The whole purpose of installing electric fans is to remove the original belt driven unit. Any speed above say 25kmh is sufficient to allow the radiator to do it's job (ie airflow through the radiator) the belt driven fan sucks the air through.
By removing the fan and using twin elec fans and wireing them through an adjustable thermostat they only kick in at a certain temp. As a bonus, the engine is quieter, "pick up" is more noticeable and fuel economy improves.
I have used elec fans on all my previous cars and if set up properly are a cost effective mod.
All the best