Operating Forces of Indicating Instrument

Operating Forces of Indicating Instrument Points : Operating Forces of Indicating Instrument, Deflecting force or torque, Controlling force or torque, Dumping force or torque Indicating instrument Consists of essentially a pointer moving over a calibrated scale and attached to the moving system pivoted on jewelled hearings. For satisfactory working of indicating instruments, the following forces or torques are required. These are:
(a) Deflecting force or torque
(b) Controlling force or torque
(c) Dumping force or torque Deflecting Force or Torque The deflecting or operating force is required for moving the pointer from its zero position. The system producing the deflecting force is called “Deflecting System” or “Moving System”. The deflecting force can be produced by utilizing any of the effects explained earlier. Thus the deflecting system of an instrument converts the electric current or potential in to a mechanical force called deflecting force. The deflecting system thus acts a the prime mover responsible for deflection of the Pointer. Controlling Force or Torque This force is required in an indicating instrument in order that the current produces deflection of the pointer proportional to its magnitude. The system producing a controlling force is called a “controlling system”. The functions of the controlling system are:
(a) To produce a force equal and opposite to the deflecting force at the final steady position of pointer in order to make the deflection of the pointer definite for a particular magnitude of current. In the absence of a controlling system the pointer will shoot (swing) beyond the final steady position for any magnitude of current and thus the deflection will he indefinite.
(b) To bring the moving system back to zero when the force causing the instrument to deflect is removed. In the absence of a controlling system the pointer will not come back to zero when current is removed. Controlling force is usually provided by springs. Damping Force or Torque When a deflecting force is applied to the moving system. it deflects and it should come to rest at position where the deflecting force is balanced by the controlling force. The deflecting and controlled forces are produced by systems which have and, therefore. the moving system can not settle at its final position but overshoots ahead of it. Suppose “0” is the equilibrium or final steady position and because of inertia, the moving system moves to position “a”. Now for any position “a” beyond the equilibrium position the controlling force is more than the deflecting force and hence the moving system swings back. Due to inertia it can not settle at “0” but swings to a position say “b” beyond the equilibrium position. At “b” the deflecting force is more than the controlling force and hence the moving system again swings ahead. The pointer thus oscillates about its final steady (equilibrium) position with decreasing amplitude till its kinetic energy (on account of inertia) is dissipated in friction and therefore, it will settle down at its final steady position. If extra forces are not provided to “damp” these oscillations, the moving system will take a considerable time to settle to the final position and hence time consumed in taking readings will be very large. Therefore, damping forces are necessary so that the moving system comes to its equilibrium position rapidly and smoothly without any oscillations.