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stalfos

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So I recently acquired a new (to me) lathe and bridgeport -- problem here is the power as I'm sure everyone in this subsection of the forum has seen 423462457724534 times (rough estimate, maybe.).

Anyway, I've been researching the pros and cons of vfd's vs rpc's and a question hit me... why can't I just use a vfd, set at a constant 60hz, wire the output to a wall socket or something and use my machines as they're intended to be if they were plugged into a regular 3 phase wall outlet..? I understand vfd's can be used to control motor speeds, etc but in my case, I'd much rather use the machines as they were intended to be used originally so as to keep things as simple as possible, at least initially while learning and gaining some familiarity with them.

Thoughts? Advice? Suggestions?

Thanks, hopefully this doesn't seem like too stupid of a question, unfortunately I'm not far down this path, only just finished getting everything home last night... Gonna have to post some pics once I get everything arranged, sorted, setup, etc. =)
 
Good questions!

If you intend to have 3 phase wall plugs to power your machines, then a RPC is the way to go. A VFD needs to be directly connected to motor, so this would preclude operating the lathe or mill in the normal manor.

Compare the cost of 2 VFDs vs. a RPC, I'm not sure how that comes out. My personal preference is VFDs due to the speed control capabilities
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I think a lot of VFD's will give you variable speed whether you want it or not, there is no reason you can't ignore it though just set it once and forget it. There are pro's and con's to the VFD vs RPC question.

RPC - Expensive, noisy, takes up a decent chunk of shop space. One RPC however will power multiple machines, if sized properly it will power them concurrently. Some lathes have 2 to 3 three phase motors, spindle motor, coolant motor, oil pump. A single RPC will power them all. You would need 3 VFD's for such a lathe, one for each motor. The rule is the VFD needs to be connected directly to the motor with nothing in-between. Now some may suggest going with a budget RPC they are out there cobbled together with used bits from wherever. Personally I won't risk my life or home to some amateur RPC job but that's me. In my research I found the only RPC manufacturer that is approved for use with Haas CNC machines and that's the company I plan to use if need be.

VFD - Much smaller, generally less expensive but not cheap still several hundred dollars and you would need one for each machine. They can be noisy the cooling fans on them are pretty loud. If you mount it in an electrical cabinet then you have another fan for the cabinet. A VFD may require ripping out some of the factory electrical controls and replacing them. I have a 3hp mill for example and ordered it with a 3 phase motor and VFD. They did not wire it up properly, instead they wired it to the cheap China barrel switch that came with the machine. VFD to barrel switch, then from there to the motor. This was incorrect. I removed the factory barrel switch and replaced it with a Forward, Reverse, Stop button and Speed knob (pot) and wired these 4 controls to the VFD and let the VFD control those as is proper. It works brilliantly and even though I have a gear head mill with 6 speeds I love the variable speed and use it often. It even has the grunt to tap holes at very low RPM. Having owned both a single phase 6 speed mill and this variable speed I would opt for the variable speed again.
 
Thanks for the reply Jim,

After posting my question, I came across a couple comments here and there stating that having some sort of interruption on the output, such as a drum switch like I currently have on my little logan lathe would somehow damage the VFD. Is that correct?

As for the cost comparison, the RPC may actually be the cheaper option here. There appears to have been some sort of RPC installed in the garage I pulled this equipment out of, though I haven't had the chance to inspect any wiring yet because the box that everything was connected to is still hanging off a wall 30 miles away. I killed power to the circuit, took pictures and clipped the mill out of the box for transport, leaving stubs of wire so I knew where everything went in the event circumstances dictated that I reuse the whole thing. (The input from the home circuit was clipped out as well, after verifying the circuit was dead. I wire nutted the ends of the lines and taped them on for safety purposes. No one likes getting shocked or throwing sparks when flipping switches!) This RPC, if that's what it actually is, appears to be a smaller motor driving a larger motor, both with pulleys and a belt between the two. Does this sound familiar to you at all? If so, that equipment that's still installed is mine to claim as well, I simply have to go rip it out. That'd definitely be "cheap", though figuring out what's currently powering the circuit may be a bit more complicated and time consuming and time is not something that's readily available at the moment. The garage that was housing this stuff is unfortunately part of a property that will no longer be the seller's property come the 8th. Long story, I'll write all about it when I've finally had the chance to inventory my haul.

If I were to rip this stuff out and provide pictures to document what goes where in the existing circuit, do you think it'd be possible for someone here to help make sense of it and figure out what I need to do to get everything going?

If it helps at all, here's a previous thread where hvontres identified my mill head as a variable speed 2J: http://www.hobby-machinist.com/threads/resources-for-series-i-information.36682/#post-313437
Speed control capabilities don't seem to be a huge necessity in this case as the mill seems to have that functionality from the factory as well, however there may be some other advantage a VFD holds here that I'm not aware of. If so, please feel free to laugh and point, just so long as you tell me why you're doing so afterward, heh. =)

Thanks again for the reply, I really appreciate the input!
 
Thanks for the input as well Coolidge!

Fortunately this lathe isn't super big. As far as I can tell, it just has the one drive motor with no secondary features as far as coolant pumps or anything else goes.

As a side note to my reply to Jim's post above, I should also mention this garage I'm pulling this equipment from has motors (some are very large actually) scattered all over the place. I have no doubt that some could be repurposed from gathering dust to providing power to all of my equipment pretty easily, should they be in workable condition. Fortunately I'm no stranger to working with power and though I'm unfamiliar with the RPC concept, I'm sure it's something I can figure out with an assist from the forums and have something functional and safe at the end of everything. Also, to simplify things a bit, I'm not working with any CNC equipment quite yet. I may convert the mill later on down the road as I have a friend that works with 3d printers and I'm sure plenty of those skills are adaptable, however, for the time being, old school is the way to go for me. In the event of a conversion later on, I'd most likely work with a VFD setup to control motor speed, tied into some opto-isolator type relays to provide the actual control, being run by a separate 120v AC CNC system that's electrically separate from the drive so as to provide the least amount of issues possible. I figure that way I can run stepper motors through their paces without actually having a motor running on the head and kinda "test run" things before trying stuff live. No one likes wrecking machines nor good material on a test run.
 
I have five machines that operate on three phase power. When I moved from California (where 3 phase was available) to Washington (where 3 phase was NOT available), I wired everything to an RPC that I inherited with my Bridgeport CNC. Since it is only me in the shop... only one machine would be used at a time, so a small RPC is fine.

After having everything running a few years, I am converting each machine to VFD.

I say:
If you already have an RPC available, get started with it first.

It looks like you have some cleanup work to do on the machines so you can retrofit VFDs later if you want (like I'm doing).

Oh, and believe me... you will want....;)
 
Lol, see aerohawk, the thing there is why though? That's the thing I'm not really understanding. I'm not seeing what the advantage is with a VFD having never seen one used live, in person vs a variable speed J2 head operating as designed. I'm not against having or using them. The cost isn't an issue either, it's simply a question of what benefit does a VFD bring to the table when powering my mill that my mill head with variable speed functionality doesn't have already, you know?
 
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stalfos said:
Lol, see aerohawk, the thing there is why though? That's the thing I'm not really understanding. I'm not seeing what the advantage is with a VFD having never seen one used live, in person vs a variable speed J2 head operating as designed. I'm not against having or using them. The cost isn't an issue either, it's simply a question of what benefit does a VFD bring to the table when powering my mill that my mill head with variable speed functionality doesn't have already, you know?
Click to expand...

A few random thoughts on VFD vs. RPC on a mill. I have both a VS head and a VFD, and for most operations have the VFD set to 60Hz and use the mechanical speed control.

Soft motor start vs. instant on. Less shock on the mechanical system
Controlled breaking/deceleration vs. coast to stop
Nice for drilling & tapping. Change the speed with a knob vs. crank speed down or or even go into back gear for tapping speed.
If converting to CNC, spindle speed can be controlled by the program vs. manual speed change.
In a CNC conversion, wiring is simplified for FOR/OFF/REV vs. rather complex and expensive contactor controls.
Motor overload protection vs. no overload protection on most mills.

Even if I had 3 phase power available, I would probably install a VFD on the mill just for convenience.

On a lathe, the VFD is less advantage, but all of the above applies. I am running a static phase converter on my lathe but have been thinking putting a VFD on it just to be able to fine tune the spindle speed between the geared speeds.
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My manual mill has the mechanical vari-speed head. The bushings in the cone were getting worn to the extent that the drive was very noisy. Plus, the belt had 'taken a set' because it sat around for a year during my move. I took out all the monkey-motion mechanical speed control and replaced it with a micro-v serpentine belt and pulleys. I then installed a VFD.

OMG, WHAT A DIFFERENCE!

Granted, the mechanical vari-speed drive was in need of repair. But the VFD eliminates so much hardware it is MUCH smoother and quieter than the mechanical system will ever be.

Your mill has been sitting for years... so don't be surprised if the vari-speed drive is noisy.

I also installed a VFD on my 6x12 surface grinder. It is a small enough motor that I got a 110 volt single phase VFD. I can now plug into any standard outlet, avoiding the need for additional 220 volt wiring.

What I discovered:

A VFD changes the A/C to D/C internally, and then from that, it generates the three separate phases electronically. The phases are perfectly 120 degrees apart.

An RPC starts with two phases 180 degrees apart (single phase is really two phases 180 degrees apart), and adds a third phase (usually at a lower voltage). But to get the sine waves phased properly, it depends upon inductance and capacitance to shift them to approximately correct angles. It isn't nearly as accurate as the VFD. The RPC is also much noisier than the VFD (which only has a cooling fan to make noise).

The accuracy of the phases makes a difference in starting torque (and therefore power), efficiency and audio noise. My surface grinder is smoother and quieter (along with now having variable speed).

When I had three phase power (when I was in California), I got used to how my lathe sounded when it started and stopped. Here in Washington, with the RPC, the lathe really groans when starting in the high speed range. Plus, is accelerates up to speed slower. This is telling me that the RPC isn't giving me an accurate three phases.

To be fair, an RPC will do the job just fine. I just like the VFD much better. It takes a little effort to get set up. But I think it is worth it! It's one of those things that, knowing what I know now, I wish I did long ago.
 
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WNY supply RPC on my DoAll DH612 (Precision balanced 3ph 1hp motor) and I have never had a problem. No chatter when grinding and it operates smoothly.

The Bridgeport and South Bend both have motor swaps.....and personally I believe that's the best way to go. (but then again I didn't do the swap!)
 
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