Classical Electromechanical Instruments

Deflecting, Controlling and Damping Forces

Towards the full scale deflection angle

<p>True (A)</p> Signup and view all the answers

<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

<p>True (A)</p> Signup and view all the answers

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

<p>True (A)</p> Signup and view all the answers

<p>True (A)</p> Signup and view all the answers

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

<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

<p>True (A)</p> Signup and view all the answers

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

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

<p>True (A)</p> Signup and view all the answers

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

<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

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

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

<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

<p>True (A)</p> Signup and view all the answers

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

A deflection instrument is a type of measuring device that uses a pointer moving over a scale to indicate the quantity being measured. It relies on the interaction of three forces: deflecting, controlling, and damping forces.

The deflecting force causes the pointer to move, and its magnitude and direction are directly proportional to the input quantity being measured.

The controlling force acts to stop the pointer at its final position, preventing it from overshooting. It also returns the pointer to its zero position when the input quantity is removed.

The damping force helps to quickly bring the pointer to a resting position without excessive oscillation. It acts against the pointer's movement, reducing the oscillations.

The PMMC (Permanent Magnet Moving Coil) instrument is a common type of deflection instrument. It uses a coil that moves in a magnetic field generated by a permanent magnet.

In a PMMC instrument, the coil is suspended using either jewel bearings or taut bands. Jewel bearings are cone-shaped cuts in jewels where the coil pivots. Taut bands are metal ribbons held under tension that support the coil.

Jewel bearings provide precise movement and support for the coil in a PMMC instrument. They are designed to absorb shocks and maintain accuracy.

Taut bands are a type of suspension in PMMC instruments that uses flat metal ribbons under tension to support the coil. They also act as electrical connections.

Taut band suspension offers higher sensitivity than jewel bearing suspension. It allows for very small currents (as low as 2 µA) to produce full-scale deflection.

The deflecting torque in a PMMC instrument is generated by the interaction of the magnetic field from the permanent magnet and the current flowing through the coil.

The magnitude of the deflecting torque in a PMMC instrument is directly proportional to the current flowing through the coil. This relationship ensures linear scaling.

The controlling torque in a PMMC instrument is generated by the elastic properties of the spiral control springs or taut bands. It opposes the deflecting torque and ensures the pointer returns to zero.

The magnitude of the controlling torque in a PMMC instrument is directly proportional to the angle of deflection of the pointer. This ensures a linear relationship between the angle and the current.

The relationship between the deflecting torque and the controlling torque in a PMMC instrument determines the final position of the pointer. When they are balanced, the pointer stops at its correct position.

The scale of a PMMC instrument is linear because the deflecting torque is directly proportional to the current, and the controlling torque is directly proportional to the angle of deflection. This creates a linear relationship between the current and the pointer position.

A galvanometer is a type of PMMC instrument designed to be extremely sensitive to low currents. It can be used as a null detector, with its zero deflection point in the middle of the scale.

To increase the sensitivity of a galvanometer, taut band suspension is often used, allowing for smaller currents to cause a noticeable deflection. Light beam pointers are also incorporated for more precise readings.

Critical damping in a galvanometer ensures that the pointer settles quickly without excessive oscillations. It is achieved by adjusting the damping resistance to the optimal value.

Current sensitivity (SI) in a galvanometer is the amount of current required to produce a one-millimeter deflection on the scale. It is measured in microamperes per millimeter (μA/mm).

Voltage sensitivity (SV) in a galvanometer is the amount of voltage required to produce a one-millimeter deflection on the scale. It is measured in microvolts per millimeter (μV/mm).

Megohm sensitivity (SMΩ) in a galvanometer is the resistance that needs to be connected in series to produce a one-scale division deflection with a one-volt potential difference. It is measured in megohms (MΩ).

A galvanometer is protected from potentially high currents by using a protection circuit. This circuit limits the current flow to a safe level and prevents damage to the sensitive instrument.

A galvanometer is a delicate instrument and needs to be handled with care. It is important to avoid subjecting it to strong magnetic fields, shocks, or extreme temperatures to ensure its accuracy and longevity.