprocessor.
Lead compensator improves:
A)
Steady-state error
B)
Phase margin and transient response
C)
Power output
D)
Current flow
A lead compensator is a high-pass filter that adds a positive phase (phase lead) to the open-loop transfer function of a system.
1. **Phase Margin**: By adding positive phase at the gain crossover frequency, the phase margin is increased, which directly enhances the relative stability of the system.
2. **Transient Response**: An increase in phase margin typically leads to a reduction in peak overshoot and a decrease in settling time, thereby improving the overall transient response. It also tends to increase the bandwidth of the system, resulting in a faster response (shorter rise time).
Why other options are incorrect:
- **Option a (Steady-state error)**: Lead compensators generally do not improve steady-state error; in fact, they may slightly degrade it because they do not increase the low-frequency gain. Improving steady-state error is the primary function of a 'Lag compensator'.
- **Option c (Power output)**: This is a parameter related to power electronics or electrical machinery, not a characteristic of a control system compensator.
- **Option d (Current flow)**: Similarly, current flow is a physical electrical quantity and not a control system performance metric improved by a lead compensator.
State rules for block diagram reduction technique.
Define load factor, capacity factor, and diversity factor.
Load frequency control ensures:
A)
Constant voltage
B)
Constant frequency
C)
Constant power factor
D)
Maximum fault current
Load Frequency Control (LFC) is a system used in power system operation to maintain the system frequency at a specified value (e.g., 50 Hz or 60 Hz) and to maintain the scheduled power exchange between different control areas.
1. **Mechanism**: In a power system, the frequency is directly linked to the balance between total power generation and total load demand. If generation exceeds demand, the frequency increases; if demand exceeds generation, the frequency decreases. LFC adjusts the mechanical power input to the generators (via governor control) to balance this equation and keep the frequency constant.
2. **Why other options are incorrect**:
- **Option a (Constant voltage)**: Voltage is maintained by Automatic Voltage Regulators (AVR) and reactive power compensation (like capacitor banks or SVCs), not by LFC.
- **Option c (Constant power factor)**: Power factor is managed by controlling the reactive power flow at the load or generation end, not by frequency control.
- **Option d (Maximum fault current)**: Fault currents are determined by the system impedance and the nature of the fault; they are not 'ensured' or controlled by LFC, which is a steady-state/dynamic stability control mechanism.
Voltage control in power systems is mainly achieved using:
A)
Circuit breakers
B)
Tap changing transformers
C)
Fuses
D)
Lightning arresters
Option B is correct: Tap changing transformers.
The ratio of the primary to secondary voltage of a transformer is 2:1. The saving in terms of weight of copper required if an autotransformer is used instead of a two-winding transformer will be:
A)
50%
B)
33.33%
C)
66.67%
D)
97%
Option B is correct: 33.33%.
For a system with poles at 0, -2, -4 and zero at -1, the centroid of asymptotes is:
A)
-1
B)
-1.67
C)
-2
D)
-3
Option B is correct: -1.67.
Economic operation of a power system aims at:
A)
Maximizing losses
B)
Minimizing total generation cost
C)
Increasing voltage
D)
Reducing frequency
Economic operation (or Economic Dispatch) of a power system is the process of allocating the required load demand among the available generating units such that the total cost of operation is minimized while satisfying all operational constraints.
1. Why 'b' is correct: The primary objective of economic dispatch is to minimize the total fuel cost of generation. This is typically achieved when the incremental fuel costs of all operating units are equal (the Equal Incremental Cost Criterion).
2. Why 'a' is incorrect: Maximizing losses would lead to inefficiency and increased costs, which is the opposite of economic operation.
3. Why 'c' is incorrect: Increasing voltage is related to voltage regulation and stability (Reactive Power Management), not the economic cost of generation.
4. Why 'd' is incorrect: Reducing frequency would indicate an imbalance between generation and load, leading to system instability; frequency must be maintained at a constant rated value (e.g., 50Hz or 60Hz) for system reliability.
Explain per unit system. Describe positive, negative, and zero sequence components.
Explain the purpose of load flow studies in power systems. Distinguish between slack bus, PV bus, and PQ bus used in load flow analysis.
Define steady-state error and derive expressions for steady-state error for step, ramp, and parabolic inputs using the static error constants Kp, Kv, and Ka.
State Routh-Hurwitz and Nyquist stability criterion.
Transient stability refers to:
A)
System stability under small disturbances
B)
System stability under large disturbances
C)
Long-term voltage variation
D)
Frequency control
In power system analysis, stability is categorized based on the magnitude of the disturbance.
1. **Transient Stability** refers to the ability of the power system to maintain synchronism when subjected to a severe or 'large' disturbance. Examples of such disturbances include three-phase faults, the sudden loss of a major generating unit, or the switching of a large load/line. This is typically analyzed using the 'Equal Area Criterion' or by solving the swing equation.
2. **Small-Signal Stability** (Option a) refers to the ability of the system to maintain synchronism under small, gradual disturbances (such as small changes in load), which can be analyzed using linearized models.
3. **Voltage Stability** (Option c) relates to the ability of a system to maintain steady voltages at all buses after a disturbance, which is distinct from rotor angle (transient) stability.
4. **Frequency Control** (Option d) is a function of the governor system to balance generation and load, rather than a definition of transient stability.