I suspect it has to do with the load. If you only have a few frames in it or most are not full, it may not matter. If you have fully loaded frames and you try to run at full speed you will probably heat the motor beyond it's design temperatures. I always start slow and slowly increase the speed. I listen to the motor. As the honey extracts you can hear the motor speed up. Then I increase the speed. I keep the lid down but as I am finishing a load, If it tests above 18%, I sometimes open it to allow a little bit of drying. I do not leave it open for long. Feel the temperature of the motor with your hand. If you cannot keep you hand on it, it is running too hot.
I have never had the motor to hot to lean the back of my hand on it for extended length of time. Your palm is far less sensitive than the back of your hand.