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OT- I need suggestions for a DC motor soft-start

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  • OT- I need suggestions for a DC motor soft-start

    I need to 'soft-start' a 24VDC high current motor which directly drives a hydraulic pump. Ramp up from 0-full should be between one and two seconds. The motor is fused at 150A and I would like to build a circuit that can handle that same current or at least 120A.

    The application is a custom man-lift and right now the motor spins up nearly instantly and jolts the rider pretty good starting up.

    Please, no counsel or what-ifs on liability, just stick to the question. If it might be better or easier to modify the hydraulics I'm all ears. A simple restrictor won't work though, because the lift rate after start is perfect.

    Thanks,
    Gordon

  • #2
    One big-ass capacitor with a drain resister might do it. Otherwise, I think I'd add a small air-gap tank on the hydraulics side. There will be a slight delay in full pressure as the air is compressed. The tank will have to handle the pressure but only needs one inlet--Like a welding tank. Oil goes in from the bottom and compresses the air in the top of the tank.

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    • #3
      Would an accumulator like this help? http://www.surpluscenter.com/Hydraul...C-P-9-9637.axd
      When I get Time... I'll...

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      • #4
        It does sound as if a hydraulic side approach is more direct and simple. (and has fewer liability issues).

        The one question about an accumulator deal is whether that would lead to an undesirable "springiness" in the system. The air might have that effect, and the amount of restriction you can apply to the tank as a damper system might be pretty limited if it is supposed to work well.
        1601

        Keep eye on ball.
        Hashim Khan

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        • #5
          That example only goes to 1000psi. I'd make one out of a hydraulic cylinder. Oil on one side and cap the other.

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          • #6
            Needs to be either a diaphragm,bladder or piston accumulator.It's not good to mix a high pressure air volume with high pressure oil.The three types I mentioned all have a barrier between the gas and oil and behind that barrier is Nitrogen so no chance of a loud bang.

            https://www.ebay.com/itm/PARKER-AD03....c100754.m4842
            I just need one more tool,just one!

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            • #7
              Thank you gentlemen. I believe the accumulator idea could work, however space is severely limited. The uniqueness of this lift is its ability to maneuver into very tight spaces and it has a very small footprint. The current motor control is a simple solenoid switch mounted on the motor exactly like an automotive starter. Might there be a way to ramp-up a current/voltage control device capable of doing what I need elecrically? I can think of easy ways to generate the signal, but I am not familiar with how to manage such high current. I believe that many power-transistors can be paralleled but I don't know how to manage load distribution between the transistors.

              I will look into the feasibility of fitting an accumulator though.

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              • #8
                Originally posted by J Tiers View Post
                It does sound as if a hydraulic side approach is more direct and simple. (and has fewer liability issues).

                The one question about an accumulator deal is whether that would lead to an undesirable "springiness" in the system. The air might have that effect, and the amount of restriction you can apply to the tank as a damper system might be pretty limited if it is supposed to work well.
                Great points, Jerry. Springiness would be bad, and it seems to me that 'tuning' the accumulator idea could be difficult in part because the riders weigh between 130 and 250 lbs.

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                • #9
                  You could do it electrically, yes. The size, for that current level, will likely be larger than an accumulator approach.

                  It sounds as if you are really trying to mitigate the abrupt acceleration, and do not really need a lot of slow start. A fairly small accumulator might convert a sharp starting acceleration into a more acceptable start, without being much if at all larger than a pop can or at worst, a malt liquor oversize can.

                  The electrical approach I would expect to be about the volume of a malt liquor can, at least, just due to heatsinking and electrical connections.
                  1601

                  Keep eye on ball.
                  Hashim Khan

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                  • #10
                    Ok. I can get creative with heat-sinks and airflow if the circuit is not horribly complex. There is a space about 10" cubed, but most of it will be taken up by the yet-to-be-installed battery charger.

                    I've got to hit the sack so I won't be responding until tomorrow, but please keep the ideas coming!

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                    • #11
                      Start with a resistance in series with the motor and short the resistance when the motor is running.

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                      • #12
                        Springiness won't be a factor if the volume of the accumulator is kept small.
                        I just need one more tool,just one!

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                        • #13
                          Originally posted by CCWKen View Post
                          That example only goes to 1000psi. I'd make one out of a hydraulic cylinder. Oil on one side and cap the other.
                          Or even better a coil spring on the other,no problems with compressed air and the spring could be sized so that it compresses fully before max system pressure is reached.
                          I just need one more tool,just one!

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                          • #14
                            Given that It's DC and runs a pump, it should be able to use a PWM controller to start it up slowly. Normally a PWM at lower speeds would introduce some cogging, but the fluid should provide enough buffer that the pulsing will not be noticed.

                            If that works, all you need is to be able to control the PWM controller.

                            Dan
                            At the end of the project, there is a profound difference between spare parts and extra parts.

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                            • #15
                              Originally posted by wierdscience View Post
                              Springiness won't be a factor if the volume of the accumulator is kept small.
                              I was seeing the thing as a side tap to the pressure line, potentially with some sort of a restriction on its input to damp any springiness. When pressure comes up, it gets partially relieved into the accumulator. That volume of hydraulic fluid does NOT go to the cylinder, so the start is slow, but the pressure will come up to normal.

                              The issue than, is the restriction on the input, so that it will be open enough to relieve the pump pressure fora bit, but not so open as to either fail to damp the springiness, or to end up as just a delay on the start. With the proper restriction, the pressure will come up enough to begin movement, but the accumulator will still be taking fluid, so the actual movement will be less. Might want an adjusting valve on the connection to get it right.

                              The PWM would do anything you want, but is quite a complicated solution. There are high current controllers, from companies like Curtis http://curtisinstruments.com/product...t-controllers/ but I think they will be both overkill and expensive.

                              An accumulator system is likely to be fine for the need, small, and far less expensive.
                              Last edited by J Tiers; 12-03-2017, 01:41 AM.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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