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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Induction type instruments are used for
measurements
measurements
Resistance measurements
Voltage measurements
measurements
Quick Summary: 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.
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.
τd∝Φ1Φ2sinα — Deflecting torque produced by interaction of two alternating fluxes
τc=Ksθ — Control torque provided by spiral springs
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 τd∝Φ1Φ2sinα, where α is the phase difference between the fluxes.
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.
Effective damping due to eddy currents
Long scale length (up to 300 degrees)
Less affected by stray magnetic fields
Readings are frequency and temperature dependent
Higher power consumption compared to PMMC instruments
Limited to A.C. only
Energy meters (Watt-hour meters)
A.C. Relays
A.C. Switchboard instruments
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.
A is correct — Induction type instruments rely on electromagnetic induction, which requires time-varying (A.C.) magnetic fields to produce a driving torque.
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.