In electrical engineering, a wattmeter is a device used to measure the amount of electric power in watts in a given circuit. There are three methods in measuring wattage, the one, two and three wattmeter methods. The two wattmeter method is commonly used on three-phase loads or circuits.
In three-phase currents, all three pairs of the line-to-line voltages have to be measured accurately in order to determine how much power can be consumed by the three-wire load. The two wattmeter method measures the current in two of the three lines and two voltages from the said lines with respect to the third line. From there the total power of the lines, regardless of balance of current, voltage or load variations, can be accurately derived.
When using the two wattmeter method, the power of a three-phase circuit drops below zero. This is due to the behavior of the voltage, which leads the current on one wire by 30° and delays the current by 30° in another wire. The correct sum will be obtained when two single-watt transducers are used in measuring the power they currently have instead of just one.
Any of the points in a three-phase system can be used as a reference angle when using the two wattmeter method. By means of using a reference point, establishing the measurement of total power by using only two out of three meters is made possible. Many studies show that using this method is only effective on balanced systems, however, whenever a reference point is chosen, it will measure zero, and thus the power obtained from the two remaining meters will continue to be true.
When using the two wattmeter method, it is important to take note of Kirchoff's Laws in order to avoid confusion in power calculation. In his first law, Kirchoff states that when two of three currents are already known, the third current must be equal to the sum of the first two, but carries a different direction. Whereas, the second law states that if two of the three voltages are known, the third should also be equal to the sum of the first two, also also in a different sign or direction.
The two wattmeter method is used in determining total power derived from the three phase load which is always constant, regardless of variations in phase power as well as angular displacement. Thus, the power output from a three-phase motor is also constant based on the findings using the said measurement.