Construction of Electrodynamic or Dynamometer Type Instruments

Construction of Electrodynamic or Dynamometer Type Instruments Points : Construction of Electrodynamic or Dynamometer Type Instruments The points of construction of dynamometer type instrument are described. i) Fixed Coils The field is produced by a fixed coil. The coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft. The instrument illustrated in the fig may by a milliampere, or may become a voltmeter by addition of a series resistance. The fixed coils are wound with fine wire for such applications.

Field coils (fixed) are usually wound with heavy wire carrying the main current in ammeters and wattmeters. The wire is stranded where necessary to reduce eddy current losses in conductors. The coils are usually varnished and baked to form a solid assembly. These are then clamped in places against he coil supports. This makes the construction rigid so that there is no shifting or change in dimension which might effect the calibration. The mounting supports are preferably made out of ceramic, as metal pats would weaken the field of the fixed coil due to eddy currents. ii) 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 can not be used as eddy currents would be induced in it by the a.c. field.. Light hut rigid construction is used for the moving coil. If should be noted that both fixed and moving coils are air cored to avoid hysteresis losses in iron core. iii) Moving System The moving coil is mounted on an aluminium spindle. The moving system also carries the counter weights and truss type pointer. Sometimes a suspension may be used in case high sensitivity is desired. iv) Control The controlling torque is provided by two control springs. These springs acts as leads to the moving coil. v) Damping For these instruments the air friction damping is employed. It is provided by a pair of aluminium vanes, attached to the spindle at the bottom. These vanes move in sector shaped chambers. vi) Shielding The field produced by the field coils is some times weaker than in other types of instruments. It is nearly 0.005 to 0.006 Wb/m². In d.c. measurements even the earth’s magnetic field may affect the readings. Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields. Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing in a casing of high permeability allay. Double casing is highly effective in the case of precision instruments. The outer casing is made up of a material of high saturation density and low. Coercive force, while the inner casing is made up of a material having high initial permeability. vii) Cases & Scales Laboratory standard instruments are usually contained in highly polished wooden cases. 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.