Aggressive flying – or in some cases, even docile flying – can lead to your motors getting hot. In some cases, this is normal, in others, this is bad. Most of the time, it is easily fixable. Let’s dive in.
How hot is too hot?
First off, just because your motors are getting hot does not mean there is DEFINITELY something wrong. In quadcopters, and especially miniquads, you’re putting the motors under a lot of stress, and motors may get hot.
So the first thing really to see is if your motors are too hot – in which case something is wrong and you want to fix it in order to keep your motor healthy and not smoke it.
My rule of thumb is that if you can comfortably grip the motor with your fingertips, it’s not too hot.
It’s as easy as that.
If they’re so hot that you can’t even hold them without burning yourself, then there is something wrong.
With that out of the way, let’s dive into potential causes.
Cause 1: the motor tries to spin but something is in the way
You’ll commonly see this after a crash where your propeller gets jammed or bent out of shape to the extent that it hits your quadcopter’s arm.
If you don’t immediately disarm upon crashing, the motor will continue to try and spin – even though the prop is being physically blocked by something. In this case, the ESC will try to supply more and more current to it to try and get it to spin, heating the motor up.
Unless you disarm within a few seconds, you may burn your ESC or motor due to the extreme current.
A LOT of current probably goes through at this point, so you may find that the motor and ESC is still hot even after you walk over to the quad and go fetch it.
In this case, just wait a few minutes for the motor to cool down, fix/replace the prop, and you’re good to go back in the air.
Cause 2: the bolt is touching the motor windings
If you screw the bolt in too far to the point that it touches the motor windings, you may find that the motor gets hot.
However, more likely than not, your motor will probably stutter, too, so you’ll have bigger problems. The solution is really easy – use a shorter bolt!
Cause 3: D term is too high
Another cause for hot motors is often an over-active D term.
The D term works by dampening noise received by the gyro – that’s why it balances out the tiny oscillations you see on an aggressive P term.
However, if the setup is very noisy, or for a number of other reasons, a high D term can lead to hot motors.
If it’s not cause 1 or cause 2, a high D term is likely the reason behind your hot motors. The first thing you can do – if you like your PIDs and don’t want to mess with them – is to soft mount your flight controller using rubber bobbins or using O rings. This will help reduce vibrations going to the gyro, and your quadcopter should not only have cooler motors, it should fly better overall, too.
If you’ve already soft mounted, you can try reducing the D term to see if that helps.
Again, this should only be a cause for worry if your motors are coming down too hot to touch.
If they’re hot, but bearably so, you can live with and your quad should not have any issues.
Cause 4: Filters are not configured properly
The final(legitimate) cause for hot motors is misconfigured or over-applied filtering.
Betaflight, Raceflight, and Butterflight all use some degree of filtering to help reduce noise in the gyro. However, removing filtering on a clean build can really supercharge the performance and smoothness, so the Dynamic Filter in Betaflight and the Fast Kalman filter in Butterflight take advantage of that.
If you have a noisy setup, you may not be able to afford removing the filters. If you’ve got all the notch filters turned off and are landing with crazy hot motors, you can first try to:
- Reduce the D term
- If that doesn’t work, start re-enabling the filters one by one until the motors cool down
I know this isn’t an ideal solution, since that will affect performance, but you can retune and get your quad flying relatively satisfactorily again.
Cause 5: It’s really hot outside
The last cause for hot motors is flying in really hot weather with the sun bearing down.
If you have black or dark colored motors, they’ll absorb a lot more sunlight and get really hot to the touch, regardless of filters, D term, or whatever.
If you’re trying to diagnose or set up a build, don’t do it where the sun is cooking up your electronics. You won’t be able to tell if it’s the sun that’s causing it or if it’s an issue with your quad!
I’ve found that these are the most common causes for hot motors. How have you found hot motors and diagnosed or fixed them?
If all else fails, you can of course replace your motors. It’s the most expensive of the options, but if you fly really rough, you may just have to do that!