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  • Hypothetical electric motor question

    I'm ordering a VFD for the lathe I just bought and was surprised to see that even though it's a 13" it only has a 1 hp motor.

    What would be the disadvantage, and I'm not actually going to do this, of putting a single-phase motor on it? 1 hp single-phase motors are readily available.

  • #2
    VFD = Soft start, variable speed and better surface finish. You won't get plug reversal capability with a vfd, but...

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    • #3
      Single phase does not "plug" reverse at all, you have to generally let it come to a stop, then start it in the other direction. So no loss leaving it behind.....

      Little lathes are very sensitive to single phase vibration.... Atlas any size, SB 9", Logan 10", etc. They can chatter like crazy in tune with the single phase. As you get bigger, that problem decreases. I do not know how much less of an issue it is with a considerably heavier (we hope) 13" machine.

      All I know is that 3 phase gives very smooth power.... And a VFD has advantages.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        I have a stout 13" lathe with a 2hp motor. I use all of that hp fairly often, and 3hp would be better. Unless you have a very lightweight 13" lathe, get a bigger motor. I also use the 'instant' reverse a lot. You get 100lbs of chuck and work spinning and it takes forever to stop without it. The quick reverse is also essential to me for tapping. I've never used a VFD, but if it means giving up the instant reverse, I never will. I suppose you can set up a VFD to brake to a stop, but I wonder if it is as quick as a 3ph reverse. Soft start? Why? Get it going and get to work.

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        • #5
          It's a South Bend 13 so it's pretty stout. I'm going to try the motor I have now but I'm certainly not opposed to getting a bigger one. I had a 9" Junior before and while it was a nice little lathe I got tired of taking wispy little cuts. I want to see a big, blue, steel chip for a change.

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          • #6
            A bigger motor, or a 3 ph motor, is going to have more torque at slower speeds- which you'll need if you're operating the motor using a speed control and need to use slower speeds often enough. It stands to reason that using a vfd and a 3 ph motor is going to give a smoother result- what is unclear is if your 1 ph motor would rob you of some of your expectations from the machine. It may not- but it's still true that a 3 ph motor and vfd will give smoother operation. You must also consider the individual resonances that could arise, and how well that can be controlled using whatever setup you use to control the motor. It is quite possible that a particular lathe would behave very well, using a single phase motor and control box- if the resonances don't reinforce. At the same time there's no guarantee that using a 3 ph motor will improve things, although in most cases it will.

            If you want or need the ability to hog material out and get a job to a customer in a timely fashion, then you should avoid any motor not able to produce a couple horse at least. But having said that, I think that many of us here are satisfied with hanging a 3/4 horse on the lathe- that's what mine came with and I was happy enough (but it did burn out, which I was less than happy about- but which also gave me the opportunity to change the drive system). If you own a single phase motor of at least 1 horse, then it's probably going to do a decent job for you. It would not be a crime to use it, let's put it that way.
            I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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            • #7
              I didn't mean to infer single phase can reverse instantly.. more a comment about three phase verse single. But... if you want plug reversal in a single phase motor, get a motor for a slide gate. 3/4 is the biggest I've had but they change direction "instantly" to reduce damage if the the side gate hit a vehicle.

              Personally... I hate plug reversal. My 7.5 hp does it, but it's really hard on the lathe, and scares the crap out of me when it happens by operator error.

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              • #8
                I figured they would do a clutch for that. Did not know they did a plug reverse. There is a system for doing that with single phase, a special relay control deal.
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  I would put the biggest 3phase motor that would fit on the lathe. Coupled to a VFD and live life happily ever after! Not too many people I would think stand at their lathe and think, "I wish I didn't have so much power available." Probably the exact opposite.
                  If you are a hobbyist then you more than likely don't run your lathe all day long so the added cost of a higher HP motor is not relevant. If you do work for money then the you for sure need to be able to work faster which equates to more HP. I have two lathes. One with 3hp and the other with 14hp. I hate using the 3hp lathe. No power.
                  How to become a millionaire: Start out with 10 million and take up machining as a hobby!

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                  • #10
                    I'll agree with everyone, definitely don't put a 1HP single phase on it, and you'll likely find over time you'll want more than 1hp. You'll be tempted to dial the speed up or down from 60hz with the VFD rather than changing belts and in both cases you'd be operating the motor at a disadvantage.

                    I'd get the 3HP Lapond VFD on Amazon, which is what I have on my 12" lathe with a 2HP motor.That way when you decide to upgrade the motor you'll be all set with the VFD. It's far and away the cheapest VFD with braking resistor capability, and they work more than adequately for hobby use.

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                    • #11
                      Well, like I said I'm going to see how I feel about the 1HP before I change anything, although I've become obsessed with seeing a big, blue chip curling off a piece of steel

                      If I decide to go for more power I can move the 1hp motor and the VFD over to my bandsaw. Having speed control there would be pretty sweet.

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                      • #12
                        A vfd is great for variable speed, but remember, at quarter speed you only have 1/4 hp, so you will still need to use your belts/gears. The advantage of a larger motor is a wider range of acceptable power.

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                        • #13
                          Originally posted by lakeside53 View Post
                          A vfd is great for variable speed, but remember, at quarter speed you only have 1/4 hp, so you will still need to use your belts/gears. The advantage of a larger motor is a wider range of acceptable power.
                          I have a VFD on my milling machine and for that I did away with the stock forward/reverse switch. In addition to the power issue I really dislike the waiting for the machine to stop so I can change cutters. Could I set this up so that the VFD is wide open all the time?

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                          • #14
                            Originally posted by Commander_Chaos View Post
                            Well, like I said I'm going to see how I feel about the 1HP before I change anything, although I've become obsessed with seeing a big, blue chip curling off a piece of steel

                            .....
                            Depends on what you call "big".

                            I can get significant sized blue chips off the Logan, powered through a flat belt by a little 1/3 HP motor. depth of cut maybe 0.060 or a bit more (1/8 off the diameter per pass) depending on type of steel. While I can get up to almost 0.25" wide chips, I do not get them in blue....and they are not very thick They do not "clang" when they hit the floor, but what you get with 3 HP on the 13" is not going to "clang" either.
                            1601

                            Keep eye on ball.
                            Hashim Khan

                            Comment


                            • #15
                              Originally posted by Commander_Chaos View Post
                              I have a VFD on my milling machine and for that I did away with the stock forward/reverse switch. In addition to the power issue I really dislike the waiting for the machine to stop so I can change cutters. Could I set this up so that the VFD is wide open all the time?
                              I don't know what you mean by wide open all the time, but you should be able to set the VFD to brake yes. It's a standard parameter to set stopping speed in number of seconds. The quickness with which the VFD will be able to do so without a fault depends on the RPM and rotational inertia of whatever is spinning. If you try to slow too fast you'll get an overvoltage and the VFD will trip. Units with an external braking resistor can stop things faster because they dump excess energy to the resistor. My lathe required a braking resistor but lathes typically have a much higher inertia than milling machines. Even without a resistor you should be able to do much better than a simple coast down.

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