Question on VFDs and Motor Horsepower
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    Default Question on VFDs and Motor Horsepower

    I have a large metal lathe that came with a 5 HP 3 PH motor on it. I am hoping to build a rotary phase converter for it eventually. In the meantime I would like to get it up and running temporarily with an inexpensive VFD.

    My Question: Can I power up this 5 HP lathe motor with a VFD that is rated for 2 HP motors?

    I understand that I won't get 5 HP from this setup, but my question is, will it work at all? Or will it just instantly fry the VFD? I don't need a lot of power. I just want to get the lathe up and running to check out how well it functions and what might need to be repaired on it.

    Thanks

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    My suggestion would be to try a simple, inexpensive static phase converter. They really do work well and you could incorporate it into the start circuit of your future rotary if you sized it accordingly. Static's are shunned on this site by many folks, but I have used them and they work very well.



    Stuart

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    2HP is likely pushing it even with slow ramp speeds. It may not even let you enter the nameplate amperage for the motor. I went right for this and it works great. This did involved a substantial rewire of the machine, but your idea to wire one temporarily will not be trivial either.
    https://www.amazon.com/LAPOND-Perfor...=lapond&sr=8-5

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    Quote Originally Posted by atomarc View Post
    My suggestion would be to try a simple, inexpensive static phase converter. They really do work well and you could incorporate it into the start circuit of your future rotary if you sized it accordingly. Static's are shunned on this site by many folks, but I have used them and they work very well.



    Stuart
    Stuart is right on! I use a static " converter" to start the RPC. Works great. (Multi motor Mill)

    It will be tough to do anything but trip the over current warning if trying to make a 2hp VFD power a 5HP motor. Current mismatch both starting and stopping.

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    Yeah, static converter will do it. And, if rating is sufficient, will serve to start the RPC idler when you have one.

    These folks have a 4 to 8 HP which will work for the 5 HP, and also start a 7.5 HP idler to form an RPC. They have (or had) application notes to do so. Look around on the site.

    Static Converter Models & Prices On Phase-A-Matic, Inc.

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    Static converter for now...

    We have 7.5 hp lathe that came with static converter.

    Works okay...

    Low speeds good but high speeds the oil slosh in gearbox friction on cool days causes protection unit to trip.

    You loose about 1 third of power but the converters are cheap so you can get working now and do upgrade later.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Certainly not the right solution and it may not work, but worth trying as an experiment with very long ramp up speeds since lathes have lots of inertia.
    You didn’t say what VFD you have.
    If it fails, it will most likely fault out. Nothing should fry/die. If it does, then you’re better off, as you have eliminated a really crappy VFD

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    Quote Originally Posted by JST View Post
    Yeah, static converter will do it. And, if rating is sufficient, will serve to start the RPC idler when you have one.

    These folks have a 4 to 8 HP which will work for the 5 HP, and also start a 7.5 HP idler to form an RPC. They have (or had) application notes to do so. Look around on the site.

    Static Converter Models & Prices On Phase-A-Matic, Inc.
    Their PAM1200HD had a predecessor labeled 7 to 10 HP "Heavy Duty".

    That went to the landfill, having contributed to the demise of the Euro-spec 7 HP motor in the Cazeneuve HBX-360-BC.

    Got the lathe for a lower price with the dead motor than what a new motor PLUS a Reliance-idlered RPC and a far better Phase-Craft 10 HP rated RPC controller cost me.

    No foul.

    Static "converter" (not really, it isn't) actually did me a favour. I wasn't a big fan of Euro-spec AC motors to begin with!


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    Quote Originally Posted by rpseguin View Post
    Certainly not the right solution and it may not work, but worth trying as an experiment with very long ramp up speeds since lathes have lots of inertia.
    I don't know anything about VFDs other than they can be complicated in the sense of offering a billion options in terms of programming. Which is good actually. Although I don't know how to program one yet.

    Quote Originally Posted by rpseguin View Post
    You didn’t say what VFD you have.
    I don't yet own a VFD. I'm looking at one on eBay for $89. It's 1.5 Kw I take it that's equivalent to 2 HP.


    Quote Originally Posted by rpseguin View Post
    If it fails, it will most likely fault out. Nothing should fry/die. If it does, then you’re better off, as you have eliminated a really crappy VFD
    Falling out would be no problem. But obviously a total fry would instantly destroy $89. If it doesn't fry I could either recoup the money later by reselling it, or possibly use it for an actual 2 HP 3 PH motor in the future.

    Here's the LONG STORY:

    I just purchased a Warner & Swasey No. 3 lathe at basically scrap metal price. This way if it turns out to be scrap metal I haven't lost much.

    The lathe appears to be in fair condition actually. It also appears to be complete although without any tooling.

    Short Term Plans:

    I would like to get the lathe up and running, at least enough to make it go through all its motions so I can check to see what works, and what might need to be repaired in terms of automatic feeds, and other functions.

    I'm torn between two options.

    Option 1: Try to fire it up on a cheap VFD as per the topic of this post.

    Option 2: Replace the original 3PH 5 HP motor with a single phase 1 HP motor, just for this initial run and inspection of what works and what doesn't.

    Long Term Plans:

    If the lathe proves to be in fair shape and usable I plan to eventually build a homemade Rotary phase converter for it. In the meantime I can actually work on restoring and/or repairing anything on the lathe that needs attention.

    More Information:

    I love restoring old machines. The ultimate long term plan is to potentially buy more machines to clean up and restore. I'd love to have a milling machine and a surface grinder too. So building the 3PH Rotary converter will be well worth it as I could then use it to power other machines in the future.

    In the meantime, for right now, I'd just like to get this lathe "turning" if you'll pardon the pun. I'd like to be able to see what kind of running condition it might be in so I can decide whether to go forward with a full restoration, or part it out and scrap what's left. In any event, this won't be the last machine I buy. But hopefully it will turn out to be a good one, at least for the price which was pretty much rock bottom. Any less and it would have been below scrap metal prices.

    Final Note:

    I'm actually more interested in rebuilding old lathes and milling machines than in actually using them. Although I'm sure I will use them once I get them up and running.

    In any case, the current goal right now it just to get this thing turning. I've even thought about running it will a small garden tractor engine in a pinch. I just want to make it go for now to see if it even works at all. The main goal right now it to just check the functionality of the machine. I'll come up with a better power supply solution later once the machine is ready to actually use. The $89 VFD seems like a quick and dirty solution if it works! Of course, if it fries itself then it's $89 down the toilet. If it just doesn't work but doesn't hurt itself, then I can either continue to use it for smaller motors, or resell it and recoup some of the money I paid for it.

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    Sounds like you have a plan, but to answer your first question on using a 2HP VFD on a 5HP machine, here is how that works.

    The VFD is rated for an amount of CURRENT it can safely handle. A motor is rated for a maximum current it will DRAW under load. Your 5HP 230V motor is ;likely rated around 15A Full Load Current (FLC). Your 2HP VFD is likely rated to handle about 7A at full load. So you can set up the VFD to do what's called "Current Limit" at the 7A maximum rating, and it will not burn up, it will just not deliver more than 7A to the motor. How it accomplishes that is that it will limit the maximum speed the motor can operate at. What that speed will end up being is going to depend on the actual physical LOAD on the motor from your machine. If the motor is completely unloaded, 7A may allow it to get to full speed, but as soon as you put a tool to it, it will slow down to whatever speed it can handle and still keep the current at 7A. You can't predict what that will be.

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    Quote Originally Posted by Jraef View Post
    Sounds like you have a plan, but to answer your first question on using a 2HP VFD on a 5HP machine, here is how that works.

    The VFD is rated for an amount of CURRENT it can safely handle. A motor is rated for a maximum current it will DRAW under load. Your 5HP 230V motor is ;likely rated around 15A Full Load Current (FLC). Your 2HP VFD is likely rated to handle about 7A at full load. So you can set up the VFD to do what's called "Current Limit" at the 7A maximum rating, and it will not burn up, it will just not deliver more than 7A to the motor. How it accomplishes that is that it will limit the maximum speed the motor can operate at. What that speed will end up being is going to depend on the actual physical LOAD on the motor from your machine. If the motor is completely unloaded, 7A may allow it to get to full speed, but as soon as you put a tool to it, it will slow down to whatever speed it can handle and still keep the current at 7A. You can't predict what that will be.
    This sounds good to me for now. I'm not worried about doing any actual machining with the lathe right now. All I want to do is get it up and running so I can test all its functions. I just bought the thing at scrap metal prices. Although the lathe looks to be in fairly good shape. If it's as good as it looks it should be a steal. If it turns out to be scrap metal I won't have lost much. Between scavenging parts from it and scraping the rest I should be able to retrieve money spent and potentially come out ahead in the deal.

    My main goal right now it too see if I can power up the lathe at all. If it runs and all the feeds and other functions actually work, then I'll consider "restoring it". Although that might not be the best term to use. Perhaps "refurbish" it might be a better description.

    In short, if it turns out to be a usable lathe I'll keep it and it will become a permanent member of my shop. And if not, I'll have fun recovering my investment. I'm sure I'll be able to either sell, or use, various parts from it.

    So yeah, for now all I want to do is get it up and running so I can put it through all its motions and see where we're at. If it's worth refurbishing I'll build a rotary phase converter for it over the long haul as I refurbish the lathe.

    No rush. I'm a hobbyist more interested in restoring old machines than in actually using them.

    Although if I get it all up and running I'm sure I will put it to good use as well.

    For now I'm just looking for a CHEAP way to play with it and see what works and what doesn't.

    So if I can just get it running with no machining load, that will be just fine.

    I would imagine that if I get that far I could probably at least make some very light cuts also just to see what kind of tolerances it can hold, etc. Very light cuts shouldn't bog it down.

    I'm not concerned with doing any heavy machining with it for now.

    I'm currently looking at this cheap VFD for $86 and free shipping: Cheap VFD

    I don't want to pay $300 for a VFD just to test out this lathe. So I figure the $86 might be worth the gamble. If it will run the lathe just good enough to test everything out I'll consider that to be money well spent. And if it survives this overload condition I might be able to use the cheap VFD to run smaller 3 PH motors later too. That would be very nice indeed.

    My other option is to try to stick a 1 HP single phase motor on the lathe just to test things out. But I'd rather run it with its original motor if possible. I just want to see if it all functions properly.

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    First, a rotary converter is a static converter with an idler motor that increases the usable load range by helping transfer energy from one input to another. You can test your lathe by hooking up a capacitor to generate the third phase and by playing around with different capacities find the one that works best. A source of run capacitors is a HVAC contractor who throws away run capacitors every time they scrap an air conditioner. Single phase air conditioners usually do not have centrifugal switches because they are sealed units that expose the works to Freon and oil. The capacitors and potential relay from a similar sized compressor would be a good starting place. If the motor runs the right speed and does not make a loud whine or overheat, you should be close enough to try out the lathe.

    Bill

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    Quote Originally Posted by Sweet Dreamer View Post

    My Question: Can I power up this 5 HP lathe motor with a VFD that is rated for 2 HP motors?

    Thanks
    No load current is typically about 50% of full-load current. It won’t fry the VFD, it will just trip out. But a 2
    hp vfd won’t deliver enough current to run a typical 5HP motor.

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    I'm missing something I guess. You want a quick cheap and easy way to run the lathe just to make sure it works. That solution truly is a static converter, but in all your verbiage above you have not mentioned it..keep talking about a VFD. Why not a simple static?

    Stuart

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    Quote Originally Posted by atomarc View Post
    I'm missing something I guess. You want a quick cheap and easy way to run the lathe just to make sure it works. That solution truly is a static converter, but in all your verbiage above you have not mentioned it..keep talking about a VFD. Why not a simple static?

    Stuart
    I've decided to go with a full RPC. It's easy enough to build and will continue to serve as the final power supply permanently. I was considering a VFD because of lack of knowledge of what the other options were. Now that I have a better understanding of RPC I'll just go with that. So I'm no longer concerned with questions about a VFD.

    Can a thread be marked "solved" or "finished" on these forums. I don't need this thread on VFDs anymore. I'm no longer interested in VFDs.

    Thanks.


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