DC Voltmeter and PMMC Instruments

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Questions and Answers

Which force is NOT required to move the pointer in a PMMC instrument?

Deflection force

Damping force

Friction force (correct)

Controlling force

What is the primary function of PMMC instruments?

To measure resistance

To measure only DC current

To measure only AC current

To measure both AC and DC current (correct)

What type of measurements can PMMC instruments be calibrated for after modifications?

Low-voltage measurements only

Only high-frequency AC measurements

AC currents and voltages (correct)

Only voltage measurements

Which statement about PMMC instruments is true?

They provide accurate measurement of both AC and DC when properly calibrated. Signup and view all the answers

What type of errors are associated with PMMC instruments?

Both random and systematic errors Signup and view all the answers

For what purpose are CTs (current transformers) used in electrical systems?

To sense an increase in current for fault detection Signup and view all the answers

Which of the following is NOT a component involved in the PMMC deflection mechanism?

Spring Signup and view all the answers

What effect does a 1% change in coil resistance have when the swamping resistance is nine times the coil resistance?

0.1% change in total resistance Signup and view all the answers

Which of the following is NOT a reason for errors in Permanent Magnet Moving Coil (PMMC) instruments?

Improper pivoting Signup and view all the answers

What is a key limitation of PMMC instruments?

They are suitable for direct current only Signup and view all the answers

How can frictional errors in PMMC instruments be reduced?

By proper pivoting and balancing of weight Signup and view all the answers

What is the practical advantage of using manganin in series with the coil of a PMMC instrument?

It reduces temperature effects Signup and view all the answers

Which statement best explains the function of shunt resistance in a PMMC ammeter?

It permits the measurement of large currents Signup and view all the answers

Which of the following is an advantage of PMMC instruments?

High linear and uniform scale Signup and view all the answers

What factor contributes to a significant error in readings of PMMC instruments over time?

Aging of the instrument Signup and view all the answers

What effect does adding a series multiplier resistance to a PMMC voltmeter have?

It increases the voltmeter range. Signup and view all the answers

Given a PMMC instrument with a full-scale deflection (FSD) of 100 µA and a coil resistance of 1 kΩ, what is the required multiplier resistance to measure 50 V at full scale?

49 kΩ Signup and view all the answers

How does a voltmeter's resistance affect the measured voltage in a circuit?

Higher resistance reduces the voltmeter's impact on the circuit voltage. Signup and view all the answers

What is the consequence of using a voltmeter with low resistance in a circuit?

It causes a significant voltage drop across the measurement points. Signup and view all the answers

If a PMMC voltmeter is modified to measure a voltage higher than its original rating, what is essential for proper measurement?

Additional series resistance to limit current. Signup and view all the answers

What is the ideal characteristic of a voltmeter to ensure accurate voltage measurements?

Infinite resistance. Signup and view all the answers

What factor does a multiplier resistance of nine times the coil resistance have on the voltmeter's range?

Increases the voltmeter range by a factor of ten. Signup and view all the answers

What is the likely relative error when a galvanometer is used to measure the voltage across a resistor in a circuit?

It introduces a significant relative error. Signup and view all the answers

What is the main purpose of the ammeter circuit in measuring current?

To provide a shunt resistance that influences current measurement. Signup and view all the answers

What does the abbreviation FSD stand for in the context of ammeters?

Full Scale Deflection Signup and view all the answers

How is the percentage error calculated from measured and expected current values?

((Measured Value - Expected Value) / Expected Value) * 100 Signup and view all the answers

Why is a make-before-break switch used in a multirange ammeter?

To ensure the circuit is not left open while switching ranges. Signup and view all the answers

What component in the ammeter circuit has a resistance of 99 Ω when measuring at FSD?

Coil resistance Signup and view all the answers

What modification allows a PMMC meter to measure higher voltage ranges?

Using a series resistance known as multiplier resistance Signup and view all the answers

What is the value of the total current passing through the ammeter circuit at FSD?

10 mA Signup and view all the answers

In the context of an ammeter, what does the term 'shunt resistance' refer to?

Additional resistance used to bypass some of the current. Signup and view all the answers

What material is primarily used for the permanent magnet in PMMC construction?

Alnico Signup and view all the answers

What is the preferred ratio of length to breadth (L/b) for the coil in PMMC to achieve high torque efficiency?

1.3 to 1.5 Signup and view all the answers

Which material is used for the hair springs in the controlling system of PMMC?

Phosphorous Bronze Signup and view all the answers

What is the significance of the deflection torque in a PMMC?

It generates force on the current carrying conductor. Signup and view all the answers

What is the type of coil geometry preferred for PMMC construction?

Rectangular coil Signup and view all the answers

In the force equation for PMMC, what does 'N' represent?

Number of turns in the coil Signup and view all the answers

Which of the following features is NOT commonly associated with PMMC construction?

Fluid-based cooling system Signup and view all the answers

What is the radius range of jewel used in PMMC?

0.01 mm to 0.02 mm Signup and view all the answers

Study Notes

DC Voltmeter

PMMC (Permanent Magnet Moving Coil) meters can be adapted to measure voltage by adding a series resistance.
A multiplier resistance that is nine times the coil resistance expands the voltmeter's range by a factor of ten.
Example: For a PMMC instrument with a full-scale deflection (FSD) of 100 µA and a coil resistance of 1 kΩ, one can calculate the required multiplier resistance to measure up to 50 V.

Effect of Voltmeter on Circuit

A voltmeter connected in parallel introduces its resistance (RV), affecting the measured voltage (Vab).
The voltmeter draws current (IV), which alters the voltage drop across circuit components.
To ensure accurate measurements, the voltmeter's resistance must be significantly higher than the circuit resistance (ideal is infinite resistance).

Permanent Magnet Moving Coil (PMMC) Instruments

PMMC meters can measure both DC and AC currents/voltages with appropriate circuitry modifications.
Deflection requires three forces: deflection force, controlling force, and damping force.
Construction includes a permanent magnetic system (Alnico, U-shaped), a moving system (rectangular coil), and a control system (hair springs).

Deflection Torque

The total deflection torque (F) acting on a coil is given by F = BILN, where:
- B = magnetic flux density (Tesla)
- I = current (Ampere)
- L = coil length (meter)
- N = number of turns

Errors in PMMC Measurements

Common errors stem from weakening magnetism due to vibrations or stray fields.
Proper pivoting, balancing, and temperature control can reduce errors.
Swamping resistance helps mitigate temperature effects, ensuring better accuracy.

Advantages of PMMC Instruments

Offers a linear and uniform scale with low power consumption (25 µW to 200 µW).
High torque-to-weight ratio aids in achieving high accuracy (typically 2%).
Minimal error from stray magnetic fields and no hysteresis loss, suitable for multi-range measurements.

DC Ammeters

PMMC instruments can function as ammeters, measuring DC currents.
A shunt resistance permits the measurement of larger currents by allowing most current to bypass the coil.
Basic equations estimate current and voltage in the ammeter circuit.

Multirange Ammeter

Utilizes a rotary switch to select among various shunt resistances for measuring different current ranges.
A make-before-break switch ensures continuous parallel connection of the shunt during range switching.

Constructional Features of PMMC

Moving coils are made from lightweight materials to enhance torque efficiency.
Phosphor-bronze hair springs control the movement and are designed to handle minimal current.
Special attention is given to the coil's shape and dimensions for optimal performance.

Practical Applications

PMMC meters are essential for accurate electrical measurements in testing, monitoring, and maintaining electrical systems.
Instruments can be calibrated specifically for DC measurements and can also handle AC with rectification.

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Description

Explore the principles and applications of DC voltmeters, specifically focusing on Permanent Magnet Moving Coil (PMMC) instruments. Learn how to adapt these meters for voltage measurement, understand the effect of voltmeter resistance on circuit measurements, and calculate required multiplier resistances for accurate readings.