Engineering Q&A
Explain the essential toques in analog type measuring instruments.

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List of torques in analog instruments: 

1. Deflecting torque 

2. Controlling / Restraining torque. 

3. Damping torque. 

1. Deflecting torque: To create deflection proportional to the quantity to be measured, this is normally current. - In PMMC instruments it is produced due to interaction of magnetic fields due to permanent magnet and current coil placed in it. Deflecting force is proportional to the permanent magnetic field and the current in the coil. 


-Moving iron instruments: The current in field coil induces similar fields in the two iron vanes that repel each other to give the deflecting torque proportional to square of current in coil. 


- Moving iron instruments with one coil producing magnetic field while the iron piece is attracted towards the coil where the force of attraction is proportional to the square of current in the coil


2. Controlling / restraining torque: - To restrict the motion of pointer/spindle and stop the pointer at the relevant position to get correct reading. - To bring back pointer to zero position when the quantity under measurement is removed. This is provided by springs normally made of phosphor bronze that are used to hold the moving member along with spindle in the magnetic field producing the deflecting torque/force. - This is also provided by control weights shown in figure above.


3. Damping torque:

- To stop pointer/spindle at the final deflected position. 

- Bring the pointer to stand still quickly. 

- To minimize oscillations about final position. 

 For air damping: air trapped in the chamber works as damping medium for the piston movement connected to the spindle. The piston moves in the air chamber. The clearance between piston and air chamber wall is very small. When the pointer system moves in either direction the piston arm experiences an opposing force due to either compression action on one side and opposition to expansion on the other side. Thus the oscillations of the pointer system are damped by the opposition by the damping system. The damping torque is directly proportional to the speed at which the piston (pointer/spindle) moves. Hence greater the speed higher will be the damping torque bringing the pointer to the equilibrium position quickly. 


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