A Note on PMMC Instruments

1. DEFLECTING TORQUE:- The deflecting torque is produced by making use of one of the magnetic, chemical, electrostatic and electromagnetic induction effects of current or voltage and causes the moving system of the instrument to move from its zero position when the instrument is connected in an electrical circuit to measure the electrical quantity. The method of producing this torque depend upon the type of instrument. In attracting the type of instrument, this torque to equal to Td = 1/2 I2 dL/dθWhereas in Pmmc instruments Td = Bilur Where B - magnetic density i - current flowing l - length of coil u - number of turn r - radius of coil 2. CONTROLLING TORQUE:- The magnitude of the movement to the moving system would be somewhat indefinite under the influence of deflecting torque unless some controlling torque exist. This torque opposes the deflecting torque and increases with increase in deflection of the moving system without controlling system the irrespective magnitude of current and moreover, once deflected it would not return to its zero position on removing the current. In attraction type instrument it is produced by spring control and in PMMC too it would be produced by spring control. 3. DAMPING TORQUE:- This torque is also necessary to avoid oscillation of the moving system about it's final deflected position owing to the inertia of the moving parts and to bring the moving system to rest in it's final deflected position quickly.