Soft-starting DC motor - advice needed

Discussion in 'The Projects Forum' started by djgyro, May 2, 2011.

  1. djgyro

    Thread Starter New Member

    May 2, 2011
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    Hi all, fairly new to messing about with electronics so please bear with me [​IMG]
    I need a control device to in effect 'soft-start' a 24v DC permanent magnet motor rated at 375w and 22.5A. It doesn't need to take a set amount of time, just prevent the motor cooking the batteries on start or overtorquing the drive components.
    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 or whether it will do what I require.
    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' a DC motor of this size?

    Rgds, Ben
     
    Last edited: May 2, 2011
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Is the 24v motor actually going to draw 22.5 Amps all the time when running? Or in other words what is the typical and max running currents when it is at 24v?
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
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    24V @ 22.5A = 540W. That aside, the PWM won't be a problem but designing the drivers to handle 22.5A will take some care.
     
  4. djgyro

    Thread Starter New Member

    May 2, 2011
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    I wondered about the motor power rating and rated amps but never reached a conclusion as to why that was.

    Model 110ZYZT101-110ZYZ156: Torque1274-2230(mN.m), Speed1500-3000(r/min), Power245-700(W), Votage24-220(V),Current1.5-45(A)and Speed Tolerance100-200(r/min).

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

    Above is the link for the motor, because I haven't sorted out any control gear for the motor I can't see what the running current will be. I dont think the load will actually be 71Nm but need to make all the associated electrical and mechanical components with enough capacity. I will fuse the electrical supply at 30 amps, so am hoping I can base everything on that.
     
  5. djgyro

    Thread Starter New Member

    May 2, 2011
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    No idea what the start and running current will be because until I have some sort of control gear on the motor I don't want to risk damaging the drive or batteries. The website states 12vDC PM 375w and 22a but I appreciate these figures don't tally - I just don't know enough to understand why this is! I've included the motor url below.

    I'm using the motor to shift a 600kg load which I've calculated will require about 20Nm of torque (for initial acceleration) but would like the set-up to have capacity to move 1200kg. Not sure how the extra load will affect rolling resistance and therefore torque required though.

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

    My basic understanding is that the most torque will be required to accelerate the load from rest to running speed and the motor speed will obviously therefore be low with high torque and therefore high current draw. Given that my force calculations are for acceleration I presume the motor will still not draw current at its rated torque?
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Back in the good old days, before high-power permanent magnets were available, a motor this size would have a wound-field stator, and to start a heavy load they'd switch on with series excitation and maybe cut over to parallel excitation once the load was moving.

    Now yes, you'd most likely do it with electronics. Maybe you could try a bunch of series resistors, to be shorted out as the speed builds up? It's a crude way to do it, though.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    Doing so would seriously rob the motor of starting torque. Besides that issue, it's inefficient and extremely (I think this statement is redundant).... wasteful. :rolleyes: You'd be burning up power in the form of heat dissipated from the very large power resistors.
     
  8. djgyro

    Thread Starter New Member

    May 2, 2011
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    I had come to the same conclusion with regard to resistors. What about using a 555 timer circuit and solenoid to 'hammer' the supply? That would be extremely crude but might provide a step to increase to say half-speed first? I rang RS today who (do) make a circuit that does what I want but its out of stock and they aren't sure if it will ever come back into stock.......

    Rgds, Ben
     
  9. gerty

    AAC Fanatic!

    Aug 30, 2007
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  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    BTW, we have members who are well versed in the use and application of high power MOSFETS for PWM.

    The attached schematic is not really a schematic in the true sense. It's meant to be a conceptual diagram of what I believe your requirements to be. It intentionally lacks any detail.
     
  11. djgyro

    Thread Starter New Member

    May 2, 2011
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    Gerty, just visited the product page and have emailed them so thanks for that.

    CDRIVE, yes that appears to be what I am aiming for although I suspect a true circuit diagram would be immensely more complex than the simple solution that appears to be!

    There is one thing - i've learned more about DC and PWM in the last 2 days than I thought possible!
     
  12. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can't design the entire system from scratch when the only real electrical spec you have is the motor rating!

    I would hook up the motor somehow, even with a large series resistor (like a Jug element) etc or large variable DC power supply if you have one and see how many volts and amps it takes to start the load moving, how many volts and amps to keep the load moving and test some movement speeds etc.

    Then for the final application I would probably use a PWM driver controlled with closed loop feedback based on motor current (to control start torque). If it also has closed loop voltage regulation that will set your top speed once the load is moving.

    Check out some of the "combat robot DC motor driver modules" from the big robot stores, many brands will handle that size motor and they usually have trimpots to set the maximum current (start/stall torque), and max voltage (max speed).
     
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