# AHU blower overload



## MechanicalDVR (Dec 29, 2007)

What's the nameplate FLA of the motor?


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## gottspeed (Mar 8, 2010)

MechanicalDVR said:


> What's the nameplate FLA of the motor?


Sorry, I could have been clearer. Motor FLA is 10A.


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## scotch (Oct 17, 2013)

Could be it's oversupplying air flow ; doesn't take much to overcorrect ; so you could adjust the v-belt sheaves to get it down to 10A ...the FLA....and see what happens on the air balance .


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## gottspeed (Mar 8, 2010)

scotch said:


> Could be it's oversupplying air flow ; doesn't take much to overcorrect ; so you could adjust the v-belt sheaves to get it down to 10A ...the FLA....and see what happens on the air balance .


I never thought to adjust the pulley, great advice.


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## telsa (May 22, 2015)

gottspeed said:


> I'm dealing with an air handling unit that is tripping its supply air fan overload. The OL is set at 10A (FLA), but the motor is drawing a steady 12.5A while running so it takes a few minutes, but still trips.
> 
> This unit is out of warranty because it was delivered over a year ago, but still hasn't been commissioned. My boss says that if the system is out of balance it can overload a blower.
> 
> ...


You should suspect mechanical issues.


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## gottspeed (Mar 8, 2010)

telsa said:


> You should suspect mechanical issues.


First thing I checked was if the shaft spun freely. It does.


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## scotch (Oct 17, 2013)

Sorry ; my Apple seems to like to use it's own words....I was saying 
"overcurrent ".


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## gottspeed (Mar 8, 2010)

I think another relevant question here is: If the code states that you can adjust OL settings to +15% or +25% on motors with SF lower or higher than 1.15, how can it be that you just set an electronic overload to motor FLA?

There would be no way to accommodate for motors with different service factors.

I guess this is a reason not to match IEC and NEMA.


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## scotch (Oct 17, 2013)

There's a difference between thermal overloads and electronics overloads in their accuracy ....and obviously ;what you use to measure the motor current as well comes into play ...all that RMS and Average reading stuff ; and distorted waveforms blah blah blah !
Really after you've got the airflow problem fixed it's normally single phasing you're protecting against ....so set the electronic and be happy !


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## Bird dog (Oct 27, 2015)

gottspeed said:


> I think another relevant question here is: If the code states that you can adjust OL settings to +15% or +25% on motors with SF lower or higher than 1.15, how can it be that you just set an electronic overload to motor FLA?
> 
> There would be no way to accommodate for motors with different service factors.
> 
> I guess this is a reason not to match IEC and NEMA.


You may need to take a look at 430.32(A) &(C). 2014 NEC


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## Big John (May 23, 2010)

Overloads are already designed with a % over nameplate, that's why yours is going out at ~125%. You set them at your precise FLA for best protection.

I agree, upsizing is a BS answer. I also agree with the belt idea: First thing I'd do is check tension, then roll those off and check unloaded amps.

90% of the time this is a driven load issue.


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## MechanicalDVR (Dec 29, 2007)

Take an amp reading with the unit closed up and then again with the access door open fully.


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## micromind (Aug 11, 2007)

If in fact, the O/L is set higher, maybe the new motor will be one size larger than the existing one...........


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## Cow (Jan 16, 2008)

I agree with the other posters, I would rather change a pulley temporarily and get the current down to 10 amps than go the other direction and upsize the OL with the potential for it never to be corrected.

Whoever is responsible needs to understand there is a very real chance they may smoke a motor running it harder than it was designed for while they sort the HVAC balancing out.

If the person signing your paychecks says to replace the OL though, then that is what I would do.


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## gottspeed (Mar 8, 2010)

Cow said:


> If the person signing your paychecks says to replace the OL though, then that is what I would do.


I think I'm gonna do both, take due diligence on my part with mechanical adjustments to try and drop the current. Then proceed to do what I'm told after that.

Thanks for all the troubleshooting advice guys!


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## JRaef (Mar 23, 2009)

In a centrifugal blower, load = flow, so if the blower was designed for a specific flow rate and it changed, the motor will overload. Flow rate is affected by things like dampers, duct size, elbows etc., and as previously noted, belt sheave ratios. 

That said, unless someone made a design mistake it's unlikely the sheave ratio is wrong, especially if it was working for a year and just began overloading. Since you believe you have eliminated mechanical binding of the motor, it's then far far more likely that someone opened or even removed a damper that was necessary for the flow design or has tapped into an air duct for something without telling anyone, or a duct seam has split open. That drops the static pressure, which again affects the flow and increases the load on the motor. Bottom line, I don't think it's a good idea to _*start *_trying to fix a mechanical problem with electrical solutions that might involve damaging equipment. Have the HVAC guys fully vet the ducting / damper system for leaks/changes etc. first, then go back to the supplier and ask why the motor might be overloading, THEN start thinking about the motor itself.

As to the NEC section "allowing" 115-125% on OLs, this is a commonly misunderstood and misapplied issue. The NEC is addressing overall design requirements, *but this must be compared to what your SPECIFIC OL relay says in it's instruction manual*. Some mfrs tell you to make the adjustment in the selection / setting, some do it ahead of time for you already. So if you are using an OL relay that _*already *_adds the 115% as it's pick-up point, it will tell you to set the OL at the nameplate FLA value. If you then add ANOTHER 25%, you are setting the OL pickup point at 125% _*of 115%*_, so it is effectively at almost 144% and you lose the motor. 

RTFM...


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## MechanicalDVR (Dec 29, 2007)

This could be as simple as clogged construction filters. That's why the door test is a good place to start.


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## frenchelectrican (Mar 15, 2007)

I know many mention pulley sleeve size that can make the differnce on that and also check the supply voltage both when motor not running and running so see how much voltage it do drop.

Especally if you are on 208 volt system that you need to watch it closely. 

I know someone else mention balancing on HVAC I think one of the damper or ductwork is not properly hooked up correct. if that is related to ductwork get the tinkonckers back there and deal with it. 

Of course the O/L that do not go over the motor nameplate rating. so get the pulley sized to get the running amps down a bit.

However if that is direct driven blower then check the discarge side of blower to see why if no restriction on that. with direct driven blowers the curpit useally unthrottled suction or discharge side. 

If it did have VSD you may want to try to turn the blower speed down a little to get under the nameplate rating.


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## JRaef (Mar 23, 2009)

Just to reiterate, blocked filters or other restrictions actually REDUCES the load on the motor, because... 

Load = flow. 

Less flow, less load / more flow, more load.


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## MechanicalDVR (Dec 29, 2007)

JRaef said:


> Just to reiterate, blocked filters or other restrictions actually REDUCES the load on the motor, because...
> 
> Load = flow.
> 
> Less flow, less load / more flow, more load.


Assuming it is actually going out on the overload and not just tripping the thermal cutouts.

If it's been running a year and now there's a problem I'd look at other things than the motor.


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## PlugsAndLights (Jan 19, 2016)

MechanicalDVR said:


> Assuming it is actually going out on the overload and not just tripping the thermal cutouts.
> 
> If it's been running a year and now there's a problem I'd look at other things than the motor.


From the op:
This unit is out of warranty because it was delivered over a year ago, but still hasn't been commissioned.


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## MechanicalDVR (Dec 29, 2007)

PlugsAndLights said:


> From the op:
> This unit is out of warranty because it was delivered over a year ago, but still hasn't been commissioned.


From years of dealing with HVAC/R if it's there it will be run.

Warranty is from start up date, not install date.


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## gottspeed (Mar 8, 2010)

So much great information in this thread, thanks so much for the replies.

I think JRead is on the money, the general contractor probably added some ducts, or the system is just out of balance- this is part of a huge facility with many AHUs. When I opened the door of the blower compartment the current shot up to almost 20 amps.

We're gonna have the HVAC guys come troubleshoot. Thanks again for the advice everyone!


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## JRaef (Mar 23, 2009)

MechanicalDVR said:


> Assuming it is actually going out on the overload and not just tripping the thermal cutouts.


Yeah, I get it, that actually occurred to me about 4 hours after I posted... AHU motor designed in expecting to be in the air stream to keep it cool,so blocked filters = less air flow and the motor over HEATS, not over LOADS. 

But then I went back and read the OP, he said it trips the _overload _and he noticed the current was 25% higher than the FLA. I'm still thinking it's a mechanical issue, just not blockage.


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## MechanicalDVR (Dec 29, 2007)

JRaef said:


> Yeah, I get it, that actually occurred to me about 4 hours after I posted... AHU motor designed in expecting to be in the air stream to keep it cool,so blocked filters = less air flow and the motor over HEATS, not over LOADS.
> 
> But then I went back and read the OP, he said it trips the _overload _and he noticed the current was 25% higher than the FLA. I'm still thinking it's a mechanical issue, just not blockage.


Oh I agree 100% it's mechanical, just need a starting point to go from. 

I realize the OP is just concerned from just the electrical standpoint but I'm used to finding the problem and getting it fixed or bypassed and getting the unit back on if I can, billable time is billable time.:thumbsup:


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## Bird dog (Oct 27, 2015)

When troubleshooting equipment, you have to think "system" not just electrical. Had a piece of equipment that would blow control fuses. It ended up being a grease fitting for a carriage that wasn't greased(it was harder for the motor to move it & drew excessive current).


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## MechanicalDVR (Dec 29, 2007)

Bird dog said:


> *When troubleshooting equipment, you have to think "system" not just electrical.* Had a piece of equipment that would blow control fuses. It ended up being a grease fitting for a carriage that wasn't greased(it was harder for the motor to move it & drew excessive current).


*Exactly*, but many seem to focus on just what they consider 'their job'.


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