Using screws that are too long is a common mistake among FPV drone builders. FPV drone motors only come with one set of screws while FPV drone arms might have different thickness can often causes incompatibility. When the motor screws are too long, they can come into contact with the motor windings, causing various problems.
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What Happens When Screws Touch Motor Windings?
Since carbon fiber is conductive, screws touching the motor windings can cause short circuits. Here are some of the issues you might face:
- Motor Overheating: Some motors may run significantly hotter than others without obvious reasons.
- Motor Desync: Persistent oscillations or vibrations remain which is not related to PID or filter settings.
- FPV Video Noise: Even with additional filtering capacitors installed, interference may persist in your video feed.
- Damaged Motor: Motors may burn out without any apparent cause.
Using screws that are too long can also physically damage the motor’s stator coils, leading to permanent damage.
How to Avoid Screws Touching Motor Windings
To prevent this issue, select screws based on the thickness of the arm and the motor base. As a general rule, screw length should be approximately 2mm longer than the thickness of the arm. For instance, for 5mm arms use 7mm screws.
How to Check if Screws are Shorting Motor Windings
Using a Multimeter, set it to Continuity Test Mode. The multimeter I am using: https://oscarliang.com/fpv-tools/#Multimeter
In this mode, the multimeter will emit a warning beep if it detects a short circuit (or display “OL” and show resistance in ohms). If your multimeter doesn’t have this feature, you can manually check for zero resistance.
Place one probe on a motor screw.
Place the other probe on one of the three motor wires. (You only need to test one wire, as all three are connected internally.)
If the motor wires are soldered to the ESC, you can test the solder joints instead.
If the screw is touching the winding, the multimeter will detect a short circuit. Repeat this process for all motor screws.
How to Determine the Correct Screw Length
The screw length depends on the thickness of the carbon fiber arms, for instance, with 3mm arms use 5mm screws, and with 5mm arms, use 7mm screws. No shorter screws? Use washers to adjust the screw length.
Another good way of checking is described here: http://intofpv.com/t-determine-the-length-of-motor-screws
Final Tip
Taking the time to ensure your motor screws are the correct length can save you from headaches down the line.
13 comments
Hello, I have recently purchased an Emax Babyhawk r 2″ and right away i noticed overheating on two motors. Only a short test hover. Both counter clockwise motors. i have checked all screws and none appear to be touching winding. I back out each screw and checked just in case there was something i was not seeing. The short still exists. The short exists through multiple screws on each motor. I removed the stack so only the four in one esc and motors attached. Still shorts on all screws.Then removed 4 in 1 from frame so only motors attached to frame because it appeared esc components were touching in spots. Still shorts at those screws. I have thoroughly checked all the soldering and seems clean. The short also exists through frame to esc solders on the same two motors only. I am completely stumped at this point. Any Ideas what could cause this? Thanks in advance.
Clearly a defective product based on your description, I’d probably take it back for a replacement.
If you have flickering OSD issues… this^^^ triple check your motor screws are not shorting.
My OSD would start flickering as soon as I armed my quad and video would intermittently black out for 1 second. Finally found the issue when I armed on the bench and let the motors spool up. Motor2 was hot to touch after 5seconds with props off! Motor screw was shorting on winding’s.. This article finally resolved my flashing OSD issue. First quad ive built and the last time I make this mistake. Thank you!
I tightened the screws on my stock hglrc batman220 and at the end of my flight motors started stuttering and got really hot. When trying to arm again it flipped on me – then worked again for some LOS testing.. turned out on two motors the screws were touching.. and that with a bnf drone… well…
Is it normal to have some resistance from the esc wire and the motor screws?
no they shouldn’t be connected.
I’ve noticed today that the flight controller screws also have an effect on the motors for some reason. My Motors were getting so hot I replaced every screw beneath the motors and still got very hot. It wasn’t until I change the flight controllers 4 screws to those plastic ones and now my Motors don’t even get hot after torture Punch outs. I think I’m the first person who’s ever had this happen to them LOL
It was a rmrc newt frame with Emax 1106 motor and a F3 Magnum stack from a baby hawk r
The original screws were metal? My guess is the original screws were probably passing more vibration through to the FC.
1st thanks for this resource and all the work helping the comunity. I checked this on my quad and am confident that i did not have the screws too long, however i realized my vtx antenna mount does ground to the frame and i can detect some resistance from the frame to my motor leads by way of the vtx mount. I am thinking this may be the cause of my desync and that i should insulate the antenna mount from the frame? The way i had mounted the antenna actually only grounded to the frame intermittently most likely during the hard turns which are when my desyncs occur. Thanks
I did this test on my quad. I ended up removing all the screws and the motors from the frame entirely and still had continuity between the motor base and the motors wires!
how do you do the test if you have removed all the motor screws? LOL
were you testing with the frame? then it probably means your electronics is somehow shorting the frame…
Another ill effect of this, it causes interference in video that can’t be removed by any ordinary means. No LC filter etc helps. I have found this by a very long and painful process.
yes very true!