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Thread: Issue finding proper relay/some motor capacitor questions.

              
   
   
  1. #1
    Member 4dollarjoe's Avatar
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    Issue finding proper relay/some motor capacitor questions.

    So for my bike project I am using a alltrax spm 72300. The controller uses the b+ to wire to the ignition. This means my ignition switch is 72v.


    My minibms and lighting/audio system would work great off a relay, but I cant seem to find the one I need.


    I would need a 72v dc input and 12v output, correct?

    What is another way I can get around this if I can't find the proper relay?

    Honestly it really annoys me that the minibms has to have a ignition input. I had a separate switch for it and it always made me paranoid to flip it after charging.


    In addition I was wondering if anyone has tried experiment using capacitors on a bike. I am not an ee major but I have been skeptical about the motor ratings many manufacturers provide. Many older motors used to be rated for 36 volts, but often times people run them at higher voltages. When we are starting off to get from 0-60 why not pump 600 amps for 6 seconds into a rated 300 amp motor (@ 1 minute) to get the insane torquey acceleration you need? Bypass the controller and have the capactiors wired in parallel directly on the motor line; have a switch when you want to use them. Is it even feasible to get large enough capacitors to discharge that many amps for that long?

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    Empulse R #24 frodus's Avatar
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    Do you have a DC-DC?

    Aux battery?

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    Member 4dollarjoe's Avatar
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    I have a crappy non isolated dcdc which I am hoping to replace with an isolated dcdc.


    So would just wiring in parallel after the keyswitch from the 72v be a solution? That way the entire 12v dc system will shut off once the main keyswitch is disengaged I guess. However the dc-dc I have can output 10 amps and the ignition recomends a 5a fuse. However since they are in parallel this should not be a problem, correct?

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    Empulse R #24 frodus's Avatar
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    Do you have an enable on your DC-DC?

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    Member 4dollarjoe's Avatar
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    my old one doesn't, just 3 wires hi V+, low V+, common ground.

    The new one I just ordered is this:
    http://www.ebay.com/itm/231640765516?ul_noapp=true

    It says on the pdf online that this dc-dc converter has 1 remote on/of input. Is that what you mean by the enable?

  6. #6
    Empulse R #24 frodus's Avatar
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    Then connect the remote on/off to your keyswitch, then the DC-DC comes live and can drive a 12V relay to do what you need.

    Not sure if the controller needs full pack voltage on it's KSI input, but if you do, use a 12V relay from tyco to switch pack voltage (tyco KUEP-3D15-12 goes to 150V on the contacts, and has a 12V coil.)


    Better to have 12V on your handlebar than pack voltage.

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    I'd probably do a DPST rocker switch. DP stands for double pole, meaning it's got 2 separated connections internally. The ST stands for single throw, meaning a simple "on/off" connection. DPST basically lets one physical switch activate 2 different circuits in 1 motion. Figure out what you want to power your low voltage system (DC-DC vs aux battery). Your "on" switch can then turn on both high voltage and low voltage systems simultaneously with isolation at the switch. If you choose a battery, it would allow low voltage systems (emergency lights and brakes) to stay on in the event your high voltage fuse pops (either from your key-on fuse shorting or main pack fuse blowing).

    In regard to capacitors after the controller, don't. Controllers already have a massive capacitor bank inside, serving to preserve the MOSFETS that drive it all. Assuming you PERFECTLY timed a disconnection of your own capacitors to allow the controller to take over, it's still terrible for the motor. Blasting a huge surge of current to the brushes can end things quickly. In addition, exceeding a certain level of current delivery gains you no more torque as the motor components reach magnetic saturation. Never mind the difficulty of getting a trickle charge system implemented on your cap bank.

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    Empulse R #24 frodus's Avatar
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    Don't use a 12V switch for pack voltage. That's a good way to weld the contacts or decrease the life of the switch. Use something rated for the voltage it is switching. Not many pack-voltage DPST switches out there.

    What I've found is the best solution, and others on here have too, is to enable the DC-DC with a switch (low voltage and low current), and let the DC-DC output switch a relay for you. You can use the ignition switch in line with the kill switch to enable the DC-DC.

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    Quote Originally Posted by frodus View Post
    Don't use a 12V switch for pack voltage. That's a good way to weld the contacts or decrease the life of the switch. Use something rated for the voltage it is switching. Not many pack-voltage DPST switches out there.

    What I've found is the best solution, and others on here have too, is to enable the DC-DC with a switch (low voltage and low current), and let the DC-DC output switch a relay for you. You can use the ignition switch in line with the kill switch to enable the DC-DC.
    I just knew he was using Alltrax with a peak of 90v. It's pretty much just a key-on signal at minimal current, so I saw no reason why a standard 110v A/C rated switch couldn't handle it, which are super common and cheap.

    That being said, I like your suggestion better. Just make sure that DC-DC is built to survive significant voltage sag under max acceleration. Depending on your situation, it could suck to lose motor control via a DC-DC failure.

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    Member 4dollarjoe's Avatar
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    See I've always wondered about that but I called alltrax and they said it was completely fine and it is their wiring diagram to just hook the ignition to the battery. I mean the thin wire acts somewhat as a fuse so is there really going to be that much current drawn to weld the contacts? When I took apart my bike a few weeks ago there was no signs of damage.

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