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Variable frequency drive transition system

Few manufacturers provide any kind of a transition system in LV VFD bypass systems. Not common. They are almost universally manual (at least in the US) and are used as a backup, ie. emergency operation. Granted, I have seen and sold automatic systems where if the VFD fails the motor is automatically put across the line for continued operation. I always tried to talk my customers out of that option.

If you are talking about MV VFDs then they almost all provide a synchronous transfer solution so that one very expensive VFD can be used to start many motors and transfer them to utility power. It just so happens I sell an MV drive that can do a closed transition synchronous transfer. The VFD I sell is a voltage source drive. The sync reactor is installed between the VFD motor terminals and the output contactor so that only one reactor is necessary, not one per motor. The VFD is designed to match the output voltage and frequency to the input voltage and frequency, close the utility contactor and then open the VFD output contactor. What you get is a bumpless transition because of the true match. This system is not current sensitive like a current source. I have yet to see a current source system in the US that uses a closed transition because of the current loop used for control.

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