Induction type instruments are used for

Question

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A.C. measurements

Explanation: Induction type instruments operate on the principle of electromagnetic induction and are exclusively designed for A.C. measurements. They function by creating a rotating magnetic field which induces eddy currents in a metallic disc, producing a deflecting torque.

Working Principle: The instrument uses two electromagnets: one carrying the current (or voltage) and another acting as a control. The alternating flux from these magnets induces eddy currents in a light aluminum disc. The interaction between the eddy currents and the alternating magnetic fluxes results in a driving torque $	au_d \propto \Phi_1 \Phi_2 \sin \alpha$, where $\alpha$ is the phase difference between the fluxes.

Key Formulas: $	au_d \propto \Phi_1 \Phi_2 \sin \alpha$ — Deflecting torque produced by interaction of two alternating fluxes; $	au_c = K_s 	heta$ — Control torque provided by spiral springs

Key Points: Induction instruments are generally used as A.C. ammeters, voltmeters, or wattmeters.; They are restricted to A.C. because D.C. would produce zero eddy current torque due to lack of flux change.; The disc must be made of non-magnetic conducting material like aluminum to allow eddy current flow.; Gravity control is often used in these instruments as they are usually mounted in a vertical position.

Advantages: Effective damping due to eddy currents; Long scale length (up to 300 degrees); Less affected by stray magnetic fields

Disadvantages: Readings are frequency and temperature dependent; Higher power consumption compared to PMMC instruments; Limited to A.C. only

Applications: Energy meters (Watt-hour meters); A.C. Relays; A.C. Switchboard instruments

Additional Info: Frequency dependence is a major drawback as a change in frequency alters the flux and phase angle, causing errors in calibration.. Option B (D.C. measurements) is incorrect because D.C. does not produce the required time-varying magnetic field to induce eddy currents.. Option C (Resistance measurements) is typically handled by bridges or ohmmeter circuits, not induction-type instruments.. Option D (Voltage measurements) is a subset of A.C. applications, but the question asks for the primary category, which is A.C. instruments.

Answer: A is correct — Induction type instruments rely on electromagnetic induction, which requires time-varying (A.C.) magnetic fields to produce a driving torque.

Exam Tip: Always remember that induction-type devices (like induction motors or induction meters) require a time-varying signal to induce current via Faraday's Law, making them inherently unsuitable for steady-state D.C. systems.

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