The rotation of a microwave's turntable stops when the door is opened after it has been turned on as a safety feature. This safety mechanism is designed to prevent any potential harm or accidental exposure to the microwaves that are generated when the microwave is in operation.
When the microwave door is opened, the interlock switches, also known as safety switches, are triggered. These switches are located around the door and latch area and are responsible for interrupting the power supply to the magnetron (the component that generates the microwaves). When the power to the magnetron is cut off, it stops producing microwaves immediately.
The turntable rotation also stops as a result of the power interruption. The turntable is primarily intended to ensure even cooking by rotating the food during the microwave's operation. When the microwaves stop, there's no need for the turntable to continue rotating, and it comes to a halt as well.
The combination of safety interlock switches and the automatic cessation of the turntable rotation when the door is opened ensures that no microwaves escape from the microwave's cooking chamber, preventing any potential harm to the user. This safety feature is essential to minimize the risk of microwave radiation exposure and is present in all standard microwave ovens to comply with safety regulations and protect users from accidents.