In order to ensure proper operation of indicating instruments, the following three torques are required:

  • Deflecting torque.
  • Controlling torque.
  • Damping torque.

What is deflecting torque in measuring instruments?

 
One important requirement in indicating instruments is the arrangement for producing operating or deflecting torque (Td) when the instrument is connected in the circuit to measure the given electrical quantity.
 
This is achieved by utilizing the various effects of electric current or voltage. The deflecting torque causes the moving system to move from its zero position. The deflecting torque is produced by utilizing one or more of the following effects of current or voltage:

  • Magnetic effect ——————————- Moving-iron instruments.
  • Electrodynamic effect ———————- (i) Moving coil instruments, (ii) Dynamometer type.
  • Electromagnetic induction effect ———Induction type instruments.
  • Thermal effect ———————————Hot-wire instruments.
  • Chemical effect ——————————–Electrolytic instruments.
  • Electrostatic effect —————————-Electrostatic voltmeters

The table below gives information about the electrical measuring instruments in which deflecting torque is produced by utilizing the first three effects.
 
What is deflecting torque in measuring instruments
 

What is controlling torque in measuring instruments?

 
The controlling torque (Tc) opposes the deflecting torque and increases with the deflection of the moving system. The pointer comes to rest at a position where the two opposing torques are equal i.e. Td = Tc. The controlling torque performs two functions.
 
Controlling torque increases with the deflection of the moving system so that the final position of the pointer on the scale will be according to the magnitude of an electrical quantity (i.e. current or voltage or power) to be measured.
 
Controlling torque brings the pointer back to zero when the deflecting torque is removed. If it were not provided, the pointer once deflected would not return to zero position on removing the deflecting torque. The controlling torque in indicating instruments may be provided by one of the following two methods:

  1. Spring control.
  2. Gravity control.

Spring Control Method

Spring control method of controlling torque image
This is the most common method of providing controlling torque, in electrical instruments. A spiral hairspring made of some non-magnetic material like phosphor bronze is attached to the moving system of the instrument as shown in the figure.
 
Springs also serve the additional purpose of leading current to the moving system (i.e. operating coil). With that deflection of the pointer, the spring is twisted in the opposite direction. This twist in the spring provides the controlling torque.
 
Since the torsion torque of a spiral spring is proportional to the angle of twist, the controlling torque (Tc ) is directly proportional to the angle of deflection of pointer (θ) i.e. Tc α θ.The pointer will come to rest at a position where controlling torque is equal to the deflecting torque i.e. Td =Tc.
 
In an instrument where the deflecting torque is uniform, spring control provides a uniform scale over the whole range. The balance weight is attached to counterbalance the weight of the pointer and other moving parts.
 

Gravity Control Method

 
Gravity control controlling torque image
In this method, a small weight is attached to the moving system, which provides necessary controlling torque. In the zero position of the pointer, the control weight hangs vertically downward and therefore provides no controlling torque.
 
However, under the action of deflecting torque, the pointer moves from zero position and control weight moves in opposite direction. Due to gravity, the control weight would tend to come in original position (i.e. vertical) and thus provides an opposing or controlling torque. The pointer comes to rest at a position where controlling torque is equal to the deflecting torque.In this method, controlling torque (Tc) is proportional to the sin of angle of deflection (θ) i.e. Tc α sin θ.
 
Because in this method controlling torque (Tc) is not directly proportional to the angle of deflection (θ) but it is proportional to sin θ therefore, gravity control instruments have non-uniform scales; being crowded in beginning.
 

What is damping torque in measuring instruments?

 
A damping torque is produced by a damping or stopping force which acts on the moving system only when it is moving and always opposes its motion. Such a torque is necessary to bring the pointer to rest quickly. If there is no damping torque, then the pointer will keep moving to and fro about its final deflected position for some time before coming to rest, due to the inertia of the moving system.
 
This damping torque acts only when the pointer is in motion and always opposes the motion. The position of the pointer when stationary is, therefore, not affected by damping torque. The degree of damping decides the behavior of the moving system.
 
If the instrument is under-damped, the pointer will oscillate about the final position for some time before coming to rest. On the other hand, if the instrument is over damped, the pointer will become slow and lethargic.
 
What is damping torque in measuring instruments
 
However, if the degree of damping is adjusted to such a value that the pointer comes up to the correct reading quickly without oscillating about it, the instrument is said to be critically damped. The damping torque in indicating instruments can be provided by:

  • Air friction damping.
  • Fluid friction damping.
  • Eddy current damping.

Air Friction Damping

 
Air friction damping torque image
Arrangements of air friction damping are shown in fig. (a) and fig. (b). In the arrangement shown in fig (a), a light aluminum piston is attached to the spindle that carries the pointer and moves with a very little clearance in a rectangular or circular air chamber closed at one end.
 
The cushioning action of the air on the piston damps out any tendency of the pointer to oscillate about the final deflected position. This method is not favored these days and the one shown in fig. (b) is preferred.
 
In this method, one or two light aluminum vanes are attached to the same spindle that carries the pointer. As the pointer moves, the vanes swing and compress the air. The pressure of compressed air on the vanes provides the necessary damping force to reduce the tendency of the pointer to oscillate.
 

Fluid Friction Damping

 
Fluid friction damping torque image
In this method, discs or vanes attached to the spindle of the moving system are kept immersed in a pot containing oil of high viscosity. As the pointer moves, the friction between the oil and vanes opposes the motion of the pointer and thus necessary damping is provided.
 
The fluid friction damping method is not suitable for portable instruments because of the oil contained in the instrument. In general, fluid friction damping is not employed in indicating instrument, although one can find its use in Kelvin electrostatic voltmeter.
 

Eddy Current Damping

Eddy current damping torque image
Two methods of eddy current damping are generally used.In the first method, as shown in the figure, a thin aluminum or copper disc is attached to the moving system is allowed to pass between the poles of a permanent magnet. As the pointer moves, the disc cuts across the magnetic field and eddy currents are induced in the disc.
 
These eddy currents react with the field of the magnet to produce a force which opposes the motion according to Lenz’s Law. In this way, eddy current damping torque reduces the oscillations of the pointer.
 
In the second method, the coil which produces the deflecting torque is wound on aluminum former. As coil moves in the field of the instrument, eddy currents are induced in the aluminum former to provide the necessary damping torque.
 



Measuring Instruments–1 | Objective Type Question Answers

#1 A pointer of an instrument once deflected returns to zero position, when the current is removed, due to

controlling torque

#2 An instrument in which the value of electrical quantity to be measured can be determined from the deflection of the instrument, when it has been pre-calibrated by comparison with an absolute instrument is known as

secondary instrument

#3 Damping torque in instruments is generally not produced

electro-statically

#4 Which of the following materials will be preferred as a shunt for extending the range of measurement of a voltmeter?

Manganin

#5 Low temperature coefficient of the resistance used for extending the range of a voltmeter

reduces inaccuracies due to the temperature variations

#6 The deflecting torque in an instrument may be produced

#7 To take-care the change in frequency of AC current, while using moving iron type instruments,

a condenser of suitable value is used in parallel with the swamp resistance

#8 A portable instrument is likely to have

eddy-current damping

#9 In eddy-current damping systems, the disc is usually made of

conducting and non-magnetic material

#10 In eddy-current damping systems, the disc is usually made of

conducting and non-magnetic material

#11 Which type of wattmeter cannot be used for both AC and DC?

induction type

#12 A moving iron type ammeter has few turns of thick wire so that

resistance is less

#13 A repulsion type ammeter when used on AC circuits, reads

RMS value of current

#14 If damping torque is not provided in an instrument

the pointer will oscillate about its final deflected position for quite time before coming  to rest

#15 When the damping of an instrument is adjusted to enable the pointer to rise quickly to its deflected position without overshooting, in that case the instrument is said to be

dead beat

#16 When the damping force is more than the optimum, the instrument will become

slow and lethargic

#17 In a moving iron type ammeter, the coil has

few turns of thick wire

#18 In a repulsion type instrument, the force of repulsion is approximately proportional to

square of the current

#19 In moving iron type instruments because of the hysteresis in the iron parts of the moving system, the readings are

higher on descending values but lower on ascending values

#20 Magnetic shielding of the working parts is obtained by using

covering case of cast iron

#21 If the torque/weight ratio of an instrument is low, then it can be concluded that

sensitivity of the meter will be low

finish

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4 Comments

NUWASHABA GODWIN · January 31, 2019 at 8:50 am

i want alebelled diagram of damping, controlling torque

    Ajay Sharma · February 1, 2019 at 8:59 am

    Sorry friend, I can not help more.

Evaristo Chishimba · May 31, 2019 at 1:44 pm

I need the pdf book of this

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