Errors in Dynamometer Type Instrument
The main sources of error in a dynamometer type of
instruments are due to
1. Low torque to weight ratio.
2. Frequency
3. Eddy currents
4. External magnetic fields
5. Temperature change
1. Low Torque/Weight Ratio: Since the coils, are air cored,
therefore, magnetic field produced is of small strength. Hence, large number of
turns are to be provided. This results in heavy moving system and therefore,
small torque/weight ratio. Hence frictional losses are large as compared to
other type of instruments.
2. Frequency: The frequency error is due to
(a)
Change
in ration of the operating coils and
(b)
Change
in the magnitude of eddy currents set up in the metal parts of the instrument
near to the operating portion.
3. Eddy currents: The eddy currents are induced in the
metal parts of the instrument and developed a torque because of coupling
between the moving coil and the neighbouring metal parts.
This
error can be reduced by keeping the metal in the support of the coil and other
structural parts to the minimum possible.
4. External Magnetic Field: Since the operating field in the
instrument is weak, therefore these instruments must be protected from the
external magnetic fields. Portable instruments use metal shields which serve to
isolate the coils from external influences.
Precision
type of instruments are unshielded as eddy current are induced in the metal
shields which introduce errors.
5. Temperature Changes: High currents are carried by coils
which produce heat. Self-heating of coils produces errors. High accuracy
instruments contain temperature compensating resistors which tend to neutralize
the effects of temperature changes.
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