Electrical Engineering ⇒ Topic : TwoWattmeter Method

David
 
TwoWattmeter Method In fact, one of the three wattmeters used to measure 3phase power above is superfluous. We can measure power in a 3phase load (Y or A, balanced or unbalanced) by using two wattmeters only.This can be easily proved. The point x, the common connection of the potential coils See Fig. (a) can be located anywhere without affecting the algebraic sum of the three wattmeters. It is because the point x can be at any potential. Thus, if one end of each potential coil in Fig.(a) is connected to Yline, then voltage across the potential coil of wattmeter W_{2} will be zero and this wattmeter reads zero.
figure (a) Now, W_{1} + W_{2} + W_{3} = Total power or W_{1} + O + W_{3} = Total power or W_{1} + W_{3} = Total power The wattmeter W_{2 }may, therefore, be removed and the algebraic sum of the remaining two wattmeters readings is still equal to total power. As a matter of fact, 2wattmeter method has become universal method for the measurement of power in a 3phase circuit. The principal advantage is that the algebraic sum of the readings of the two wattmeters indicates the total power regardless of (i) load unbalance (ii) source unbalance (iii) difference in wattmeters (iv) waveform of the source and (v) phase sequence.  
 
David
 
TwoWattmeter Method In fact, one of the three wattmeters used to measure 3phase power above is superfluous. We can measure power in a 3phase load (Y or A, balanced or unbalanced) by using two wattmeters only.This can be easily proved. The point x, the common connection of the potential coils See Fig. (a) can be located anywhere without affecting the algebraic sum of the three wattmeters. It is because the point x can be at any potential. Thus, if one end of each potential coil in Fig.(a) is connected to Yline, then voltage across the potential coil of wattmeter W_{2} will be zero and this wattmeter reads zero.
figure (a) Now, W_{1} + W_{2} + W_{3} = Total power or W_{1} + O + W_{3} = Total power or W_{1} + W_{3} = Total power The wattmeter W_{2 }may, therefore, be removed and the algebraic sum of the remaining two wattmeters readings is still equal to total power. As a matter of fact, 2wattmeter method has become universal method for the measurement of power in a 3phase circuit. The principal advantage is that the algebraic sum of the readings of the two wattmeters indicates the total power regardless of (i) load unbalance (ii) source unbalance (iii) difference in wattmeters (iv) waveform of the source and (v) phase sequence.  
 
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