So I think I know the answer to this but want to confirm and ask further questions.
Whenever you run most microwaves in timer mode, it stops the microwave operation at 0 seconds, but continues to leave the light on, the fan running and even the tray rotating for an extra 2 or 3 seconds. Why do they do this? One manufacturer's site mentions that its to cool down the elements inside.
But if this is the case, does that mean that if you manually stop the process by either hitting clear/stop or just opening the door, are the elements not being cooled down as they should? Because manually stopping the process prevents the cool down cycle from happening. Will manually stopping the microwave shorten the lifetime of the microwave?
Best Answer
I had a microwave oven that would keep the fan, light, and turntable going until the food was rotated to its original orientation. That way it was easy to grab the handle of your tea cup :)
I've repaired microwave ovens in the past, and I don't think that a few extra seconds of airflow (after the cooking is completed) would help the longevity. They are designed so that nothing gets hot except for the food.
Unless, of course, it is a dual microwave/convection model. Then a cooling cycle would keep the hot elements from over-heating the convection pathways.
Update:
I've been doing some research and have found additional information. The microwave ovens are not actually all that efficient from an electrical power standpoint. For example, a representative oven I found online cooks at 900W but requires 1350W from the wall outlet.
This relates to your question because all that extra power is dissipated in the oven as non-functional (not-for-cooking) heat, and is removed with fans. Most of the excess heat is generated in the magnetron, and a little is used for the controller, user interface, etc.
Removing some of this waste heat from the magnetron may very well improve the longevity of the oven. The design engineers may have reduced the oven's passive venting by including extra active fan-based cooling.