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Electrical Question - Lathe control

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  • Electrical Question - Lathe control

    I am refurbishing a 14x40 lathe. Not so small that it has direct switching of the motor as you would expect to find on, say, a 9” bench lathe, but not a 10 HP or greater industrial beast either. (I'm guessing it's 2HP but the motor name plate is no longer legible.) It has several contactors for controlling 240V motor power and direction, with 24V coils and an appropriate transformer.

    Although this is an older machine, I was able to get a wiring diagram and it agrees with what I traced out.

    All switching is in the low voltage circuit, acting through the contactors. Reasonable enough. However, this implies that the primary of the transformer is constantly energized. Now, I have always understood that the magnetizing current of a transformer is small (Insignificant? Ignorable?) but this "always on" situation does not seem quite right.

    It appears I have a couple of options:

    1) Live with it, and assume that the primary current draw with no load can in fact be ignored,
    2) Unplug the thing when not in use,
    3) Install a disconnect switch in the 240V line upstream.

    The first option bothers me, the second is inelegant, and the third seems kinda redundant, since there is all this control circuitry on the lathe itself.

    So what I'm wondering is, is it a typical configuration that the primary is continually powered?
    "A machinist's (WHAP!) best friend (WHAP! WHAP!) is his hammer. (WHAP!)" - Fred Tanner, foreman, Lunenburg Foundry and Engineering machine shop, circa 1979

  • #2
    It is common with most machines that the control power is on continuously, but usually a main disconnect switch is fitted.
    If everything is in one enclosure it would be strange that it does not have one?
    If it is 240v 1ph then you would need a double pole, you can get nice 3ph panel mount disconnect switches on ebay cheap, small rotary jobs, Square D, Klockner-Moeller etc.
    Ebay 150672677492
    Last edited by MaxHeadRoom; 12-04-2011, 11:40 PM.


    • #3
      OSHA says your supposed to have a disconnect switch within 25' IIRC of the machine in question so power can be killed for service.

      Other than that the control transformers stay powered up constantly on every piece of equipment I've ever worked on.That said the control circuit is isolated and should be fused anyway.

      It won't hurt to leave it on,or if it makes you feel better install a disconnect switch such as one of these-
      I just need one more tool,just one!


      • #4
        The low voltage control circuit of a magnetic contactor draws insignificant power when the relay coil is not energized. This is because there is no load on the transformer secondary until the relay latches. Even when it is energized the load is still very low. I figure somewhere between 5 to 7 Watts.


        • #5
          Yes, as has been discussed above.

          You can use a plug as a disconnect, or use a breaker if it's within sight of the lathe. However, that's often not a convenient as an "off-on" switch.

          Ignore the transformer load, but a disconnect (off-on) switch at the machine is a good option. For larger lathes, for less then $8 you can get a Square-D 60 amp non-fused disconnect in an enclosure from Home Depot. 2hp lathe? Heck a toggle switch will suffice.