This is why most speed controllable motors are DC motors and not AC. AC motors need almost 5 times the current to start (single phase) and are not considered ideal for running at reduced RPMs of rated...that's why they won't start when the speed controller is on less than full speed. You'll need to go to a DC fan motor, and speed controller, to make this kind of set up work....it's not you, it's physics! However, there are/is more than one way to skin a cat...I would try installing dampers in the duct work and letting the fan run full speed, it will be cheaper than going to DC drive. In Home DePot or an A/C supply house can be found what's called a "dampered tab collar"...Instead of reducing the speed of the motor, reduce the airflow by using this and damping it off until you get the tent to run at the temps you desire. They come in various common duct sizes, and are used for just this purpose in branch runs of flex duct in A/C and heating systems....only cost a couple bucks too! You will get the same effect by damping off airflow as you would from reducing fan motor speeds. Hope this helps!