Operating domain of induction wattmeter: For which type of circuit(s) is an induction (AC) wattmeter suitable?

Introduction / Context:
Induction-type measuring instruments (e.g., induction wattmeters and induction energy meters) exploit the interaction of alternating magnetic fields to produce torque proportional to electrical power. Recognizing their operating limitation is necessary when choosing instruments for AC versus DC measurements.

Given Data / Assumptions:

• Induction wattmeter uses two alternating fluxes with phase displacement to generate torque proportional to VI cos φ.
• DC signals provide constant (non-alternating) fields.
• Proper operation requires time-varying flux to induce eddy currents in the moving element (e.g., aluminum disk).

Concept / Approach:

In DC, with steady current and voltage, there is no alternating flux to sustain eddy currents and hence little or no torque in induction mechanisms. Therefore induction wattmeters are inherently AC-only devices. They are widely used on both single-phase and three-phase AC, but the principle does not work on pure DC without auxiliary choppers/inverters (which are not part of the instrument itself).

Step-by-Step Solution:

Verification / Alternative check:

Ferraris-disk energy meters and induction wattmeters populate AC metering extensively; DC metering uses dynamometer or electronic wattmeters, not induction mechanisms.

Why Other Options Are Wrong:

• DC only / both AC and DC: contradict the induction principle.
• AC 3-phase only: induction wattmeters also work on single-phase AC.
• “DC with ripple”: not a defined operating mode; any torque would be erratic and not calibrated.

Common Pitfalls:

• Assuming all wattmeters are universal; only dynamometer/electronic types handle DC correctly.

Final Answer:

ac circuit only