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# Variable frequency drive power anomalies

Variable frequency drive power anomalies can be divided into following three types: phase loss, low voltage and power off, sometimes they maybe appear mixed. The main reasons for these anomalies are transmission line impact by wind, snow and lightning, sometimes it's the power supply system appear ground wire and phase short circuit. The lightning is very different due to geographical and seasonal factors. In addition to voltage fluctuations, some power grid or self-generation units will have frequency fluctuations, and these phenomena maybe appear repeated in short times, in order to ensure normal operation, the variable frequency drive power supply also need to make corresponding requirements.

If there is a direct-start motor or cooker or other equipment near the variable frequency drive, to avoid voltage decrease when these devices power on, those devices power supply should be separated with the VFD power supply to reduce influence each other.

For the applications require continues operation in instantaneous power off, in addition to select appropriate VFD drives, we also need to consider the motor load deceleration ratio. When the variable frequency drive and external control loop are adopted instantaneous power off compensation, we need to prevent over current during acceleration by detect motor speed when power on.

For the application requires continuous operation, it's better to install additional automatic switching uninterrupted power supply devices. Like adopt diode input and single-phase control power variable frequency drives, it can continue work even if in phase loss status, but individual rectifier device current is too high, and the capacitor pulse current also high, it's not good for the variable frequency drives reliability and service life in long time running, so we should handle it the early the better.

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