Electrical Engineering ⇒ Topic : Two-Wattmeter Method
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David
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Two-Wattmeter Method In fact, one of the three wattmeters used to measure 3-phase power above is superfluous. We can measure power in a 3-phase 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 Y-line, then voltage across the potential coil of wattmeter W2 will be zero and this wattmeter reads zero. figure (a) Now, W1 + W2 + W3 = Total power or W1 + O + W3 = Total power or W1 + W3 = Total power The wattmeter W2 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, 2-wattmeter method has become universal method for the measurement of power in a 3-phase 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) wave-form of the source and (v) phase sequence. | |
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David
| |
Two-Wattmeter Method In fact, one of the three wattmeters used to measure 3-phase power above is superfluous. We can measure power in a 3-phase 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 Y-line, then voltage across the potential coil of wattmeter W2 will be zero and this wattmeter reads zero. figure (a) Now, W1 + W2 + W3 = Total power or W1 + O + W3 = Total power or W1 + W3 = Total power The wattmeter W2 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, 2-wattmeter method has become universal method for the measurement of power in a 3-phase 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) wave-form of the source and (v) phase sequence. | |
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