List the different errors in wattmeter & explain compensation for it.
Errors in Wattmeter
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Compensation method
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Error due to connection method
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To overcome this error, wattmeters are provided with additional compensating winding which is connected in series with pressure coil but positioned in such a manner that it produces a field in opposition to that produced by current in current coil.
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Error due to pressure coil inductance
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A suitable value capacitor connected in parallel with pressure coil.
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Error due to Pressure Coil Capacitance
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This error can be reduced by designing pressure coil circuit such that inductive reactance of the circuit matches exactly with the capacitance reactance of the circuit i.e. XL=XC.
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Error due to mutual inductance effect
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This error can be reduced by proper design of pressure coil and current coil system so that they always remain in a zero position of mutual inductance.
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Error due to stray magnetic fields
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To avoid this error, magnetic shield is placed over CC & PC.
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Error due to eddy currents
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These are minimized by avoiding solid metal parts and using laminated core.
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Temperature error
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Using zero temperature coefficient materials for coils and components, this can be minimized.
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Error due to vibration of moving system
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It is avoided by designing the moving system such that its natural freq is greater than 2 times the freq of deflecting torque of the wattmeter.
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Error due to friction
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The weight of moving system be reduced to minimum possible.
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