Difference between the construction of PMMC and dynamometer instruments
PMMC (Permanent moving magnet coil) Instrument
(1) Moving Coil: The moving coil is wound with many turns of enamelled or silk-covered copper wire. The coil is mounted on a rectangular aluminium former which is pivoted on jewelled bearings. The coils move freely in the field of a permanent magnet.
(2) Control: The coil is supported between two jewel bearings the control torque is provided by two phosphor bronze hair springs.
(3) Damping: Damping torque is produced by the movement of the aluminium former moving in the magnetic field of the permanent magnet.
(4) Pointer and Scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer is of light-weight construction and, apart from those used in some inexpensive instruments, has the section over the scale twisted to form a fine blade.
Dynamometer Instrument
(1) Moving Coil: A single element instrument has one moving coil. The moving coil is wound either as a self-sustaining coil or else on a non-metallic former. A metallic former cannot be used as eddy currents would be induced in it by the alternating field. Both fixed and moving coils are air cored.
(2) Control: The controlling torque is provided by two control springs. These springs act as leads to the moving coil.
(3) Damping: Air friction damping is employed for these instruments and is provided by a pair of aluminium vanes attached to the spindle at the bottom. These vanes move in sector shaped chambers.
(4) Pointer and Scale: The scales are hand drawn, using machine sub-dividing equipment. Diagonal lines for fine sub-division are usually drawn for main markings on the scale.
