Figure 1 shows the basic construction of a PMMC instrument. A moving-coil instrument contains principally of a permanent magnet to offer a magnetic field and a small lightweight coil pivoted within the field. A soft iron core is included between the poles of the magnet so that the coil rotates in the narrow air gap between the poles and the core. When a current is passed through the coil windings, a torque is exerted on the coil by the interaction of the magnet’s field and the field set up by the current in the coil. The resulting deflection of the coil is indicated by a pointer that moves over a calibrated scale.
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Fig.1: PMMC Basic Construction
In addition to a deflecting force provided by the coil current and the field from the permanent magnet, a controlling force is needed. This is the force that returns the coil and pointer to the zero position when no current is flowing through the coil. The controlling force also balances the deflecting force, so that the pointer remains stationary for any constant level of current through the coil. The controlling force is usually provided by spiral springs as indicated in figure 2. The springs are also employed as connecting leads for conducting current through the coil.
Fig.2: PMMC Coil Springs
One other force, known as a damping force, is required for correct operation of a deflection instrument. When no damping force is present, the pointer swings above and below its final position on the scale for some time before settling down. In the case of the PMMC instruments, the damping force uses eddy currents. To facilitate this, the coil is wound on an aluminum frame or coil former in which eddy currents are generated by any rapid movements of the coil in the magnetic field. The eddy currents set up a magnetic flux that opposes the original movement that generates them. Thus, oscillations of the meter pointer are damped out.