There are a few things to consider when getting a fan controller, purchase one that will quiet your exhaust fans within the tent or room, then pick out a variable frequency drive.
For amore inexpensive route, start with the solid state fan speed controllers, they can be as little as $25 therefore they are very popular due to being so cheap! However for long-term farming, these are less of an ideal option; this is due to the fact of how they function, the motor hums. This is not only noisy but can lead to the failure of the fan prematurely.
These controllers are also not the most compatible with various fans as well, therefore it is not generally recommended to use them. However, sometimes to get over the initial hurdle of cost at the beginning of the grow operation, they are a viable option. Just be sure that they will indeed work with your setup, though they aren’t the best fan controller on the market.
Variable autotransformer fan controllers are a preferable choice as compared to solid state units which do not produce a hum as they give a sine wave of power voltage continuously. This makes them gentler and less likely to damage your fan like solid state controllers can.
For the top of the line fan controller, available to growers on a budget, it starts around $75. Be sure to purchase a unit with amp ratings that are well above the amp requirements in order for you to run it and plan. Keep in mind that when the fan turns on, there is a surge of power, so be sure your amp rating on the variable autotransformer is high enough for the individual type of fan that you choose to run.
Amp ratings are of concern to grows with multiple fans running through the controller. A 6-inch or 8-inch fan will be rated under 1.5 amps and therefore should be no difficulty running on a 3 amp 120-volt variable autotransformer.