Motor Question

[ddalberto]

Not sure if this the place for this , but I bet I get an answer....I had to do a start up for another contractor on an exhaust fan at a waste water treatment plant...I didn't do the install. Small fan, dont even know the HP, but it doesn't matter here. Here's the problem. Motor starter trips after about five minutes. (overloads they installed were too small)
Motor data as follows:
460v/3ph fla .55 service factor 1.0
The overloads installed were .32 to .35 amps. (Sq D BO.51...I think)
Actual amp draw .6/.7/.6 (each of the three legs).
My question is, if I put in bigger overloads on the starter to handle up to .7 or .8 amps, will I hurt the motor, or am I being too fussy? I know I would be over the service factor, and I'm not too comfortable about it.
Don't want to mess with the adjustable shieve just yet, the cfm here is really important here and the contractor doesn't want me to do that just yet. Everything is explosion proof and I guess the air exchanges are important.
What do you guys think?
Thanks!

[pecmsg]

Your first problem is the motor is over amping. Rated at .55 drawing .6 / .7 / .6 Why?

Was the blower replaced but hooked up to the existing starter?

Whoever installed / designed this should be making the decisions concerning heater size!

[ddalberto]

Your first problem is the motor is over amping. Rated at .55 drawing .6 / .7 / .6 Why?

Was the blower replaced but hooked up to the existing starter?

Whoever installed / designed this should be making the decisions concerning heater size!

Brand new building, new installation, all brand new equipment. Not sure why the motor is overamping at this point. Would like to adjust the sheive to try and lower the amperage, but they want to keep the cfm up. Didn't design it just got hired by another contractor to start it up and make sure it's right...something his guys can't do. Just wondering if that amperage will hurt the motor over time. If yes, my advice may be to re-design and install it properly at a cost no one wants to incur.

[kdocsr05]

The actual current draw on this motor is 33% over the motor full load rating. Reducing the fan speed by 10 % would be a solution; however the new current draw would still exceed the rating on your overload.

I think you need to look at design elements; system requirements and fan curve would be helpful.

[ascj]

Brand new building, new installation, all brand new equipment. Not sure why the motor is overamping at this point. Would like to adjust the sheive to try and lower the amperage, but they want to keep the cfm up. Didn't design it just got hired by another contractor to start it up and make sure it's right...something his guys can't do. Just wondering if that amperage will hurt the motor over time. If yes, my advice may be to re-design and install it properly at a cost no one wants to incur.

You state the contractor wants to keep the cfm up. What cfm is the exhaust system producing? What was the design cfm?

You are damaging the motor by over amping it.

You were contracted to start it up and make sure it's right. Well it's not right if it's over amping. So the next step is to measure airflow and compare it to specced air flow. Then you have a leg to stand on and know how to proceed.

[mrhvacmechanic]

As the other techs stated above your first problem is over current.

Yes, adjust the drive to bring your amperage to .5 or lower temporairly. Your overloads should be sized for .5 amps, no higher.

Also as stated above , find out what cfm the fan should be exhausting. Then measure your cfm.

At this point you can figure out if the fan or the motor is capable of doing the job it was designed for.

[Roof Monkey]

Make sure the rotation is correct. On a centrifugal fan, it will still move air, but the amp draw will be higher.

[btuhack]

Any chance the motor is wired for the wrong voltage? gotta ask.
Did I just brain fart? Seems right.

[ddalberto]

Fan is rotating correctly, wiring is correct. Don't know the recommended cfm. Municipal job specced by an engineer. Have to get my hands on the prints to see the recommended cfm, measure it and see if I can ahieve with the shieve, while trying to get my .55 amps. If not, the engineer will have to start from scatch...maybe a larger fan (which will entail duct work too).
My company was hired by the mechanical contractor to start up a bunch of stuff. We didn't design, spec or install it. I was just wondering if the motor would get hurt being so close to the service factor. I think I got my answer....yes. All I can do is make a recommendation at this point.
Thanks for the feedback.

[kdocsr05]

Roof Monkey,

That’s a bold statement regarding motor current and rotation. Could you elaborate a little more on your conclusions?

[jayguy]

i understand about the cfm in places like that...i have one myself that i take care of.

what was the voltage? lower voltage will equal higher amp draw.

as far as heater sizing...like others have said...pass it on to the installing contractor.

[Roof Monkey]

Kdocsr05,
Please refer to point #2 under "operation" in the attached link. This explains it better.

http://www.captiveaire.com/MANUALS/EXHAUSTFANS/BELTDRIVEDOWNBLAST_OIPM.pdf

[kdocsr05]

Reverse rotation with radial and forward curved bladed impellers will give reduced performance and power consumption but can cause severe overloading of the motor in the case of backward bladed impellers.



http://www.engineeringtoolbox.com/fan-types-d_142.html

[John Culpepper]

i understand about the cfm in places like that...i have one myself that i take care of.

what was the voltage? lower voltage will equal higher amp draw.

as far as heater sizing...like others have said...pass it on to the installing contractor.

Make sure that you have proper voltage. I have had to upsize conductors before to compensate for distance from power source.

[flange]

they want it right. they want it quick. you know youve got issues. first, make them install the right overloads for the motor. next, check airflow, its most likely high due to not being balanced yet. make them get it right. the air balancer will adjust the shieve to correct airflow if there is no other option like closing a balancing damper. if you cant keep the motor runnig, you cant balance it right? if it were me, i would change the pitch diameter on the pulley to keep it running. i dont give a damn if they dont want to do that, the issue here is its a poop plant. a running exhaust fan moving some air is better than one that trips overloads at five minutes ey?

[pauls heating &]

regardless of the engineers wants..you are going to cook the motor as is..
the best is get your amps within specs so it runs then test cfm etc...

sounds like some extra work for you $$$$$

[ddalberto]

An update...went to the job site. Put in the proper size overloads. Check the CFM...didn't like what I saw. Pulled the cover back off the fan...guess what. THE MOTOR WAS GOING THE WRONG WAY! Roof Monkey hit it. Got my proper cfm, amperage draw .5/.5/.5 Perfect!!!
Just goes to show ya (especially you guys just getting into the business)...we can all miss a simple thing (keep it simple stupid???)
Been in the business over 20 years, hundreds of installs/start-ups, lots of experience with 3 phase etc. and I miss a simple thing like fan rotation.
Don't ever be ashamed to ask if your not sure of something, or somethings just not hitting you. I could have put bigger overloads and ran, but we don't roll that way...gotta follow through.
Thanks for all the input!

[kdocsr05]

ddalberto,

Glad to hear you solved the condition. Good call Roof Monkey. Backward bladed fans can be tricky when in reverse rotation, the standard fan laws will not apply. In this case a speed reduction may not have reduced the power component of your motor.

[hvac n-j-near]

Oops fired this off before reading to the end of the thread...Glad you worked it out!

As a designer, my guess would be the static pressure was overestimated in the design and the fan is actually moving more CFM since it is seeing less static. The added CFM is causing the fan to overload. (Seems like a poor selection IMO)

I would have someone T&B the system to see where you are and adjust the fan speed to the design CFM.

If the CFM is correct I would go back to the original designer and or the fan manufacturer.

[Randy S.]

Good job going back to the basics, something not everyone does in this day of "technology".

The service factor of 1.0 is not 1.0 Amps.

What it pretty much means is that this motor cannot take the same abuse that one with a 1.15 service factor could.

You still need to go by the rated current.

[aircooled53]

need to check belt, rotation and make sure wiring is correct as you are the start up technician.