looking to get something like a variable frequency drive, but don't need something that costs $850

[CamTheNewby]

I want to get a 5 hp, 1800 rpm electric motor, and run it from 0 to 1800 at a steady rate. Variable frequency drives are all huge, very expensive units that have multiple applications. Does anyone know if there is something simpler? I literally just want a knob that I can turn from low to high frequency. Is this possible?

 

[ozmosis]

As many have pointed out, I don't understand why the discussion is referring to power (hp) all the time. I would say you need to get back to first principles and determine the torque required to do the task in hand.
A VFD is essentially a constant torque device and is linear power between the speed ranges you are looking for. You will not get "10hp" at slow speeds required.
Your opening post referred to cost as a big issue. You could be paying 3x more if you just look at power rather than torque if your speed range is 3:1.
It seems an odd test, to be honest. What are you looking to measure? Just leakage? I assume you will need some means of determining load at specific speeds, so if you are "way over-rated with the drive system" what is it you are actually testing and therefore, why are you concerned about cost?
I would have thought the major 'cost' would be the measurement equipment and, ultimately, the overall quality (or not) of her rotary valve you manufacture.