Page 839 - Basic Electrical Engineering

Although dynamometer-type instruments can be used for both ac and dc
measurements, they are mostly used for ac measurement.
The weight of the moving system of this type of a instrument is high as

compared to other types of instruments. Hence, the torque by weight ratio is
low, and therefore the sensitivity is low. Error due to friction is also

considerable. Power consumption is considerably high as both the coils will
draw currents. Thus, the loading effect will be more.

Dynamometer-type instruments have the advantage of measurement of
rms. values of ac quantities irrespective of their wave shapes.

Errors that usually occur in such diffecting-type measuring instruments are
as follows.

i. Frictional error: This is due to the movement of the deflecting system. If torque developed is
high and the weight of the moving system is kept low, frictional error will be minimized.
ii. Temperature error: Current flowing through the coil providing the magnetic field raises the
temperature of the copper wires used thereby contributing to error in measurement. Even
changes in ambient temperature (operating temperature or room temperature) adversely
increases error in measurement.
iii. Effect of stray magnetic field: The deflection of the moving system may get affected due to the
presence of any magnetic field around. Since the magnetic field produced by the measuring
instruments is not so strong, their operation may get affected if any strong magnetic field
exists nearby. Such effect of stray magnetic field (outside magnetic field) is eliminated by
proper shielding of the instrument. Shielding involves placing the instruments inside a
laminated steel container such that the whole of the instrument is shielded (insulated) from
outside the magnetic field.
iv. Error due to changes in frequency: In certain types of instruments, like in moving iron
instruments, the change in frequency causes error due to the change in magnitude of eddy
currents set up in the metal portion of the instruments. Changes in frequency causes changes
in reactance of the operating coil (X = 2πf). This causes change in the current flowing and
field produced as in the case of ac operation of instruments as voltmeters.

11.10 MEASUREMENT OF POWER

Power in dc circuit is the product of voltage and current. In ac circuit power

is a product of voltage, current, and power factor. Using an ammeter and a
voltmeter power cannot be measured in ac circuits as this method would not
take care of the power factor. For measuring power in ac circuit we use a

wattmeter.