Why do brushless motors heat up? I would like to know what makes a brushless motor heat up. I'm aware of the problems generating the high temperature such as wire contact problems or overloaded motors. I want to know what makes a normal motor in normal conditions warm up. Is it related to the Joule effect?
The motor is converting electrical energy in mechanical energy, and unfortunately all energy conversions lose some energy to heat. This site states the cause of the heat losses as:


Friction losses due to bearings etc.
Windage losses due to air resistance.
Iron losses caused by magnetic paths of the motor (hysteresis losses and eddy current losses).
Ohmic losses from current flow.
Stray losses: other