Electrical Engineering ⇒ Topic : Drysdale A.C. Potentiometer

Drysdale A.C. Potentiometer

This potentiometer was developed by C. V. Drysdale for a.c. measurements. In this potentiometer,the unknown a.c. voltage is measured in the polar form V∠θ i.e., in magnitude and relative phase.For this reason, it is called polar potentiometer.

Construction. Fig. (a) shows the basic construction of Drysdale a.c. potentiometer. The slidewire PQ is supplied from a phase-shifting circuit so arranged that the magnitude of a.c. voltage supplied by it remains constant but its phase can be changed from 0⁰ to 360°. The phase-shifting circuit consists of (i) two-phase stator winding and (ii) a movable single-phase rotor winding. The stator is supplied from a single-phase supply which is converted into 2-phase supply by using a phase-splitting device consisting of capacitor C and resistance r as shown. The two-phase winding produces a rotating magnetic field which induces a secondary e.m.f. in the rotor winding. The e.m.f. induced in the rotor winding is of constant magnitude but its phase can be changed by rotating the rotor to any desired position. The rotor moves over a graduated circular scale marked in degrees to indicate the phase of the rotor e.m.f. relative to the stator

                                                                              figure (a)

Working. The working of Drysdale a.c. potentiometer is as under :

(1) First, the a.c. potentiometer is standardised i.e., it is made direct reading. For this purpose, the slide-wire circuit is connected to d.c. supply and the standard current is obtained as usual by using a standard cell. This standard current makes the potentiometer direct reading and is measured by a dynamometer ammeter which is included in the battery supply circuit of the potentiometer.This ammeter remains connected for a. c. operation also because the r.m.s. value of cun-ent in theslide-wire must be maintained at the same value (i.e., standard current) as was required on direct current. Since dynamometer ammeter reads correctly on both direct and alternating current, the potentiometer remains direct reading with an a.c. supply.

(2) Once the a.c. potentiometer is standardised, the d.c. supply is removed and the slide-wire is connected to the rotor winding of the phase-shifting circuit. The r.m.s. value of alternating current in the slide-wire is made (through the adjustment of rheostat R) the same as on the d.c. supply. Now, the unknown a.c. voltage is applied to the slide-wire through the vibration galvanometer (detector). Balance is obtained by varying the position of slide-wire contact and position of the phase-shifting rotor. When the vibration galvanometer reads zero, it means that balance is achieved. Now, slidewire reading gives the magnitude r 'of the unknown a.c. voltage and the rotor position its phase angle θ. Therefore, the unknown a.c. voltage is represented as V∠θ.