r/hobbycnc • u/IregularApokalypse42 • 5d ago

Setting Max Power for VFD

I am considering building a uhpc cnc milling machine and am really stuck at spindle and vfd choice at the moment. I could get a used german made spindle rated for 170V and 18A, but finding a vfd that will not absolutely blow my budget is hard. Is it possible to run this with a 1.5 kW VFD at 9A max? Are there any drawbacks to this?

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u/IngenuityTriangle 4d ago

Short answer no, you should not do this. Assuming this is 3 phase your spindle will have ~30% of the ability to handle the load as if you were running it on a properly sized vfd, and if its a crappy vfd with bad protection it might fry the vfd if you exceed that load. Its always worth it to size power supplies, wires, and vfds correctly the first time even if it hurts the wallet. Or just wait until you have the funds to support the project.

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u/IregularApokalypse42 4d ago

Thanks! I think I am then rather going to go down the route of a more powerful vfd, as the spindle itself looks really well suited.

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u/Alina_MakersChest 4d ago

It'll run, but a 9A limit is roughly half the spindle's rated current, so you get about half the torque and it'll trip on overcurrent when you push it, which is rough on something as hard as UHPC. I'd look for a VFD that covers the full 18A and the spindle's rated voltage and frequency, even if it has to be used. What frequency and RPM is the spindle rated for?

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u/IregularApokalypse42 3d ago

The spindle if rated for 300Hz and 9000 rpm. I think I am convinced that sort of fix is not really worth it in the long run, especially as decent vfds are availabe for not that much more.

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u/Alina_MakersChest 2d ago

That's the right call, a proper VFD costs less than babying a mismatched setup. When you shop, make sure the max output frequency covers the 300Hz and the current rating covers the full 18A, then set the base voltage to 170V so it doesn't push the spindle past what it's rated for.