Re: Some success hacking the VFD, Was: DC Voltage of three phase rectifier greater?

What Roger said but add: You should have full RMS voltage on any phase but the circuit doesn't. that says there must be a limiting resistor feeding the small bridge and a similar one on the output. Unless all 3 phases are feeding the bridge, you don't get enough voltage out. Putting a 1x resistor in parallel with the feed would leave the circuit intact but working on single phase.

RogerN wrote:
"Ignoramus19266" <ignoramus19266@xxxxxxxxxxxxxxxxxxxx> wrote in message news:A7CdnR9iyq1HAQLUnZ2dnUVZ_gCWnZ2d@xxxxxxxxxxxxxxx
So, as the saga to try to convert my three phase VFD to single phase
input is continuing, I have had some success.

I posted a few minutes earlier that I understood how phase loss
detection works. It is very simple. Here's how it works: the input
lines L1, L2 and L3 are connected to a tiny, SEPARATE three phase
rectifier, with no capacitor on its DC output. So this is rectified,
but not smoothed out voltage. It is then sensed by the VFD and
rejected if it is too low.

Indeed, with three phases connected to the VFD, it is much above 300
volts, (with drive dysplaying "RDY") and with only single phase
connected, it is much below 300 volts (with drive faulting and
displaying "PhF").

I connected alligator clips to the wires leading from the rectifier to
another board and that's how I measured the voltage.

My next try was to try to fool the VFD into thinking that it has good
3 phase, by connecting the main DC bus to these wires.

Lo and behold, even from single phase, with the voltage on the sensor
well above 300 volts, due to connecting it to DC bus, the drive was
finally fooled and said RDY even though it was connected to only one
phase.

My next try would be to see if I can get the motor to at least spin
for 3-4 seconds from a drive fed with single phase only.

If so, I can try to address these changes in a less haphazard fashion,
by using a tiny 0.1A or some such fuse to connect it to the DC bus.
And, of course, I would need to use a bigger rectifier and plug it
into the DC bus directly.

i

You could probably put a small value capacitor on the small rectifier so it would charge up to DC bus level. 240V AC RMS single phase should have a peak voltage of 240 X 1.414 = ~ 340V, so a small value capacitor that can handle maybe 400V should charge up to 340V and let your VFD run. You may need to add capacitors to the DC bus to supply power from the missing phase. Some VFD's I've seen said you could run them from single phase if you added the capacitor option they sold.

RogerN


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