When my first electric razor stopped working, I decided to open it up and figure out why. It turns out the connection from power to the motor rotor was by graphite rods, similar to pencil leads. It's certain that the graphite was designed with a specific lifetime in mind, and once it ground down to nothing you got metal-on-metal grinding that destroyed the motor.
This assumes that you've noticed the problem before irreparable damage occurs, and you're familiar enough with motors to know what to look for and where to find parts. Neither was true in my case.
