How to identify and address multirotor motor overheating

Question

When flying my drone, I often notice my drone motors getting hot after a flight and am concerned that it could explode/blow up.

• What does a hot drone motor mean?
• Is this something to be concerned about?
• What could be done to prevent overheating?

Answer 1

Assuming that you've spec'ed a good propeller-motor match and the heat isn't generated by a tiny motor being forced to drive a giant propeller, the heat generation is likely due to a physical issue with the drone, mistaken filter settings, or a tight PID tune.

First of all, it's a good idea to determine whether your motors are actually hot enough to matter. A good rule of thumb I've heard commonly thrown around is that if your motors are SO hot to the touch that you want to take your fingers off after a few seconds, then that's too hot. If you feel like you could keep them there forever, then it's probably not something that will ever be a problem.

Physical Causes

It's possible that you have a mechanical connection issue somewhere on your drone which is causing problems, like if one of the motor fastener screws is protruding into the motor casing and making contact with the windings like in this video from Joshua Bardwell which also goes through troubleshooting steps and how to rectify the issue.

Other physical causes can include locations on the drone where parts or wires (like motor phase wires) that are touching conductive things they shouldn't be. Giving your drone a look over to make sure any of these aren't the problem is worth a try.

Also, make sure that the hot motor is still balanced. If the motor has been damaged, that is almost certainly the cause of the elevated temperature of the motor.

Filtering Issues

If you ever went into the configurator for your flight controller firmware (e.g. Betaflight, Cleanflight, Raceflight, iNav, etc.) and disabled or restricted the default gyroscope filtering settings, it's possible that these are the culprits. Insufficient filtering on the noisy gyroscope signals can result in hot motors because the flight controller tries to respond to the noise it sees in the gyroscope data and not just the pilot's inputs. The inconsistent nature of the gyro noise means that motor commands would be constantly changing up and down, which can be a cause of hot motors.

If you ever tweaked the filtering settings to try and improve their latency, it's possible that any crashes you've since experienced have beat up the drone sufficiently that there is now extra noise that wasn't there before when you adjusted the filters, causing hot motors. In that case, I'd recommend raising the filter sensitivity. The Betaflight Configurator for >4.0 versions of Betaflight makes this super easy with a slider that you could push towards increased filtering.

PID Tuning Factors

As noted in this (admittedly old, for Betaflight <2.x) Betaflight Tuning Guide, motors will start getting hot if your D-gains are too high. I would recommend checking all of the previous situations before pursuing this case because issues with the PID gains are often more subtle and difficult to track down, but a solution here would be to slowly back off on the D-gains in your PID controller until the motors cool down sufficiently.

The D term in the PID controller tries to look ahead into the future and consider if the controller is trying to reach the pilot's input commands quickly enough, attempting to slow the reaction time or speed it up as necessary to minimize overshoot. This system can be completely screwed up if excess noise enters, so checking the filtering settings to make sure nothing is wrong there first is a good idea.