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Fuse Question

  1. Dec 5, 2012 #1
    I have an application where I am using a 24V DC motor to drive a ball screw. If someone isn't paying attention and they hit the mechanical stop on the ball screw, the current is going to spike to about 200 Amps. The normal running current is about 35 amps, and the max current is 60 amps.

    I'd like to put a fuse between the motor controller and the motor. If possible, I'd like it to be a resetable fuse like a circuit breaker. I'm currently using an automotive fuse, but after looking at the data sheet, this 60A fuse will need to see 200A for 5 seconds before it breaks. I'd like the fuse to blow as soon as the current exceeds 60 amps, but I can't seem to find what I'm looking for. I'm not an electrical engineer so maybe I don't know enough about fuses and how to read the Time-Current Characteristic Curves. Hopefully you guys can help me.
     
  2. jcsd
  3. Dec 6, 2012 #2
    I don't want to pretend I know about fuse, I think you might have better luck looking into circuit breaker rather than fuse. I looked around a little, they don't tell how fast it will react. But I think you are going to have to read and call them up to find out. Here is Tyco

    http://us.telecomosp.com/pdf/1773105_05.pdf

    Also

    http://www.perfectswitch.com/downloads/Circuit%20Breaker.pdf

    Just one to start with, call them. Look for more, I would look more into solid state circuit breaker as the good old temperature breaker are too slow. Sorry I can't help you any further.
     
    Last edited: Dec 6, 2012
  4. Dec 6, 2012 #3

    NascentOxygen

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    Staff: Mentor

    But the starting current may be considerably more than 60A, much more if it is starting under load. So you may go to all the trouble of fitting a fast-response breaker only to find it is too fast (and the trip level of 60A too low) and that consequently the motor never gets started. An oscilloscope may provide the means for examining the amplitude of the typical starting current, allowing you to gauge the level of current at which the CB should be set to operate.
     
  5. Dec 6, 2012 #4
    You could fit a microswitch or similar that will trip the motor before they hit the mechanical stop.
     
  6. Dec 6, 2012 #5
    Ball screw motors need high torque and this would have a pretty high inrush current at start up. When they mechanically stop the motor is in Locked Rotor case, so typically pretty close to the current level of when you start ( I'll bet you see more then 60 amps when the motor is started). The Start current in practice may actually be as high or even higher as the locked rotor current in many start cases the windings are cold. If the motor has a locked rotor current in the rating - that is the number you should be designing around.

    So you need a Slow Blow fuse - and it sounds like the automotive one you are using is this type, for a circuit breaker - you need to look at the trip curves - perhaps allow 1 Sec at 200A and will trip in 2-3 Sec at that current.

    But I agree with Jobrag - the real fix is to shut the motor down 1st. A mechanical stop on ball screw is not just causing current problems - but causes high mechanical stress in the system - the full torque of the motor and any rotational inertia in the system are also causing damage - this is referred to as an uncontrolled system stop - and is not good industrial design.

    So in addition to the Fuse - I would find a way to get an electrical stop to be more convenient to the operators than the mechanical stop, or make the mechanical stop less convenient....
     
    Last edited: Dec 6, 2012
  7. Dec 6, 2012 #6
    I was thinking about this, the approach is not correct. You are using a +24V source for the motor. The solution is not the fuse, it's the power supply. You need to get a power supply that has current limiting.............Which.........Most regulated supply has that build in. It is pretty much instantaneous. There are easy ways to do current limiting in electronics and all power supplies that I encountered have that build in.........Not to make your life easier, but to protect their products!!!

    Look for the right power supply.
     
  8. Dec 6, 2012 #7

    jim hardy

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    2016 Award

    indeed you want this thing convenient.......

    i'd call it a bad design to use the mechanical stop to stall the motor
    even garage door openers use limit switches to prevent that.

    A supply with "Foldback Current Limiting" will give the resettable characteristic you want, so search on that term.
    Myself i'd make that be the backup to the normal position control mechanism.

    old jim
     
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