A potentiometer is an instrument which measures unknown voltage by balancing it with a known voltage. The known source may be DC or AC. The working phenomenon of DC potentiometer and AC potentiometer is same. But there is one major difference between their measurements, DC potentiometer only measures the magnitude of the unknown voltage. Whereas an AC potentiometer measures both the magnitude and phase of unknown voltage by comparing it with a known reference. There are two types of AC potentiometers:
- Polar type potentiometer.
- Coordinate type potentiometer.
Polar type Potentiometer
In such type of instruments, two separate scales are used to measure magnitude and phase angle on some reference of the unknown e.m.f. There is a provision on the scale that it could read phase angle up to 3600. It has electrodynamometer type ammeter along with DC potentiometer and phase-shifting transformer which is operated by single phase supply.
In a phase-shifting transformer, there is a combination of two ring-shaped laminated steel stators connected perpendicularly to each other as shown in the figure. One is directly connected to power supply and the other one is connected in series with variable resistance and capacitor. The function of the series components is to maintain constant AC supply in the potentiometer by doing small adjustments in it.
Between the stators, there is laminated rotor having slots and winding which supplies voltage to the slide-wire circuit of the potentiometer. When current start flowing from stators, the rotating field is developed around the rotor which induces an e.m.f. in the rotor winding.
The phase displacement of the rotor emf is equal to rotor movement angle from its original position and it is related to the stator supply voltage. The whole arrangement of the winding is done in such a way that the magnitude of the induced emf in the rotor may change but it does not affect the phase angle and it can be read on the scale fixed on the top of the instrument.
The induced emf in rotor winding by
The induced emf in the rotor winding by the stator winding 2,
From equation (1) and (2), we
Therefore, resultant induced emf in the rotor winding due to two stator winding
Where, Ø gives the phase angle.
Coordinate type Potentiometer
In coordinate AC potentiometer, two separate potentiometers are caged in one circuit as shown in the figure. The first one is named as the in-phase potentiometer which is used to measure the in-phase factor of an unknown e.m.f. and the other one is named as quadrature potentiometer which measures quadrature part of the unknown e.m.f. the sliding contact AA’ in the in-phase potentiometer and BB’ in quadrature potentiometer are used for obtaining the desired current in the circuit. By adjusting rheostat R and R’ and sliding contacts, the current in the quadrature potentiometer becomes equal to the current in the in-phase potentiometer and a variable galvanometer shows the null value. S1 and S2
Now to measure unknown e.m.f. its terminals are connected across sliding contacts AA’ using selector switch S3. By doing some adjustments in sliding contacts and rheostat, the whole circuit gets balanced and galvanometer reads zero at the balanced condition. Now the in-phase component VA of the unknown e.m.f. is obtained from the in-phase potentiometer and quadrature component VB is obtained from quadrature potentiometer.
Thus, the resultant voltage of the coordinate AC potentiometer is
And the phase angle is given by
Applications of AC Potentiometer
- Measurement of self-inductance.
- Calibration of voltmeter.
- Calibration of Ammeter.
- Calibration of watt meter.