Q7Electronic Measurement & Instrumentation

Unlike a DC potentiometer which only measures magnitude, an AC potentiometer must determine both the magnitude and phase of an unknown AC voltage.

Principle: The coordinate type achieves this by breaking the unknown voltage vector into two Cartesian components: an In-phase component ($V_1$) and a Quadrature (90 degrees out-of-phase) component ($V_2$). $V_{unknown}=V_1+j{V}_2$.

Working: It consists of two separate, identical potentiometer slide wires. 1. The first wire is supplied with an AC current $I$. 2. The second wire is supplied with a current $I$ that is passed through a phase-shifting transformer, making it exactly $90^{\circ }$ out of phase with the first wire. 3. The unknown voltage is connected in series opposing the combined voltage drops of both slide wires through a vibration galvanometer. 4. Both slide contacts are adjusted simultaneously until a true null balance is achieved. 5. The readings from the two slide wires give $V_1$ and $V_2$, from which magnitude $\sqrt{V_1^2+V_2^2}$ and phase ${\tan }^{-1}(V_2/V_1)$ are calculated.