Torque in Indicating instruments

There is three type of torque that we are needed to satisfactory operation of any indicating instrument which are as following:-

  1. Deflecting torque 
  2. Controlling torque 
  3. Damping torque


Deflecting Torque:-

  • Deflecting torque is the amount of Torque needed for the pointer to move from its zero position.
  • In the above figure an ammeter having full scale value of 2 ampere current in the above figure current is one ampere
                                           
  • In the above fig. Ammeter having full scale value of 2A. Current in fig. (I = V/R) I=10/10=1A
  •  For the ammeter to indicate a value of 1 ampere of current a torque or force is required which is called deflecting torque and is proportional to current.

Controlling torque

  • The controlling torque is in opposite to the deflecting torque at a point where controlling torque and deflecting torque is equal the pointer stops and show the measured value of the reading.
  • The controlling torque is also used to bring the pointer to zero initial position if there is a no deflecting torque.
  • The controlling torque is provided by spring or gravity control.
  • For vertically mounted instrument gravity control is used.    
     
  • Spring is made up of phosphorus bronze to reduce the heating effect.
  • If the control spring have been failed then pointer moves to the maximum position of the scale expect in PMMC.                  
  • In case of PMMC instrument the pointer comes to zero initial position due to current is passing through the spring in PMMC instruments.


Damping torque

  • Analogue instruments are of second order type which has a damping factor  Î´ 0.6 to 0.8
  • The type of damping is underdamped.
When it deflecting force is applied to the moving system it deflects and should comes to rest at a position when controlling torque equals to the opposite to deflecting Torque but due to inertia the pointer keeps oscillating about its equilibrium there for a damping torque is required to damp out the oscillation of the pointer 

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