As mentioned before, here and here, VFDs are an excellent means to saving money and extending the life of your motor. Amanda Griffith breaks out the benefits of variable frequency drives. For example:
Variable-frequency drives are the most effective energy savers in pump and fan application. In most facilities, centrifugal pumps and fans run at one speed, with an automatic value, or some mechanical means, varying fluid flow rates. With a variable frequency drive, users can change motor speeds electronically. By adjusting a pump or fan speed to meet only the desired rate needed, one can significantly reduce energy costs while also saving on equipment maintenance.
Read the whole thing.