help designing pwm motor contoller circuit

Discussion in 'Power Electronics' started by Jakesandk, Jan 26, 2017.

  1. Jakesandk

    Thread Starter New Member

    Jan 1, 2017
    Let me start off by saying. This is my first post on this site, forgive me if i am posting in the wrong category. I am not an electrical engineer. I am an electrician. However, i do have an extensive understanding of electrical theory compared to others in my trade.
    Continuing on, for christmas this year, I built my children each a power wheels buggy. Basically from scratch, the only thing "power wheels" is the plastic body No ordinary power wheels, these were designed to be able to ride alongside atvs for extended periods of time, efficiently, without getting stuck on every bump/log/mud hole along the way.

    After many sleepless nights, they do function flawlessly (mechanically anyway) as intended, however, i am finding myself in need of a motor controller that can supply more current. Everything is fine except for one really steep hill behind our house ( i mean really steep. 8% grade for about 80 yards.) When the kids go up the hill in the buggys, they can make it almost all the way to the top, but by the time they reach the crest, the controllers i have now are cooking. I havent completely fried one yet, but i know the days are numbered.

    From the research I have done, I have concluded that without spending a substantial sum of money, you cant really buy a suitable pwm controller that can withstand the lower rpm/higher current abuse that these motors are going to be forced to take.

    As everything is right now:
    24v 500w dc brushed scooter motors
    24v 500w PWM scooter motor controllers
    18:1 3 stage gear reductions

    My questions are as follows:
    Am i better off spending a couple hundred dollars on 1000w scooter controllers, or building/modifying one of my own?
    Can i modify the controllers i have now to handle more current?(im assuming by upgrading the mosfets)
    Any available circuits for this amount of current that i could duplicate?
    The scooter controllers have a "soft start" function. is this what is causing this issue? If so, any way to bypass?

    Any help will be greatly appreciated, if i have left out any crucial info please let me know.
    thank you,
  2. MaxHeadRoom


    Jul 18, 2013
    You would need to show the type or details of the present controller.
    You may be able to beef up the present contollers, OTOH it maybe the buggy is under-powered.
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
  4. rthomas12


    Dec 6, 2016
    I have the similar parts laying around and was considering a project on a custom frame. I did read a few articles about boosting the power on the motors but it seemed to make them dangerously fast. Are you running the power wheels with the original set up of two motors? Or do you have one motor and chain drive? If it's change drive, I would add another sprocket( similar to a ten speed.)
  5. Jakesandk

    Thread Starter New Member

    Jan 1, 2017
    Thanks for the replys.
    For batteries, each vehicle has 3. 2 of them are 12v 35Ah SLA wheelchair batteries wired in series for the 24V to the motor controller. The third battery is just a 12 Ah 12v SLA used for accessories.(headlights, diff-lock, under-glow leds.) i used a third battery so that i could keep the drain on the two main batteries equivalent. Everything is wired in 10AWG SEO cable.

    The controllers i am using at the moment are just regular ol chinese scooter controllers like these:
    I am sure they probably cant output the full rating being that they are cheap and chinese, but would a bigger cheap chinese controller help any with the lower RPM torque? Or should i shift my focus from "bigger" to "higher quality"? I am not entirely sure how the circuitry in these is layed out but from what i have read, they put out more of a triangle wave than an actual square wave. My thinking is that a square wave should increase my available torque at lower RPM.

    The motors are 24v DC brushed scooter motors 500W like these:
    One motor for each vehicle.

    I have hall effect throttle pedals for variable speed control.

    Also, on the output wires of the motor controller i have installed a DPDT relay on each buggy. The relay is engaged by a microswitch that i installed inside the throttle pedal. Once the pedal moves a very slight amount the relay engages and power is allowed to the motor. When the pedal is released, the motor input wires are shorted( automatic braking, my daughter is a terrible driver).

    As for the power wheels design, nothing about them is power wheels except for the plastic body. The pink jeep sits on an angle iron frame and the boxed out compartment under the seat has been rebuilt out of 1/4" plate. One battery is in the front, and the other is under the seat along with the motor and auxiliary battery.
    The green dune buggy one sits on a frame made from square tubing, and the motor/batteries in the back are mounted on a combination of angle iron and plate. One battery is in the front and the back holds the other battery and the auxiliary battery.
    The front axles were robbed out of riding lawnmowers and both of them have 14" rubber tires.
    Both of them have a solid keyed shaft for a rear axle with a 3 stage(or maybe 2 stage depending on what exactly was considered a stage) jackshaft gear reduction. The have 3 chains each. I did this so that i could reduce my gearing in smaller increments and use smaller chain, due to weight/space requirements.
    I do not believe gear ratio is my issue, the top speed is only about 8 to 10 MPH and these things have ALOT of power. Now that i think about it more, it does seem as if the issue is derived in this "softstart" function on these controllers.
    I have pondered on making them 2 speed, with a sliding collar on one of the jackshafts, but i am very limited on space and getting the linkage right to where a 4 year old can shift it seems like it would be a pain. Also i would like to avoid that for fear of it kicking out of gear under load and breaking a child's finger.
    I have also wondered about using the existing gears off of a bicycle, However the forward/reverse function is done electrically (with a drum switch) not mechanically. So the tensioner would have to work in both directions. a tensioner from a mountain bike will only work in one chain direction.
    In conclusion, i feel that the simplest solution would be to somehow increase the torque electrically.
    I will follow up with pictures.
    If i have failed to provide necessary info please let me know.
    Again thank yall for the support, any help/advice/ideas are greatly appreciated.
    Thanks jake
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