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Power issues with power drill physics help needed please

  1. Jun 17, 2010 #1
    We are rigging up a power cord for a cordless drill for work. This is necessary because the batteries are starting to die on some of the models. We rigged it up so we can plug the drill into a variable DC power supply. We can control the voltage and amperage output on this power supply. When we match the batteries amperage and voltage output to the dc power supply the batteries power seems more instantaneous. The battery power causes the drill to start instantaneously with full power while the power supply takes more time to reach to the same RPMs as the battery. Also, the power supply does not supply the same torque as the battery did, this was seem while screwing in a screw and the battery was able to screw it all the way into a board while the power supply could not make it all the way. Then we decided to install a capacitor to the power input to increase the response rate, but to no avail. The voltage of the battery is 2.4 V and we have cranked the amps and the voltage up to try and get the same response to no avail...any help in describing why this happens would be greatly appreciated. I know this is a physics problem because the wiring done it correct.
     
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  3. Jun 17, 2010 #2

    K^2

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    Use multimeter to check voltage and current from battery and power supply. My first guess would be that power supply is not providing the same current, but multimeter will tell you for sure what's happening.
     
  4. Jun 17, 2010 #3
    Tried that the multimeter reads the same voltage and amperage but the amps diminish quickly over time. The power supply is supplying the same voltage and amps. Its interesting...i am not sure what it could be. I dug out my physics book, but still have yet to find an answer
     
  5. Jun 17, 2010 #4

    K^2

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    Current depends on RPMs of motor (given fixed voltage). Torque depends on current. So yeah, as the motor spins up, the current drops. It's the peak current you are interested just as the drill starts, which might be difficult to measure.

    Here is what you can do. Try measuring current while the drill is actually doing some heavy work, like driving a tight screw. Since you are saying that battery performs better at this, you should be able to see the difference in current.

    Have you tried increasing the setting for current limiter on the power supply? If the drill is working properly, simply applying the right voltage should result in the correct amount of current. Of course, if the drill gets stuck, the current will spike, and that can burn out either the drill motor or the power supply. Since battery was connected without any limiter, it appears, the danger to motor is no greater than with battery.
     
  6. Jun 17, 2010 #5

    russ_watters

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    Batteries don't have an amperage, only a voltage. The amperage is whatever the drill wants it to be. The problem is almost certainly either:

    -Either you made a typo or are supplying the wrong voltage. It could not be 2.4 volts. Most common iirc is 18.
    -The. psu doesn't have enough power/amperage capacity to supply what the drill demands.

    Could we have the make and model of the drill so we can verify the specs?
     
  7. Jun 17, 2010 #6

    K^2

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    That's not true. Any battery will have an internal resistance, which does limit current. Maximum possible current is always battery's own voltage over own resistance.
     
  8. Jun 17, 2010 #7

    russ_watters

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    While I certainly simplified the matter and am aware of internal resistance, my explanation was more true and relevant than yours due to the particulars of the OP's problem. A battery has a nominal/rated voltage printed on the side of the battery, but no nominal/rated amperage except in rare cases such as CCA on a car battery. Certainly the battery pack of a cordless drill has no rated amperage output.

    When you use a power drill, the batter pack provides near-constant voltage with an amperage that varies with the torque and speed of the drill, but never comes anywhere close to the short-circut amperage of the battery..... so that amperage is utterly irrelevant to the OP's question.

    There is a clue in the OP as to what the issue is:
    I'd wager this quote a misunderstanding of the amp-hour rating of the battery pack and as a result, the OP is using a power source incapable of providing the necessary amperage to the drill.

    My 18V cordless drill uses a 1.2 amp-hour battery. If you use a regulated 1.5 amp power supply, it won't perform as designed. With a continuous load, you would probably only get 10 minutes of battery life out of it, which means it draws somewhere around 7.2 amps. So at the very least, I'd want a 10 amp power supply for it.
     
  9. Jun 18, 2010 #8
    How much you willing to wager? I was referring to the draw the drill pulled on the battery, which we measured with a multimeter. The drill operated much better with 30 Amps being available to the drill with 3.5 volts. The issue still stands however with the torque issue. Not too sure why the drill will start to for a lack of words die when connected to the PSU, but the battery will supply a more consistent torque. Also, there is still the issue with the start up time. The battery supplies a more instantaneous supply while the PSU takes time to "warm up" but the supply of current is constant. This power supply is an above average PSU in which we use to test electronic equipment (airplane lighting, police bars, ect..) I know the difference between amp hour and amps being drawn, and I am using a PSU that is more than capable for the job.
     
  10. Jun 18, 2010 #9

    uart

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    Well you really should have said all of that in the original post. Russ is correct, the torque problem is due to lack of current, I guarantee it. The measurement of the peak (transient) current may have been difficult with the equipment you had at hand. The short circuit battery is not limited to 30 Amps, and there in lies the difference.
     
  11. Jun 18, 2010 #10

    K^2

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    At this point, I'd check power rating of the drill itself, divide that by voltage, and see what you get for current.
     
  12. Jun 18, 2010 #11

    russ_watters

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    ...and I'm still confused about the very low voltage for the drill. Could we have the make and model? 30A at 2.4V is only 1/10 horsepower. That's a pretty small drill with an unusually high amperage and low voltage.
    I'm wrong sometimes, but jeez, there is an awful lot still unclear about what you have and are doing here.
     
    Last edited: Jun 18, 2010
  13. Jun 18, 2010 #12
  14. Jun 18, 2010 #13

    russ_watters

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    Perhaps....but that just adds to the confusion. Why say "drill" if its a screwdriver? And 30A would be an awful lot for one of those too.
     
  15. Jun 18, 2010 #14

    DaveC426913

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    Once the OP gets an answer, I'd like to add on to this; I want to do a similar thing.

    I want to put a power drill on my boat. It has a standard 12V deep cycle battery, (recharged with an 11W solar panel). It's got to run off this because there's no point in leaving a drill on the boat that is not rechargeable.

    I want to know if this is feasible. I suppose I have the luxury of choosing what size drill will work best before going and getting one from a second-hand store.
     
  16. Jun 18, 2010 #15

    K^2

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    Could you explain why you need a drill on the boat? The power consumption and torque depend on RPM and applied voltage of the motor. So without knowing an application, it's absolutely impossible to tell what sort of currents you should be expecting.

    I'm just trying to picture a use for a drill on the boat, and other than quickly scuttling it if Coast Guard catches up with you, I come up with nothing. I'm sure it's just failure of imagination.
     
  17. Jun 18, 2010 #16

    DaveC426913

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    Repairs underway. Improvements at mooring. Being owner of a sailboat, I am continually tinkering with it.

    I suppose it is significant that I do not dock my boat, I moor it - which means it is a hundred yards from land and the nearest power source.

    I don't want to have to sail it to the dock everytime I need to drill holes for new hinges on a lazarette.

    My original plan was to stow a hand-drill in my toolbox (store reps don't even know what a hand drill is. When I ask about them, I am directed to the cordless drills.)
     
  18. Jun 18, 2010 #17

    K^2

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    Heh. Last time I used a hand drill was almost two decades ago. I wouldn't know where to start looking for one either.

    I suppose, if the battery isn't enough to bring the bit up to speed fast enough, you can always attach a capacitor in parallel.

    So what you really need is to make sure that the continuous current provides sufficient torque when the drill is actually doing work. If you can find maximum continuous current/power for the motor, that'd be the best way. If your battery can provide that much current, and you also add a capacitor in parallel, you should have absolutely no trouble. Unfortunately, this is not information you typically find on a drill.
     
  19. Jun 18, 2010 #18

    russ_watters

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    Your situation is much simpler since you are still running it off a battery. The main issue of concern is the amperage capacity of the boat's wiring.
     
  20. Jun 18, 2010 #19
    If you get a 12 volt rechargable drill you could connect it directly to your 12 battery supply voltage (actually 12.5 to 13.8 volts)
     
  21. Jun 18, 2010 #20

    DaveC426913

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    I am not sure how my situation is different from the OP. But maybe I didn't read between the lines of what he's doing.

    That was the plan until I came across this thread and started to second-guess it.
     
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