AC Potentiometer

A potentiometer is an instrument that measures unknown voltage by balancing it with a known voltage. This source can be either DC or AC. While both DC and AC potentiometers work similarly, the key difference is that a DC potentiometer measures only the magnitude of the unknown voltage, whereas an AC potentiometer measures both the magnitude and phase. There are two types of AC potentiometers:

1. Polar type potentiometer.
2. 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.

A phase-shifting transformer consists of two ring-shaped laminated steel stators placed perpendicularly. One stator connects directly to the power supply, while the other connects in series with a variable resistor and capacitor. These components help maintain a constant AC supply in the potentiometer through small adjustments.

Between the stators, there is a laminated rotor with slots and windings that supply voltage to the potentiometer’s slide-wire circuit. When current flows through the stators, it creates a rotating field around the rotor, inducing an e.m.f. in the rotor windings.

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 stator winding 1 can be expressed as

The induced emf in the rotor winding by the stator winding 2,

From equation (1) and (2), we get

Therefore, resultant induced emf in the rotor winding due to two stator winding

Where, Ø gives the phase angle. You can go study similar questions to the one above in our electrical engineering MCQs.

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 are signs changing switches which are used to change the polarity of the test voltage if it is required for balancing the potentiometer. There are two step-down transformers T₁ and T₂ which isolate potentiometer from the line and give an earthed screens protection between the winding. It also supplies 6 volts to potentiometers.

To measure the unknown e.m.f., its terminals are connected to sliding contacts AA’ using selector switch S₃. Adjusting the sliding contacts and rheostat balances the circuit, and the galvanometer reads zero. The in-phase component V_A of the unknown e.m.f. is read from the in-phase potentiometer, while the quadrature component V_B is read from the quadrature potentiometer.

Thus, the resultant voltage of the coordinate AC potentiometer is

And the phase angle is given by

Applications of AC Potentiometer

1. Measurement of self-inductance.
2. Calibration of voltmeter.
3. Calibration of Ammeter.
4. Calibration of watt meter.