Connection of Gall – Tinsley a.c. Potentiometer
- This potentiometer consists of two separates potentiometer circuits enclosed in a common
case. One is called the “in-phase” potentiometer and the other the “quadrature” potentiometer.
- The slide-wire circuits are supplied with currents which have a phase difference of 90˚.
- On the first of these potentiometers, that component of the ‘unknown’ voltage which is in phase with the current in the slide-wire circuit of the potentiometer is measured. On the other
potentiometerthe component of the “unknown” voltage in phase quadrature with the current in its slide-wire circuit is measured.
- If these measured values are V1and V2 respectively, then the unknown voltage is given by V = (V12+V22 ) and its phase difference from the current in the ‘in phase’ potentiometer slide-wire circuit is given by the angle θ where tan θ = V1/ V2.
- Fig-1 shows the connections of the potentiometer. The in-phase and quadrature potentiometer circuit are shown, with their sliding contacts bb’ and cc’ and rheostats R and R’ for current adjustment.
- The supplies to the potentiometer are obtained from a single phase supply by means of the arrangement shown in Fig. 2.
- T1 and T2 are two step-down transformers supply about 6 V to the potentiometer circuits. They also serve to isolate the potentiometer from the line and are usually provided with earthed screens between the windings. The supply to
T 2 ,is obtained through a variable resistor R and variable capacitor C for the purpose of phase splitting. Quadrature phase displacement is obtained by adjusting C and R.
- Referring again to Fig.-1, V.G. is a vibration galvanometer and K is its key, A is a reflecting dynamometer instrument for maintaining the current in the two slide -wires at the standard value (50mA) S1 and S2 are two “sign-changing” switches which may be necessary to reverse the direction of the unknown emf applied to the slide wires S3 is a selector switch by which the unknown voltages to be measured are placed in the circuit. There are four pairs of terminals for the application of such voltages, the connections to only one
pair-towhich an unknown voltage V is applied – beingshown in figure. The selector switch, when in the position shown in the figure-called the ‘test position’ -allows the current in the quadrature potentiometer slide-wire to be compared with that in the in-phase potentiometer wire, utilizing the mutual inductance M for the purpose.
- The current in the in-phase potentiometer wire is first adjusted to its standard value by means of a direct current supply and a standard cell, the vibration galvanometer being replaced by a galvanometer being replaced by a galvanometer of the d’ Arsonval type for this purpose. The dynamometer ammeter is of the torsion head type and the torsion head is turned to give zero deflection on direct current. This setting is left untouched during the calibration with alternating current, the slide-wire current being adjusted to give zero deflection again. The vibration galvanometer is then placed in
circuitand the direct current supply is replaced by the alternating supplies.
- The slide-wire of the in-phase potentiometer is thus set to this calculated value of induced emf in the secondary of M ( the slide-wire current being maintained at its standard value) and rheostat R and capacitor C are adjusted until exact balance is obtained. For balance the current in the quadrature potentiometer slide- wire must be both equal to the standard value and also must be exactly 90˚ out of phase with the current in the in-phase slide – wire. This latter condition follows from the fact that the emf in the secondary of M lags 90° in-phase behind the primary current and therefore, for this emf to be in-phase with the voltage dorp across a portion of the in-phase slide wire, the current in the primary of M must be in exact quadrature with the current in this in-phase slide-wire. Any difference in polarity between the two circuits is corrected for by the sign-changing switches S1 and S2.
- These adjustments having been made, the unknown voltage is switched in circuit by means of the selector switch S3. In this portion of S3, the two slide-wire circuits are in series with one vibration galvanometer, Balance is obtained by adjusting both pairs of sliding contacts (bb’ and cc’) together with the reversal of switches S1 and S2, if necessary. At balance. the reading of the side- wire of the in – phase potentiometer, together with the position of S1, gives the magnitude and sign of the in-phase component of the unknown voltage, while the reading of the quadrature potentiometer, with the position of S, gives the magnitude and sign of the quadrature component.