AC Potentiometer

A measurement potentiometer compares an unknown emf with an adjustable reference voltage and detects the point of zero difference. At null, the detector does not draw current from the unknown source. A DC balance requires equal magnitude and correct polarity. An AC balance also requires equal frequency, waveform and phase. Traditional instruments express the AC result in one of two forms, giving two types of AC potentiometers:

1. Polar type potentiometer.
2. Coordinate type potentiometer.

Polar type Potentiometer

A polar instrument reads the unknown emf as magnitude and phase angle relative to a selected reference. Its angular scale covers 360 degrees. A precision electrodynamometer-type ammeter transfers the calibrated slide-wire current from DC standardisation to AC operation. A phase-shifting transformer then sets the reference phase from a single-phase supply.

The phase shifter has two stator windings with magnetic axes at right angles and a movable wound rotor. One stator branch is supplied directly. The other includes adjustable resistance and capacitance to produce a current approximately equal in magnitude and 90 degrees apart in phase. Together, the stator currents create a rotating magnetic field.

The laminated rotor carries the winding that supplies the potentiometer slide-wire circuit. When current flows in the stator windings, the rotating field induces a rotor emf. Turning the rotor changes the phase of that emf relative to the supply.

In the ideal two-phase field, rotor angle gives the phase displacement of the induced emf while its magnitude remains constant. A shaft scale shows the relative phase. Real instruments need careful adjustment because unequal stator currents, imperfect quadrature, frequency change and winding asymmetry introduce magnitude and phase errors.

The contribution to rotor emf from stator winding 1 is:

The contribution from stator winding 2 is:

Combining equations (1) and (2) gives:

The resultant rotor emf due to both stator windings is therefore:

Here, Ø represents the rotor angle and the corresponding phase displacement in the ideal model. Related instrument questions are available in the electrical engineering MCQs.

Coordinate type Potentiometer

A coordinate AC potentiometer resolves the unknown emf into two rectangular components. One slide wire supplies a voltage in phase with the reference, while the other supplies a voltage in quadrature. Contacts AA’ and BB’ set the two component magnitudes. The rheostat controls establish the specified slide-wire currents, and the null detector shows when the vector sum balances the unknown. Switches S₁ and S₂ reverse the signs of the two components so all four quadrants can be represented. Screened step-down transformers T₁ and T₂ provide isolated low-voltage supplies and reduce capacitive coupling from the mains.

Selector switch S₃ connects the unknown emf to the balance circuit. The operator adjusts both slide contacts until the detector reaches null. The in-phase component V_A is read from one potentiometer and the quadrature component V_B from the other, including the signs selected by the reversing switches.

The resultant magnitude indicated by the coordinate AC potentiometer is:

The relative phase angle is:

Applications of AC Potentiometer

1. Measuring inductive or capacitive impedance by resolving voltage into magnitude and phase components.
2. Calibrating AC voltmeters against a null-comparison reference.
3. Calibrating AC ammeters from the voltage across a known standard impedance.
4. Calibrating wattmeters and phase-sensitive instruments under controlled voltage, current, frequency and phase conditions.