Question

Advantage and disadvantages of potentiometre

Answer 1

The main advantage of the ac potentiometer lies in its versatility.  By employing shunts and volt-ratio boxes, its use may be extended to cover current, voltage and resistance measurements over a wide range of values.  Further, the fact that phase, as well as magnitude, is measured leads to such applications as measurement of power, measurement of inductance and phase angle of a coil, etc.  The principle of ac potentiometer is also incorporated in certain special measurement circuits; a good example is the Arnold circuit for measurement of error in CTS.

Despite these advantages, ac potentiometers are not very widely used. They suffer from the following disadvantages:

1. Slight difference in reading of the reflect dynamometer instrument either in dc or ac calibration introduces error in the alternating current to be set at standard value.

2. The normal value of mutual inductance M is affected due to mutual inductance between various parts of the instrument and so a slight difference is introduced in the magnitude of the current of quadrature wire compared to that in the in phase potentiometer wire.

3. Inaccuracy in the measured value of frequency will also result in the quadrature potentiometer wire current to differ from that in phase potentiometer wire.

4. The potential gradient of the wires is affected due to the presence of inter capacitance and mutual inductance.

5. Since the standardization is done on the basis of r m s value and balance is obtained depending upon the fundamental only so presence of harmonics in the wave form introduces operating troubles and vibration galvanometer tuned to the fundamental may not show full null position at all.

Good quality indicating instruments may give very nearly as high accuracy (and be much simpler to use) for measurement of current, voltage and power.  AC bridges can be arranged to give superior accuracy and freedom from frequency errors for measurement of the electrical properties of components