Many AC sewing machine motors have carbon brushes that rub against the armature of the motor. As the AC current moves back and forth the wire coil and armature-brushes cause the drive shaft to turn. If, however, the armature becomes coated with carbon from the brushes, the motor slows down and may fail to operate. One solution is to replace the carbon brushes.
Note the brush consists of a spring and a small block of carbon. Remove the old and insert the new.