Phase reversal detector

  
Assume the correct phase sequence to be VA-VB-VC. The circuit terminals are connected such that Tl gets connected to phase A and T2 to phase B. The capacitor advances the voltage developed across R2 due to phase `B` by — 60°, while the voltages developed across it by phase `A` is in phase with VA as shown in Fig. 69-1. The net voltage developed across R2 ~ zero, the neon lamp is not energized, thereby signaling correct phase sequence. If terminal T2 gets connected to phase C, a large voltage, K(VA + Vc 60°), gets developed across R2, energizing the neon indicator to signal reverse phase sequence. The motor terminals can be connected to the three phases in six different combinations.
Phase reversal detector  - schematic

A three-phase motor will run in the forward direction for three such combinations, while for the other three it will operate in the reverse direction. As shown in the table, the circuit detects all three reverse combinations. This circuit can be wired into any existing motor starter where the operator can see whether the phase sequence has been altered, before starting the machine.




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