There are several reasons:
1. Bad contact of the switch, there is an 80% possibility. The treatment method can take the switch apart, put two contacts on a screw, plug in the power supply to see if it can turn, and turn to indicate that the switch is broken.
2. Carbon brushes grind to the end and can not touch the rotor. Detailed removal of carbon brushes for inspection is clear at a glance.
3. The copper wire of the rotor or stator is burnt down, which can be judged by the nose. It has a heavy burning smell and must be sent to the repair shop.