Disconnect for Roof Top Exhaust Fan

[kwired]

Well I got the OK the snap switch under the hood is cool for our small >2HP exhaust fans. Actually, most modeles come pre-installed with a snap switch from Greenheck.

One last wrinkle..a select few Exhaust Fan are getting Brand new VFD with controls from the Building Automatic System to help with active test and balance. Fancy new stuff. Any specifical considerations to be made when disconnecting a motor which is powered from a VFD.

VFD's don't like to have the motor switched in/out of the output circuit. Switching motor off maybe isn't too big of a deal, particularly with modern drives but switching a motor into an active output is not good thing, though many modern drives may also better handle that than older ones did.
Chances are it will shut down with a fault code though and will need reset - can't just service the fan close the switch when done and walk away.

An option is an aux contact on the disconnecting means that will open an enable circuit in the controls to shut the drive down whenever the disconnect is open.

 

[mbednarik]

Switching off an active vfd does the most damage, there is an exception for motors on vfds or sensitive equipment. Most of the time there is a snap switch located under the center cover of the fan, which is suitable. For larger fans I have used manual motor starters without overload protection.