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Lathe project -- About DC drives

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  • 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
    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.
    3751 6193 2700 3517

    Keep eye on ball.
    Hashim Khan

    If you look closely at a digital signal, you find out it is really analog......

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    • #3
      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)

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      • #4
        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.

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        • #5
          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.

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          • #6
            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.

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            • #7
              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.
              Chilliwack BC, Canada

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              • #8
                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!

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                • #9
                  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.
                  3751 6193 2700 3517

                  Keep eye on ball.
                  Hashim Khan

                  If you look closely at a digital signal, you find out it is really analog......

                  Comment


                  • #10
                    Try a Consew servo.

                    Comment


                    • #11
                      Originally posted by briuz View Post
                      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!
                      KB models carry on running but at the set current limit, or if highly loaded go into reduced rpm or stall if current limit met.
                      There are two versions, SCR bridge or the little more superior PWM types.
                      Max.

                      Comment


                      • #12
                        Originally posted by elf View Post
                        Can anyone that has one of these post the size & approx weight of just the motor please?
                        Milton

                        "Accuracy is the sum total of your compensating mistakes."

                        "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

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                        • #13
                          Thanks for all the feedback! I have a 3/4 hp frame type 48 3 phase motor coming and will explore that option with a VFD.

                          I will also keep the KB DC drives in mind but I have a concern about how easy it was to stall that 1/4 HP DC motor.

                          Comment


                          • #14
                            Originally posted by DICKEYBIRD View Post
                            Can anyone that has one of these post the size & approx weight of just the motor please?
                            Not including the pulley and shaft, the motor is approximately a 4" cube. I'd have to uninstall it to get the weight. It's been running on my rose engine for over a year. There are quite a few accounts of internet posters using them on lathes.

                            Comment


                            • #15
                              Originally posted by elf View Post
                              Not including the pulley and shaft, the motor is approximately a 4" cube. I'd have to uninstall it to get the weight. It's been running on my rose engine for over a year. There are quite a few accounts of internet posters using them on lathes.
                              I think they would work great,I have a older style brush type still in box that can be reversed easily.I thought someone posted about the brushless style was a little complicated to reverse direction.

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