Can I install a power fan if I have a ridge vent already on my roof?

It is not recommended to mix two different types of attic exhaust vents.

Mixing a power vent with a ridge vent can short-circuit the attic ventilation system just as a gable vent can as discussed above. This happens because air follows the path of least resistance. When the power vent turns on, it can pull air from the ridge vent, which could lead to weather infiltration and unbalanced airflow along the underside of the roof deck. When the power vent turns off, it acts like a roof louver — an opening on the roof without a motor. In this scenario, the ridge vent pulls its intake air from the power fan leading to possible weather infiltration and less than optimal ventilation along the underside of the roof deck. Basically, mixing two different types of exhaust vents on the same roof that has a common attic is not recommended because it can lead to short-circuiting of the attic ventilation system.