Polar type AC potentiometer

Construction and Working Principle:

 This instrument consists of an ordinary dc potentiometer in which the resistance coils are non- inductive and the auxiliary apparatus like Drysdale phase shifting transformer and transfer instrument.

  • Drysdale Phase Shifting Transformer –

The phase shifting transformer consists of a laminated silicon steel stator which houses a two or three phase winding. The rotor is also a laminated structure having slots in which a winding is provided. There is a small air gap between the stator and rotor.

When current flows in the stator winding, a rating field is produced. inducing an emf in the rotor winding. The rotor can be adjusted through any angle. The phase displacement of rotor emf is equal to the angle through which the rotor has been moved from its zero position.  A scale and a pointer are provided on the top of the instrument to indicate the angle through which rotor has been moved from its zero position. The scale is graduated both in degrees and cosines of the angle.

A phase shifter operated from a single-phase supply can be used by means of a phase shifting device. The device is shown in the figure above.

 In this phase shifter, two separate windings are provided which are displaced by 90° in space on stator. One winding is fed directly from the supply while the other winding is connected in series with a variable resistance R and a variable capacitor C ·The values of resistance R and capacitor C are adjusted until the current through the two winding are equal with 90 phase displacement creating uniform revolving filed.

 For particular position of the rotor the induced emf due to each of the stator winding is proportional to the cosine of the angular displacement from position of maximum emf. The emf induced in rotor winding due to stator winding 1 is given by,

e1 Kl sin at cos θ and that due to statorwinding 2 is

Operation with А.С. :

e2= KI sin (ωt +90˚)Cos(θ+90˚)

    = -KI cos ωt sin θ

The resultant emf is the rotor:                                                                                   

e = KI [ sin ωtcos θ-cos ωt sin θ]

    =KI sin (ot-e)

The above relationship shows that the rotor emf has a constant amplitude but having phase angle given by rotor position θ.

When a three-phase supply is available, a three-phase shifting transformer is used which is less sensitive to small changes in frequency than the phase splitting circuit using single phase supply

  • Transfer Instrument –

 In order to standardize the a.c. potentiometer, the slide wire circuit is switched on to a d.c. supply. The standard current is obtained using a standard cell. The standard current required to make the potentiometer direct reading is measured by a precision ammeter which is included in the battery supply circuit of potentiometer. A change over switch is used to enable the potentiometer to be used either for d.c. or a.с.

 A simplified diagram of the potentiometer for use with a.c. shown in the figure above. The Kelvin- Varley slide principle is employed in the slide wire circuit. V.G. is a vibration galvanometer used as a detector for measurements at commercial frequencies. This must be carefully tuned to give resonance at frequency of circuit under test. r is the shunting resistor for the reduction of the range of the potentiometer. When this shunt is put in circuit by the switch S, the resistor R is simultaneously connected in series with the slide wire circuit in order to maintain the resistance of working portion of potentiometer constant. R is a rheostat for adjustment of slide wire current. ‘A’ is the precision ammeter.

The potentiometer is first standardized by adjusting rheostat and the standard current is noted. The switch S is then connected to battery side for standardization and the vibration galvanometer is replaced by d’Arsonval galvanometer. The switch is then connected to a.c. supply side, the standard cell and d’ Arsonval galvanometer being previously replaced by the a.c. voltage to be measured and the vibration galvanometer respectively. The stator windings of the phase-shifter are then adjusted to exact quadrature by means of variable resistor and capacitor. These instruments are then adjusted until the a.c. current in the slide wire is constant for all positions of the rotor.

Balance of the potentiometer is obtained by successive adjustment of sliding contacts of the slide wire circuit and of the rotor of the phase shifter. The readings of potentiometer dials and the slide wire, at balance give the magnitude of voltage being measured, while the readings on the scale of phase shifter gives the phase of voltage being measured relative to supply voltage.

Also Read- Measurement of earth resistance by fall of potential method

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