Sparky’s help needed. VFD won’t hold motor relay on. Milling machine.

Hi guys, I have a new to me Chinese Bridgeport that I just purchased at auction. The listing said the machine had been tested and worked as it should. It has a 1.5hp three phase motor so I connected a 3hp VFD straight to the original power cable. The coolant pump, light and power feed all work but when I try to run the motor a relay in the control box rapidly engages and disengages like its fluttering. There is a small square button on the relay which manually engages it and if I hold the button in then the motor runs beautifully but if I release the button then the motor stops again. There is a second identical relay next to the first and it does the same fluttering but I don’t know what it powers as when I hold its button in all I get is a whirring sound deep in the base of the machine. Maybe the coolant pump? So, what is causing both the relays to flutter and how do I fix it. Why would a VFD cause the fluttering? If I can fix the fluttering then everything will work as it should and I’ll be as happy as a pig in sh!t. Some info that may help: This is the third VFD I have installed and they are all the same brand and rating. The other two were plug and play and then I made adjustments to the ramp up, ramp down etc and they are working well. The mill was built in 1999 and to Australian electrical standard. We only have 230 volt single phase and 420 volt three phase. The relays are Siemens 3TB40 22-OX. One other thing I noted is the the variable speed control on the power feed doesn’t change the speed a great deal. I thought it should slow the speed of the table to almost zero. I notice the newer mills like mine are single phase now. Of course any ideas or help would be greatly appreciated. Cheers Stuart.

You cannot use a VFD as a 3 phase power source for the mill, i.e. you can't connect it to the original power cable and operate the transformer, contractor and any ancillary drives. You also do not use contactors in the motor connection path of the VFD in most circumstances. I would read the VFD manual. The VFD motor output would be directly wired to the motor and the run commands are driven by the VFD low voltage inputs. Most likely the other drive system on the lathe may be run off of single phase or off of a transformer, so that would need to receive single phase power to the transformer taps and the drive if not 3 phase. Would need more information as to a schematic and the table drive to determine if it is feasible.

Hi and thanks MSKJ. That’s just the info I needed, clear, concise and easy to understand. I’ve already started sketching out a new control box for the whole machine. The main controls are down on the knee which I don’t like and they have seen better days so this will give me the chance to correct that and as I won’t be using the motor control up on the left hand side I can remove that. My VFD has a removable control panel so the main body of that will go into the old control box and I also have a 230-110v step down transformer for the power feed that can go in there as well. I have a feeling that the cutting fluid pump is three phase so I will have to change that out completely. This plan will allow me to clean up the controls and have everything run of the one panel and I can add a digital tacho as well. Wish me luck and many thanks again. Cheers Stuart, Canberra, Australia.