What happens if I reduce the air flow for an exhaust fan?
Fans draw a load in relation to the amount of work they do.  It sounds as if you are closing the discharge duct of the fan and the fan is working to compress the air in the discharge duct.

Set your system up so the discharge duct is left open.  Close the inlet duct while monitoring the Motor load (current).  You should see the Motor load go down and the fan speed remain the same or go up a little.

Restricting or blocking the inlet results in the fan not having to do as much work.  It moves less air and any air it does move can be freely discharged in the exhaust duct.