Electrical Engineering ⇒ Topic : Wattmeter Errors

Gopal
 
It was assumed while deriving the expression for the deflecting torque that the pressure coil circuit is purely noninductive. But in actual practice, some inductance is present. This introduces an error in the wattmeter reading. If the pressure coil circuit contains some inductance, then the current I_{p} will be lagging behind the supply voltage by an angle β , as shown in Figure(a). If cos Φ is the power factor of the load, then the torque on the instrument is
FIGURE Error due to the inductance of the pressure coil and method for its compensation (1) (2) When the power factor is lagging, the angle ( Φ  β ) will be less and hence cos ( Φ  β) will be more than cos Φ and the meter will indicate a higher reading than the actual one. On the other hand, for leading power factors, cos (Φ +β ) will be less than cos Φ, and hence the meter will indicate a reading less than the actual value If z_{p} is the impedance of the pressure coil circuit, then
where, r_{p} is the resistance of the pressure coil, R is the resistance connected in series with the pressure coil and β is the impedance angle of the pressure coil. (3) Therefore, the observed reading of the instrument
However, the instrument will give the true reading when β = 0. (4) Therefore, (5) From the above equation true reading = (6) Therefore, the correction factor = (7) The error = observed reading true reading = observed reading = observed reading Error = observed reading = observed reading = observed reading (8) Compensation for the Inductance of the Pressure Coils The error due to the presence of the inductance of the pressure coil can be eliminated by connecting a high resistance R, which is in parallel with a capacitor C, in series with the pressure coil as shown in Fig. (b).If the circuit is to be noninductive then its impedance must be independent of the frequency. This can be achieved by using the procedure described earlier in (also see Fig. ) in which a capacitor is connected in shunt with the swamping resistance (r) such that the value of the capacitance is given by C = L / r.^{2}  
 
David
 
Wattmeter Errors (1). Error due to potential circuit connections. (2). Error due to potentialcoil inductance.
(3) Error due to capacitance in potential coil circuit. (4) Error due to eddy currents. (5) Error due to stray magnetic fields  
 
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