Essential Torques of Indicating Instruments

ESSENTIAL TORQUES OF INDICATING INSTRUMENTS

Indicating Instruments are those which indicate the value of the measured quantity at the time at which it is measured. Such instruments essentially consists a pointer which moves over a calibrated scale and which is attached to a moving system, pivoted in a jewelled bearing. The moving system is subjected to the following three torques:

1. Deflecting (or operating) torque

2. Controlling (or restoring) torque

3. Damping torque

1. Deflecting Torque (T_d)

This torque (T_d) is produced by utilizing one or more effects mentioned above (ie., magnetic, electrostatic, electrodynamic, thermal or chemical etc.,) This torque cause the moving system of the instrument to move from its zero position i.e., its position when the instrument is disconnected from the supply.

2. Controlling Torque (Tc)

The deflection of the moving system will be indefinite, unless otherwise controlled by some opposing torque to make the deflection constant at any particular position for a given value of quantity being measured on the scale of the dial. Such a torque is called controlling torque. The magnitude of the controlling torque should increase with the angle of deviation and the torque should be acting in a direction, opposite to that of deflecting torque.

With the two torques acting on the moving system it should come to rest at such a position at which the controlling torque is equal to the deflecting torque. The controlling torque in an indicating instrument is always obtained by a spring or much less by the gravity of the moving system.

1. Spring Control

A hair‒spring, usually of phosphor‒bronze, is attached to the moving system of the instrument, as shown in Fig. 6.1 (a).

With the deflection of the pointer, the spring will be twisted in the opposite direction. This twist in the spring produces controlling torque, which is directly proportional to the angle of deflection of the moving system T_d ∝ I. The pointer comes to a position of rest, when the deflecting torque (T_d) and controlling torque (Tc) are equal. For example, in permanent‒magnet moving coil (PMMC) type of instruments, the deflecting torque is proportional to the current. I passing through them.

∴ T_d ∝ I

As,

T_c = T_d

 θ = I

Since, deflection θ is directly proportional to current I, the spring‒controlled instruments have an uniform or equally‒spaced scales over the whole of their range as shown in Fig. 6.1 (b).

The requirements of spring material:

(i) It should be non‒magnetic.

(ii) Not subjected to appreciable fatigue.

(iii) Should have low specific resistance

(iv) Low temperature‒resistance co‒efficient.

2. Gravity Control

In gravity controlled instruments, a small adjustable weight is attached to the moving system in such a way that it produces a controlling torque when the system is deflected. This is illustrated in the fig (6.2).

In this controlling system, the controlling torque is proportional to the sine of angle of deflection,

Tc ∝ sin θ (Not θ)

If  T_d ∝ I

As,

T_c = T_d

 I = sin θ           (Not θ)

Hence in gravity ‒ controlled instruments the scale will not be uniform but are cramped or crowded at their lower ends.

3. Damping Torque

Even with the existence of deflecting torque and controlling torque acting on the moving system, it will not come to the position of rest due to the inertia possessed by it. The pointer which is attached to the moving system will oscillate about the equilibrium position, and will come to rest only after a long time. This involves time and create hardships in observing quick readings and also prevent another force which is required to dampout the oscillations occurring at the position of equilibrium for any given quantity of electricity that is being measured. Hence a third torque known as damping torque is required to act on the moving system, so as to bring the moving system quickly to the position of rest without oscillation.

The adjoining curves shows the effect of damping upon the variation of position with time of the moving system of the instrument is shown in Fig. 6.3.

When the system is under‒damped the pointer oscillates and slowly comes to rest (final deflection position). When the system is over‒damped the pointer slowly rises to final deflected position and comes to rest. Whereas if the system is critically damped the pointer quickly rises to the final deflected position and rest. Such a damped instrument is otherwise called as "Dead Beat".

The damping torque should come into operation only while the moving system of the instrument is actually moving. Damping force can be provided by

1. Air friction

2. Fluid friction [used occasionally]

3. Eddy current

Each type of damping is applicable to a particular type of instrument.

1. Air‒friction Damping

Two methods of air friction damping are shown in Fig. 6.4. In Fig. 6.4 (a) the light aluminium piston, attached to the moving system travel with a very small clearance, fixed air chamber closed at one end. The cross‒section of the chamber is either circular or rectangular. Damping of the oscillations is affected by the compression and suction actions of the piston on the air‒enclosed in the chamber. Such a system of damping is not much favourable these days, than those shown ion Fig. 6.4 (b). In the latter method, one or two light aluminium vanes are mounted on the spindle of the moving system, which moves in a closed sector‒shaped box.

2. Fluid Friction Damping

In this method the friction drag exerted on the movement of a disc inside the pot fluid is utilised. In this type of damping careful fitting is required, in the previous method it is not necessary. A light vane or disc attached to the spindle of the moving system, dips into a pot of a viscous liquid and it should be completely submerged by the oil. The Fig. 6.5 illustrates the method. There is no frictional force when the disc is stationary and a frictional drag acts on the disc when it moves and the amount of force acting is proportional to be speed of motion. The suspending system of the disc should be cylindrical and of least diameter where it penetrates the oil surface. So as to minimise effects due to surface tension.

In second system instead of a disc a pair of vanes rigidly fixed at right angles is made to rotate into a cylindrical pot of oil. An increased damping when compared to the previous system is obtained by using vanes.

Principle

The principle requirements of the oil used for damping are:

(i) It should not evaporate quickly.

(ii). Do not have corrosive action on metals.

(iii) Sufficient viscous.

(iv) The viscosity should not change appreciably with temperature,

(v) Should be a good insulator.

Advantages

(a) Due to more viscosity of fluid, more damping is provided.

(b) The oil can also be used for insulation purposes,

(e) Due to up thrust of oil, the load on the bearing is reduced thus frictional errors, are reduced.

Disadvantages

(a) This can be applicable only for instruments which are vertical in position.

(b) Due to oil leakage, the instruments cannot be kept clean.

3. Eddy Current Damping

On the above types of damping the most efficient form of damping is the eddy current damping. The damping utilised is a critical damping of its nature and hence such instruments using eddy current damping are called "Dead Beat" instruments.

In this method, a thin disc of a conducting non magnetic material like copper or aluminium mounted on the spindle which carriers the moving system and the pointer of the instrument. The disc so positioned that its edge, when in rotation, cuts the magnetic flux between the poles of a permanent magnet. Hence eddy currents are produced in the disc which flow and so produce a damping force in such a direction so as to oppose the cause producing them (Lenz's law). Since the cause producing them is the rotation of the disc, these eddy currents retard the motion of the disc and the moving system as a whole. Comparison between the types are given below:

Table 6.2 Comparison of Air Vs Fluid Vs Eddy damping