Explain the error occurred due to pressure coil inductance of electrodynamometer type wattmeter. How this error is compensated?

Explain the error occurred due to pressure coil inductance of electrodynamometer type wattmeter. How this error is compensated?
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1 Answer

Answer :

For power measurement assume that the pressure coil of wattmeter is purely restitive. Hence current taken by pressure coil Ipc is in phase with voltage across the pressure coil Vpc 

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Hence the wattmeter reads more power than the actual power consumed by the load. The error occurred due to pressure coil inductance of electrodynamometer type wattmeter can be compensated as follows: To reduce this error, capacitance is connected in parellel with pressure coil of the wattmeter. By making the value of capacitive reactance Xc equal to inductive reactance XL the pressure coil circuit is made purely resistive and the error is eliminated.

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