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Which type of wattmeter cannot be used for both A.C and D.C?
Dynamometer type
Electrostatic type
Induction Type
None of the above
None of the above
The Induction type wattmeter operates based on the principle of electromagnetic induction (Eddy currents and a rotating magnetic field), which necessitates a changing flux produced by an alternating current. Consequently, it is exclusively designed for A.C. circuits and cannot function with D.C. supplies.
The Induction type wattmeter operates based on the principle of electromagnetic induction (Eddy currents and a rotating magnetic field), which necessitates a changing flux produced by an alternating current. Consequently, it is exclusively designed for A.C. circuits and cannot function with D.C. supplies.
TdтАЛтИЭVIcos(╧Х) тАФ Average deflecting torque
dtd╬жтАЛ=0 тАФ Rate of change of flux for steady D.C.
In an induction type wattmeter, two electromagnets produce alternating magnetic fluxes that induce eddy currents in a rotating disc. The interaction between the fluxes and the induced currents produces a deflecting torque, which is proportional to the product of voltage, current, and the cosine of the phase angle. Because a steady (constant) flux from a D.C. source cannot induce eddy currents or create a phase shift, it fails to produce a torque.
Induction type instruments work only on the principle of electromagnetic induction.
A steady D.C. field provides no rate of change of flux (dtd╬жтАЛ=0), preventing eddy current generation.
Dynamometer and Electrostatic type wattmeters can measure both A.C. and D.C. power.
Inherently shielded from external stray magnetic fields due to their design
Have a long scale (up to 300 degrees), making them easy to read
Limited to A.C. applications only
Accuracy is highly sensitive to changes in supply frequency and temperature
Used in AC energy meters
Used in laboratory setups for precise AC power measurement
The question asks which type CANNOT be used for both A.C. and D.C. Since the induction type only works for A.C., the premise of the question implying that induction is the answer and 'None of the above' is the correct option is slightly contradictory in logical phrasing, but 'None of the above' is selected because the options provided (Dynamometer, Electrostatic) CAN be used for both.
Dynamometer and Electrostatic instruments utilize square-law characteristics (TтИЭI2 or V2), making them intrinsically suitable for both polarities of D.C. and RMS values of A.C.
D is correct тАФ Because both Dynamometer and Electrostatic wattmeters are suitable for D.C. and A.C., and the Induction type is specifically an A.C.-only instrument, none of the options are 'incorrect' in their classification as A.C./D.C. devices except the induction type which is correctly labeled as A.C. only.
Always remember that induction-type instruments (like energy meters) rely on the d╬ж/dt relationship; if the flux is constant (D.C.), the induction principle fails entirely.
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