700E Crash / BEC Burst!

[Destin]

I knew when I was building it, that the location I chose for the BEC probably wasnt very good because it would not get much cooling. I was right. :arggg: Next time, I'll be sure to listen to that little voice in my head.

During my first flight of the day I had lost engine power while a few hundred feet up. I was able to keep control of it, and actually did my first successful emergency autorotation! Sweet! :thumbup:

I thought for sure that the ESC had overheated and cut off because I had changed my throttle curve from a flat 71 to 73 (I wanted a bit more head speed). Since I still had control of it, it didn't even occur to me that the BEC was the problem.

So, I brought my throttle curve down to flat 69, and decided to take it easy with a test hover. During that hover, about 50 feet up, I lost complete control.

On inspection the BEC was obviously ... ummm ... fried.

I should note that the BEC was Western Robotics Hercules Super Mini BEC (10A/16A Peak) running JR 8717 cyclic servos and JR G7703D Gryo and 8900G tail servo. Pitch curve was -10 to +10. I do very basic sport flying - no 3D yet. The BEC was wired so that it tapped off one of two packs in series. It was getting only 6S of power, not the full 12S. I made sure of that.

My guess is the location did not allow it to cool, so it blew. But this is where I seek expertise advice... I am not sure that a simple cooling problem would cause so much damage to the BEC (see picture). I am open to suggestions on how to avoid a repeat of this.

Since the tail took the brunt of the crash, the heli withstood the impact fairly well. Main blades, the head, and even the flybar were not damaged. I still need to inspect for hidden damage, by my initial estimate comes to just under $200 for repairs - the most expensive part being a new BEC... (Maybe!)

[helicraze]

If the BEC is up to the task it should not really need cooling, they only get warm.

Also do not run off one of the flight packs, if you run off 12S run it off 12S! Or if the BEC can't handle 12S get one that will.
Otherwise you are making one pack draw more current than the other.
Hyperion mentions this in there instructions as a no no, there could be other reasons.

[rcmarty]

I wonder if it has something to do with the series adapter ...

Where did you tap off the 1 pack ?

If you were to connect the bec to a 12s pack then it would be in parallel to the source battery. (12s acting as 1 source)

If you tapped into 1 of the 6s packs , then the bec could have some power curculating in it. due to the 1 pack being the source and the second pack being another source.

Maby some back currents or something that would overload the bec.

Electrician by trade , we get some weird things happen to buildings with lots of transformers. My instructor was telling me about the lotteries building. Will all there computers running it causes an icreased load on the neutral wire. They ended up passing on the newer generation lighting due to circualting currents it would induce into the system. There are also several other situation where there are curcualting currents that are hard to read or figure out.

That looks good and fried though. Mind you all bets are off when conected to a 6s pack.

I know that there are people running 2 bec's in parallel for increased output.
Might consider there HV version in parallel for extra capacity. I know WR has a good rep for there BEC's

Hope you get it figured, my 700 is the slimer version ..

MArtin :)

[Destin]

I followed the attached instructions for connecting my BEC to the electrical system.

I wont be doing that again. Instead I will use the new BEC that handles up to 14S. This way it wont draw more current from one of the packs, as helicraze suggested. I will also be placing it where plenty of forced airflow is on it.

Had I minded the instructions, I dont think this would have happened. It clearly says, with Warning, that it requires direct air flow cooling or failure could occur.... :oops: That was mistake #1. The second mistake was attempting to fly again after the first failure - thinking I had it all figured out already... not! :arggg:

I'd like to think I learn from my mistakes. And I made a few which led up to this crash. Mechanical failure or not, it's my fault all the same for making the wrong decisions. At least I knew better to fly well enough away that it was no where near anyone when it went down. Loss of control of a bird this big is scary. :shock:

[helicraze]

I would ignore that CC instructions, I have run UBEC of main flight ESC leads for years and most do the same.

[cbdane]

I think that you did everything fine and there is no way it was a matter of poor airflow. There are not even cooling fins protruding from the heat shrink. I have read of several failures of this CC switching regulator design and unfortunately, I think yours gets added to the list.

Here's just one post below from many for this poorly designed BEC. The failure is just like yours. It looks like it's just on the edge of handling 6s, in my judgement.

http://www.helifreak.com/showthread.php?t=105611

[MrMel]

It was a WR SuperBEC that failed?

Contact WR right away, you shouldnt see any of those kinds of load, even with 8717 that should be an issue for that BEC, even without cooling/airflow.

I measured my Vbar T700N with 8717 and there isnt that much load so you would think.

[Destin]

This is a Western Robotics Super Mini BEC that failed, not a Castle Creations BEC.
I have shown the CC instructions for connection a to multi-pack electrical system only as an example of how I have it connected in my setup.

I have contacted Western Robotics about the issue, as suggested. If I shouldn't have been getting a load to cause such heat, then perhaps there is some other cause of the BEC's failure, or the BEC itself was defective?

What is odd is that the first failure seemed to be power loss to the motor only, as I was still able to control the heli and do an autorotation to land it safely. That is why I did not suspect the BEC before attempting the next flight. I do remember touching the BEC after that landing and it was hot, but not alarming in my opinion.

For the second failure I had lost all controls. The motor died and all cyclic controls went dead.
And keep in mind, I was only hovering when it happened, so there was very little load on it.

[MrMel]

Electronics can of course always fail, but my guess would be that the first shutdown you saw perhaps was a brownout for some reason, and second failure was a complete one.

Do indeed check all cables, the ESC, everything since it can have been a short somewhere else that contributed to it.