As many of you know, I'm an HVAC (mechanical) engineer. I do a lot of fan, motor and variable frequency drive specification. As you go from fan to motor to drive, the device goes from purely mechanical, to purely electrical, and my level of knowledge/understanding drops off... Many building owners, electricians and maintentance guys are still skittish about VFDs, for issues such as minimum and maximum drive frequency. I'm less so, but there are a lot of minutae about what they do and what side effects they have that I am not clear on. Most vendors of both say they are safe to operate from close to zero up to 120hz. But there is a catch: They have to be compatible with the fan at the right speed and power output. If the fan is belt drive, life is relatively easy because you can specify sheaves (pulleys) to make the motor run near nominal speed to get the fan to run near nominal speed. But what about direct drive? I have a direct drive fan specified to operate at 2350 rpm and 11.1 horsepower. I've selected a 15hp motor, but at what rpm; 1800 or 3600? Ordinarily, 11.1/15=0.74 is a nice comfortable 26% safety factor. But if I select the 3600 rpm motor it will run at 39hz. VFD output voltage is a linear function of frequency (up to nominal), so instead of 480V it will run at 312V, which also means - assuming consistent power factor and efficiency - the motor is now only capable of 9.8 horsepower. Oops. So I've selected an 1800 rpm motor, which will then run at 78hz. My understanding of the standards (is thin) is that as long as the motor is rated properly for VFD operation it will be fine, but I'm not really 100% sure of that. A few potential issues: 1. Running at higher rpm should mean more friction and shorten motor life, right? 2. Electrical signal issues; harmonics, spikes, etc? I'm not one of those who equates operation of a motor at above 60hz as "overspeeding", but at the same time, I'm not totally clear on what issues may occur with operation above nominal speed. Anyone able to help clarify?