Phase indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Indicators, detectors
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Some three-phase products, especially those with three-phase motors, require correct phase rotation: phase B must lag phase A by 120°, and phase C must lag phase B by 120 °.
If the phase sequence is incorrect, the switched on motor will rotate in the opposite direction to the correct direction. This will lead to a deterioration in its cooling if it is equipped with an impeller, and in some cases, when a gear transmission with a ratchet is connected to the motor shaft, the mechanism is very likely to break. In industrial workshops, three-phase power supply with the correct phase sequence is provided. In rural areas, only two voltage phases or a phase and zero are connected to the consumer, and all three phases are brought in only if necessary. In these cases the correct phase sequence should be checked and a phase indicator can be of assistance.
The schematic diagram of the pointer is shown in the figure.
Terminals 1, 2 and 3 connect the pointer to a three-phase network. The circuit contains two neon lamps, which are supplied with voltages of two phases relative to the third through phase-shifting chains C1, R4 and C2, R6. Resistors of the same resistance R1, R2 and R3 are designed to create an artificial zero point.
With the correct phase sequence of the voltages connected to the terminals, the L1 lamp lights up, and the L2 lamp does not light up. If the alternation is incorrect, on the contrary, the L2 lamp lights up, and L1 does not light up. In this case, you need to swap any two phase wires. In the event that both lamps light up, this indicates a break in one of the phases, and if none of the lamps light up, then two of the three phases are cut off.
The double button KN1, KN2 and the resistors Rml and Rm2, shown in the diagram by dashed lines, serve to use the pointer in the case of a three-phase voltage with a frequency of 400 Hz. If such control is not expected, these elements can be removed from the scheme.
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