Fluid is fluid, mass is mass, air and water are exactly the same. Mass x acceleration is force.... force over distance is work, work over time is power. When you take the door off a AH, air flow increases... it either now has a place to blow out and bypass duct work, or it has a place to suck in unrestricted by filters. Air flow increases, power use increases. Glad you have seen this first hand. Just because a fan overheats, does not mean it is over amping. It could be that you redirected cooling flow across the motor... or it's over amping moving more air.... over working. I'm not a HVAC guy... but physics is the same for you as it is for me. 25 years in the power industry starting with Naval Nuke Power school pretty much explained it for me. Yet to find a flaw in the equations all this time. Heat transfer, fluid flow, power conversions have not changed. Ask any Mechanical or Electrical engineer you may know. You don't have to take some guys word for it on the internet. Just look into it. You'll learn something new. A simple wiki search will help you. Just study power use, fluid flow, and work. All the equations are there for you.