Connect with us

Dual DC motor wiring help

Discussion in 'General Electronics Discussion' started by salmirabile, Apr 8, 2020.

Scroll to continue with content
  1. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    Hi everyone,

    i need some advise if i may. I'm converting my boy's 12v car to 28v but it seems i need to change quite a few things.

    I've created a little wiring diagram with all the parts i've purchased. Can anyone validate it for me?

    Many thanks,
    Sal
     

    Attached Files:

  2. WHONOES

    WHONOES

    1,099
    309
    May 20, 2017
    I think it would make more sense if you could produce an annotated block diagram of your proposal along with an idea of what you wish to achieve.
     
  3. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    Hi,

    I’m unfortunately rubbish at block diagrams but isn’t the diagram clear enough?

    I just need to know if the wiring makes sense?

    cheers,
    Sal
     
  4. hevans1944

    hevans1944 Hop - AC8NS

    4,562
    2,134
    Jun 21, 2012
    It appears to make sense to me. And it appears to be correct, too.

    I wonder if the two 30,000 rpm motors are geared down enough to safely propel your son around the commons, or perhaps inside your home on carpeted floors? Do you have chargers for your two batteries? Will the speed controller supply enough current to operate a stalled (locked-rotor) motor without it or the motors burning up? While your wiring may be correct, who knows if your selection of components is adequate to the task? The only way to find out (since you already purchased the parts) is the traditional "smoke test." Power it up and see if anything smokes! Good luck with your project. This looks like a fine father and son adventure, perhaps eventually leading to gasoline-powered motors and go-carts...
     
  5. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    Haha thanks for your message :)

    so yeah, you’re right, I’m not gonna know much until I do a bench test next week once the parts arrive. This car will definitely be only for outside as I’m envisaging a speed of around 12mph which is around 4 times his speed now lol.
    I’ve tried to match the component currents as much as possible so hopefully I shouldn’t see any smoke.
    The controller should have ample current to supply the motors and the batteries are rated at 100a so the only weak link will be the motors. I can't see the motors needing more than 20-30a under load but we'll see i guess!
    the real test will be under load!

    He's only 3yrs old but man, he loves tinkering so can't wait till he's older to get him into a go kart
     
  6. hevans1944

    hevans1944 Hop - AC8NS

    4,562
    2,134
    Jun 21, 2012
    My kids are all grown-up adults now, but when they and I were younger, the company I was working for was discarding a bunch of obsolete computer peripherals. This included a 9" reel-to-reel tape drive with vacuum columns, which allowed the tape to move really fast between the reels. Like, warp speed! I salvaged the large relay-rack style cabinet the tape drive was mounted in and removed two huge DC reel motors from the tape drive, with the idea of building a go-cart for my two sons. Later, I acquired (for free!) a 24 volt lead-acid battery, brand-new, never had electrolyte (sulfuric acid solution) installed. I thought this battery would be good for powering up the go-cart... and perhaps also good for adding a few hundred pounds close to the ground for stability.

    Unfortunately, I never did learn how to weld either steel or aluminum tubing to build a go-cart chassis, although I did purchase a welding helmet, intending to take classes at a local community college, and eventually to purchase a TIG (tungsten inert gas) welder. I still have the helmet in its original box, but the motors and the battery were left in Dayton, Ohio when we moved to Florida. And I never did purchase the welder or sign up for hands-on training, which IMHO is the only way to learn how to weld, or for that matter to learn anything requiring haptic skills and eye-hand coordination. Printed instructions and videos are a big help, but in the end you have to do it yourself and learn mostly by trial and error.

    About three-year old tinkerers... that is exactly how I was at that age, eager to examine and learn about everything. Mom taught me the alphabet using Little Golden Books and flash cards. Then, after that, she taught me how to read before I reached age four. By the time I hit kindergarten I was so far ahead of the other untutored children that it became a struggle to stay attentive. Borrrring! Moral of this story: always be there for your kids, to nurture, to teach, to love and to explore. If you do it right, they will grow up to be exceptional human beings. Well, there will be outliers like me, not exceptional at all, but I still had a lot of fun growing up.
     
    Last edited: Apr 9, 2020
  7. Bluejets

    Bluejets

    4,497
    957
    Oct 5, 2014
    If the supply is a tad over double and the motors are the same, where does 4 times the speed originate..?
     
  8. Audioguru

    Audioguru

    3,085
    692
    Sep 24, 2016
    The over-powered car might do a wheelie and the boy falls out the rear on his head. Or the sudden acceleration might break his neck
    I wonder how quickly the motors, wiring and speed controller will catch on fire. The car was designed for much less current.
    Don't do it.
     
  9. hevans1944

    hevans1944 Hop - AC8NS

    4,562
    2,134
    Jun 21, 2012
    Woah! We know exactly zip, nothing, nada about this kiddie car nor its putative driver. The OP needs to provide some more details, like the length of the wheel-base, the un-sprung weight of the vehicle, the turning radius, the type of braking employed and how far the vehicle will travel if brakes are fully engaged at 12 mph, and (most important) the weight of the 3 year-old driver, including the weight of his crash helmet, leather boots, leather jacket, and leather chaps, all absolutely necessary for protection against inadvertent ejection from the vehicle that could lead to a painful case of "road rash." Don't ask me how I know personally about the importance of these protective items, especially with regard to largeish 1100cc Honda ACE cruiser-style motorcycles.

    It is quite common for relatively inexperienced drivers to use BOTH feet, one on the accelerator pedal and the other on the brake pedal. This can lead to a condition where BOTH pedals are pressed down fully ("pedal to the metal") during an emergency, panic, situation. The result can be a stalled or "locked rotor" condition of the motor(s) and uncontrolled skidding of the brake-locked wheels. Electric DC motors with locked rotors have zero back emf (electromotive force) to limit the armature current, which can lead to burned out motors, melting insulation on connecting wiring, and possibly fires. The vehicle should be equipped with suitable fuses to protect the wiring, and an approved ABC fire extinguisher, located within reach of the driver, to ameliorate these, admittedly unlikely, but extremely dangerous conditions.

    But, most important of all, remember to have FUN! :D
     
  10. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    sorry, the speed won't be 4 times! original speed is actually max of 5mph so 12mph should be reachable.
    nothing will catch fire! the only thing that might happen is the motors give up far quicker that first thought.
    Anyway, i've amended my original diagram to be much more detailed in my wiring and i've also now added a brake pedal which might, might not work very well.
    The car will not be capable of wheelies as i've added weight near the front of the car. Also, the accelerator pedal is not just an on/off switch. he can go as slow or as fast as he wants. the worst that'll happen is he slams on the accelerator and does a little burnout.

    Hevans1944, you've touched on something i need clarity on! The brake pedal is basically there to short out the motors for dynamic braking but i'm not sure if my wiring from the relay is at all correct. (I hope the diagram makes things clear on how i'm wiring it up)
    if it's correct, should i install a resistor in between the short to dissipate the heat from the braking? or is there another way of doing it?

    thanks for your help so far
    Sal
     

    Attached Files:

  11. Bluejets

    Bluejets

    4,497
    957
    Oct 5, 2014
    Dynamic braking is normally a good idea on freerunning motors however, with the gear ratios involved here and the load of the passenger, it will not even be noticeable

    As far as nothing will catch fire, when dealing with LiPo batteries, especially those with the sort of capacity you show in the diagram, never say never.

    I doubt very much your project will have the necessary power or traction to perform wheelies.

    Ok, so the 4 times speed has been withdrawn, where does the 5 to 12 mph conversion come from?
    As I said previously, there is an increase in voltage supply from standard 12v to 29.6v which may be 2.5 times but there are also losses which double within any speed controllers etc.not always in a linear fashion, not to mention those motors can only run at a certain speed under load.

    No idea how the excess in energy levels due to the changes were calculated either.
    My guess it is like many Op quoted outcomes here, just a stab in the dark.

    You really must learn to draw schematics as, what initially went from being recognisable, has now become a dogs breakfast ( rats nest to some)
    However, that said, connections do appear at first inspection to be somewhat correct. A redraw would be necessary to eliminate any problems within the circuit as a whole.

    Second look, no interlocks between power on and braking to avoid activation of both at the same time. No fire hazard, really??

    What is the device in the red lead of the top battery pack..??
    Might be a good time to point out that some of these LiPo packs, especially if designed for rc aircraft, do not have protection circuits built in.
    Protection circuits are there to eliminate the possibility of over-discharge or over-charge rates and critical voltage levels.
    Monitoring of the rc units is normally achieved via the balance plug on a dedicated charger and by critical discharge voltage level monitoring via telemetry.

    As is said so many times, alright if you say it quick.
    In reality there is normally more to it than meets the eye.
     
    Last edited: Apr 11, 2020
    hevans1944 likes this.
  12. Audioguru

    Audioguru

    3,085
    692
    Sep 24, 2016
    Yesterday I stood on the rear axle of a shopping cart and rode it downhill fairly far in the parking lot. No brakes and no steering! Yes, it was FUN.:)

    My RC airplanes pulse the motor as a warning when the battery voltage is becoming low then shuts off the motor (but allows the servos to work for a steered landing) when the voltage is too low. If the propeller gets stuck in the grass and stops the motor when it is running then the motor squeals for a moment and an over-current-protection circuit turns it off. I programmed the speed controller to short the motor as a brake so that the propeller is not a free wheeling air-dam when the airplane is gliding at zero throttle.
     
  13. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    Hey Bluejets, thanks for making sound like a right retard!
    Anyway, you highlighted some good points so I appreciate it.

    I have no idea how much free running the motors will do so that's why i've added a brake. I'll be bench testing the motors and will find out before installing the brake. if i don't need it, great, if i do, I have the parts to do it.

    As far as schematics go, i already said i'm no good at them and i tried to lay it out the same way it's going fit in the car so it's easier for me to visualise the install. I agree it could be much neater. I appreciate your patience in looking at it!

    29.6v is the nominal charge of the batteries. Fully charged, it's actually 33.6v. The original motors have a max RPM of 13,000 at 12v. the new motors have a max rpm of 30,000 at 12v. each motor will be fed 16.8v fully charged so will reach more than 30,000RPM, hence my speed calculation. yes there may be small losses in the circuit but i doubt enough to severely drop the speed.
    i don't want to get into speed calculations, as you say, it's a stab in the dark and i'm much more concerned about the circuitry.

    I've now rewired the relay so when he presses the brake pedal, the speed controller is disconnected from the motors so it shouldn't matter if he keeps his foot on the accelerator. Any thoughts?

    The device connected to the top battery is a circuit breaker with a max amperage of 40a. this should give some protection if the motors try and draw more than that. The batteries are capable of a discharge rate of 90a so the circuit breaker should be enough to protect the batteries, and the motors for that matter. The only thing i will need to add is a voltage monitor to shut off the batteries when they get too low. I'll have a look at what i can get.

    Would be great to have more feedback!

    Many thanks,
    Sal
     

    Attached Files:

  14. Bluejets

    Bluejets

    4,497
    957
    Oct 5, 2014
    What we supply is fact.
    How you decide to take it is entirely up to you.
     
  15. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    You took that out of context. It was mean't as a joke.
    So you have no more feedback then?
     
    Last edited: Apr 11, 2020
  16. Audioguru

    Audioguru

    3,085
    692
    Sep 24, 2016
    Nothing limits the current to the LEDs so they will instantly burn out.
    30,000 RPM is a VERY high speed, isn't that when the motor has no load? Then when loaded, the RPM of the motors and speed of the car will be much less than you calculated the gearing to do.
     
  17. salmirabile

    salmirabile

    7
    0
    Apr 8, 2020
    the distribution board on the left side of the drawing has built in resistors to limit the current. That's the original part of the car and will not change. the only thing i'm doing with that is connecting it to a buck converter to get the 12v it needs. I already know i need to do the same for the rear lights, just haven't added it to the drawing.

    The current loaded speed is approx 3-5mph at the current max motor speed of 13000rpm. The new motor rpm will be more than 30000 as it's been given over 4v more than the 12v it needs to reach that RPM. So even at load, it won't be much less than what I calculated. Plenty of youtube vids to show you the speed of these things with a very similar power output to what i'm installing. Like i said, i don't wanna get into a speed debate as i really don't care. My original question is about will my setup work and what am i missing of which i've had some good feedback so thanks.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-