Motor not turning freely

[anku]

Hi, I have got a quad with ReadyTosky 2212 920kv motors. I experience an issue with one of the motors from the beginning and the same problem developed with the second motor later on. The motor rotates freely before the flight, but immediately after the flight it shows some resistance when I rotate the propellers by hand. The motor is rotating smoothly with no friction. It is slightly warm but not hot. Several minutes after the flight the motor stops showing any resistance when rotated, it is turning freely and smoothly. I am concerned that this motor behavior is not normal and can significantly reduce copter flight time. There is no explanation I could find so far and there is nothing I could do to reduce this effect. I tried to clean the bearings with gasoline and re-lubricated but this did not help.

Any ideas?

Thanks       

[Dickw]

Very odd.
If you definitely can't feel anything rubbing when you rotate it, try disconnecting the motor from its ESC and see if that will free it up.
I have seen resistance to rotation after a flight on some of my fixed wing gliders but that is linked to the ESC "prop brake" function. A bit of a long shot perhaps, but might as well eliminate that possibility.

Dick

[Bad Raven]

Hi Andrey,

Re:  2212 920kv motors

Is that correct? 920 is a VERY low kV for a 2212 on a quad!

What battery are you running?

What props?

[anku]

Guys, many thanks for your valuable replies and suggestions. The quad is Fire Wheel F450 from AliExpress. I carried out a small investigation and can confirm that Dick was right about "prop break" function. However, it turned out to be more tricky in my case: this function was still working for some time after the power was turned off. That is why I did not consider the option of "prop break" for the first time. It looks like some power still remains in ESC after the power is cut off. 

[Bad Raven]

Guys, many thanks for your valuable replies and suggestions. The quad is Fire Wheel F450 from AliExpress. I carried out a small investigation and can confirm that Dick was right about "prop break" function. However, it turned out to be more tricky in my case: this function was still working for some time after the power was turned off. That is why I did not consider the option of "prop break" for the first time. It looks like some power still remains in ESC after the power is cut off. 

Not met that with quads but have with planes. I once had a plane eject the battery to ground at the top of a loop, taking the canopy with it, so plane was inverted.  The ESC capactors still fed the Rx enough power to give the time to half roll to get right way up and steer back onto landing patch and with an by then uncontrolled but shallow angle arrival, no damage!!

[anku]

Unfortunately, it was not the end of my story about the motor not turning freely. I went to fly my drone recently and took off with no suspicion of anything bad can happen to it. I ended p by falling from about 20 meters height and in this fall the smoke was following the drone. When landed it caught fire - it was an ESC burning. To my great surprise, the motor, disonnected from the burned ESC, WAS NOT TURNING FREELY but with some resistance, not a mechanical one. To satisfy my curiousity I disassembled the motor and checked all of the mechanical parts - everything  was OK, the bearings were turning freely. The only explanation of the accident I ve is that some wires in the motor were shortcut and the current induced in the shortcut wires created a resistance force preventing the motor to turn freely. The shortcut could happen in the air and caused increase of power consumption of the motor to raise rapidly far above the limit of ESC, that is why it was burned. I have checked the drone fight logs and just before the moment of crash the current raised above 30A. My conclusion in this story was that the quality of cheap outrunner motors from china is really bad and they are dangerous to use. What do you think?

[Bad Raven]

I've used very cheap brushless motors for near 25 years, inc very very fast flying wings and helicopters before quads.

I have come to trust Suppo motors, though these are sold unbranded and with many labels added by importers like RobotBirds.

I have come to trust HobbyWing (not HobbyKING) ESCs, often taking out a supplied ESC in an ARTF straight away to install one.

It sounds more to me that the combination of load (prop dia/pitch), motor size, kV, and ESC were incorrect.

It's not a simple matter to get the correct set up, for example, if you trawl the internet you will find many posts claiming authoritatvely that a Suppo 2300kV 2212-6T motor on a flying wing cannot run 6x4 props it will burn out. Well I have decade plus 2300kV 2212-6T's on 40A ESC happily running 6x4 and even 7x5 Speed Props, taking planes into three figure speeds, so thats clearly rubbish!

I have long ago learnt that what a manufacturer supplies as a "combo" is very rarely a properly compatible set!

[Dickw]

.............................. The only explanation of the accident I ve is that some wires in the motor were shortcut and the current induced in the shortcut wires created a resistance force preventing the motor to turn freely. .............................. What do you think?

Sorry to hear of the loss.
Bad Raven had covered most of it, so the only thing I would add is that shorting the motor wires is basically how the motor "brake function" works.
Normally this is done by the ESC deliberately if it is set to "brake", but you would get the same result if the wires shorted in a fire.

Try an experiment –
Take a motor that is not connected to anything and spin it with the wires/connectors not touching.
Now hold the bare wires/connectors together and try spinning it again.

Dick

[anku]

Hi, thanks for your replies and suggestions. Just to add to my story that one of the motors turned out to be faulty with with some wire shortcut and ended up in a bin. Three other survived. But most important is that I found what was the problem - it was in low quality and weak bullet connectors between the motor and ESC. After replacing these connectors I have no problems any more. Learning from my mistakes I recommend soldering motors and ESC.

Thanks again for your help!