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'Soft starting' a DC motor

Discussion in 'Sensors and Actuators' started by djgyro, May 2, 2011.

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  1. djgyro

    djgyro

    5
    0
    May 2, 2011
    Hi all, fairly new to messing about with electronics so please bear with me
    I need a control device to in effect 'soft-start' a 24v DC permanent magnet motor rated at 375w and 22.5A and don't really know how to go about it.
    If the motor is rated at 22.5A, what will it draw on start-up? To my thinking current will be higher than the run current as with AC?

    SK Pang do a Pololu High-Power Motor Driver 18v25 CS, the link is below. I have contacted them but they are rubbish at replying and various searches on Wikipedia haven't helped me understand anymore about the electronics in the unit.
    Do I need some extra equipment to 'program' this board or is it done with dip switches?

    http://www.skpang.co.uk/catalog/product_info.php?cPath=159_161&products_id=662

    If anyone has other thoughts about what I could use instead I would be very grateful, I assume PWM is the only way to 'soft-start' DC?

    Rgds, Ben
     
  2. poor mystic

    poor mystic

    1,061
    30
    Apr 8, 2011
    Hi djgyro! :)
    Please tell us what kind of load the motor is to drive... just how hard is it to get the mechanism moving?
     
  3. djgyro

    djgyro

    5
    0
    May 2, 2011
    Hi,
    The motor is to move a load weighing 600kgs but I would like it to have capacity to move 1200kgs. With help from a uni-buddy we have worked out that based on rolling resistance and the size of drive-wheel, it will require about 22Nm of torque to move the load. I have included the motor url below. I have asked the supplier for motor performance graphs but they have as yet failed to supply anything, even when they sent the motor!

    http://www.alibaba.com/product-gs/324828797/24VDC_375W_DC_Gear_Motor.html

    It has been pointed out that 375w at 12v is much less than 22a. Any ideas why the two do not correlate?

    Rgds, Ben
     
  4. poor mystic

    poor mystic

    1,061
    30
    Apr 8, 2011
    Yes the current at start-up might be very large, especially if there is significant stiction. I believe (though I am not a motor specialist) that it is during start-up that motors are most often damaged, because there is little back-emf to limit current, as there would be once the motor is moving. So there might be lots more torque required at start-up, and hugely more current.
    I hope a motor specialist comes along to help you with this one.
     
  5. djgyro

    djgyro

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    May 2, 2011
    poor mystic, yes the problem with back emf does appear to be critical to successful application. I am convinced that there is a unit out there that will do exactly what I want to achieve. RS advertise a circuit but apparently it is out of stock and may not actually come back into stock..........which is useful!
     
  6. poor mystic

    poor mystic

    1,061
    30
    Apr 8, 2011
    Hi again Digyro
    It's a well-known problem, that the stiction part of the load cycle comes at the worst possible time.
    I couldn't get anything from the link you posted, but it sounds like you've spent some real money already, and to protect your investment (and get the job done) I think it might be good to consult a mechanical engineer with experience in electric motor driven plant equipment.
     
  7. djgyro

    djgyro

    5
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    May 2, 2011
    Think it will get to that poor mystic. Another electronics forum has suggested a PWM driver that should do significantly more than what I want it to do and is rated at 50amps so I'm going to invest in that, do some trials and see exactly what my start and run currents are and then I will have more defined parameters to either get something off the shelf or custom made.
    Incidentally the SK Pang unit appears to need a module that sets the PWM frequency so I've ruled it out.

    Thanks all for your input, gratefully appreciated

    Rgds, Ben
     
  8. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    The motor will have an effective resistance when under full load of about 1 ohm, If a resistance of about 0.5 ohm is placed in series, then the motor should still start. The resistance could be by-passed with a relay after a suitable time. Alternatively, A powerful MOSFET (IRF540?) placed in series with the motor could be gently turned on with a resistor feeding a capacitor. There will be significant power dissipated in the FET during start up unlike a PWM circuit and it may be necessary to use a heat sink.
     
  9. Resqueline

    Resqueline

    2,848
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    Jul 31, 2009
    Is the load linear or rotating, and what speed is it to attain? If linear load what pulley diameter, and if rotating mass what figure of inertia does it have?
    The 375W is the shaft output (mechanical) power. Divide this by the 528W electrical power that the motor draws and you get a 71% efficiency figure.
     
  10. djgyro

    djgyro

    5
    0
    May 2, 2011
    Ahhh thanks Resqueline, that makes sense, there is a 4-stage epicyclic gearbox between the motor and output shaft, the efficiency loss will be due to this.
    The load is linear and I have calculated that the load requires less than 400N (using several 'worst-case' scenarios) to accelerate the load to its constant speed of approx 1mph. The pulley diameter is effectively 300mm so I calculated the motor would require 60Nm of torque.
    I hope I went about all that correctly!

    To be honest I looked more at the output shaft diameter of 22mm and reckoned allowing for a suitable safety margin, the motor looked 'beefy' enough to do the job!

    Rgds, Ben
     
  11. motronic

    motronic

    4
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    Nov 11, 2013
    SS-1 DC Soft Starter

    I can help you there. But looking at the date of this message I suspect the window of opportunity has well and truly passed. Check out our Web Site and see the icon for 'DC SOFT STARTERS' and it will take you to what you need.
    Regards
    Motonic.
     
  12. johntucker

    johntucker

    3
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    Nov 20, 2013
    Hi Motronic,
    what is the URL of your website?
    I need soft start for a Motenergy ME0708 DC motor - a PWM unit that will allow speed control would be great. Can you help?
    Cheers,
    John
     
  13. motronic

    motronic

    4
    0
    Nov 11, 2013
    Hi John, we have the SS-1 which is a soft start only, with over current etc protection. No variable speed, but we have a variant called "PUP20" that does have variable speed.
    See these at http://www.motronic.com.au/MotorControls.htm
    Cheers mate.
     
  14. johntucker

    johntucker

    3
    0
    Nov 20, 2013
    Hi,
    thanks for your reply.
    I could not find the PUP20 on your website - could you please advise navigation.
    Best wishes,
    John
     
  15. motronic

    motronic

    4
    0
    Nov 11, 2013
    Hi John, it was on the page I linked you, but it's skilfully disguised :)

    It's called an SS-1Light duty variable Speed Controller (Uni Directional) but the order code uses the term PUP20

    http://motronic.com.au/MotorControls.htm just scroll down.
     
  16. johntucker

    johntucker

    3
    0
    Nov 20, 2013
    I found the PUP20 - thanks for that.
    The motor I wish to control is a Motenergy ME0708 at 48 - 52V. It will draw 200amps at full load - 10kW and 800 at startup. I will only need half power. Is there some way I could modify your product - like swapping its power mosfet with an IRFP2907 so that it could work in this situation?
     
  17. motronic

    motronic

    4
    0
    Nov 11, 2013
    Hi John, no, this model PUP20 is not designed for such large currents or that voltage.
    We have another model that can do 48v but not at that current. (SDX50). Sounds like you need to talk to Curtis Controls for your need.
    Cheers mate.
     
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