DJI E800 3510 350KV BLDC Motor Failure

Well I now looked more into the motor failure of my DJI 3510 motor and found some strange happenings inside.

_DSC6395

If you will notice in the above image that the wire failed about 3mm from the solder joint, wait, WHAT? A solder joint on a high current motor lead? Now I think that this is piss poor engineering! And also notice that the wires are not even twisted around each other to help make a low resistance joint. Instead the lead in wire was simply laid upon the other two coil wires (appears to be delta wound) and as such the current must travel directly through a large portion of relatively high resistance solder, and no this is not silver solder since it melts at a relatively low temperature. I also noticed that the other two windings solder joints were also obviously over heated since the heat shrink tubing was also very well cooked. Obviously this is the weak point in this motor.

Good grief, I don’t know what the standards are in the Radio Control (RC) world are but crimp connectors should have been the obvious choice here. Silver solder may be acceptable but only if the joint can be annealed after soldering. (This is why using solder joints in modern aircraft is highly frowned upon) There is also a complete lack of any strain relief making highly susceptible to failure from vibrations in these wires so be damn careful when handling the lead in wires to this motor folks. I now don’t know if the motor connections failed causing the ESC to become smoked or did the failure mode of the ESC overloaded the motor causing it to fail, chicken and egg conundrum. The motor was operating fine before I removed it and the bad ESC, so it must have broke while handling the leads during removal.
_DSC6396Now I will be able to repair the connection and slop some new Gliptol on the burned windings to create a workable motor but this one will not ever be supporting anything valuable like a heli or a camera. I’m sure that portions of the windings have to be hardened and brittle to the point that it will most likely fail again quite prematurely.

I also don’t know why the wires failed where they did, perhaps the wires were nicked causing a high resistance choke point, I just can’t say for sure.

Update, 2015-03-22:

Well, I reattached the lead in wire and with the exception of one of the leads being shorter it functions perfectly. I even did a “Identify” from the ESC Assistant software and it passed all tests, even with the crispy overheated wires. Again this motor will never be used as a heli motor, perhaps I will use it to do some development as a position control motor for something like a gimbal motor, we’ll have to see…