All questions of Electrical & Electronic Measurements for Electrical Engineering (EE) Exam

Sensitivity of an LVDT (Linear Variable Differential Transformer) refers to its ability to accurately measure and respond to small changes in displacement. It determines the smallest measurable change in displacement that the LVDT can detect. The sensitivity of an LVDT is mainly due to the exact cancellation of the secondary voltage.

Explanation:
1. LVDT Operation:
An LVDT consists of a primary winding and two secondary windings, placed on a cylindrical core. When an AC excitation voltage is applied to the primary winding, the core becomes magnetically saturated, resulting in a zero net voltage output on the secondary windings.

2. Displacement Measurement:
When the core is displaced along its axis, it induces a varying magnetic field in the secondary windings. This variation leads to an output voltage proportional to the displacement. The LVDT measures the difference between the two secondary voltages, known as the differential voltage.

3. Secondary Voltage Cancellation:
The secondary windings of an LVDT are constructed in such a way that they cancel each other out when the core is perfectly centered. This exact cancellation is achieved by winding the secondary coils in opposite directions. As a result, the differential voltage is zero when the core is in its null position.

4. Sensitivity and Secondary Voltage Cancellation:
The sensitivity of an LVDT is directly related to the accuracy of the secondary voltage cancellation. If the secondary windings are perfectly balanced and the cancellation is precise, even a small change in displacement will result in a measurable change in the differential voltage. This high sensitivity allows the LVDT to accurately measure small displacements.

5. Other Factors:
While the exact cancellation of the secondary voltage is the primary factor determining the sensitivity of an LVDT, other factors can also affect it to some extent:
- Magnetic shielding of the core helps reduce interference from external magnetic fields, improving the accuracy and stability of the measurement.
- The permeability of the core material affects the magnetic flux and the induced voltage, but it is not the main factor affecting sensitivity.
- The insulation used in the windings ensures proper electrical isolation and protection against environmental factors but has minimal impact on sensitivity.

In conclusion, the sensitivity of an LVDT is mainly determined by the exact cancellation of the secondary voltage. This cancellation allows even small changes in displacement to be accurately measured by the LVDT.

Understanding the Loss of Charge Method
The loss of charge method is a widely used technique for measuring insulation resistance in electrical systems. Here’s how it applies specifically to option 'C', the insulation resistance of cables.
What is Insulation Resistance?
- Insulation resistance refers to the resistance offered by insulating materials against the flow of electric current.
- It is crucial for ensuring that electrical systems operate safely and efficiently, preventing leakage currents that can cause faults or hazards.
How the Loss of Charge Method Works
- The loss of charge method involves charging a capacitor through the insulation under test and then measuring the time it takes for the capacitor to discharge.
- This process reveals the insulation's ability to hold charge, which directly correlates to its resistance.
- The rate of charge loss indicates how well the insulation is performing; a rapid loss suggests poor insulation resistance.
Why It’s Suitable for Measuring Insulation Resistance
- The method provides a practical and accurate way to evaluate the integrity of insulation materials.
- It is particularly effective for cables, which are prone to degradation over time due to environmental factors.
- The loss of charge method can detect issues like moisture ingress, damage, or aging in insulation.
Conclusion
In summary, the loss of charge method is specifically designed to measure the insulation resistance of cables. It provides valuable insights into the condition of the insulation, ensuring safety and reliability in electrical systems. This makes option 'C' the correct answer.

To reduce the loading effect, an instrument must possess a high input impedance. This is because the input impedance of an instrument determines the amount of current that flows into the instrument when connected to a source. A high input impedance means that the instrument draws very little current from the source, minimizing the loading effect.


The loading effect occurs when the impedance of the instrument connected to a source is significantly lower than the source impedance. In such cases, the instrument draws a significant amount of current from the source, causing a decrease in the source voltage and affecting the accuracy of the measurements.


Having a high input impedance is crucial for accurate measurements in many electrical and electronic applications. Here are some reasons why a high input impedance is desirable:

1. Minimizes Current Flow: A high input impedance means that the instrument draws very little current from the source. This minimizes the loading effect and ensures that the source voltage remains relatively unaffected.

2. Prevents Signal Distortion: When an instrument has a low input impedance, it acts as a significant load on the source, altering the voltage levels. This can distort the signals being measured and introduce errors in the readings. A high input impedance avoids such signal distortion.

3. Preserves Signal Integrity: High input impedance ensures that the instrument does not significantly alter the voltage or current being measured. It allows the instrument to 'listen' to the source without interfering with or altering the signal, preserving its integrity.

4. Wide Range of Applications: Instruments with high input impedance can be used in a wide range of applications, including voltage measurements, impedance measurements, signal analysis, and data acquisition. This versatility makes them suitable for various scenarios where accurate and non-intrusive measurements are required.


In summary, an instrument with a high input impedance is essential to reduce the loading effect. It minimizes the current flow from the source, prevents signal distortion, preserves signal integrity, and enables accurate measurements. By ensuring that the instrument does not significantly alter the source voltage or current, a high input impedance allows for precise and non-intrusive measurements in a variety of electrical and electronic applications.

Electrodynamometer Type Wattmeter

The electrodynamometer type wattmeter is an instrument used for measuring the power or wattage of an electrical circuit. It operates on the principle of the interaction between the magnetic fields produced by the current-carrying coils.

Components of an Electrodynamometer Type Wattmeter

The electrodynamometer type wattmeter consists of two coils:

1. Current Coil: The current coil, also known as the series coil, is connected in series with the load. It carries the load current and produces a magnetic field proportional to the load current.

2. Pressure Coil: The pressure coil, also known as the potential coil, is connected across the load. It is designed to carry a small current proportional to the voltage across the load, which produces a magnetic field proportional to the load voltage.

Working Principle

The working principle of an electrodynamometer type wattmeter is based on the interaction between the magnetic fields produced by the current coil and the pressure coil. When current flows through the current coil, it produces a magnetic field. Similarly, when voltage is applied across the pressure coil, it produces a magnetic field.

The magnetic fields produced by the two coils interact with each other, resulting in a torque on the moving coil. This torque is proportional to the product of the current and voltage. By measuring this torque, the wattmeter can determine the power or wattage of the circuit.

Fixed Current Coil

In an electrodynamometer type wattmeter, the current coil is made fixed. This is because the current coil is designed to carry the load current, which can be quite high. Making the current coil fixed ensures that it can handle the current without any mechanical movement.

By making the current coil fixed, it also simplifies the design and construction of the wattmeter. The fixed current coil is typically wound on a soft iron core, which helps to concentrate the magnetic field produced by the coil.

Advantages of Fixed Current Coil

1. Handles High Current: The fixed current coil is designed to handle high currents, making it suitable for measuring power in high-power circuits.

2. Simplified Design: The fixed current coil simplifies the design and construction of the wattmeter.

3. Improved Accuracy: By making the current coil fixed, it reduces the chances of mechanical movement and improves the accuracy of the wattmeter.

In conclusion, the current coil is made fixed in an electrodynamometer type wattmeter to handle high currents, simplify the design, and improve accuracy.