A method for determining the sign of the smaller: Wattmeter reading in balanced three-phase power measurements

WHEN three-phase (three-wire) power is measured by the two-wattmeter method, the smaller reading may be either positive or negative, so that the total power, which is the sum of the two wattmeter readings, may be obtained by adding the smaller reading to the larger in one case, and by subtracting it in another case. On balanced load this reading is positive, and must be added, when the power factor is greater than 50 per cent; it is negative and must be subtracted when the power factor is lower than 50 per cent. However, unless the power factor is known, it is necessary to make sure by some other means which is the correct sign for this smaller reading; failure to do so has sometimes led to curious if not serious error. For balanced load, there are in use several methods of determining the sign of this reading at the time of test. These require that either the connections or the load be altered and that additional data be taken. Often enough, however, this sign is questioned only after the test is completed and the apparatus scattered, and in this case it is apparent that a determination which depends upon a simple consideration of the usual instrument readings would be useful. Such a method is the one to be described. It is applicable only to balanced three-phase loads and sinusoidal wave forms, but in itself gives an indication of the extent to which results are affected by variation from these conditions.