Fundamental Electrical and Electronic Principles, Third Edition

Electromagnetism 161Fig. 5.22Damping Torque When current is passed through the coil, thedeflecting torque accelerates the pointer away from the zero position.Now, although the coil and pointer assembly is very light, it will stillhave sufficient inertia to ‘ overshoot ’ its final position on the graduatedscale. It is also likely to under- and overshoot several times beforesettling. To prevent this from happening, the movement needs to beslowed down, or damped. This effect is achieved automatically by thegeneration of eddy currents in the aluminium coil former as it rotatesin the magnetic field. The full description of eddy currents is dealtwith later in this chapter. However, being induced currents means theyare subject to Lenz ’ s law. They will therefore flow in the coil formerin such a direction as to oppose the change that produced them; thatis the rapid deflection of the coil. If the dimensions of the formerare correctly chosen, then the result will be either one very smallovershoot, or the overshoots are just prevented from occurring.In the latter case the instrument is said to be critically damped, or‘ dead beat ’.The main advantages of the moving coil instrument are:1 Good sensitivity: this is due to the low inertia of the coil and pointerassembly. Typically, a current of 50 μA through the coil is sufficientto move the pointer to the extreme end of the scale (full-scaledeflection or fsd).2 Linear scale: from equation (5.7) we know that T BANI .For a given instrument, B,A , and N are fixed values, so T I.Thus the deflecting torque is directly proportional to the coilcurrent.The main disadvantage is the fact that the basic meter movement so fardescribed can be used only for d.c. measurements. If a.c. was appliedto the coil, the pointer would try to deflect in opposite directionsalternately. Thus, if only a moderate frequency such as 50 Hz isapplied, the pointer cannot respond quickly enough. In this case onlya very small vibration of the pointer, about the zero position, might beobserved.The complete arrangement for a moving coil meter is illustrated inFig. 5.23 .