Question

The wattmeter method is used to measure power in a three-phase load. The wattmeter readings are 400W and -35W. Find the reactive power​

Answer 1

Answer:

The statement in the question suggests getting a negative value for the reading of the wattmeters indicate something is wrong. But that’s false. Recall active power (a.k.a. real power or average power) is the average rate of flow of energy, and this flow of energy has direction. A positive or negative value for the reading of the wattmeter tells you the direction of average flow of energy. The pointer of the wattmeter deflects towards the positive direction when the average flow of energy (a.k.a. active power) is flowing from the input terminals to the output terminals.

It can be mathematically proven that, if the two wattmeters are connected as shown in any of the following three images, and if the three-phase system is three-wire (balanced or unbalanced) or balanced four-wire, then the total (or three-phase) active power absorbed/consumed by the device (the gray box) is

PT=P1+P2

where  P1  is the active power read by the wattmeter  W1  and  P2  is the active power read by the wattmeter  W2 . Please note in the above equation you must include the sign of  P1  and  P2 .

Caption: Using phase A as common for both wattmeters.

Caption: Using phase B as common for both wattmeters.

Caption: Using phase C as common for both wattmeters.

The symbols “ ± ” indicate the positive reference terminal of the voltage coil and the current coil of each wattmeter.

If you’re using digital wattmeters, or you’re using analog wattmeters that can show negative active power, then you can proceed as already explained. However, if you’re using analog wattmeters that can’t show negative active power, and if you get any wattmeter reading a negative active power, then you’ll have to swap the two terminals of either the current coil or the voltage coil (but not both) of such wattmeter. After that, now such wattmeter will deflect with a positive reading. However, since you swaped two terminals, you’ll have to add the negative sign to that reading before using the equation  PT=P1+P2 .

(I made all the images using Falstad’s circuit simulator.)

470 viewsView upvotes

Step-by-step explanation: