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Which of the following types of instrument canтАЩt be used fo D.C
Moving iron-attraction type
Moving coil permanent magnets type
Hotwire type
Induction type
Induction type
Quick Summary: Induction type instruments operate on the principle of electromagnetic induction, which requires an alternating magnetic field to produce eddy currents in a rotating disc. Since a D.C. source produces a steady, unidirectional magnetic field, it cannot induce the necessary currents to create a torque, making these instruments suitable only for A.C. measurements.
Induction type instruments operate on the principle of electromagnetic induction, which requires an alternating magnetic field to produce eddy currents in a rotating disc. Since a D.C. source produces a steady, unidirectional magnetic field, it cannot induce the necessary currents to create a torque, making these instruments suitable only for A.C. measurements.
TdтАЛтИЭ╬ж1тАЛ╬ж2тАЛsin╬▒ тАФ Torque produced by induction type instruments
IeddyтАЛтИЭdtd╬жтАЛ тАФ Eddy current induction principle
The instrument consists of two electromagnets energized by the source. These magnets produce alternating fluxes ╬ж1тАЛ and ╬ж2тАЛ that differ in phase. These fluxes induce eddy currents in a metallic disc, which interact with the fluxes to produce a torque TdтАЛтИЭ╬ж1тАЛ╬ж2тАЛsin╬▒. In D.C., the rate of change of flux is zero, meaning no eddy currents are induced and TdтАЛ=0.
Induction instruments rely on electromagnetic induction (Faraday's Law).
They require a changing magnetic field to generate an eddy current.
D.C. results in a constant flux, yielding zero torque.
These instruments are widely used as watt-hour meters in A.C. systems.
Long scale of deflection (up to 300┬░).
High operating torque-to-weight ratio.
Operates only on A.C.
Errors occur due to frequency and temperature variations.
A.C. Energy meters
A.C. Wattmeters
Moving Iron instruments work on both A.C. and D.C. because torque depends on I2.
Moving Coil (PMMC) instruments are specifically for D.C. because they use a permanent magnet and polarity sensitive torque.
D is correct тАФ Induction type instruments require a changing magnetic field (A.C.) to produce torque via eddy currents, which is absent in D.C. systems.
Always remember that induction-based components (like transformers and induction motors) fail on D.C. due to the lack of flux change, leading to zero reactance and excessive current.