Electric Go cart + Controller build thread

Discussion in 'The Projects Forum' started by strantor, Sep 25, 2011.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    Ok, I'm serious enough about this to start a build thread. I've bought a Go cart frame and a motor. Now I just need to get my act together on designing the controller.
    here's the hideous go cart (50$, craigslist local):
    [​IMG]

    It doesn't have brakes, a seat, or a live axle.
    It's too small for a real person, so I will cut in half and add a couple of feet to it.
    and paint it (if it's lucky)

    here's the motor (Ebay, used 380$, >500$ new):
    Mars ME0709
    • Voltage 24-72 Volts
    • 8 continuous & 18 Peak Hp
    • 300A peak
    • 125A continuous
    • Speed 3050 rpm at 72V Unloaded
    • Size 8" OD, 6.75" long (w/o shaft)
    • Shaft 7/8"x 1-5/8", 3/16" key
    • Weight 36 lbs
    Advertised "slighty used, for 30min for a science fair project"
    [​IMG]

    So, if you haven't been keeping up with my recent flurry of motor contol circuit topics, the purpose of this build is only to learn how to make a high power motor controller. I eventually want to make a brushless controller, but starting out simple(r).

    Oh, I forgot to give credit. Just to give an idea just to how much of an extent that you fellow forum members are carrying me through this project, these are all the related posts that have gotten me to the point I am now:

    http://forum.allaboutcircuits.com/showthread.php?t=59767
    http://forum.allaboutcircuits.com/showthread.php?t=59738
    http://forum.allaboutcircuits.com/showthread.php?t=59764
    http://forum.allaboutcircuits.com/showthread.php?t=59528
    http://forum.allaboutcircuits.com/showthread.php?t=59654
    http://forum.allaboutcircuits.com/showthread.php?t=59460
    http://forum.allaboutcircuits.com/showthread.php?t=59148
    http://forum.allaboutcircuits.com/showthread.php?t=59320
    http://forum.allaboutcircuits.com/showthread.php?t=59310
    http://forum.allaboutcircuits.com/showthread.php?t=59195
    http://forum.allaboutcircuits.com/showthread.php?t=58843
    http://forum.allaboutcircuits.com/showthread.php?t=58689
    http://forum.allaboutcircuits.com/showthread.php?t=58926
    http://endless-sphere.com/forums/viewtopic.php?f=14&t=32199
    http://endless-sphere.com/forums/viewtopic.php?f=7&t=32231
    http://endless-sphere.com/forums/viewtopic.php?f=30&t=32172
    http://endless-sphere.com/forums/viewtopic.php?f=30&t=32073
    http://endless-sphere.com/forums/viewtopic.php?f=30&t=32040
    http://endless-sphere.com/forums/viewtopic.php?f=7&t=31842
    http://www.diyelectriccar.com/forum...ttempt-build-brushless-controlleri-63430.html
    http://www.diyelectriccar.com/forums/showthread.php/why-pay-big-bucks-batteries-my-63437.html
    http://www.diyelectriccar.com/forums/showthread.php/best-controller-and-whyi-62972.html
    http://www.diyelectriccar.com/forums/showthread.php/ev-dc-controller-setup-62822.html
    http://www.diyelectriccar.com/forums/showthread.php/ev-dc-controller-setup-62822.html
    http://www.diyelectriccar.com/forums/showthread.php/diy-hybrid-generator-62670.html
    http://www.electro-tech-online.com/robotics-mechatronics/121825-motor-load-sharing.html
    http://www.electro-tech-online.com/general-electronics-chat/121752-generating-pwm.html
    http://www.electro-tech-online.com/general-electronics-chat/121672-driver-hiperfet-module.html
    http://www.electro-tech-online.com/...-does-kelvin-terminal-mosfet-module-work.html
    http://www.electro-tech-online.com/...10-what-true-amp-capacity-isotop-package.html
    http://www.electro-tech-online.com/...2-exceeding-relay-contact-current-rating.html
    http://www.electro-tech-online.com/general-electronics-chat/121487-pwm-dc-motor-controller-ev.html

    and here is a list of good websites and documents I have been reading:
    http://powerelectronics.com/power_m...nt-limiting-eases-design-motor-drives-PET.pdf
    http://www.ti.com/lit/ml/slup169/slup169.pdf
    http://homepages.which.net/~paul.hills/SpeedControl/SpeedControllersBody.html
    http://robots.freehostia.com/SpeedControl/SpeedControllers.html
    http://robots.freehostia.com/SpeedControl/Mosfets.html
    (and many others which I didn't print out, or printed out and lost, etc, can't remember them all)

    and for the brushless design that I decided against doing, I read:
    ww1.microchip.com/downloads/en/AppNotes/00899a.pdf
    ww1.microchip.com/downloads/en/AppNotes/00885a.pdf
    ww1.microchip.com/downloads/en/AppNotes/00887a.pdf
    http://www.drivetechinc.com/articles/curbldc3.pdf
    http://www.irf.com/technical-info/whitepaper/ir2137igbt.pdf
    http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=5206
    http://www.atmel.com/dyn/resources/prod_documents/doc7658.pdf
    http://krex.k-state.edu/dspace/bitstream/2097/1507/1/JamesMevey2009.pdf - huge BLDC book
    http://www.instructables.com/id/Mak...ctric-Hub-Motor/step2/The-Brushless-DC-Motor/
    http://ocw.mit.edu/courses/mechanic...trol-ii-fall-2007/lecture-notes/lecture05.pdf
    http://www.4qd.co.uk/serv/MOSFETfail.html#dvdt - MOSFET failure modes
    http://forum.allaboutcircuits.com/showthread.php?t=11499&page=4
     
    Last edited: Sep 25, 2011
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Awesome motor! Congrats. :)

    Did you get the nice speed controller with the motor?

    Can you get manufacturer's datasheet (charts) for the motor?
     
  3. SgtWookie

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    Jul 17, 2007
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  4. strantor

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    Oct 3, 2010
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    I have seen that, and forgot about it. it addresses everything I'm trying to right now (all over again) in plain English. Thanks!
     
  5. Smoke_Maker

    Active Member

    Sep 24, 2007
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    Here is a list of goals to shoot for, good luck and I will follow this thread because it something would like to do some day.

    http://www.kellycontroller.com/khb7270124-72v700aopto-bldc-controllerwith-regen-p-838.html

    They (people who have built controllers) say to get your self a old Oat Meal can and when it is just about full of burnt parts it's almost finished :D
     
  6. strantor

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    Well, just made another big ticket purchase. I have been reading copius amounts of APP notes from all the big MOSFET manufacturers about parameter matching, circuit layout, and all the tons of other crap that goes into paralleling several MOSFETs to make a high power switch. I have seen multiple failures in the the commercial EV motor controllers which employ the multiple paralleled FETs. I have come to the conclusion that slapping 40 FETs into an extruded heat sink and trying to make them share the load is a kludge in the presence of something like this.
    [​IMG] So I bought one. 200$, 100V, 1200A MOSFET. I won't even need a power PCB; I can bolt bus bar with caps & my motor/battery leads directly to it. I think this will save me time and money.

    PS I would like to acknowledge that a few people here suggested this option to me in the past but I was hard headed. I had to convince myself. Its hard to convince yourself when you look at what the "professionals" are doing (all of them) and do something else, but I really don't know what these guys are doing.
     
  7. THE_RB

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    You didn't buy two for when you blow the first one up? :eek:

    Just kidding (re our conversation in another thread). :D

    Fuses are your friend. Well designed high speed analog current limiting (at the highest level of your control heirarchy) is your friend.

    The good news is that that part is big, so you won't need to put too many in to fill your "oat meal can" to finish the project. ;)
     
  8. strantor

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    Lol. at 190$ I had to really will myself into buying the first one! The reason I think this will save me money is that I am expecting the 1200A capacity to be very forgiving. My motor has a terminal resistance of .09Ω, so even if I forget to use a fuse and my current limiting fails entirely it will draw 800A @ 72V stalled, which is well within the current limit of the FET module even at 80C. It's only a low side switch so no shoot-through possible. I expect I will fry my motor before I fry this mosfet module. So that saves me the cost of filling a jar full of FETs and fail PCBs, now I just have to worry about diodes and caps. I have 300A fast blow semiconductor fuses, and I plan to use them.
     
  9. SgtWookie

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    Something you're going to have to consider is: Where is all that current going to go when you turn off the low-side switch? :eek:

    Unless you provide a path for it, you will get a voltage spike that will continue to increase until it makes it's own path, probably through something that's rather expensive.

    You might consider using rather large power diodes across the motor, but you will get a lot of power dissipation in the diodes; as that will really be the main place where power will be dissipated.

    I suggest that what you will need is a synchronous switching arrangement, where you have one MOSFET module sinking current from the motor, and another module that switches the path back through the motor. MOSFETs can be used as "ideal diodes". If ever the two are turned on at once, you will have smoke. However, synchronous switching, if properly implemented, will eliminate practically all of your power waste.
     
  10. shortbus

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    SgtWookie, thats also called synchronous rectification. To do it on a motor like this you need a high side and low side to control the motor and a second high side switch for the synchronous rectification. The synchronous rectification also heps out when the motor is put into a 'coast mode', like in a vehicle. In coast mode the motor is a generator and that energy has to have some place to go. Diodes drop more voltage than the high side mosfet.
     
  11. strantor

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    I've been eyeballing a some diodes out there that are physically set up like that MOSFET module, with the bus bar terminals. Rated >1KA and voltage drop <.5V, excellent thermal characteristics. I could just bolt it to the heat sink at the end of the MOSFET module, make my bus bar longer, and bolt it straight together. super modular, pricey. Giving the synchronous recitification thing a once over before I buy anything else. I think I would need another of the same MOSFET module for that, but not sure.

    EDIT: apparently my memory exaggerated the numbers for me. It's .8V and 582A (continuous) 2880A pulsed repetetive
     
    Last edited: Oct 12, 2011
  12. SgtWookie

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    This. :)
     
  13. strantor

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  14. iONic

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  15. SgtWookie

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    Yep, that's what I'm suggesting. But only if you want the maximum performance and least losses in wasted power dissipation.

    Not bad. So, if you have, say, a 1V drop across that diode module, with say, 300A current through it - that's 300 Watts of power to dissipate.

    Plug in your actual current and calculated Vf; because that's how much power you'll have to dissipate from the module. Houston, Texas gets pretty toasty in the summer, which decreases your margin for error.

    Not trying to discourage you - but you need to consider this type of thing.
     
  16. strantor

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    Your prodding is soft compared to the gauntlet I've run on another forum. You won't discourage me, I welcome and appreciate any and all input.
    I think that I'm starting to think myself out of a good idea, but I can't shake it... Here's what I'm thinking now: the diode is just a transient supressor. there's no work being done, just redirecting the flyback voltage back to the capacitor bank/battery, so there should not really be any amperage through the diode at all; maybe a very fast high current pulse if the dv/dt is fast and the diode is too slow, but otherwise there should be no current to speak of. If that's the case, then I could probably get away with something even smaller than what I linked to. So, am I right or am I talking myself back up the tree?
     
  17. SgtWookie

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    I ain't on here to berate; just to help edu-cate ;)

    Good. :)
    Let's say you're at 75% PWM, so you have ~3/4 of the maximum current flowing through the motor.
    How much current is that?

    When the low-side switch turns off, you STILL have 3/4 of the maximum current flowing through the motor, and you better do SOMEthing with it mighty quickly. If you have, say, 200A flowing through there, you might get away with a diode rated for 50A average current.

    However, if that diode fails, you will toast your low-side current sink in a flash.
     
  18. strantor

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    Ah, right, inductors try to keep current the same. so, I do need the beefy diode. I'm sure you are right about the synchronous rectification being a much better option, but unfortunately is just isnt an option for my pocketbook at the moment. I shouldn't have bought the first module, much less a second one. Max rated current on the motor is 300A, which I will adhere to at first, but I'm sure later on I will get bored with that and abuse the motor @ ~500A.
     
  19. strantor

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  20. SgtWookie

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    Precisely. Inductors resist a change in current through themselves, just like capacitors resist a change in voltage across themselves.
    Yep.
    While your pocketbook may be irritated with you at the moment, imagine at it's outright loathing if you should turn the module into a paperweight.

    You are going to need to implement safety devices, so that if you're getting close to reaching the units' maximum ratings that it gets shut down by most expeditious means.
     
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