Engineering Q&A
List the different errors in wattmeter & explain compensation for it.
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Errors in Wattmeter
Compensation method
image Error due to connection method
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.
Error due to pressure coil inductance
A suitable value capacitor connected in parallel with pressure coil.
Error due to Pressure Coil Capacitance 
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.
Error due to mutual inductance effect
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.
Error due to stray magnetic fields
To avoid this error, magnetic shield is placed over CC & PC.
Error due to eddy currents
These are minimized by avoiding solid metal parts and using laminated core. 
Temperature error
Using zero temperature coefficient materials for coils and components, this can be minimized.
Error due to vibration of moving system
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.
Error due to friction
The weight of moving system be reduced to minimum possible.


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