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P3A: Minor crash. Now one motor won't start.

Discussion in 'Pro/Adv Discussion' started by indoctrin8ed, Jun 27, 2016.

  1. indoctrin8ed

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    Minor crash, no visible damage, props not broken.

    Now when I take off, one of these happes.
    Either a) One motor won't start and I see an error "motor obstruction", or b) One motor won't start but the drone will try take off and immediately tip over and crash again. If it does not crash, it shuts down and will not attempt a spin up again unless I power cycle the drone.

    All motors spin freely and I don't see any debris or physical indicator that the bad motor is damaged.

    I took the lid off - I don't see anything obvious, but I don't know what to be looking for other than something broken or dislodged.

    Please help on how I could fix this myself if possible. I'm an electrical engineer, so I'm comfortable doing surgery.
     
  2. alokbhargava

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    Are you able to start spinning all the motors without props mounted?
     
  3. indoctrin8ed

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    Yes - I just did that an hour ago actually. Learning here. If I do a CSC takeoff, they will keep running and the controller behaves as if it's flying.

    However, the dead motor does nothing.

    Are there ways to electronically test the motor, perhaps with a multi-meter or look for signalling to the motor with an oscilloscope? I dug out my scope but I couldn't find my probes - I will get some tomorrow.
     
  4. Vertigo

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    A scope wont tell you much; even a good working esc that is connected to a bad motor, chances are you wont see much, as the esc needs counter-electromotive force of a good motor.

    Since you appear to be no stranger to electronics, just swap 2 motors and see if the problem stays with the motor or the esc.
     
  5. indoctrin8ed

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    That makes sense on the inductive/reactance side, but should I at least see a spike in input voltage on startup? I'm not an expert in motor controllers.

    Swapping motors does seem like a logical troubleshooting step, just hesitated prior to desoldering because the motor physically looks pristine and mechanically feels normal - do these motors fail like this with minor physical shock?

    Thanks for the help
     
  6. Vertigo

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    I have no idea, but its going to be either the mainboard/esc or the motor and so you are going to have to desolder at least one motor regardless. Might as well do that, and see if you can spot something wrong with the motor. You could then also try to measure its resistance, but as you're probably aware, that is a little tricky to do properly.

    If you have any other RC equipment, you could also test the motor with a regular ESC.
     
  7. indoctrin8ed

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    Ok, I will swap motors and see what happens.

    Are you saying that there are only two serviceable parts in this thing (for the flying portion of it)? The mainboard and the motors?
     
  8. Trackman1

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    Yes. Basically just motors and mainboard. If it were me I would look at how long it will take to get a replacement motor. Time being more concern than a $20 motor.
     
  9. indoctrin8ed

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    Ok - are motors a regular "wear item" in these things? Good idea to have spares on hand? Are they often damaged in crashes? Just asking because maybe I should order several of them? I'm surprised DJI doesn't have a connector harness for the motors.
     
  10. N017RW

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    Phantoms are not considered durable goods and there's no reason to have such things user serviceable (in their apparent opinion). Adding connectors increases cost and could be argued to reduce reliability (I won't).

    BLDC motors are very reliable with the only moving parts to be the bearings.

    I've said it here before I have 600+ hours and counting on a BLDC motor in a CP-Heli. I have others that have been trouble free as well but did not document the usage. I have never had a BLDC fail. Ever.
     
  11. Vertigo

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    There are plenty more serviceable parts, but I cant for my life think of any that would make just one motor not spin.

    As for motors going kaput in a crash; it can happen, but more likely is bearings wearing out. Doesnt hurt to have some spares, but its not something that you should replace every x months either.

    Lastly about the connector; thats just extra weight, completely understandable DJI left that out. You should feel how light (and flimsy) the motor bells are, or the gimbal arms. I dont know the exact formula, but isnt it something like ~1 second of fight for every extra gram? If anything, Im surprised the cables are so long!
     
  12. Vertigo

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    And how many crashes did it survive? With normal use, brushless motors can last very long, but in a crash, with the prop directly attached to the bell, especially the super light weight bells and extremely thin axles of the phantom motors can easily be bent. And of course, bearings can be damaged, and good luck replacing those on the latest revision.

    And FWIW, I (obviously) use brushless motors on my racing mini quads. I dont think they last 6 hours on average. Or more aptly: more than 60 crashes each :p
     
  13. N017RW

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    In your scenario the BLDC did not fail, it was damaged - external trauma would be the cause.
     
  14. Vertigo

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    Yeah, but its not just my scenario, he did ask "Are they often damaged in crashes? ".
     
  15. indoctrin8ed

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    So I have not yet had time to swap in a new motor, or put in a replacement motor, but I did put a scope on the red/black leads of a working motor and observed the signal (see picture attached). This makes total sense as the voltage is (+) when pulling the motor magnet towards the winding, then reverses (-) to push it away. This signal would be the same for any two leads and their timing would be offset by 60 degrees to create a three phase x 2 sides rotation cycle per revolution.

    Motor-Scope.JPG

    I'm not seeing any signal on the motor that is "down". So my inclination is that this a bad board and not a bad motor, being that the motor turns freely and identically to the others which really sucks because this was a minor crash - not even any visible damage.
     

    Attached Files:

  16. indoctrin8ed

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    So my next questions is - assuming this is a bad board, what are my best options for getting a replacement? This is a P2A 2.7k with LightBridge. I'd like to avoid paying top dollar from DJI - are there reputable/reliable options for purchasing refurbished? Is it simply a desolder remove and replace exercise?
     
  17. Trackman1

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    The reason I was suggesting you buy a new motor is because if the motor does have a problem and that damaged the mainboard do you really want to take a chance attaching that motor to a possible replacement board? If you have to go that route.
     
  18. hunch

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    if it was a bad board you'd get an ESC error on your screen and NONE of the motors would start.
    put the ohm-meter between the black-yellow, black-red, yellow-red and tell us what's the resistance is there?
     
  19. Trackman1

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    Another route is to ship the P3a back to DJI and let them fix it.
    At least it is guarantied to be working properly then.
     
  20. indoctrin8ed

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    Hunch: I will measure resistance per your suggestion when I get home in a couple hours - what should I expect to see?
    Trackman1: What will that typically cost to send to DJI to repair?

    I plan on swapping in a good motor but hoping to learn and share these diagnostic steps to help resolve these failures more easily, assuming someone has a scope (they are so cheap these days), etc.