Minimum fan speed for vfd

[MeatWagon]

I am evaluating the energy savings for installing a vfd on a 30 hp cooling tower fan. Can someone tell me what is the minimum speed I can expect the VFD to run reliably?

 

[MintJulep]

The VFD itself will happilly try to run the motor as slow as you care to.

The limiting factor will most likely be motor cooling.

Ask the motor manufacturer.

 

[RossABQ]

There is another very important factor on CT fans with gear drives, and that is that the gears and bearings are usually lubed by splash lubrication. At speeds less than ~ 25%, there is inadequate splashing and the gears./bearings burn out. Some gearbox vendors offer a pressure lube option.

Another consideration for the large fans is "cogging", also related to the gearbox.

Stay above 30% if you want to be assured of long life.

 

[sterl]

At some combination of velocity, discharge humidity, outdoor temperatures, as well as local wind and wall effects, you will run into plume reversal....

This can be a minor detail and self managing if the moisture laden air simply turns around and goes back through the tower. But it can lead to some real damages if the CT is winter time operational or if you have confines that can lead to mold growth or similar.

If the cooling tower is in the middle of a field and well above ground, you could experiment to establish the limit; or you can undertake the psychrometrics based on your "season" of operation and limit or potentially instrument accordingly.

Those who operate VFD's 24/7/365 have found that pumped volume becomes an accompanying topic at O/D temperatures below 45-deg. F.

 

[marcoh]

I was told by a motor supplier motors should not run below 20% speed to ensure adequate cooling and prevent overheating.

 

[ozmosis]

Below 25%, a cooling tower fan will probably not be doing anything anyway.
There are VFD's on the market that also include a function called Sleep/Hibernation mode. This, in combination with a minimum speed will automatically shut the VFD down if the speed is idling at minimum for any length of time. If it switches off automatically, then this is where the big savings in energy, low acoustic noise and wear/tear will be minimised even more. Very common on cooling tower applications to use something like this.

 

[absrbrtek]

I'm with Rossabq, 30 Hz is a good minimum speed. The motor fan can't cool the motor properly below that speed in most aplications. If the motor is driving a gearbox, lubrication issues come to mind.