potentiometer II.pptx

Potentiometers

1. DC & AC Potentiometers

2. Vernier Potentiometer Slide wire is eliminated. Two ranges- Normal range of 1.6V down to 10 μ V Lower range of 0.16V down to 1 μ V

3. Three measure dials Measures up to 1.5V in steps of 0.1V-XI range Measures up to 0.1V in steps of 0.001V has 102 studs Measures up to 0.0001V to 0.001V in steps of 0.00001V has 102 studs.

4. The resistance of the middle dial shunts two of the coils of first dial. The moving arm of middle dial carries two arms spaced two studs. The vernier potentiometer reads to increment of 10 μ V on range of X1 and has readability of 1 μ V on X0.1 range

5. If a third range of X0.01 is provided, the readability becomes 0.1 μ V. Measurements are subjected to stray thermal and contact emfs in the potentiometer, galvanometer and the measuring circuits. These emfs can be minimised by properly selecting metals for resistors, terminals and connecting leads.

6. AC Potentiometer Its principle is same as a d.c . potentiometer. One very important difference between the two. In d.c . potentiometer, only the magnitudes of the unknown e.m.f . and slide-wire voltage drop are made equal for obtaining balance. But in an a.c . potentiometer, not only the magnitudes but phases as well have to be equal for obtaining balance. To avoid frequency and waveform errors, the a.c . supply for slide-wire must be taken from same source as the voltage or current to be measured.

7. Types POLAR Polar potentiometers in which the unknown voltage is measured in polar form i.e. in terms of magnitude and relative phase. COORDINATE Co-ordinate potentiometers which measure the rectangular co-ordinates of the voltage under test.

8. DRYSDALE POLAR TYPE AC POTENTIOMETER

9. DRYSDALE POLAR TYPE AC POTENTIOMETER It is basic d.c . potentiometers along with some auxillary components such as, drysdale phase shifter and electrodynamometer type ammeter. Drysdale phase shifter is also known as phase shifting transformer. It consists of a ring shaped laminated steel stator.

10. DRYSDALE POLAR TYPE AC POTENTIOMETER This sector is wound with either a two phase or three phase winding. Inside it there is a laminated rotor keeping some air gap between it and stator. The rotor consists of a winding provided in the slot which supplies voltage to slide wire circuit of potentiometer.

11. When the current flows through stator winding, a rotating field is produced inducing e.m.f in the rotor. The phase of rotor current can be changed through any angle relative to stator supply voltage by rotating rotor. Thus the change in the phase of secondary e.m.f . is equal to the angle through which rotor is moved from its original zero position. It is very important to arrange windings such that even though the magnitude of emf induced in rotor is changed, phase remains unchanged. The phase angle can be directly read with the help of scale fixed on top of the instrument.

12. The variable capacitor and resistor shown are so adjusted to obtain exact quadrature component between the two stator winding currents.

13. Standardization of Polar AC Potentiometer Both d.c . as well as a.c . standardization is done. The d.c . standardization is done first by replacing vibration galvanometer by D’arsonval galvanometer. A standard cell such as Weston cell is used for d.c standardization. Then by adjusting sliding contacts null deflection in galvanometer is achieved. The reading of a precision ammeter included in battery supply is noted. During a.c . standardization again vibration galvanometer is used.

14. Measurement of Unknown E.M.F. An emf to be measured connected across terminals A-A’. Sliding contacts P1 and P2 and the position of rotor in phase shifter are adjusted simultaneously till the balance is obtained as indicated by the null deflection of vibration galvanometer.

15. At balance, the magnitude of the unknown emf is obtained from P1 and P2 and the phase angle is measured from the scale reading which is mounted on the top of the instrument. Thus the unknown emf can be expressed in polar form as E∟Ө°.

16. GALL TINSLEY CO-ORDINATE TYPE AC POTENTIOMETER

17. GALL TINSLEY CO-ORDINATE TYPE AC POTENTIOMETER The in-phase and quadrature potentiometer consist of sliding contacts BB’ and CC’ respectively. Rheostats R and R’ are also provided in the respective potentiometers for the adjustment of current.

18. GALL TINSLEY CO-ORDINATE TYPE AC POTENTIOMETER By using different arrangement, the supply for the potentiometer are obtained. A vibration galvanometer VG is tuned to the supply freq and it is connected in series with a switch K and electrodynamometer type ammeter.

19. S1 and S2 are the sign changing switches which are necessary for reversing the direction of unknown emf . The unknown voltage is introduced using selector switch S3 which is having 4 pair of terminals.

20. T1 and T2 are step down transformers and they supply about 6 to 8 volts to potentiometers. By using variable resistor and capacitor, the supply for T2 is obatained . By adjusting R & C ,quadrature is obtained.

21. Standardization The d.c . standardization is done first by using standard cell. Then without disturbing this setting, ac standardisation is done by adjusting slide wire current to give zero deflection. Then previous galvanometer is replaced by vibration galvanometer and also DC supply is replaced by AC supply. The rheostat is adjusted till the current in the quadrature potentiometer wire is same as that in the inphase potentiometer magnitude wise. Also these two currents must be exactly in quadrature.

22. Measurement of Unknown EMF The emf to be measured is connected across the terminals A-A’ using selector switch S3. Sliding contacts of both the potentiometers are adjusted till the contacts of both the potentiometers are adjusted till the null deflection is obtained.

23. Measurement of Unknown EMF Under the balance condition, the in-phase component of the unknown emf is obtained from inphase potentiometer while the quadrature component of the unknown emf is obtained from quadrature potentiometer. If needed the polarity of the test voltage may be reversed by using sign changing switches S1 and S2 to balance the potentiometer.

24. The following types of instruments are used as detector in bridge and potentiometers • Vibration galvanometer • Head phones (speaker) • Tuned amplifier Vibration galvanometer A vibration galvanometer is connected to indicate the bridge balance condition. This A.C. galvanometer which works on the principle of resonance. The A.C. galvanometer shows a dot, if the bridge is unbalanced. Head phones Two speakers are connected in parallel in this system. If the bridge is unbalanced, the speaker produced more sound energy. If the bridge is balanced, the speaker do not produced any sound energy. Tuned amplifier If the bridge is unbalanced the output of tuned amplifier is high. If the bridge is balanced, output of amplifier is zero.