If you have no ducts in the attic, increase your attic insulation and call it good.

 

Are you implying that the fan will not increase the air flow? Are there ways to decrease the temperature in the attic without a fan?

 

As you have found out ridge vents are basically worthless. Promoted heavily on the Internet, but doesn't really work well. If yours uses a "filter" see if it's clogged with dirt. If so maybe cleaning them out with an air compressor might help. If the filter is clean that means no air is getting through. If yours doesn't use filters try creating a smoke/fog/dust cloud (don't use anything flammable !!) and see if the smoke/fog/dust makes it out of the ridge vents.

If it was my house I'd forget about the ridge vent (remove and cover up with standard shingles) and install wind turbines (in climates with a good breeze most of the time) or and electric fan (in areas with little breeze).

 

I do not like hot attics.Even with good insulation you still have that great temp differance between the attic and the house.
If you have an extremely hot attic then your roof vent is not working.
If you size a gable fan correctly and run it off a line volotage stat and humidistat ,you will decrease that temp differance.
Be aware that you may have to increase the size of the opposite gable grill.
I have never seen too much ventilation in an attic.

 

I have never seen too much ventilation in an attic.

I have, and it was sucking the suspended ceiling tiles off the grid and causing infiltration in a restaurant.

 

With ducts in the attic, reducing attic air temps can provide limited benefit. Reducing where the ridiculous amount of heat gain originates, at the roof deck, makes far more sense for ducts in an attic. This can be had by either radiant barrier or an insulated roof deck. One not so expensive and the other very expensive.

With no ducts in the attic, having sufficient insulation on the attic floor accomplishes more than forced attic ventilation to reduce heat transfer into the dwelling below. Forced attic ventilation is not recommended if you cannot assure there is an airtight barrier between attic and house. You can end up sucking air you paid to cool right out of your house and into the attic. Even with soffit and gable venting, this can happen.