Construction of Permanent-Magnet Moving Coil (PMMC) Instrument

Construction of Permanent-Magnet Moving Coil (PMMC) Instrument Points : Construction of Permanent-Magnet Moving Coil (PMMC) Instrument The general construction of this instrument is shown. The main components are as follows: 1. Moving System Moving Coil The moving coil is wound with many turns of enameled or silk covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coil moves freely in the field of a permanent magnet. Most voltmeter coils are wound on metal frames to provide the required electro- magnetic damping. Most ammeter coil, however, are wound on non-magnetic formers, because coil turns are effectively shorted by the ammeter shunt. The coil itself, therefore, provides electromagnetic damping. 2. Magnet System Simple U-shaped permanent magnet is mot widely used. This magnet is made of Alnico and has soft-iron end- pole pieces which are bored out cylindrically. Between the magnetic poles is fixed soft iron cylinder whose function is (i) to make the field radial and uniform (ii) to reduce the reluctance of the air path between the poles and hence increase the field strength. 3. Control System The controlling torque is provided by two phosphor bronze hair springs, either helical or spiral, coiled in opposite directions, normally equal in strength. Constancy of spring performance is essential to maintain accuracy. The thickness of spring is controlled in manufacturing process. The control springs also conduced the operating current in to and out of the moving coil. 4. Damping System In this instrument the damping torque is provided by movement of the aluminium former moving in the magnetic field of the permanent magnet. This is actually eddy current damping. 5. Pointer and Scale The pointer is carried by the spindle and moves over a graduated scale and indicates the angular deflection of the coil and, therefore, the current flowing through the coil. The pointer is made of a light aluminium tube flat at one end to form a vertical knife-edge.