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Small DC motor run by a super capacitor - Is it possible?

  1. Jan 15, 2016 #1
    I'm doing a project where I need to hand power one small, dc motor (1.5-12V) and charge a 1F 5.5V super capacitor, and then use it to drive another small 1.5-12V dc motor.

    Does anybody have any suggestions for motor specifications or circuit elements that will help me get the most efficiency out of such a system? The amount of energy input into the generator motor is a fixed amount (as in, a certain amount of revolutions as our car rolls down a hill). The drive motor will need to drive a roughly 1.5kg car
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  3. Jan 15, 2016 #2


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    One F, 5.5 V into a 1.5 kg car? Fifteen joules? Operating between 1.5 and 12 V?
  4. Jan 15, 2016 #3
    We haven't selected a motor yet so i dont know the voltage, but i assume itll be on the scale of 1.5 to 12V. The amount of joules is around mgh * 70% efficiency ~ 12-15 joules. We were going to use a gear ratio to charge the capacitor through a "generator" motor as our car rolls down the hill, then use an arduino to switch to the drive motor and use the energy stored in the capacitor. Our problem is just in the numbers and specifics and if its even going to work as i'm a mechanical student and not that familiar with electronics stuff
  5. Jan 15, 2016 #4


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    That gets tough --- "squeezing the last bit of goody" from a sponge that's already been wrung dry.
  6. Jan 16, 2016 #5


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    Most supercaps have a max voltage around 2.3-2.8V so you may need several in series which will reduce their capacity.

    Is your cart allowed to leave some weight behind? eg carry a brick down the slope, drop it off at the bottom, and have the now lighter cart accelerate away?
  7. Jan 16, 2016 #6


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    I think you need to do some estimates for the power consumption and required run time. Some back of the envelope calculations for you...

    Lets say the Arduino and drive motor works over the voltage range 4 - 9V (check because I'm not familiar with the Arduino boards). To store 9V you need 4 x 2.3V super caps in series so the capacity is reduced from 1F to 0.25F.

    So lets say at the bottom of the slope you have a 0.25F capacitor that is charged to 9V. Now we calculate how much current you can draw from it. Firstly you need to know how long you have to run the drive motor? Lets say 10 seconds...

    The Capacitor equation is..
    Q = CV
    I = dQ/dt = CdV/dT

    dV = 9-4 = 5V
    dt = 10 seconds
    C = 0.25
    I = 0.25 * 5/10 = 125mA
    So the max current you can draw is around 125mA for 10 seconds. That will discharge the capacitor from 9V down to 4V in 10 seconds.

    I found someone on the web that said their Arduino draws around 25mA when running code so it appears you would have around 100mA for the drive motor and any interface electronics. Your mpg may vary!

    In the above I assumed that the motor would cut out when the voltage falls below 4V. With careful design that need not be the case. You could arrange for the motor to carry on running even when the voltage falls below the 4V needed to keep the Arduino running, This could increase the distance travelled.

    You also need to consider what happens at the start. If they insist you fully discharge the capacitor then the cart will be nearly half way down the hill before the capacitor voltage reaches 4V. Some way of ensuring the Arduino doesn't power up too early (when the voltage is too low) may be needed.

    I'm sure there are a bunch of practical issues to be considered. Make sure you have something working well in advance of the official event so you can iron out the bugs and optimise everything.

    PS: Life might be easier if you are allowed to use a separate battery for the Arduino.
  8. Jan 16, 2016 #7


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    Since "motor" and "generator" are the same thing for small PMDC motors, you might just use the same device for both. You could even try simply wiring the capacitor to the motor, rather like adding a flywheel to a mechanical arrangement. All you would need is to add a parallel voltage limiter like a Zener to ensure the capacitor could not get charged above its rating.
    If you did power it by hand, it would work like a traditional flywheel ("friction") drive: push it forward several times to charge the capacitor, then release it to run from the stored energy.

    I don't know how efficient this could be. The only limitation I can see, is that you could not use a "one way" gear like a worm.
  9. Jan 18, 2016 #8

    Thanks for the reply. We are allowed to use a separate battery for the arduino. All energy for propulsion must come from gravitational potential energy harvesting

    How would you propose to keep the motor running after the voltage drops below 4V if the motor takes 4V to run?
  10. Jan 18, 2016 #9


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    The motor should work down to nearly 0V although it will get slower as the voltage falls.
  11. Jan 18, 2016 #10
    My motors effectively stop by 1.5v
  12. Jan 23, 2016 #11
    From the description I've seen so far and my understanding, I too think you can do without the Arduino and second "generator" motor...

    If your capacitors are not charged at the top of the hill, scrap the motors all together and the car will go much farther much faster.

    The long and the short of it is your total potential energy the force of gravity x weight x height of slope. No amount of adding motors, capacitors, and controllers will give you any more energy. The more times you convert the form of energy the less of it you'll have in total. The ratio of energy to the weight of the vehicle and parasitic friction will determine how far it goes

    @CWatters... That would be a good idea.. you'd get the benefit of a lot of energy from the added weight of the brick on the downhill to charge capacitors, etc, and if you can drop the brick, that energy would be applied to a much lighter vehicle.. that means farther and faster :)
  13. Jan 23, 2016 #12

    It's not about distance. I want to effectively take as much energy as I can while going down the hill while braking, stop at the bottom and then use that harvested energy to start my car again.
  14. Jan 23, 2016 #13
    OK, then I guess you do need some energy storage... Does it have to be electronic? Mechanical may be more efficient. If you do go the electrical route, you can do it with just one motor, and I'd choose mosfet transistors since they are a bit more efficient, especially at lower voltages.
  15. Jan 23, 2016 #14

    Mechanical would definitely be more efficient, but unfortunately I've already put together my car with the dual motor setup. Right now I have a voltage regulator, schottky diode etc but the problem im finding is that the motor charges too slow to get enough voltage to drive any significant motor. Right now I can generate about 1.2-1.5 volts on the ramp down, but I can't find any motor that can actually operate on the voltage for a few revolutions that has enough torque to actually move my car.

    Any suggestions? I have about 10 electric motors ive tried and they all seem to "shut off" at around 1 volt across my capacitor as its discharging. THe only one that keeps running is a super weak one that isnt strong enough to drive my RC. so essentially I need a motor with enough torque to move my car, but can also run down to at least 0.5 volts or so. I only want to get a couple revolutions (if i can move a foot ill be happy).

    The ramp is about 1.5m long and at 45 degrees

  16. Jan 23, 2016 #15
    Getting any motor to drive under 1 V is probably going to be pretty hard.

    How much voltage do you have at the bottom of the hill?

    Do you have any bipolar resistors driving the motor? Inline diodes?.. That could explain why you don't get any drive under 1V.. (a .7v drop somewhere)

    If you can raise the voltage of the whole system and run at less amperage you may get better efficiency..
  17. Jan 24, 2016 #16
    The only diode is a Schottky. We have a voltage regulator to try to fix the voltage problem but it doesn't seem to help the charging rate at all.


    and the pic of our regulator circuit (box)

  18. Jan 24, 2016 #17
    Most charge pumps can only supply small currents.. this could be a problem because it can't supply the current to charge the capacitor in time.

    If you're really going to use 2 motors, one to generate the power and the other to drive the vehicle, perhaps choosing a 12V motor as a generator will provide the desired 5V to charge the capacitor much easier since it won't have to turn as fast...

    I think the first thing to be done is determine which side is causing the problem.. check the voltage across the cap at the bottom of the hill, and check the specs for how much current the charge pump can deliver.. If it's 10ma and your trip down the hill only lasts 2 seconds, it's no wonder you aren't able to drive anywhere after..
  19. Jan 24, 2016 #18


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    that resistor will cause problems.

    also the part you are using is not designed to work for up to 12 V.

    what is your plan if the generator produces 12VDC?

    take a look at this, and look up parts like it. http://www.ti.com/tool/TIDM-BUCKBOOST-BIDIR
  20. Jan 24, 2016 #19


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    one thing you can do is provide a boost converter (might need to be a buck boost depending on the voltage you want and the cap voltage). This will provide a slightly more linear power supply to the drive motor until the capacitor depletes. which is why you can use a bidi converter to save parts.

    alternatively you can use one converter single direction from the generator, and one converter single direction to the drive motor
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