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Thread: Lathe project -- About DC drives

  1. #1
    Join Date
    Jul 2019
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    4

    Question Lathe project -- About DC drives

    Hello,

    I am working on a special small CNC turret lathe and have it mostly built. However, I've just discovered that using a DC drive with a 1/4 hp DC motor may have been a bad idea. (I chose a 1/4 HP DC motor because Sherline uses a 1/12 HP DC motor and it works quite well....for it's size. I also wanted to fit a small motor in the case.)

    The problem is that I have an Iron Horse DC motor controller from Automation Direct. When the current limit is exceeded, it doesn't just limit the current (and slow the motor some) it shuts down the drive entirely. (Sherline motors just slow down, they don't have this problem as the drive is different....which was why I never considered this to be a problem.)

    I am thinking that I should be considering changing the motor out to a 3 phase motor and use a VFD so I can avoid this problem. Unfortunately, it's a huge change at this point, but it may be necessary. So, the other issue is that most motors, in this class, are of a type 56 frame type and I'd really like to use a smaller frame such as a type 48. (I think my Tormach uses a type 48.) Finding those motors is a lot harder, though.

    Has anyone run into similar problems with motors, know of a DC drive that will just limit the current and not shut down, etc.?

    Thanks!

  2. #2
    Join Date
    Jan 2004
    Location
    Missouri
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    Many VFDs also have the "cut the wheels off the car" overload protection also. Some do not. Find out which do not before you buy.
    1601

    Keep eye on ball.
    Hashim Khan

  3. #3
    Join Date
    Jun 2007
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    Anderson SC
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    1,507

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    A lot of people out there have used treadmill motors and controls for such things. The motors are quite a bit smaller than a nema 56. I have one on my drill press and it works great. Generally speaking though, the horsepower is usually quite a bit more than you would like. (1-3 being common)

  4. #4
    Join Date
    Aug 2010
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    Flint, Michigan
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    715

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    On a small machine with proportionately sized chips, a 1/4 horsepower would work. But unless you are using gearing to change motor speed to work speed, you will only see a fraction of the 1/4 horsepower. 1/2 speed is 1/2 horsepower, for example.

  5. #5
    Join Date
    Mar 2013
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    Hightstown, NJ
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    I have had good results with KB dc drives. Used one running under current limit continuously pulling in a large hose on a reel which changes diameter and speed.

  6. #6
    Join Date
    Jul 2006
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    Dracut, Massachusetts
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    I'd second that you check out the KB DC drives. If the project warrants the effort many of them can be set up with tachometer feedback that will allow the drive to maintain the speed over a very wide RPM range. It's not all that difficult to set up, really. I did this on a vertical bandsaw, using a small DC motor as a tachometer generator, and can vary the speed from about 150 rpm to 1700 rpm and virtually never stall the thing at any speed.

  7. #7
    Join Date
    Dec 2015
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    Chilliwack, BC, Canada
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    Perhaps double check the manual to see if the overcurrent action is something settable? For a motor controller there are times where the user would want one or the other so I could see a jumper being used or a selector switch on the board to allow the user to select the desired option.

  8. #8
    Join Date
    Jul 2019
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    4

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    I checked with the vendor and the overcurrent action isn't settable....only the overcurrent itself.

    Do the KB DC drives shut down if the current limit is exceeded or do they keep running but at the current limit?

    I have an encoder for the spindle...

    Thanks!

  9. #9
    Join Date
    Jan 2004
    Location
    Missouri
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    31,565

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    The VFDs that have the feature will hold the current under the trip point, and keep going.

    Ones that don't, they just shut down and require a re-start.
    1601

    Keep eye on ball.
    Hashim Khan

  10. #10
    Join Date
    May 2011
    Posts
    1,265

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