Can I Add A Vfd After The Disconnect?

John Caven

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Ok so I purchased and had installed a 3 phase rotary converter. I'm thinking I'd like to control my mill only with a VFD. it's a 1/2 HP bridgeport M head. I have a 3 phase load center box that is wired to a disconnect and the disconnect has a plug on it for my milling machine.

Could I some how go from the machine to a VFD to the plug? I don't need the phase converter features. I just want the ability to control my motor speed. Do I just ignore the 1 phase input side and use only the 3 phase input and output on a VFD?
 
VFDs can use either 3 phase or 1 phase input. Simpler is better, you could go from 1 phase to 3 phase converter to VFD, but i would not recommend it. That said, I've done just this on one of my mills. It had several 3 phase motors and only one for speed control.

I think it works best for the VFD to be a part of your machine. I generally mount them inside an electrical cabinet. This means needing a remote switch to start/stop the machine. And a pot to control speed. You can just use the run/stop buttons and speed pot on the VFD itself, but they are not durable. Break them -> buy a new VFD.

Let us know what we can do to help. You'll like a VFD upgrade.
 
Should be fine, but if you connect or disconnect the mother while the vfd is running, the drive will fault.


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Both single phase and three phase input VFDs convert the input power to DC and then convert that back to AC. Every VFD is a phase converter! The main difference between 3 phase input VFDs and 1 phase input VFDs is the number of rectifiers used to convert AC to DC. The 3 phase input VFDs would have more parts to fail :). How well balanced is your RPC? I'm not sure how tolerant VFDs are to unbalanced input. Personally if you want the VFD for speed control I would just go with a single phase VFD, I don't really see any benefit from using a 3 phase input VFD for such a small motor.
 
Ah this is making my brain hurt. I'm not understanding the 1 phase vs 3 phase. All I know is that my RPC turns the power to 3 phase and sends it to the 3 phase load center that sends 3 phase power to both my machines that are on a disconnect. Every time go to look at getting a VFD I get totally confused and I drop the issue of doing anything with a VFD. I feel like a toddler trying to run backwards.

So instead of using single phase or 3 phase... Can someone just tell me in terms a 3rd grader can understand please? IE: Does hooking up the power leg from the pigtail on my 3 phase disconnect that already has 3 phase power to a VFD work and in simple to understand English and realizing your most likely talking to an idiot .. (haha) how would one do it?


Karl: I know you said I could but it's a bad idea.. why? I might have made my bed if that's the case and will just have to stick with RPC only. ugg
 
I must have misunderstood the question altogether. In my eyes, just get rid of the rpc and put in a 120v (or 240v if that's what you have)1/2 hp vfd. Simple and done. Don't worry about the behind the scenes stuff. Single phase in, three phase out.


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...

Karl: I know you said I could but it's a bad idea.. why? I might have made my bed if that's the case and will just have to stick with RPC only. ugg

Its just wasteful. More componets to fail and give trouble. KISS is best (keep it simple stupid) It will work.
 
+1 what mzayd3 said. You don't really need both a RPC and a VFD for low power motors.
 
I just spent $900 on my RPC unit. I don't think I'm getting rid of it. I also use it to power my 5 HP lathe. It's a 10HP american rotary RPC. VFD's keep leaving me feeling very very frustrated .
 
I just spent $900 on my RPC unit. I don't think I'm getting rid of it. I also use it to power my 5 HP lathe. It's a 10HP american rotary RPC. VFD's keep leaving me feeling very very frustrated .
I think that everyone was assuming that the mill was the only thing you were powering with the RPC.
 
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