Classical Electromechanical Instruments
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Questions and Answers

What are the three forces that operate inside a deflection instrument?

Deflecting, Controlling and Damping Forces

What is the type of instrument where the deflecting force is produced by the current in the moving coil?

  • PMMC instrument
  • Electrodynamic instrument
  • Moving Iron instrument
  • All of the above (correct)

    • What is the direction of the deflecting force in a deflection instrument?

    Towards the full scale deflection angle

    The magnitude of the deflecting force is proportional to the input quantity to be measured.

    <p>True</p> Signup and view all the answers

    What is the function of the controlling force in a deflection instrument?

    <p>To stop the pointer at its exact final position and to return the pointer to its zero position</p> Signup and view all the answers

    The magnitude of the controlling force is proportional to the angle of deflection.

    <p>True</p> Signup and view all the answers

    What does the damping force do in a deflection instrument?

    <p>Prevents the pointer from oscillating excessively and settling down quickly with a very small oscillation.</p> Signup and view all the answers

    The magnitude of the damping force is proportional to the pointer acceleration.

    <p>True</p> Signup and view all the answers

    The direction of the eddy current damping force opposes the motion of the coil.

    <p>True</p> Signup and view all the answers

    What are the two main types of suspension systems used in deflection instruments?

    <p>Jewel bearing suspension and taut band suspension.</p> Signup and view all the answers

    What makes the taut band suspension system more sensitive than the jewel bearing suspension system?

    <p>Taut band suspension instruments can give full-scale deflection (FSD) with a coil current as low as 2 µA, whereas the most sensitive jeweled-bearing instruments give FSD with a coil current of 25 µA.</p> Signup and view all the answers

    The PMMC instrument is the most common type of deflection instrument.

    <p>True</p> Signup and view all the answers

    What is the primary limitation of a PMMC instrument?

    <p>All of the above</p> Signup and view all the answers

    What type of instrument is a galvanometer?

    <p>A PMMC instrument that is very sensitive to very low currents</p> Signup and view all the answers

    Galvanometers are designed to have a mid-scale zero deflection.

    <p>True</p> Signup and view all the answers

    Which of the following is used to increase the sensitivity of a galvanometer?

    <p>All of the above</p> Signup and view all the answers

    What is the critical damping resistance value?

    <p>It is the damping resistance value that gives just sufficient damping to allow the pointer to settle down quickly with a very small oscillation.</p> Signup and view all the answers

    What is the current sensitivity of a galvanometer?

    <p>The amount of current (μA) flowing through the instrument to give a deflection of 1 mm.</p> Signup and view all the answers

    What is the voltage sensitivity of a galvanometer?

    <p>The amount of voltage (μV) applied across the instrument to produce a deflection of 1 mm.</p> Signup and view all the answers

    What is the megohm sensitivity of a galvanometer?

    <p>The resistance connected in series with the instrument to restrict the deflection to one scale division for 1 V potential difference between its terminals.</p> Signup and view all the answers

    The galvanometer can be protected against accidental damage by a shunt resistor.

    <p>True</p> Signup and view all the answers

    What is the primary application of a galvanometer?

    <p>As a null detector, particularly in bridge circuits.</p> Signup and view all the answers

    Study Notes

    Classical Electromechanical Instrument

    • Classical electromechanical instruments use a pointer that deflects over a scale to indicate the measured quantity.
    • There are three forces acting within the instrument: deflecting, controlling, and damping forces.
    • Examples include Permanent Magnet Moving Coil (PMMC) instruments, electro-dynamic instruments, and moving iron instruments.

    Deflection Instruments Fundamentals

    • The deflecting force causes the pointer to deflect, with the deflection angle proportional to the input quantity.
    • The direction of deflection is towards the full-scale deflection angle.
    • The magnitude of the deflecting force is proportional to the input quantity.

    Controlling Force

    • The controlling force stops the pointer at its final position.
    • It returns the pointer to its zero position.
    • Its magnitude is proportional to the angle of deflection.
    • This force can be generated using spiral springs or taut bands.

    Damping Force

    • Damping forces prevent oscillations of the pointer.
    • The correct damping gives a fast and zero oscillation of the pointer movement.
    • The damping force is generated by eddy currents induced in the coil former.
    • The magnitude of the damping force is proportional to the pointer's acceleration.
    • The direction of the eddy current damping force opposes the coil's motion.

    Methods of Supporting the Moving System of Deflection Instrument

    • Various support methods exist for the moving system of deflection instruments.
    • Pivot and jewel-bearing suspension
    • Spring-supported jewel bearing
    • Some jewel bearings are spring-supported to absorb shocks more easily.
    • The most sensitive jeweled-bearing instruments reach full-scale deflection with a coil current of 25 μA.

    Taut Band Suspension

    • Two flat metal ribbons (phosphor bronze or platinum alloy) help support the coil and are held taut by springs.
    • The ribbons exert a controlling force as they twist.
    • They can be used for electrical connections to the moving coil.
    • Taut-band suspension instruments can achieve full-scale deflection with a coil current as small as 2 μA.

    Permanent Magnet Moving Coil (PMMC) Instruments

    • PMMC instruments are the most common deflection-type instruments.
    • The construction includes a permanent magnet with two soft-iron pole shoes.
    • A cylindrical soft-iron core is positioned between the shoes.
    • One of the two controlling spiral springs is shown.
    • One end of this spring is attached to the pivoted coil, and the other end connects to an adjustable zero-position control.
    • The current in the coil must flow in one direction to move the pointer from the zero position over the scale.
    • Terminals (+) and (-) indicate the correct polarity for the connection, polarizing the instrument.
    • They cannot directly measure alternating current; rectifiers are needed.
    • A mirror below the pointer aids in accurate readings by removing parallax.

    Torque Equation & Scale

    • The deflecting torque on the coil is directly proportional to the magnetic flux density, dimensions of the coil, and the coil current.
    • The force acting on each side of the coil is proportional to the current and the magnetic field.
    • The area enclosed by the coil gives the instrument a linear scale.
    • The force on both Coil sides is proportional to the current and the magnetic field with N number of turns.
    • They produce a deflecting torque Tdef - proportional to BILND, where B is magnetic field, I is current, L and D are dimensions.
    • Tdef = BILND

    Galvanometer Instrument

    • A galvanometer is a PMMC instrument designed to detect very low currents.
    • It acts as a null detector and has its zero deflection at mid-scale.
    • To improve sensitivity, taut band suspension is used.
    • A lightweight beam pointer improves response time.
    • A shunt damping resistor controls eddy currents.
    • Critical damping resistance gives quick pointer settling with minimal oscillation.
    • This instrument has a current sensitivity.
    • Voltage sensitivity is expressed, relating to the current sensitivity.
    • The megohm sensitivity is the resistance required to restrict the deflection to a single scale division for 1V potential.

    Protection of the Galvanometer

    • High initial currents can damage a galvanometer during null-detection use (e.g., with a Wheatstone bridge).
    • An adjustable shunt resistor protects the coil by diverting excess current.

    Examples and Solutions

    • Various example problems involving PMMC and galvanometer calculations will be provided, showcasing their application and problem-solving.

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    Related Documents

    Classical Electromechanical Instrument PDF

    Description

    Explore the fundamentals of classical electromechanical instruments, including their operational forces: deflecting, controlling, and damping. Learn about different types of instruments such as PMMC and moving iron. This quiz will test your understanding of how these instruments function and their underlying principles.

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