AHU Fan Over-Amping

[BronYrAur]

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?

 

[MintJulep]

What type of fan? What is its curve?

 

[BronYrAur]

I'll have to find a curve, but it is a Carrier AHU, with a AirFoil Standard Wheel AFMV01301

 

[imok2]

It seems to me that you should determine if in fact you are only delivering 20,00 CFM by accurately taking a series of velocity pressures as per traverse on round or square ducts.
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

 

[BronYrAur]

I have a balancer working on the air flows, but do you agree with my thinking on amp draw v.s. static resistance?

 

[imok2]

That could very well be the case>

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

 

[BronYrAur]

Imok2,

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.

 

[MintJulep]

If all you know is what you have told us, then we can all make guesses for as long as you want.

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.

 

[VEEKRISH]

Assuming that everything else is as per design, and, that the AHU is new, motor is new, installation is proper, then it is very likely that the AHU Pulley may be sized for higher speeds. One easy way to check - if your Return is not ducted, just block the RA side and see if the increased static solves the problem. I had a simila experience once with a CARRIER Air Handler. Finally had to change the pulley, slow down the fan and everything fell in place. Turned out the guys at the factory figured a higher static for the fan while selecting the pulleys and speed. Happens often, I guess.

 

[krb]

I have also seen cases where the pulleys were reversed, they put the motor pulley on the fan and the fan pulley on the motor. Doesn't happen often, but you can't always assume they did everything right at the factory.

 

[Jabba007]

Sometimes high duct resistance can bump the fan off the curve and make it over speed and over amp as well. Not sure about how that would work with a VFD in place... but on a regular fan/motor without a VFD it'll do that sometimes.

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