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Explain single phasing preventer with neat sketch diagram.

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Single phasing preventer:


Single phasing preventers are generally used for small / medium capacity motors. Single phasing preventers are connected in secondaries of line CTs. These mainly contains a negative sequence filter. The output of negative sequence filter is fed to the level detector, which further sends tripping command to starter or CB. When one of the three fuses blows off, two-phase supply is provided to motor. This is called “Single-phasing”. These two healthy phases get overloaded. The unbalanced condition causes negative sequence currents. The negative sequence currents cause reverse rotating magnetic field, leading to overheating of rotor and finally damage to the motor. Thus presence of negative sequence currents is considered as indication of single phasing condition. The negative sequence filter is used to detect the negative sequence current and so also the single-phasing. The output of negative sequence filter is fed to level detector which measures the negative sequence currents to ensure singlephasing. When the negative sequence currents cross the particular set level, the singlephasing is ensured and tripping command is given to circuit breaker or starter. Thus it protects motor from damage.

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