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Thread: Getting 120v in a 240v machine

  1. #1
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    Getting 120v in a 240v machine

    A few years ago the VFD on my Jet 1640 lathe fried. A replacement was $800, but people on line steered me to a VFD on Amazon for $80. Only problem is that it was 240v and my lathe was 120v. Turns out it doesn't matter because the motor is 240v. The 120v VFD used a voltage doubler which made it less durable, and it couldn't supply all the power the motor was capable of handling, so I got 20% more power off the lathe with the 240 vfd.

    BUT... the tach is 120v. I "fixed" it temporarily by simply running a 120v circuit to the lathe, but that is a nuisance to plug and unplug. I could put a switch on the cord, but that wouldn't be much better.

    My original thought was to simply run the tach off of one leg of the 240v to ground. The obvious objection to that is that there would be voltage on the lathe, and a possibility of a shock if the neutral broke. I have never experienced a broken neutral and the worst case is 30ma that the tach uses (reduced significantly since I wouldn't be at all grounded...) but still. I considered running a wire from the lathe to a drill press next to it on a different circuit so even if the neutral broke it would still be grounded through the drill press, but of course you aren't supposed to ground in two places.

    My next thought was to put a 4000ohm resistor in series with the tach so that the tach would see 120v, but someone told me that a resistor can fail in a short and that would fry the tach. I've never seen a resistor fail, but still.
    My next thought was to use a 3.3w light bulb instead of a resistor, but wiring that in would be a pain.
    Final thought was to run a multiwire circuit with the tach on one side and a resistor on the other. That would be like the second thought unless the resistor failed, and then it would be like the first thought. The resistor is unlikely to fail, especially in a short, but if it did, the tach would be protected by the neutral.

    So, what do you think? Well, other than the obvious that I have spent way too much time planning this.

  2. #2
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    A 240 to 120 step down transformer is an obvious solution, but even a small one might be too expensive. But a search for 240/120 volt travel converter will find you a bunch of options in the $20-30 dollar range...
    --I had my patience tested. I'm negative--

  3. #3
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    Quote Originally Posted by Paul F Franklin View Post
    A 240 to 120 step down transformer is an obvious solution, but even a small one might be too expensive. But a search for 240/120 volt travel converter will find you a bunch of options in the $20-30 dollar range...
    How good are they? It is one thing to risk a $50 shaver and another to risk a $250 tach.

  4. #4
    The 120 volts in your house is from one hot to neutral. When you wire a 120 volt outlet, you have a black wire that is hot and a white wire that is neutral. That white wire is also grounded at the main box.

    I would have no problems connecting one hot and using the neutral to get 120 volts. A lot of appliances, such as a dryer, do this. The heating coils are connected hot-to-hot and the rest is hot-to-neutral.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #5
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    Install a couple of primary fuse and a small control transformer or determine what DC voltage the tachometer uses and install a 240 volt input power cube of the correct voltage.

    Regards, Rod

  6. #6
    Re-wire the lathe and supply circuit with 4 conductor wire and plug so you have a neutral and a ground. Protect your 120 v tap with a small fuses?

    I have a circuit brancher I call my power thief. I connect it to the service disconnect of the outdoor air conditioner condensing unit at a few of the homes I work on when a 120V outdoor outlet is not available and I do not have access to inside the home. It has a 20A single pole breaker and an outlet. The connection wires are a black to one side of the 240, a white for neutral that I connect to the ground of the air conditioner disconnect, and a long green wire that I hook to a ground rod if one is available. It is not an ideal situation. I have to wire it hot. It does allow me to get to work. I put my connections on the line side of the Air conditioner disconnect and leave the load side off so the air conditioner won't run while I am stealing power from one side.

  7. #7
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    Quote Originally Posted by Maurice Mcmurry View Post
    Re-wire the lathe and supply circuit with 4 conductor wire and plug so you have a neutral and a ground. Protect your 120 v tap with a small fuses?
    This would be my solution.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  8. #8
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    Buy a 120/240 "control transformer" from the usual suspects. Buy some 240 volt LED bulbs for the task lights.
    Bill D

  9. #9
    Do you have a 3 wire or 4 wire connection to the lathe?

    If 4 wire, there is no problem connecting the tach between one of the hot legs and the neutral - that's what's done to power the light bulbs in ovens and electric dryers, for example.

    If 3 wire, while you can get 120V between either hot and ground, you shouldn't, as the ground is only intended for fault currents. The schemes you suggest (resistor, light bulb) are scary and you should forget those.

    If 3 wire, there are two good paths:
    1) The tach has an AC/DC converter in it to power the electronics (the electronics run at low voltage DC). That converter may already be "universal input" (meaning able to take anywhere from 90-264VAC input). If you investigate, you might find that you can just connect the tach input to 240V, as-is. Or, you could replace the AC/DC converter with one that accepts 240V input and produces the same DC output (e.g., 5VDC) (this is what I'd do, if I were you).

    2) You can buy 240V-120V transformers. Here's an example: https://www.elecdirect.com/control-t...20-240-50-60hz
    Last edited by Dan Friedrichs; 06-16-2024 at 12:18 PM.

  10. #10
    For my 20" Rockwell wood/metal bandsaw I used a 1 KVA 240X480V pri, 120/240V sec transformer for 120V control power & for a work light, wired 240V in 120V out, and grounded one of the secondary conductors which became the neutral, it was a surplus SQ D, although the saw has seen no use until a lubricant issue for the gearbox is settled, it works fine, the other way would be to use a 4-wire 125/250V plug & receptacle to supply a neutral for 120V accessories, in my case it could have been done with ease with that method with a existing short conduit run, been easy to repull conductors & change receptacles, but for anyone contemplating that with circuits over 20A plans will need to be made for overcurrent protection for the 120V circuit.

  11. #11
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    Lots of good suggestions on how to correct the problem electrically. Now, I will, at my peril, give my workaround.

    This starts with, "who really needs a tach?". I just set the speed on my 20" General VFD to a comfortable setting for the task at hand and really don't care what the "number" is.
    If you really need to know a number, then set up your tach with the extra 120V power source and make a graph/table of the high, low and mid-point speeds of your speed pot setting. Do this for each belt setting if applicable and maybe put a typical blank on the lathe for a typical load. When you want to know a number refer to your table.

    BTW, the General has no tach but it did have a metal table attached giving the range of speeds for each belt setting.

  12. #12
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    Im with the 4 wire people. The 4 wire should do what you need to do as described above. If you have this run 3 wire it would be cheaper to rewire than to replace anything else.


    how was the original tach run? Was it seperate from the vfd/motor? Can your vfd display your rpm?

  13. #13
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    I have vfd and no tach on my drillpress. I know 60 hz is rated rpm. 30 Hz is 1/2 that and 90 is 50 % higher. Everything else is just an approximation. good enough for me.
    Bill D

  14. #14
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    Quote Originally Posted by George Yetka View Post
    Im with the 4 wire people. The 4 wire should do what you need to do as described above. If you have this run 3 wire it would be cheaper to rewire than to replace anything else.


    how was the original tach run? Was it seperate from the vfd/motor? Can your vfd display your rpm?
    The VFD does display the RPM, but it is on the back facing away from me.

  15. #15
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    Potential problem with VFD tach is that will display motor RPM which may not be tool/spindle RPM if going through pulley system although this would be easy to calculate a fudge-factor for belt settings.

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