Dynamometer Type Instrument
ElectroDynamometer Instrument
The electrodynamometer type instrument is a transfer
instrument. A transfer instrument is one which is calibrated with a D.C source
and used without any modifications for A.C measurements. Such a transfer
instruments has same accuracy for A.C and D.C measurements. The
electro-dynamometer type instruments are often used in accurate A.C voltmeters
and ammeters, not only at the power line frequency but also in the lower audio
frequency range with some little modifications, it can be used as a wattmeter
for the power measurements.
Principle: Electro-dynamometer type instruments are very similar to PMMC
type instrument in which the operating field is produced, not by a permanent
magnet but by another fixed coil (usually two fixed air cored coils are used).
The PMMC instrument cannot be used on A.C currents or
voltages. If A.C supply is given to these instruments, an alternating torque
will be developed. Due to moment of inertia of the moving system, the pointer
will not follow the rapidly changing alternating torque and will fail to show
any reading. In order that the instrument should be able to read A.C quantities,
the magnetic field in the air gap must change along with the change in current.
This principle is used in the electro-dynamometer type instrument.
Construction: The below fig shows the construction of the
electro-dynamometer type instrument.