I'm still thinking Larry will run it faster than 63% when using the machines anyway. that big Unitronex will need 2000 cfm to handle the dust. Big shapers with big cutters generate chips a mile away from the dust port so he will use more of that 10 hp than he thinks. Based on my experiments I'd guess 50 hz is about as low as he will go unless just cleaning the air like an ambient. The extra energy cost isn't as significant as the saving from the slow start available with the vfd. Dave