Measurement of Voltage and Current

Podcast

Play an AI-generated podcast conversation about this lesson

Questions and Answers

What is the symbol used to represent current?

I (correct)

How should an ammeter be connected in a circuit to measure current?

In series with the load (correct)

In parallel with the load

In series with the power supply

At any point in the circuit

What is the main consequence of connecting an ammeter in parallel with a circuit component?

It measures the current inaccurately

It measures the voltage instead

It might cause a short circuit (correct)

It increases the circuit resistance

What is the purpose of a voltmeter in a circuit?

To measure the potential difference Signup and view all the answers

What is the loading effect associated with using a voltmeter?

It can interfere with circuit operation Signup and view all the answers

How should a voltmeter be connected in a circuit?

In parallel with the component Signup and view all the answers

What design feature allows a voltmeter to minimize the loading effect?

High internal resistance Signup and view all the answers

What does the moving iron instrument measure?

Flow of current or voltage Signup and view all the answers

What is the primary purpose of earthing in electrical systems?

To provide a path for shortcircuit current Signup and view all the answers

Which of the following methods involves using a copper plate for earthing?

Plate earthing Signup and view all the answers

What is the recommended maximum earth resistance for effective earthing in a powerhouse?

0.5 ohm Signup and view all the answers

What should you avoid when working with electrical equipment to ensure safety?

Touching components with wet hands Signup and view all the answers

In pipe earthing, how deep should the GI pipe be placed in the ground?

4.75m Signup and view all the answers

What is the main function of the moving iron instrument?

To measure current or voltage magnitude Signup and view all the answers

What is characteristic of the attraction type moving iron instrument?

The moving element is a flat disc of iron core Signup and view all the answers

What advantage do moving iron instruments have in terms of current direction?

They are universally suited for both AC and DC Signup and view all the answers

What is a disadvantage of moving iron instruments?

They are prone to hysteresis errors Signup and view all the answers

What type of torque does the spring provide in a repulsion type instrument?

Controlling torque Signup and view all the answers

Which of the following statements is true regarding the design of the electrodynamometer wattmeter?

The fixed coil is air-cored to avoid hysteresis effects Signup and view all the answers

How does the damping effect work in the context of tools like the Electrodynamometer Wattmeter?

It reduces undesired vibrations of the pointer Signup and view all the answers

What is one of the specific roles of the moving coil in the electrodynamometer wattmeter?

It is responsible for producing a mechanical force Signup and view all the answers

What causes waveform error in the moving iron instrument?

Non-linear deflection torque concerning current Signup and view all the answers

What is a primary component of the electrodynamometer wattmeter’s control mechanism?

A resistor in series with the moving coil Signup and view all the answers

What is a key advantage of a dynamometer type wattmeter?

It has a uniform scale and high accuracy. Signup and view all the answers

Which part of the energy meter is primarily responsible for reducing the speed of the aluminium disc?

Braking system Signup and view all the answers

What does the twisting of the moving coil measure in a PMMC instrument?

Current flowing through the coil Signup and view all the answers

What causes serious errors in the reading of a dynamometer type wattmeter?

Stray fields acting on the moving coil. Signup and view all the answers

Which of the following components can be found in the driving system of an energy meter?

Shunt electromagnet Signup and view all the answers

What determines the power consumption in an energy meter?

Number of rotations of the aluminium disc. Signup and view all the answers

What type of current does the series electromagnet in an energy meter respond to?

Load current flowing through the current coil Signup and view all the answers

What type of materials are used for the permanent magnets in the PMMC instrument?

Alnico and Alcomax Signup and view all the answers

What aspect of the coil design in PMMC instruments allows for a low measurement of current?

Aluminium former rotates into jeweled bearings. Signup and view all the answers

What ensures high accuracy in a dynamometer wattmeter?

Careful design and high-quality components. Signup and view all the answers

What materials are the control springs in a PMMC instrument made of?

Phosphorus bronze Signup and view all the answers

What role does the damping torque play in a PMMC instrument?

It helps to reduce fluctuations in pointer movement. Signup and view all the answers

What phenomenon in the PMMC instrument helps to achieve steady-state deflection?

Eddy current Signup and view all the answers

In a PMMC instrument, what does the pointer's deflection indicate?

The magnitude of the measured current Signup and view all the answers

What are the advantages of PMMC instruments?

High torque to weight ratio Signup and view all the answers

Which of the following is a disadvantage of a PMMC instrument?

Fragile construction Signup and view all the answers

Which application is NOT typically associated with PMMC instruments?

Oscilloscope Signup and view all the answers

What is the purpose of earthing in electrical systems?

To protect equipment from short circuits Signup and view all the answers

Why is galvanised iron commonly used for earthing?

It provides a low resistance path to earth Signup and view all the answers

What effect does temperature change have on PMMC instruments?

Causes errors in readings Signup and view all the answers

What characterizes the internal resistance of an ammeter?

Very low resistance to avoid affecting circuit current Signup and view all the answers

What is the primary reason for a voltmeter to have a very high internal resistance?

To prevent it from loading the circuit under test Signup and view all the answers

Which component specifically serves to move the measuring element in a moving iron instrument?

A soft iron vane Signup and view all the answers

In measuring current, what must be considered to avoid damage to the ammeter?

Never connect it in parallel with any circuit element Signup and view all the answers

Why does an ammeter need to be inserted in series within a circuit?

To measure the current without altering circuit conditions Signup and view all the answers

What could happen if an ammeter is inadvertently connected in parallel with a supply?

It could create a short circuit that damages the ammeter Signup and view all the answers

What is the term used for the current drawn by a voltmeter that may affect accuracy?

Loading effect Signup and view all the answers

What is the defining feature of the moving iron instrument's construction?

It contains a freely moving soft iron plate or vane Signup and view all the answers

What is the purpose of embedding the earthing plate in layers of coal and salt?

To lower the earth resistance of the earthing system Signup and view all the answers

Which aspect is an essential characteristic for the efficiency of pipe earthing?

Water must be periodically poured to ensure conductivity Signup and view all the answers

What is a critical reason for using insulated tools when working on electrical circuits?

Insulated tools minimize the risk of electric shock Signup and view all the answers

What should be the primary consideration when ensuring electrical appliances are properly earthed?

The connection to the ground should be securely fastened Signup and view all the answers

Why is it recommended to keep the earth resistance below 5 ohms in earthing systems?

To maintain a close approximation to earth potential Signup and view all the answers

What mechanism provides the controlling torque in a repulsion type instrument?

Spring tension Signup and view all the answers

Which type of error is NOT typically associated with moving iron instruments?

Thermal error Signup and view all the answers

What is one disadvantage of using a dynamometer type wattmeter at low power factors?

The reading can be affected by stray fields. Signup and view all the answers

Which part of the energy meter directly interacts with the electromagnetic fields to induce rotation?

Moving System Signup and view all the answers

Why is the electrodynamometer wattmeter considered suitable for both AC and DC?

It has a non-polarized design. Signup and view all the answers

What component is essential in the construction of a PMMC instrument to create a stable magnetic field?

Permanent magnets Signup and view all the answers

What characteristic of the scale in moving iron instruments affects their accuracy?

Non-uniform design Signup and view all the answers

What component in the fixed coil of an electrodynamometer wattmeter helps avoid hysteresis effects?

Air-core construction Signup and view all the answers

How does the eddy current in the aluminum disc affect its movement in an energy meter?

It generates a braking torque to control the rotation. Signup and view all the answers

What role does the permanent magnet play in the braking system of the energy meter?

It adjusts the braking torque by altering its position. Signup and view all the answers

How does the moving coil in a dynamometer type wattmeter primarily measure voltage?

By being parallel to the supply voltage Signup and view all the answers

Which factor contributes to the robustness of moving iron instruments?

Simplistic construction Signup and view all the answers

Which part of the energy meter is responsible for recording the total energy consumed in kilowatt hours?

Registering System Signup and view all the answers

What is the nature of the moving element in an attraction type instrument?

Flat disc of iron Signup and view all the answers

What factor primarily influences the torque exerted on the disc in an energy meter?

The interaction of eddy currents and magnetic fields Signup and view all the answers

What typically helps minimize friction error in moving iron instruments?

High torque weight ratio Signup and view all the answers

What is a key characteristic of the PMMC instrument's moving coil design?

It rotates freely within the magnetic fields. Signup and view all the answers

What describes the function of the shunt electromagnet in the energy meter's driving system?

It carries current proportional to the supply voltage. Signup and view all the answers

What happens to the pointer of an electrodynamometer wattmeter when the deflecting torque equals the controlling torque?

It comes to rest at a specific position. Signup and view all the answers

What mechanical process is primarily involved in the operation of a Permanent Magnet Moving Coil instrument?

Movement of the coil in a magnetic field Signup and view all the answers

What characteristic of the PMMC instrument indicates that the controlling torque is opposite to the pointer deflection?

The controlling torque is opposite to the deflecting torque Signup and view all the answers

What phenomenon contributes to the damping torque in a PMMC instrument?

Movement of the aluminium coil in a magnetic field Signup and view all the answers

Which statement correctly reflects the relationship between the pointer deflection and the current in a PMMC instrument?

Pointer deflection is directly proportional to the current Signup and view all the answers

What is a key drawback of using PMMC instruments compared to moving iron instruments?

PMMC instruments are less durable Signup and view all the answers

How does the eddy current developed by the aluminium former affect the pointer in a PMMC instrument?

It opposes the motion of the coil Signup and view all the answers

What does the linear and uniform scale of the PMMC instrument allow for?

Easier and more accurate quantity measurements Signup and view all the answers

Which of the following is NOT a necessity of earthing in electrical systems?

To connect equipment to a higher potential Signup and view all the answers

What primarily influences the cost of PMMC instruments?

The materials used in construction and the delicate design Signup and view all the answers

Which component connects the body of electrical equipment to the earth?

Galvanised iron wire Signup and view all the answers

Study Notes

Current and Voltage Measurement

Current (I) is the rate of electric charge flow, measured in amps (A).
Voltage (V) is the electrical force pushing current between two points, measured in volts (V).

Measuring Current: Ammeters

An ammeter must be connected in series with the circuit to measure current.
Ammeters are designed with low internal resistance to minimize impact on circuit resistance.
Connection of ammeter in parallel can cause short-circuiting, risking damage and injury.

Measuring Voltage: Voltmeters

Voltmeters are connected in parallel to measure voltage (potential difference) between two points.
Voltmeters draw a small current, known as the 'loading effect', which can affect accuracy.
High internal resistance (typically in megohms) is essential in voltmeters to reduce loading effects.

Moving Iron Instruments

Utilizes soft iron vanes that move in a magnetic field created by stationary coils.
Attraction Type: Iron plate moves from a weaker magnetic field to a stronger one, deflecting proportionally to the current.
Repulsion Type: Two magnetized vanes repel each other, allowing measurement of AC and DC currents.

Advantages of Moving Iron Instruments

Universal usage for both AC and DC.
Low friction error due to high torque-weight ratio.
Cost-effective and robust construction.

Disadvantages of Moving Iron Instruments

Non-uniform scale leading to potential accuracy issues.
Errors from hysteresis, frequency, and stray magnetic fields.
AC and DC calibrations differ due to inductance effects.

Electrodynamometer Wattmeter

Used for measuring both DC and AC power with fixed and moving coils.
Fixed coil connected in series with the load; the moving coil connected parallel to the supply voltage.
Reading indicated by the pointer, dependent on the mechanical force between the coils.

Advantages of Electrodynamometer Wattmeter

Suitable for both AC and DC measurements.
High degree of accuracy and a uniform scale.

Disadvantages of Electrodynamometer Wattmeter

Prone to errors at low power factors due to the potential coil's inductance.
Affected by stray fields, requiring magnetic shielding.

Energy Meter

Measures total energy consumed by an electrical load over time, typically in kilowatt-hours (kWh).
Main components include a driving system (electromagnets), moving system (aluminum disc), braking system (permanent magnet), and registering system (counting mechanism).

Working of the Energy Meter

Shunt and series electromagnets create a magnetic field influencing the disc.
The disc’s rotation indicates energy consumption, opposing motion regulated by braking torque from a permanent magnet.

PMMC Instrument

Stands for Permanent Magnet Moving Coil, measuring current via pointer deflection.
Constructed with moving coils, permanent magnets, control springs, and dampening mechanisms.

Working Principle of the PMMC Instrument

Current flow through coils produces mechanical torque, deflecting the pointer proportionally to the current.
The controlling torque from springs maintains equilibrium at steady state.

Advantages of PMMC Instrument

Accurate and consumes low power with high torque-weight ratio.
Uniform scale and efficient eddy current damping with minimal external interference.

Disadvantages of PMMC Instrument

Limited to DC measurements; AC usage is not feasible.
Fragile design and higher cost compared to moving iron instruments.

Applications of PMMC Instrument

Commonly used as ammeters, galvanometers, ohmmeters, and voltmeters.

Earthing

Transfer of electrical energy directly to the earth using low-resistance wiring, protecting equipment and personnel.
Ensures neutral and equipment remain at ground potential, mitigating shock risks during insulation failures.
Galvanized iron commonly employed for earthing connections to achieve effective discharge of leakage currents.### Earthing Overview
Earthing connects electrical equipment to the ground using low-resistance wire, often linked to a copper plate buried 2.5 to 3 meters deep.
Essential for maintaining safety and protecting electrical systems.

Necessity of Earthing

Protects personnel from electric shock in case of contact with charged equipment due to insulation failure.
Ensures constant line voltage under unbalanced load conditions.
Secures equipment operation and longevity.
Shields large buildings and machinery from lightning strikes.
Diverts short-circuit currents safely, minimizing risk of injury or equipment damage.
Provides an effective route for short-circuit currents post insulation failure.
Guards against high voltage surges and lightning discharges.

Methods of Earthing

Key methods include plate earthing and pipe earthing.
Effective earth resistance values:
- Copper wire: 1 ohm
- G.I. wire: less than 3 ohms
Target earth resistance levels:
- Powerhouse: 0.5 ohm
- Substation: 1 ohm

Plate Earthing

Involves a copper or G.I. plate (60cm x 60cm):
- Copper plate: 3.18cm thick
- G.I. plate: 6.35cm thick
Plate is installed vertically at a 3m depth, surrounded by alternate layers of coal and salt (15 cm thick).
Water is added to maintain earth resistance below 5 ohms.
Earth wire is firmly attached to the plate.
A cement chamber with a cast iron cover facilitates maintenance.

Pipe Earthing

Utilizes a G.I. iron pipe, 38mm in diameter and 2m long, with surface holes for conductivity.
Installed upright at a depth of 4.75m in perpetually moist ground.
Surrounding area (15 cm) filled with salt and coal to manage resistance.
System's efficiency improved by periodically adding water through a funnel.
G.I. wires run through a 12.7mm diameter pipe positioned 60cm below the surface.

Electrical Safety Precautions

Avoid water when working with electricity; wet surfaces increase conductivity.
Always turn off the mains before working on household electrical receptacles.
Use insulated tools to minimize risk of shock.
Wear insulated rubber gloves and goggles while handling electrical circuits.
Never handle electrical devices with wet hands to prevent severe shocks.
If someone contacts a live wire, do not pull them away; seek non-conductive support instead.
Ensure work areas are dry and avoid contact with water.
Utilize well-insulated wires in electrical circuits.
Maintain tight connections at switches and plugs; fuses should be on live wires.
Properly earth all electrical appliances to lessen risk of accidental shock.
For short circuits or live wire contact, immediately turn off the main switch and provide support using non-conductive materials.