Thread: More VFD problems.

I am still surprised that, seeing the motor was relatively unloaded, the motor did not start especially if a slow acceleration time is used.
I have run an unloaded 3HP motor with a 2HP VFD without any problems, and both of the 3HP motors I have running with a 3HP VFD will produce 3 real HP @50Hz without any problems.

Bob, it is not only the limited current rating of the single rectifier diode, compared to the 3 diodes sharing that current in a 3-phase setup. A 5.5kW VFD is designed for 3 phase input (regardless if its a 220V or 380V model). It is designed for the input current being divided among the 3 phases. The OP vas connecting it up single phase, so all the current flows through one diode (pair). That would overload the single diode pair and over time it would fail due to overheating if the VFD power is not derated. Diodes with higher current rating cost more, that is why the makers do not go one or two sizes larger. Here a basic circuit diagram:

vfd1.png

However, the overloading of the diode would not prevent the VFD from working initially. A 3-phase VFD is designed with a bus filter capacitor value to smooth out the ripple from a 3-phase rectifier. This is how the unfiltered voltage from a 3-phase rectifier would look like on an oscilloscope:
vfd3.png

If the VFD is hooked up only single phase, this is how the unfiltered voltage from the rectifier looks like:
vfd2.jpg
Again, this could be overcome if the manufacturer did use a larger filter capacitor. But capacitors are expensive and large. So for a 3-phase VFD, to keep cost down the designer has to choose a capacitor size appropriate for 3-phase operation, not single phase. Therfore the voltage between the peaks can fall below the trigger voltage for the bus undervoltage alarm.

This is different for the smaller VFD's. The designer will choose both a diode rating and capacitor size to allow safe single phase operation without derating- even when the VFD is always fitted with a rectifier that allows for a 3-phase hookup too. So when purchasing a VFD to be hooked up single phase, make sure it is rated for single phase operation (nearly all VFD's below 3HP are by default). Or else choose a 3-phase VFD that is 2 or 3 sizes larger.