No fuses inside. When a motor can`t turn or is working too hard it experiences a current rise through the windings. Usually you have a couple of seconds before this gets critical in a full stall but the high current load is too much heat for either the wire size or the shellac insulation on the wire to withstand for long.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.