AHU Fan Over-Amping
AHU Fan Over-Amping
(OP)
I have an AHU supply fan that is drawing too many amps. Don't know particulars, but it is a 40 HP fan on a VFD. I am told that they cannot turn it up past 50 Hz without over-amping. 50 Hz, however, only gives me 20,000 CFM and the system was sized for 24,000 CFM.
Am I correct in thinking that I do NOT have enough static loss in the ductwork and therefore not enough resistance against the fan? If so, can I just throttle back dampers in order to increase the static, which will allow me to go up to 60 Hz and hopefully 24,000 CFM?
Am I correct in thinking that I do NOT have enough static loss in the ductwork and therefore not enough resistance against the fan? If so, can I just throttle back dampers in order to increase the static, which will allow me to go up to 60 Hz and hopefully 24,000 CFM?





RE: AHU Fan Over-Amping
RE: AHU Fan Over-Amping
RE: AHU Fan Over-Amping
In round ducts, velocity pressure readings should be taken at centers of equal concentric areas. At least 20 readings should be taken along two diameters. In rectangular ducts, a minimum of 16 and a maximum of 64 readings are taken at centers of equal rectangular areas. Actual velocities for each area are calculated from individual velocity pressure readings. This allow the readings and velocities to be inspected for errors or inconsistencies. The velocities are then average.
By taking Pitot tube readings with extreme care, air velocity can be determined within an accuracy of ±2%. For maximum accuracy, the following precautions should be observed:
Duct diameter should be at least 30 times diameter Pitot tube.
Located the Pitot tube section providing 8-1/2 or more duct diameters upstream and 1-1/2 or more diameters down stream of Pitot tube free of elbows, size changes or obstructions.
Provide an egg-crate type of flow straightener 5 duct diameters upstream of Pitot tube.
Make a complete, accurate traverse.
In small ducts or where traverse operations are otherwise impossible, an accuracy of ±5% can frequently be achieved by placing Pitot tube in center of duct. Determine velocity from the reading, then multiply by 0.9 for an approximate average. This could tell you whether the drive is acurate
RE: AHU Fan Over-Amping
RE: AHU Fan Over-Amping
OVERLOAD ON MOTOR
a. Speed too high.
b. Fan over capacity due to existing system
resistance being lower than original rating.
c. Specific gravity or density of gas above design
value.
d. Wrong direction of wheel rotation.
e. Shaft bent.
f. Poor belt alignment.
g. Wheel wedging or binding on fan housing.
h. Bearings improperly lubricated.
i. Motor improperly wired.
j. Defective motor. Motor must be tested by motor
manufacturer’s authorized repair shop.
6
XI. TROUBLE SHOOTING
RE: AHU Fan Over-Amping
Thanks for your suggestions. It's a new unit, so hopefully most of those possibilities can be ruled out. I will verify rotation, but i am suspecting that your item "b" is the problem.
RE: AHU Fan Over-Amping
Imok has provided a good list of possibilities. I would expand on his item a to suggest incorrect pulley sizes.
Get the fan performance and power curves. Measure the actual RPM. Measure the actual power the motor is making. Measure the fan delta-P.
RE: AHU Fan Over-Amping
RE: AHU Fan Over-Amping
RE: AHU Fan Over-Amping
Take a squirrel cage fan and turn it on. Then block up the fan discharge. It'll speed up, and amp draw will go up.
Just another possibility.
Let us know when you figure it out.
Jabba