If an electric motor starts smoking a coil has shortend.
Unfortunately that is not repairable, you can only replace the motor or the saw.

The sympoms of a worn brush are:
1- some lighning effects
2- power loss

IF the brush doesn't make good contact it can't conduct enough electicity, so it can't smoke (required a lot of current)

A shortend coil gives sunndenly a lot of (not very healty) smoke, and melts the brushes.

Electic tools for home use are not made for contimuous use, usually 10 minutes use and 50 for cooling down. Professional tools have a much better work cycle.
If you overheat them they will burn out.

Rob