3 phase 2 wattmeter method

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The system is not balanced if phase voltages and currents are different, nor is it balanced if loads have different impedances.

If you have actually performed a measurement of power in a laboratory experiment when the loads are equal and similar, I would suspect that one or both wattmeters is out of calibration.

Three phase Two Wattmeter (3P 2W) method can measure both balanced and unbalanced loads. If load is balanced both the meters read same value provided voltage is also same in all phases. If load current is different or Line voltages are not balanced or PF of each phase is different, the both meters Read different readings. Total Power is equal to Sum of both meters. You should not make mistake in current coil and Voltage coil connections. As long as neutral is not connected the meter takes care of unbalanced currents.

If voltages are balanced the current drawn by three phase motors are balanced, if the voltages are not balanced the currents are also unbalanced.

J.N. Karamchetti, Hyderabad.

The 2 watt meter method is very complexed. The vectors between the phase and the reference point, the power factor of the load has an influence on the reading by each watt meter.

In some instances (low power factor) you may even have a negative reading on the one watt meter, in this case you subtract this reading from the reading obtained in the second meter (Typical example is a motor operating at a low load with a low power factor)). Most important on this method is to ensure the correct connection between the meters and the supply, because if you use the incorrect phase as the reference you will get a negative reading on the incorrect wattmeter.

I suggest you google first this topic before sending your meters for calibration.

The load never be balanced that flow in 3-lines(3-phase) in actual scenario specially in the commercial and or apartment buildings because of the kw/wattage ratings of various connected loads for axample lamps of different types(having different power factors) that hook-up in the circuits. Probably you've got the right readings....But if in three phase circuit experiments if you hook up 3-loads of the same ratings(example heating loads), the readings of 2 wattmeters are equal no doubt about it. In actual practice i designed power distribution system of shopping malls building but never get a balanced loads , example Line1 = 5000w, Line 2=5000w, Line3=5000w. There is always a diffrence and neglegible such as Line1 = 5000w Line 2= 50100m Line 3= 5050w. Therefore readings in elect. meters may have a little diffrerence.

Try to read the book AC/DC circuits. Author: by SCHAUMS. This book contents problem samples and solutions(from simple DC/AC and advance AC circuits thru transient circuits).