Electrical Engineering ⇒ Topic : Three-Phase Watthour Meter

Three-Phase Watthour Meter

In a 3-phase system, energy, like power, can be measured by means of two single-phase watthour meters. The total energy supplied will be equal to the algebraic sum of the two readings (a negative sign is used for the reading of the meter which runs backward). However,this is never done commercially as it would be more expensive and more troublesome than the use of a 3-phase meter. A 3-phase meter is merely a combination of two single-phase meters See Fig. (a), with their moving elements mounted on the same spindle.The total driving torque is equal to the sum of the torques exerted by both the moving elements. Thus only one counting mechanism is required which will directly indicate the energy being supplied to the 3-phase circuit. Fig.(a) shows how a 3-phase watthour meter is connected in a 3-phase circuit to measure energy. The current coils are connected in any two lines and each potential coil is joined to the third line. In fact, the connections are similar to 2-wattmeter method used to measure power in a 3-phase circuit

                                                     figure (a)

It is very important that the two elements are "balanced" i.e.,amounts of power flowing through each. If this is not done, the meter will not indicate correct reading on unbalanced load. The balancing adjustment is most conveniently made with the potential coils connected in parallel and the current coils in series opposition. If the elements are *balanced, there will be no rotation of the disc for this condition. The usual lag, load and power factor adjustments are made independently for each element.

Note. In some 3-phase meters, a single disc is used and the two elements drive this disc. The disc is slotted radially to prevent interaction between eddy currents produced by one element with the flux produced by other element