Need a current surge limiter
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  1. #1
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    Default Need a current surge limiter

    A customer asked me to modify a magnetic base drill. This is the type you can mount on a steel plate and drill like it was a drill press. They were using it to drill and tap 1/2-13 holes. The problem was that it did not reverse so they could tap, but then had to release the tap from the chuck and unwind it by hand. The motor is a series wound so called universal motor with a commutator and brushes. Reversing was only a matter of running the brush leads to a DPDT switch with a center off position. The problem is that an operator can throw the switch through off, to the reverse position, meaning that the back EMF is now in series with the supply, causing a large flash from the brushes. You can't trust employees to let the motor stop before reversing, so I need either a timer to delay or a surge limiter that will not allow the large current. I have solid state timing modules I can put in series with the brushes, but running on AC, it would be possible to switch at a zero crossing and not giving the module time to react. I also have a size limitation. They would like to have the device mounted on the drill. A cursory search hasn't turned up anything. I can make an inductor that will limit current, but there would always be some voltage drop.

    Bill

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    I have seen DT switches that would mechanically lock when moved from one side to the central, off, position and needed to be pushed a touch back again to unlock and the move to the other side - in effect forcing the operator to delay the reversing and to avoid accidental switching. This will be the ideal solution for you as there is no need for any additional parts or electronics. I am not sure what is the proper name of a switch like this and if still made today. The mechanism to achieve this action is quite simple so it might be possible to add it on top of the existing toggle if a commercial one cannot be located.

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    In fact I seem to have one of those switches. Here it is already wired for reversing. Made by A-H&H, though I am not sure if still in production.

    hh.jpg

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    Maybe a Metal Oxide Varistor (MOV), the things they put in surge suppressors? They are available in various shapes, sizes, and ratings. The problem with MOVs, though, is that they "wear out" after they have eaten some (fairly large) number of voltage spikes. When they're done, they fail shorted, so circuit protection is a must. Just a thought. Don't know if they'll fit the bill (but they're cheap).

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    I have a couple of the same switches but they would not fit in the space available. The designers of the drill allowed enough space in an aluminum casting for their components, but no more. I had to cut out a lot of a sidewall to get the new switch in. I haven't looked for smaller versions of the one you show because it takes several seconds for the drill to stop and some people would actuate the switch, even with the stop in the middle, in much less time. If I were doing this for use by the shop owners, there would be no problem, but it will sit on a shelf where anyone can use it. I know you can't totally idiot proof things, but we need to do the best we can. The cavity that holds the electrical controls is completely full, no extra space, so whatever I do will have to be on the outside, meaning the unit will be wider, limiting its ability to get into tight spaces.

    Thanks for the effort.

    Bill

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    For a mechanical solution, I'm thinking a movable blockade, that allows the switch to move halfway, but must then be moved over to allow it to move the rest of the way. Maybe set up so when you move it to the REV position, you can move it too far, so that it blocks the lever and will not switch to either "run" position. That way the operator has to be careful in order to allow the reverse, and will not be able to do it quickly.

    I do not really like that solution, because it is mechanically clumsy, and probably has a safety hazard.

    A "soft start", that resets when power is cut, would slow the start, and could be made to operate the same in either direction easily, as it could be wired in the power cord, and not on the drill itself.

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    The obvious choice for me would have been a tapping head mounted in the mag base drill press which would have made power in & out a cake walk. I'll hazard a guess there wasn't room for this option.

    Stuart

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    Quote Originally Posted by atomarc View Post
    The obvious choice for me would have been a tapping head mounted in the mag base drill press which would have made power in & out a cake walk. I'll hazard a guess there wasn't room for this option.

    Stuart
    Correct. The Jacobs chuck barely clears the magnet and associated housing.

    Bill

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    Quote Originally Posted by JST View Post
    For a mechanical solution, I'm thinking a movable blockade, that allows the switch to move halfway, but must then be moved over to allow it to move the rest of the way. Maybe set up so when you move it to the REV position, you can move it too far, so that it blocks the lever and will not switch to either "run" position. That way the operator has to be careful in order to allow the reverse, and will not be able to do it quickly.

    I do not really like that solution, because it is mechanically clumsy, and probably has a safety hazard.

    A "soft start", that resets when power is cut, would slow the start, and could be made to operate the same in either direction easily, as it could be wired in the power cord, and not on the drill itself.
    Something that stops the travel and requires a significant amount of time would do the job, but I agree that it would likely be clumsy and there is a certain personality that would make a game out of beating it. A soft start would be perfect except that it couldn't be on the cord because it is also supplying magnet current. Even something that limited to normal starting current would be fine. I think what is happening is that the back EMF gets added to the supply voltage because the armature is now hooked up backwards and the cores may well be saturating, sending the current even higher.

    While typing, it occurs to me that a saturable reactor with the DC control current adjusted to make it pass the starting current would do the trick, but it might be too large to mount on the drill. I could put it in s box like a wall wart and add an extra wire to the cord to separate the magnet supply. That way the operator could do what he wanted to the switch and it still would only draw the rated current.

    Bill

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    Quote Originally Posted by 9100 View Post
    ..... A soft start would be perfect except that it couldn't be on the cord because it is also supplying magnet current. ....

    Bill
    Well, that should have been obvious, considering that I have used and fixed mag drills.......

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    Add a fuse holder and give a manager a 25 pack of fuses.

    Big warning label: "Allow motor to stop before reversing. See [manager] for a new fuse if you tried it anyway"

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    Wouldn't a 'delay on make' relay wired through only the motor circuit work? I found several very small solid state units but the glitch was the limited ampacity of the device..only 10a. If you could find the proper relay, they're small and could be affixed to the outer case of the drill motor. If you put it in the motor circuit only, the mag feature wouldn't be affected.

    I watched a few videos of folks tapping with a mag drill and they switched from FWD to REV on the fly..no smoke! I'm thinking these were not universal wound motors though, but I don't know. The drill was a Milwaukee brand.

    Stuart

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    Quote Originally Posted by Strostkovy View Post
    Add a fuse holder and give a manager a 25 pack of fuses.

    Big warning label: "Allow motor to stop before reversing. See [manager] for a new fuse if you tried it anyway"
    Exactly how you don't want to solve the problem. By the time the fuses were used up the brushes and commutator would be trash. Besides, that approach ends up with signs all over saying "Do not run if the moon is in the third quarter or the outside air temperature is below 40 F." They had an alleged plant electrician whose fixes were almost that absurd. When he left, I spent a couple of months putting things in usable shape. He left machines where the operator had to push the start button to light up a VFD, then program the speed he wanted and start and stop it on the VFD keypad. I made automatic controls so the operator pushed start to run, turned a knob for the desired speed and pushed stop when done. Then he could walk away without having to do a shutdown procedure. Much of it was not my normal sort of work but they pay promptly and otherwise treat me very well.

    Bill

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    Quote Originally Posted by atomarc View Post
    Wouldn't a 'delay on make' relay wired through only the motor circuit work? I found several very small solid state units but the glitch was the limited ampacity of the device..only 10a. If you could find the proper relay, they're small and could be affixed to the outer case of the drill motor. If you put it in the motor circuit only, the mag feature wouldn't be affected.

    I watched a few videos of folks tapping with a mag drill and they switched from FWD to REV on the fly..no smoke! I'm thinking these were not universal wound motors though, but I don't know. The drill was a Milwaukee brand.

    Stuart
    Such a relay would be fine. I looked through a bunch of them, some rated 15A, which would be plenty. The problem is that the recycle times on the ones I looked at were on the order of 50 ms. You can throw the switch over faster than that. Some sort of trigger pulse lengthening circuit would be needed. A switch with the center off delay like Wlodek showed would do the job if I could find one that would fit the space. Possibly a regular solid state relay with a couple of transistors to shut it off for a couple of seconds. They are thin enough to mount on the side of the drill.

    Bill

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    Does the drill have Variable speed? Normal drill speed would be too fast for tapping, would drill a hole with the tap! If it does maybe you can modify the speed controller.

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    Quote Originally Posted by Froneck View Post
    Does the drill have Variable speed? Normal drill speed would be too fast for tapping, would drill a hole with the tap! If it does maybe you can modify the speed controller.
    It doesn't have variable speed but is geared down to a speed that I would not be afraid to tap with. On the last job they were using a 1/2-13 three flute spiral ground chromed tap. One time someone wanted a hole drilled and tapped in cold rolled. I had an identical tap and after drilling, changed to the tap without changing belts on the mill spindle. When I reached for the switch, the customer jumped back, expecting the tap to explode. Instead, it motored on through, pushing the chip out in front of it. On through holes they can be run at amazing speeds. You just have to be quick on the switch.

    I have been looking at Crydom solid state relays. They don't specifically say that the control input is isolated, but on some of them it appears to be. A one shot multivibrator might be all else I need.

    Bill


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