When a motor is overloaded in order to generate more power, it consumes additional electrical current through the windings. Because the copper windings have resistance, most of the heat generated within the motor comes from this current.
For instance, if you double the current in order to increase power the torque and power from the motor you will in turn increase the amount of copper heat being generated by a factor of four.
The motor warms up as a result of this heat. Motors are big, heavy chunks of metal, so they absorb a lot of short-term heat without increasing excessively in temperature. However, if the heat builds up in the motor windings faster than it can be dissipated to the air outside, you risk insulation burning off the copper enamel, nylon gears softening and ripping, or magnets losing their magnetism. At this point, this is where the motor can become “burnt up.” This happens when the high motor currents flow through the motor over longer periods of time. We see this in A LOT of cheaper ebike motors. They tend to increase power inefficiently causing high heat and lower performance long term.
