First let me state that I am not an electrician, so when it comes to the technical aspects of this subject, I rely on those who are.
In the Spring of 2016, I purchased a heavy duty 3-speed metal floor standing model electric fan. I used it all through the summer to help keep my shop cool, and at times it ran constantly. I can remember touching the back of the electric motor itself and found it very hot to the touch. By the end of the summer, it no longer worked, although I could hear the motor trying to make revolutions, with a humming sound, and once by pushing on the blade I could get it to turn. However after that, it would still hum, but could not turn.
When I investigated the consumer reviews of this fan, I came across someone who had the same problem, and it turned out to be the fact this fan can not be used with any extension cords. In my case, I had used extension cords, all very heavy duty "yellow" cords, but still classified as extension cords. Naturally, I then read the owners manual that had come with the fan, and sure enough, there was the extension cord warning.
Long story short, I replaced the fan and now only plug it into a direct plug. While the fan motor will get warm, it never gets hot anymore, even after extended use on high speed. My old fan got extremely hot, and just seized from the heat. I just don't understand the science behind not being able to use an extension cord (even an expensive heavy duty cord), but there you have it!