Drysdale Polar potentiometer

A Polar potentiometer is a kind of AC potentiometer. There is another type of AC potentiometer that is a coordinate potentiometer.

An AC potentiometer compares both the magnitude and phase of an unknown voltage with a known voltage.

What is a polar potentiometer?

A polar potentiometer measures the unknown voltage in the polar form.

‘Polar form’ refers to the indication of unknown voltage in magnitude and the relative phase of the quantity.

One scale of the potentiometer indicates the magnitude while another scale indicates the phase w.r.t. some reference axis.

Drysdale polar potentiometer is an example of a polar potentiometer.

Drysdale polar potentiometer

In the Drysdale polar potentiometer, the phase-shifting transformer indicates the phase of unknown voltage, and the position of the slide wire indicates the magnitude.

A Diagram of the Drysdale polar potentiometer is shown below:

Diagram of a polar potentiometer (Drysdale polar potentiometer).
It measures unknown voltage which is applied to the slide wire through a vibration galvanometer.
The vibration galvanometer indicates the magnitude of the unknown voltage.
The rotor which is connected in series with the variable resistor and variable capacitor indicates the phase of the unknown voltage.
Drysdale polar potentiometer

Construction

Ammeter is an electrodynamometer-type ammeter because it works both on AC and DC.

D’arsonval galvanometer is used for standardization of the potentiometer, and later during the measurement of the unknown voltage vibration galvanometer is used.

The slidewire should be non-inductive. The polar potentiometer also consists of 2 stator windings and rotor winding.

Working

Drysdale potentiometer is an AC potentiometer i.e. it uses AC supply for the measurements.

First, the potentiometer should be calibrated or standardized.

The standardization of polar type ac potentiometer is done by DC supply and a standard cell. For standardization, a standard cell is connected with slide wires through the d’arsonval galvanometer.

The slide wire is fixed at the same voltage as the standard cell (1.0186V).

On the other side, an ammeter is connected with a rheostat through a dc supply. Now the rheostat should be adjusted in such a way that the galvanometer indicates zero reading.

At this point, the value of standard current at ammeter should be noted.

The phase shifter circuit remains constant for unknown supplied voltage while the phase can be varied through 360°.

The phase shifter consists of 2 stators and rotor winding. Some air gap is present in between the windings. 90° phase shift is present between both stator windings.

If necessary, the capacitor and the resistor should be adjusted to keep the 90° phase shift between both stator windings. These are being adjusted until the ammeter shows an exact similar current value which it shows during standardization for all positions of the rotor.

These adjustments of slide wire and the rotor helps in achieving a balance of the potentiometer.

After standardizing the potentiometer and tuning the phase shifter, the unknown voltage should be measured.

The unknown voltage is measured by adjusting the slide wire and the rotor. By adjusting the slide wire, the vibration galvanometer indicates the zero reading which gives the magnitude of the unknown voltage. And slight adjustment in the rotor dial gives the phase of the unknown voltage.

Quick summary

  • The polar type potentiometer is an ac potentiometer, which measures both magnitude and phase.
  • It consists of stator and rotor windings, galvanometer, slide wire, rheostat, etc.
  • A potentiometer is standardized first, then the phase-shifting transformer is tuned and at last, the unknown voltage is measured.

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