Adding a power vent to an attic with a ridge vent is not recomended by any ventalation manufacturer that I know of. The power vent will serve as the exhaust vent and the ridge vent will be your intake, thus there will be no air moving betweem the power vent and the soffit vents. If there is little air moving now, then you might inspect your soffit vents and make sure that they are not covered with insulation or stopped up with some other debris. If your ridge vent has a filter in it, you should check to see if it is clogged with dust and debris, or if it has insects nesting in it. I have seen this before.
Next, you should check to see if you have the proper amount of ridge vent and soffit vents. For 1000 sq. ft. of attic space you should have 13.32 lineal feet of ridge vent ( 18 sq. in. of net free area per lin. ft. ) and 240 in of net free area in the soffit. Be carefull of perferated vinyl soffit because most that I have seen do not have the amount of net free area to have balanced system with ridge vents and soffit vents.
In my opinion the best system is with a high quality,filterless ridge vent and adequate soffit vents.