Problem 85-1    Source:   Adapted of the Example 11.6 - page 270 - NILSSON, James W. & RIEDEL, Susan A. - Book: Circuitos Elétricos- 5ª Edição - LTC Editora - 1999.

In the circuit of the Figure 85-01.1 we have V_fase = 120 V   and   Z_A = Z_B = Z_C = 8 + j6. Calculate:

a) The reading in the two wattmeters;

b) Check that the total power measured by wattmeters is equal to the power supplied to the load.

Solution of the Problem 85-1

Item a

Using the impedance value in polar form: Z = 8 + j6 = 10∠36.87°. Then the current value will be:

To find the values read by wattmeters, we use equations 85-3 e 85-4, Remembering that V_L = √3 V_fase = 120√3. Then:

O valor de W₂ será:

Item b

We know that wattages are read by wattmeters, and only resistors dissipate that power. Then, we can calculate the power dissipated in each resistor per phase and multiply this value by three, to find the total power.

Now, adding the powers measured by the wattmeters:

Therefore, it is verified that the power dissipated by the resistors is equal to the power measured by the wattmeters.