Torque in indicating Instrument (Deflecting, Controlling, Damping)

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  δ 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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