Could a suddenly-detached motor wire cause a prop to fly off?

[Monkeyleg]

I had a crash on Thursday from 60 feet. I saw a propeller go flying off, then watched with a sickening feeling as the Phantom headed to the ground.

When I did a post-mortem on it, I found that the center yellow wire had detached from the clockwise motor's ESC. I didn't make a mental note of which motor was missing the prop, so I'm not sure if that was the one.

I don't know if the wire came off in flight, or as a result of the impact. My guess would be the impact, as I don't think the prop would fly off if the wire detached in flight. The centrifugal force would keep it spinning clockwise.

But, I've been wrong many times before. Is it possible that the wire detached and somehow the prop then flew off?

 

[RoyVa]

From thenwaynep are describing the issue it sounds as though when the wire broke loose the motor reversed very quickly causing it to spin the prop off. I realize these motors are clockwise or counter clockwise rotation but they are also DIrect Current motors and by reversing polarity you can reverse to rotation. Possible senerio???

 

[Mark The Droner]

I think it's possible the prop could come loose if the motor stopped suddenly in flight. After all, the prop is spinning because the motor is pushing it. The resistance is air. The running motor keeps the prop tight because air is pushing it tight constantly. But let's be serious, we all know that prop isn't on terribly tight. So when the motor stops abruptly, or tries to, the opposite happens. The force of the now-trying-to-stop-spinning motor is now working against the props. The mechanism is spinning for a couple seconds, but it doesn't want to. It's trying to stop while air is pushing against the prop, trying to loosen it rather than tighten it. The resistance of the air on the prop will spin that prop right off.

On the other hand, if the prop just flew off in flight because of some other reason, then yes, the wire could have broken off the ESC when it crashed. I am not aware of this being a problem on Visions but I've heard of it on the Inspires.

I think it's odd that the yellow wire would just break off the ESC in mid-flight. DJI does an excellent job of securing those wires to the board. Maybe you had a crash before and the solder could have cracked and you didn't know it?

 

[N017RW]

Loss of a single wire would sure cause an abrupt change in rotation speed and could 'throw' a prop. It is even possible for rotation to continue but at a much reduced performance.

These BLDC motors are actually more like 3 phase AC motors and rotation reversal is not possible by a polarity change. That would in fact destroy the ESC.

The only item that 'polarizes' these Phantom motors is the shaft's prop threads. A BLDC motor will spin either direction just fine with the proper wiring sequence from the ESC.

 

[Monkeyleg]

Thanks for the replies. What I was thinking of was the long thread about the light weight of the v3.0 wires, and the fact that they're sold rather than stranded. There were reports in that thread of wires breaking.

This wire certainly broke at the solder joint, and I think it's possible that the vibration in flight broke it. Just to be on the safe side, I'm going to replace the thin solid wires with 18 gauge stranded.

While the bird was in flight, and long before it crashed, I saw what looked to be a prop stopping while watching the video on my tablet. That would have been a front prop, though, and the disconnected wire was on a rear motor. Weird. Maybe it was a momentary freeze in the video feed.

 

[Mark The Droner]

I think sometimes the frames per second of the video feed will align with the revolutions of a particular prop and make it look like it has stopped...

My first drone was a toy - a HAK909, and the ignition wire did break right off the motor at the solder joint during flight due to vibration. My drone fell from about 100 feet or so. However, I did crash that drone a few times beforehand. But those solders were crap relative to the good solders on our Phantoms.

Good luck

 

[Monkeyleg]

If the wire breaks at the solder joint, it's not the joint that's bad. If part of the wire with solder came loose from the rest of the solder joint that would be a bad joint.

The issue of the 2312 motors having 22 gauge solid core wires was discussed extensively early this year. Solid core wires will break more readily than stranded. I was going to replace all of the 2313 motor wires with 18 gauge stranded, but I was always too busy. Now that will be at the top of my to-do list when the new Phantom arrives.

 

[yorlik]

So sorry to hear your loss!

It has been a long time since I had my p2v+ apart, but I swear the wire size was like 14 gauge, no way 22 gauge. 22 gauge is good for, w/o looking it up, about 2 amps - we hit it with 15-20 amps rms, with peaks to 30 amps. You might want to look up on the forum what the expert replacers use - I doubt it is small 18 gauge - which is only good for 5 amps rms!

Whether the motor totally stops, or continues to run with stutter, is totally dependent on the inertia of the blade. I have not done the math: I would guess that the low inertia of the plastic prop, with the added minus toque of windage, would NOT keep it running like an Ac induction motor missing a phase.

In EITHER case, if the prop came off, it is your fault: you did not use your wrench to tighten it.

If you use your wrench to tighten it, you will tighten to MORE torque than the motor is capable of producing to spin the prop off.

If you do not use your wrench but simply hand tighten, you leave yourself wide open to props flying off anytime a glitch or something like this happens.

THAT said, would tightening the prop with your wrench have saved your quad?

YES, if the wire simply disengaged at crash.

NO, if wire came off first. Even in motor did not fully stop from low inertia, the change in lift would have crashed it: THIS type of crash is a shame also, because dji could EASILY add a subroutine to allow one motor to fully stop and NOT crash. NO extra cost to them - other than less new sales.

Your post is a good reminder to anyone NOT using their wrench to use it or face the same potential prop flying off issue.

 

[yorlik]

If it was mine, and less than 1 year old, I would be contacting dji for warranty repair. Never know - they did warranty for one of ours when the smart battery decided to shut off in flight by itself and we destroyed the quad....

 

[Monkeyleg]

yorlik, do you have any idea how huge 14 gauge wire is? I doubt there's a circuit board anywhere with that heavy a wire soldered to it. Welded, maybe.

I swapped out the 22 gauge wire for 18 on motors I had previously on this Phantom. I should have done it when I changed the motors again.

Here's a discussion about the wires from last May, started by hunch and Burlbark: If you just purchased a brand new Phantom 2 Vision+ V3 READ! | DJI Phantom Forum

 

[syotr]

I don't know if the wire came off in flight, or as a result of the impact. My guess would be the impact, as I don't think the prop would fly off if the wire detached in flight. The centrifugal force would keep it spinning clockwise.

But, I've been wrong many times before. Is it possible that the wire detached and somehow the prop then flew off?

A detached wire will stop the motor quickly. The prop will try to continue spinning in the direction it was turning because of inertia. If it was not tightened with a wrench, it can unscrew and fly off. I have seen this happen twice.

 

[Monkeyleg]

That makes sense, syotr, and it's what I was thinking. I wish I'd taken note of which motor it was that had lost the prop so I could be sure. I'm pretty certain the missing prop was a clockwise (black hub) prop, and so was the motor with the disconnected wire.

The motor that was missing the prop had mud in it, and I think that's the one with the disconnected wire. The arm that really hit hard was bent at the end, and two of the LED's came off the ESC. That motor worked, though, and the prop was still on. If any wire was going to break off from impact, I would think it would have been one on that motor.

If the wire disconnected in flight, then the bird would have gone down regardless. I'm not taking any chances, though. I'll be changing the wire gauge, and also torquing the props down good before each flight.

 

[yorlik]

A detached wire will stop the motor quickly.

this is a very incorrect statement. And totally wrong.

N017RW correctly suggested it would respond similar to an ac induction motor - aka, asynchronous ac motor.

Our motors are synchronous ac motors. They will respond exactly the same as the induction motor with a missing phase.

Many folks have made cheap 3phase converters from 1 phase by simply wrapping a rope around a motor shaft, pulling, and applying 1 phase power. The motor will spin, inertia will run it thru the missing phase, and it will run just fine.

Our synchronous motors are similar. I have designed, applied, and sold these exact same type motors for over 35 years. I guarantee you, the motor will just have a missing section of torque where the wire fell off. Since these are like 10 pole motors, that means there will be small 12 degree sections of no torque, followed by 24 degrees of full torque. As long as inertia is sufficient to rotate thru those dead spots, the motor will continue to rotate - at a slower speed maybe - again, depending on load friction and inertia.

There will be NO instant stopping of the motor.

 

[Mark The Droner]

this is a very incorrect statement. And totally wrong.

...the motor will continue to rotate - at a slower speed maybe - ...
There will be NO instant stopping of the motor.

At a slower speed MAYBE?

Are you saying the motor will continue running?

I assume you mean the motor will continue spinning, but will gradually slow down and stop.

 

[N017RW]

Polyphase motors will run with a dropped phase and depending on the load will do so until they fail due to overheating as the current increases to near double..

 

[syotr]

this is a very incorrect statement. And totally wrong.

N017RW correctly suggested it would respond similar to an ac induction motor - aka, asynchronous ac motor.

Our motors are synchronous ac motors. They will respond exactly the same as the induction motor with a missing phase.

Many folks have made cheap 3phase converters from 1 phase by simply wrapping a rope around a motor shaft, pulling, and applying 1 phase power. The motor will spin, inertia will run it thru the missing phase, and it will run just fine.

Our synchronous motors are similar. I have designed, applied, and sold these exact same type motors for over 35 years. I guarantee you, the motor will just have a missing section of torque where the wire fell off. Since these are like 10 pole motors, that means there will be small 12 degree sections of no torque, followed by 24 degrees of full torque. As long as inertia is sufficient to rotate thru those dead spots, the motor will continue to rotate - at a slower speed maybe - again, depending on load friction and inertia.

There will be NO instant stopping of the motor.

I disagree. Brushless motors are different from AC synchronous motors in one very important way. The AC motor runs at a constant speed that is synchronized with the frequency of the AC current. It is designed so that the switching of the phases is automatic and will always lead so that the motor will continue to rotate. The brushless motor depends on the ESC for this synchronization. Only two of the three phases are energized at any time. The third phase is used like a position sensor to send a signal to the controller to switch phases so that the coil ahead of the magnet will be attracting it and the one behind will be repelling it. If one of the three wires is disconnected, there is no signal to the controller so the phase does not change and the motor will stop, often abruptly. I have had this exact failure with a prop flying off in air because of a loose bullet connector on a ESC.
Here is a very detailed explanation of the type of controller chip circuit used in ESC's for brushless motors:
http://ww1.microchip.com/downloads/en/AppNotes/01305A.pdf
DJI changed their ESC's to have active braking so that they could slow down a rotor quickly for faster response. This caused some props to unscrew and fly off in mid air just by quickly braking the motor. Because of this, they added prop locks to the Inspire and composite hubs (which have more thread friction) to the P3.
If you wish to see this happen, you can place the Phantom on a table with only one prop loosely installed. Slowly speed up the motor and then reach under the prop and grip the motor bell. The prop will unscrew and take off.
There is another way a disconnected wire would cause the motor to stop quickly. If the exposed end of the wire touches another phase or some other electrical part on the ESC board, it could cause this problem or ever destroy the ESC.
If you have a wire disconnected in flight, the Phantom will crash even if the prop does stay on. DJI had a rash of bad solder joints at one time with the earlier Phantoms. Sometimes it was even the main battery wires. There were instances where users had all four props come off in flight.
After having two prop loss incidents,I now tighten my props by holding the motor bell and spinning the prop as fast as possible until it stops against the shoulder. A failed ESC will still crash the bird but at least I won't have to go look for the props.

 

[yorlik]

ok syotr, I capitulate; there are too many variables with so many potential designs; it may be that the motor might just stop depending on the design. Just for the record though, our p2 motors ARE 100% true AC synchronous motors, not something different. How an AC synchronous motor is commutated (or run open loop) can be on/off with 2 windings getting power at a time or all 3 getting sine power at same time. But hey are pure AC synchronous motors.

Bottom line I think is what all agree: wire off one motor in flight and phantom will fall. Shame, since not necessary, but true. One day they will correct this failure mode with improved software.

 

[syotr]

A simple test for your enjoyment

 

[Monkeyleg]

In order to get the prop to fly off, he had to keep putting it on ridiculously loose.

 

[syotr]

In order to get the prop to fly off, he had to keep putting it on ridiculously loose.

Exactly. It was hard to duplicate at slow speed. At higher speed it was possible even with a snug prop but took several tries. With the ESC disconnection though, it was absolute, instantaneous and very repeatable.