Electric Go cart + Controller build thread

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

  1. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    OK, so just suppose that the motor has 100A current flow (and nothing to sustain it), it has an inductance of 10H (I'm probably quite a bit low there) and your six 2uF caps in parallel with a super-duty diode to use to charge the caps with, then after about 20mS, you'd have ~91kv on the caps.

    Oops, they're only rated for 1kv.

    I see smoke and flames in your future.
     
    strantor likes this.
  2. Georacer

    Moderator

    Nov 25, 2009
    5,154
    1,281
    May I make a plea for ensuring safety on this thread? Currents and voltages seem to escalate quickly.

    I 'd hate to see strantor worry about his missing finger, apart from his missing buck. Especially since all of the veterans are gathered here, it would be bad publicity if we slipped somewhere :p

    Bah, I know you 're all old pros, it's just me being worrying too much...
     
    Last edited: Oct 12, 2011
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,201
    1,809
    The only smoke I want to see is from Strantor's tires as he peels around a corner. ;)
     
    strantor likes this.
  4. Smoke_Maker

    Active Member

    Sep 24, 2007
    126
    15
    What I know about the MOSFET controllers is that you measure and match the fets and use very good fast switching diodes, I had some part numbers but could not find them.

    I do have a theory of operation and schematic for a IGBT controller, figure out how this controller don't spray material all over the work bench before you make smoke :D :D

    http://www.evalbum.com/tech/motorola_controller.pdf

    Good luck, unless you have done this before I see more IGBTs in your future. But don't give up, you and us learn more from you making smoke, Thanks.
     
    Last edited: Oct 12, 2011
    strantor likes this.
  5. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    These are very low inductance motors. The manufacturer does not provide the the inductance for the ME0709 (what I have) but they do provide it for the ME0708 (which is the exact same motor, except wound for 48V, where mine is wound for 72V. the performance is exactly the same)

    from the manufacturer's specs on the ME0708
    Ok, So I've spent the past 3 hours trying to figure out how you figured out the voltage spike and I'm not coming up with anything intelligent. First searched the internet for a formula, empty handed. tried simulating in spice (which I suck at, total noob) and couldn't input my own FET or diode parameters, so had to use what was available.
    What I saw in spice while trying to get the numbers that you gave (without a diode) was a voltage spike peak at 40KV with 10H. With the .00006H provided by the manufacturer, the voltage spike was little over 100V. When I placed the flyback diode in parallel with the motor in both cases, the voltage spikes were eliminated entirely (perfect diode).

    I thought the diode is what is supposed to prevent the caps from seeing whatever voltage spike caused by the motor. does it not do that properly in real life? Im missing something I know it, what is it?
     
  6. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    Yes sir, safety first.

    Thanks, that's a good reference. I will print that out for the next mission; making a comparable (or higher) power brushless controller.

    As far as matching MOSFETs, I've just finished a 3 week plunge into the study of paralleling MOSFETs and concluded that it's just not for me. When dealing with the current levels I'm after, Getting the FETs (we're talking >10, probably 20 or more) to all switch at the same time has a lot to do with measuring and matching the fets as you say, but it has even more to do with circuit itself (layout symmetry, location/proximity of components, length of traces, stray inductances, parasitic resistance, etc) so that means no breadboarding. every prototype has to have it's own unique PCB. A set of perfectly exact matched FETs could have their switching times all over the place depending on the PCB. I would probably go through dozens of PCBs (keep in mind these aren't Radioshack off the shelf etchable blank PCBs, they're super thick special order ones that probably have to me milled on a CNC which I don't have) before I got one that worked worth a darn and considering my indication of a sub-par PCB design is a board full of blown FETs, that's a lot of money. Also considering in order to get 20 perfectly matched, I might need to buy bulk hundereds of fets (not sure). So, that's why I decided against paralleling FETs; despite the fact that this module costs more, I think it will cost less in the long run, and give me instant gratification (or instant heartache, hopefully not).
     
  7. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    SGT I suspect that value of .06mH is erroneous. These are very low inductance motors, but I have looked at the inductance of its competitor's motors and 30-100mH is the norm. I think the value is probably .06H, not .06mH.
     
  8. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    I found a formula but it doesn't give me the 91KV that you got
    [​IMG]
    using your numbers (10) * (100A/.02s) = 50KV
    So I guess you were factoring in the capacitors or the diode or both, I don't know how to do it.

    I was thinking of using a switching time of 354nS
    Using my numbers (.06H) * (300A/.000345A) = 50KV
    How do I factor in the capacitors?
     
    Last edited: Oct 13, 2011
  9. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    just received Thor's MOSFET at my door. Thank you Mouser for your 1.5 day shipping... now if I were only ready to use it.
    [​IMG]
    For a little perspective, some common objects, an Arduino, a TO-220 transistor, a TO-247 MOSFET, and a 9V battery.

    [​IMG]

    Look at that sexy electrically isolated 0.053 Rθjc C/W MOSFET tab. I can bolt that straight to my heat sink
     
  10. shortbus

    Expert

    Sep 30, 2009
    6,875
    4,047
    Make sure the heatsink is flat and smooth at the mounting area. Most aren't from the factory. And use thermal paste for best heat transfer.

    How many of them to fill up a coffee can? :)
     
  11. shortbus

    Expert

    Sep 30, 2009
    6,875
    4,047
  12. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
  13. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    Last edited: Oct 14, 2011
  14. shortbus

    Expert

    Sep 30, 2009
    6,875
    4,047
  15. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    5,025
    3,115
    From the datasheet it seems it's only a current sensing IC, no outputs to drive mosfets. The app note seems to agree with the data sheet that it's only a current sensing IC:
    Since I don't want the microcontroller to have anything to do with the current limiting (I'm planning pulse-by-pulse limiting, microcontroller is too slow) I'm not sure how I could work this into my plans.

    I am going to play around with the IRS2113S, PFET/NFET buffers, and PNP/NPN emitter follower totem poles and also some of the high current drivers next week when I get paid. Until then I will be working on my control circuit. Here's a rough idea of my plan:

    fixed 20KHz 99% duty cycle PWM from a 555 or some other PWM generator
    .001ohm sense resistor (or other) - 500mV @ 500A (250W @ full power, may need a lower value)
    0-5V analog reference voltage from microcontroller (DAC) corresponding to throttle position (et.al. *)
    An opamp will turn the 0-500mV (0-500A) signal from from the sense resistor into a 0-5V (0-500A) current feedback signal
    A Comparator will compare the microcontroller analog reference to the current feedback signal
    If at any time the current feedback signal becomes greater than the microcontroller analog reference, the PWM will be terminated for the remainder of the period (latch off, to prevent multiple turn-on attempts within the same period, accomplished by the shutdown pin of the IR2113S ot the enable pin of other ICs)
    At the beginning of the next period the latch and process will reset

    *there will be a hall current sensor on the controller output which will monitor average current
    There will be a thermister in the motor and one on the controller heat sink
    the microcontroller will monitor the average current and the 2 thermisters.
    If the controller or the motor gets too hot, or if the motor draws too much current for too long, the 0-5V output will be reduced.
    This will allow high current bursts (above rated motor current) during acceleration and allow the motor to realize it's 300A for 1min rating without having the driver monitor a stopwatch

    If the MOSFET itself is to be used as the "current sense resistor" (I'm currently not planning to do this) then an additional circuit will be needed:
    the mV output of the "MOSFET acting as a sense resistor" will become a 96V output when the MOSFET is OFF, so there will need to be some sort of switch to open the signal at any time when the MOSFET is not fully ON
    This will require a set delay to allow full turn-on to occur, and the signal will need to be opened before turn-off begins to prevent signal voltage spike. I have no idea how to do this as I suspect it would require "seeing into the future"

    Any input?
    I'm currently trying to put these ideas into circuit form in LTSpice, but having trouble. learning to use LTSpice as I go. I will post up what I got once I have something.

    Thank you for taking the time to help me out.
     
Loading...