Runing a 3-Phase Motor VFD from Single Phase - Page 1
Forum section: electronics / projects

author: EHT
author_rank: Frequent Contributor
timestamp: December 02, 2018, 08:12:16 pm
content: Hi All,I have a machine with a large 600W AC motor which contains its own variable frequency drive (VFD) which allows the motor to run at a range of speeds. It has a 3-phase input but I need to run it from single-phase. It seems to me that I can do this very simply and without an external converter. The VFD circuit rectifies the incoming line AC to DC and then has a digitally controlled variable frequency driver circuit which runs the motor from that. i.e. as simply depictedhere.The AC input goes into a PCB which is a set of filters (X caps, MOVs and a choke) which then go into a panel mounted bridge rectifier, from there into the main PCB (see pics). These particular models are usually sold as single-phase; mine is unusual. It is rated “200-440V AC 3 Phase”, allowing it to run in US or Europe. I’m in UK so 240V single phase (440V 3-phase if I had it). I suspect that the main PCB is the same between the single phase input and 3-phase input units, with the difference being the line filter and the bridge rectifier. I attach a picture of the filter PCB, the bridge and the main VFD PCB.So, I think I can simply disconnect one of the 3 line inputs (or maybe tie two of the 3 together) and run it on single phase 240V AC. My thinking is that the bridge rectifier is rated for the higher current that would be needed for 3-phase in the USA and it will just rectify the single phase “as normal”.I wonder:- Is that correct?- If so, does it matter if I leave the spare live input floating or connect two of the 3 together?- am I missing anything?Thanks!

author: ogden
author_rank: Super Contributor
timestamp: December 02, 2018, 09:25:18 pm
content: Quote from: EHT on December 02, 2018, 08:12:16 pmModern 3-phase devices shall have phase fault detection so it either will not mind 1 phase and just work or (most likely) will not. Definitely not self-destruct.

author: Benta
author_rank: Super Contributor
timestamp: December 02, 2018, 09:33:07 pm
content: Connect all three phases together and run it from a single phase. No problem at all. You may need to use a thicker wire for neutral, but 600 W is not a lot.

author: langwadt
author_rank: Super Contributor
timestamp: December 02, 2018, 09:47:00 pm
content: the biggest issue with 3 vs. 1 phase is that 3 phase needs much less capacitance after the rectifier, if it was made for single phase that should taken care of

author: Benta
author_rank: Super Contributor
timestamp: December 02, 2018, 10:01:02 pm
content: @Langwadt: good point, I'd forgotten about that. Don't think it's an issue here, though.

author: EHT
author_rank: Frequent Contributor
timestamp: December 02, 2018, 10:02:42 pm
content: OK, thanks for the encouragement. I will get hold of the (unusual) connector and try it out. Given that this model is usually single phase and I can't imagine the complex main PCB is different, I'm hopeful. I'll connect 2 of the 3 wires together outside the unit.Quote from: langwadt on December 02, 2018, 09:47:00 pmI have just checked the main PCB which contains the smoothing caps. There are two sets (a pair of 100uF near the rectifier and a pair of 330uF near the output) . In each case there is no unpopulated space nor is there any room to fit larger caps. Hopefully that's good news. There are a few jumpers on the board.BTW, I can power it up without setting the motor to run. There is a control switch to start/stop the motor. If the microcontroller powers up I may be able to see if it gives an error code through its RS485 interface. More work but safer than starting the motor I suppose..Thanks again.