AC Potentiometer - Definition, Types, Applications & Comparison

What is an AC Potentiometer?

     An ac potentiometer measures the unknown ac voltage or emf similar to dc potentiometers used for measuring dc voltage. The balance condition is obtained when the unknown voltage is equal to the drop across the sliding wire of the potentiometer. It measures the unknown voltage both in terms of magnitude and phase angle by comparing and balancing it with the known reference voltage.

     Although the working principle of the ac potentiometer is similar to that of the dc potentiometer (i.e., comparing unknown voltage with a known voltage). But dc potentiometer cannot be used for measuring the ac voltage.

     In dc potentiometers, the unknown dc emf is measured by taking a dc voltage reference and the only condition for the balance is the magnitude of unknown emf should be same as that of the drop across the part of the slide wire. Whereas, in case of ac measurement the conditions that must be satisfied are,
  • Both the voltages should have same frequency.
  • Their phases should be same.
  • Their waveforms must be same.
  • Also, their magnitudes must be same at all instants.

     By taking a separate source as reference (as in the case of dc potentiometers), the balance can never be achieved as we cannot estimate the above mentioned quantities of unknown emf. Hence, dc potentiometers cannot be used for ac measurements directly.

The following modifications that are needed in a dc potentiometer to be used for ac applications are,

  • Since, the magnitude, phase, frequency, and waveform must be same, instead of some external source, the unknown emf itself is taken as reference.
  • The basic D'Arsonval galvanometer is replaced with a vibration galvanometer. As it is a tunable device, it can even compare the harmonics present in the waveforms.
  • As the stray magnetic fields are likely to affect the measurement due to weak fields, they are to be eliminated or compensated.
  • All the resistors and the slide wire are to be replaced by standard non-inductive resistors and slide wire so that, their impedance does not vary with frequency and waveform.
  • The voltage across the potentiometer (i.e., the reference voltage) should be known with high accuracy. In the case of dc, either a standard cell or zener reference is considered which is not the case with ac.

Types of AC Potentiometers :

The ac potentiometers are mainly classified into two types. They are polar type and coordinate type ac potentiometer.

Polar Type Potentiometer :

     The Drysdale Tinsley AC Potentiometer is a polar type potentiometer, which measures the magnitude (V) in one scale and relative phase angle on another scale i.e., in the polar form.

Polar type AC Potentiometer

Coordinate Type Potentiometer :

     It is the combination of two potentiometers. In Phase Potentiometer which carries the current in-phase with the supply voltage and Quadrature Potentiometer that carries current in quadrature with the supply voltage. This type of potentiometer measures the unknown emf in rectangular coordinates.

Co-ordinate Type Potentiometer

Difference Between AC and DC Potentiometer :

AC Potentiometer

DC Potentiometer

In the case of an ac potentiometer, both the magnitude and phase of the two voltages should be same to obtain the balance.

In the case of a dc potentiometer, only the magnitude of the two voltages must be same to obtain the balance.

There is no reference source present in the ac potentiometer.

The reference source is present in the dc potentiometer.

AC potentiometers are less accurate.

DC potentiometers are more accurate.

AC potentiometers are more complex.

DC potentiometers are less complex.

A vibrational galvanometer is used as a null detector.

A galvanometer is used as a null detector.

Obtaining a balance point is very difficult.

The balance point can be obtained easily.

Applications of AC Potentiometer :

The various applications of ac potentiometers are,
  • Measurement of voltage.
  • Measurement of self reactance or inductance of a coil.
  • Testing of magnetic materials (i.e., ac magnetic testing) at low flux densities.
  • Calibration of voltmeters.
  • Calibration of ammeters.
  • Calibration of wattmeters.
  • Calibration of energy meters etc.

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