Runaway brushless motor

[Joe Beavis]

I had a scary moment recently. While doing a motor run check in the workshop on my electric glider, with a folding prop, motor, ESC and battery, all of which had been used together before without incident - as I throttled up, the revs rose - and rose - and rose - and BANG, the prop burst and the motor was half ripped off the fuse with shrapnel flying all around. One blade had failed at the mounting hole. The other was about to part company, and the pivot bolt was bent into a 'u' shape, so plainly it was an overspeed not just a prop fault..

I got a better prop and gently tried again (after repairs!) - no overspeed and no problems since. The set-up has been flown a number of times now.

This was very dangerous, and I would welcome any explanation

[ken anderson.]

hello joe - props are rated to a certain RPM.....after that they are dodgy...could be the prop had a knock some time or the cold temps affecting the material...thats what the LMS explained to me when one of mine got thrown in the air.........(EP)

ken anderson ne...1...... thrown blade dept

[Peter Beeney]

I’m inclined to agree with the folder prop being a bit suspect. I think it’s very unlikely that the motor would overspeed. Assuming it’s a standard permanent magnet brushless motor then it will run at it’s fastest with no load; and that will be proportional to any given voltage source connected to it. Exactly the same principle as appertaining to a brushed motor. Because of the way in which the motor functions, it cannot turn any faster, it’s self limiting. The only way to make it run faster overall is to apply a higher voltage.

So maybe something became slightly loose and then the prop started to vibrate, which then caused it to disintegrate. I saw something similar very recently, although not quite as severe, with an electric model in flight. This landed with the prop damaged and the motor broken out of it's mounting.

So perhaps the explanation is that the prop was not really intended to turn at the revs the motor is capable of?

PB

Edited By Peter Beeney on 02/03/2013 11:41:52

[Joe Beavis]

Thank you for your thoughtful comments, gentlemen. The performance of these tiny motors is a mystery to me, and I wondered if it was possible for it to sort of 'miss a beat', and get kicked into 'hypermode'.

In the light of what you say, I agree the root of the problem was probably prop damage. The very convincing impression of 'overspeed' being actually a cracked eye opening up on one of the blades - increasing vibration appearing to be increasing prm, until the blade flew.

The alarming thing is that these folding props carry no maximum rpm warning. This 8 by 4 one had only M2 pivot bolts, threaded all the way (not ideal), but was only on an 830kv motor with a 3S Lipo ("only" 9200 rpm?), but still the surviving bolt was bent. They really need a health warning.

I now take folding props much more seriously - I see that my LMS has some carbon ones - I will give those look! Thanks again, Joe Beavis

[Erfolg]

Joe, I do believe you have learnt the hard way, that if using folders, you need a good set of blades, such as Aeronaut or Graupner.

Good quality folding blades are not cheap, that is compared with some from the Far East.

You are not the first to have the same happen to them, this is a me too, situation.

[Peter Beeney]

No problem, Joe, I think the more you gradually learn about these things, the easier it becomes to sort out any problems and snags.

Regarding the propellers, especially the folders, I would certainly buy the best quality reputable brand types. As you’ve described, the prop can become a very quick total focus of attention if it decides to breakup, but in generally terms I’ve seen very few failures indeed.

As for the motors, as stated, the permanent magnet motor can’t overspeed. But there are many different types, and these can very infrequently get into that situation. If the magnetic field is created by a separate winding and current then it could be possible. When the magnetic field strength is reduced the motor speeds up, and in the event of the field current going open circuit the motor can runaway. I guess this must be a pretty rare occurrence, though. It might be possible that if the magnets got hot and lost some magnetic force the brushless motor speed could increase, but I’d say this would be a very special case indeed……

Another example is your car starter motor; that’s designed to generate maximum torque at very low speed, car starting revs, and it’s speed is controlled entirely by it’s load, the engine. Thus with no load it just speeds up to a maximum rev, which is said to be self destructive, apparently they start shedding bits and pieces. I’ve tried it a couple of times, that didn’t happen, but they certainly do leg it somewhat! A bit intimidating if you’re not ready for it. I made a glider winch using a starter motor years back, and it was very successful, to my great surprise.

Hope this is of some interest.

PB

[Joe Beavis]

Thank you all, I left aeromodelling in the heyday of the ED Bee, and getting back into the 'current' scene, having no electrical background is exposing my many areas of weakness! So all help is welcome.

As it happens I have a further ESC related problem. I recently bought two identical 40a controllers, and was a bit put out to find that they both exhibit a big 'black hole' in the throttle curve at about 3/4 throttle on the model I intended them for (one was a spare - my glass is always half empty!). The motor works perfectly with a different ESC, I have tried all three timings, and medium is the least hopeless, and though I flew it once, it is really no good. The supplier talks of 'motor incompatibility', and will probably change them if I push, but am I overlooking something? I have tried several 3S 2200 Lipo's with no difference.

Joe Beavis

[PatMc]

Joe, if you mean that there's no increase in revs after 3/4 throttle it's usually because the throttle range hasn't been set so that the Tx & ESC match. The instructions below cover a number of popular types of ESC.

[Joe Beavis]

Thanks, Pat. Sorry, I didn't explain very well. I had set the throttle range as described. What happens is that when the throttle opening gets to about 3/4, the motor stumbles, 'misfires' and just about stops, then at 7/8ths throttle - Wham, normal full chat!! It doesn't make for polished flying! But its OK with a different make of ESC. JB

[John Olsen 1]

I had a fun moment yesterday with my Electrik Superstik. All my own fault of course...I had just changed the motor to the one it should have had from the start, a Turnigy G60 500 kv instead of the 300 kv it started with. That meant it needed a smaller prop, a 13 by 10 gave 63 Amps maximum which is within the rating of the motor and ESC, so off we went. Takeoff was a bit more lively than before, throttled back more than before to cruise, and got around the circuit to the point where I thought I would do a loop...opened the throttle and there was a bit of a bang and all the power went away. The dead stick landing went fine, better than most of my planned ones in fact. At least I had plenty of height when it died.

On inspection the prop, spinner and collett were not present. The motor and firewall were hanging from the wires. There was a nasty crack down one side of the fuselage under the wing and the rudder hinges had pulled away from the fin at the top. The ESC had been wrenched forward in its mount, but not damaged.

After a considerable search of the cow paddock next door by three of us, my son found the prop and spinner, somewhat further away than I would have guessed. All together, but the collett had worked its way off the shaft. It has bell mouthed at the end. Evidently I cannot have tightened it enough, although it seemed pretty good. Oh well, another lesson, at least not too expensive...the fuselage damage is all gluing together fine, there will be scars but not too bad. I guess the out of balance forces as it came off did the damage to the fuselage. The prop and spinner are fine, I will of course replace the collett. I think I was quite lucky that the electrics stayed intact and left me with controls..it does have a separate BEC since the ESC is opto coupled.

Funnily enough an IC powered Mustang did much the same thing at nearly the same part of the field a few weeks back, but he lost the entire engine. He landed the plane OK, it was a bit floaty and hard to control but the landing was at least soft. I got a line on the engine of that and my son found it OK as we followed the line. The prop blades floated down separately, we think throwing a blade started his problem.

John

[Glyn R]

Posted by Joe Beavis on 03/03/2013 23:47:46:

Thanks, Pat. Sorry, I didn't explain very well. I had set the throttle range as described. What happens is that when the throttle opening gets to about 3/4, the motor stumbles, 'misfires' and just about stops, then at 7/8ths throttle - Wham, normal full chat!! It doesn't make for polished flying! But its OK with a different make of ESC. JB

I have a similar problem with a 1400 watt Turnigy motor. Runs perfectly with a Turnigy Plush ESC 80amp. The model is using Digital servo's so I bought a new Turnigy 85amp with a 5 amp SBEC to give more pwer for the servo's. The motor will only run with batteries of less than 6s, its fine with a 5s but with a fully charged 6s it will only cog over and never gets really going. I've tried all the settings I use card but no luck.