Electronics > Mechanical & Automation Engineering

VFDs for home machine tools

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smithnerd:
Hi all,

I’m currently looking at ebay VFD inverter drives for a machine restoration project. I think I need some help with understanding the basics.

Firstly, I’d like to run an old 60s Hoover 240V, 3ph, 3/4hp 1425rpm motor, using a single phase supply. It’s a rather hefty drilling machine which will probably only see light, infrequent use and <10mins of duty at a time.

3/4hp is approx 0.56KW, how much headroom should one allow in this figure?

Secondly, it would be nice to also provide some degree of speed control to my lathe motor (240V, single phase, 3/4hp, 1425rpm, capacitor start w/ centrifugal switch) using the same VFD (not at the same time, obviously). Would I choose a VFD with 3 times the rated power of this motor and run it off a single phase? Or do I need a dedicated VFD with single phase output? Another option is to swap the motors over. I believe this would be possible as the spindles are the same diameter (although I haven’t checked the keyways yet). If I did this, then I wouldn’t need to worry about the single phase motor at all, as the drilling machine has the usual step pulleys plus a back gear for low speed.

Thirdly, should I be looking at used ‘quality brand’ units, or are the cheapo Chinese VFDs good enough for hobby use?

Benta:
All your ideas are bad. Running single-phase motors off a VFD is troublesome to say the least, and if it's a centrifugal switch type it tends to be impossible.

If you want to pursue the VFD path, I suggest you replace the motors with three-phase types.

But you talk about "restoration": Where's the restoration in adding a VFD?

smithnerd:
Perhaps I was unclear. I have restored a 1950s/60s drilling machine which has a three phase motor fitted. I'm primarily interested in driving this with a VFD. For this machine I'm not especially interested in speed control, just 3ph drive (if I keep this motor installed).

Speed control for the single phase lathe motor was just an idea - would be nice to have. Thanks for confirming my suspicion that the centrifugal switch will be a problem. The motor isn't original equipment for this lathe and over powered, so I have been thinking of changing it anyway.

jpanhalt:
Both 3ph and DC will work for electronic speed control on a lathe.  I converted my small lathe to DC and use a KB Drives DC controller.  Some really classy American toolroom lathes also use DC, namely Monarch 10EE, while the Hardinge HLVH used 3ph and variable speed pulleys.  I have 3ph with VFD on my drill press.  There is not much difference. 

Three phase does have a lag from hitting the start button to the spindle turning.  DC doesn't.  As for cogging at very low speeds, the 3ph is probably worse, but since both drives were retrofitted to tools that also had belt drives, that is not an issue.  That is, just go to the lowest belt drive speed and you will not see cogging at any reasonable speed.

EDIT:  I would let total cost and how things fit together guide me.  For the small lathe I got a good price for a Baldor DC motor that fit in the head with almost no changes, and the DC drive fit inside the bed.  For the Drill press, the 3ph motor fit well, but the drive was considerably larger.  My large lathe already is 3ph, so I will certainly go VFD for it.

SmallCog:
I'm not super across typical UK domestic wiring but in the back of my head your standard wall sockets are 16A

Get the biggest one you can with that in mind, never know when you'll need it!

Provided you're starting the motors up unloaded (and unless we use these machines very differently you are) the drill wont need a lot of "head room" in the VFD especially if you start it a bit slower than full speed, or if the VFD unit supports ramping up

I'd suggest starting at about 45Hz and exercise caution going below 40Hz, I've never had much luck below 35Hz

For the single phase machine... convert it to 3 phase.

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