Electrical Engineering ⇒ Topic : Phase Sequence Indicator
Phase Sequence Indicator
The fact that the currents in the three phases of an unbalanced 3-wire Y-connected load change with the reversal of phase sequence of the 3-phase supply can be employed to determine the phase sequence of the supply i.e. whether the phase sequence is RYB or RBY . Fig. (a) shows the circuit of a simple phase sequence tester. Each lamp in the R-phase and Y-phase has a filament resistance of 50 Q while the B-phase has known inductive load. Suppose the line voltage is 200 V, 60 Hz.
When phase sequence is RYB. When the phase sequence is RYB as shown in Fig. (a), it can be shown mathematically that the currents in the two lamps are
Current in lamp A, IR = 1.55A
Current in lamp B, IY = 2.46A
Therefore, lamp B will be brighter than lamp A.
When phase sequence is RBY. When the phase sequence is RBY (i.e. phase sequence is reversed), it can be shown mathematically that currents in the two lamps are
Current in lamp A, IR = 2.46A
Current in lamp B,IB = 1.55A
Therefore, lamp A is brighter than lamp B.
Thus, when lamp B is brighter than lamp A, the phase sequence is RYB. However, when lamp A is brighter than lamp B, the phase sequence is RBY.
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