One common issue quadcopter pilots tend to have with mini quad motors are using screws that are too long, and they can make contact with the winding/wires inside the motor.
What happen when screws are touching motor winding?
Due to the fact that carbon fibre is conductive, if screws are touching motor winding it could cause short circuits. One or more of the following problems might happen:
- Motor overheat – some motor(s) run hotter than others even COG is perfectly centered
- Motor desync – no matter what PID numbers you try, you just can get rid of oscillation/vibration
- Causing interference and noise to FPV video even if there is LC filter or filtering capacitor in the system
- Motor Burn-out / smoke without any obvious reason
If the screws are too long it can also permanently damage the coils in the stator, so be aware!
How to check if screws are shorting motor winding?
Simply use a multimeter, and turn on Continuity Test Mode. It’s a handy feature on Multimeters and it makes a warning beep when a short circuit is detected (and display OL and Ω). Cheap Multimeter doesn’t have this feature but you can instead check for zero resistance for short circuits.
Use one probe to test the motor screw
And use the other probe to test one of the 3 motor wires (just need to test 1 because all 3 motor wires are connected inside the motor). If the motor wires are soldered on the ESC, simply test the solder joints.
If the motor screw is touching winding, a short circuit should be detected. Repeat this for all the motor screws.
How to determine the correct length of screws?
Generally speaking, the length of screws is determined by the thickness of the carbon fibre arms. For 3mm arms, I use 5mm screws; for 4mm arms, I use 6mm screws. If you don’t have shorter screws, you can also use 1 or 2 washers.
Another good way of checking is described here: http://intofpv.com/t-determine-the-length-of-motor-screws
Check here for more useful tips on quadcopter building/flying.