Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


AHU motors running at FLC

AHU motors running at FLC

AHU motors running at FLC

Hi all:

We have a couple of AHUs in our property that are not achieving design flow and the contractor proposed to replace pulleys.

When we did the first couple of units, and we managed to achieve design flow but now the motors are running at FLC (Full load current).

Contractor is saying that motors are designed to operate continuously at 100%FLC and that there is no standard regarding limiting the time that an AHU motor can run at 100%.

I'm not comfortable with the situation as these tests are done with clean filters and once filters start to collect dust, I'm forecasting that the motors will be going above FLC.

I was looking for a standard or anything technical where I could support my argument of not accepting continuous duty at 100%, but couldn't find anything.

Any hints on how to address this issue?

Thanks a lot.

RE: AHU motors running at FLC

In the IEC world a motor with a S1 duty should be capable of running 24/7/365 at FLC at an ambient of 40°C, because that is the conditions it is rated for. Many motors don't have much margin in the design, so a slight increase in load or ambient temperature could tip them over the brink. A slight drop in supply voltage will also tend to increase the current, because to a first approximation the motor behaves as a constant power electrical load over a narrow range of voltage variation.

I personally don't think good design should operate the motor at the limit of its capability, and there's almost certainly an efficiency penalty because motors are generally optimised for maximum efficiency at about 75% - 80% of rated output. Over a few years the additional energy consumed because of poorer efficiency will probably exceed the value of the motor.

You'll need to look at the fan curves to determine how the absorbed power changes as the filters clog up. Some fan designs will see the absorbed power drop as airflow reduces.

RE: AHU motors running at FLC

Super response and can't add much more. Unless you can show that you have a fan power curve where dirty filters will increase power from clean then I think you will find it difficult to prove. It's far from good design practice but officially it will work, just. But just is good enough.

The margins on over current are much tighter now than 10 or 20 years ago so I think you will just need to monitor the current, operating temperature and any failures.

What sort of guarentees/ warranty have you got. If it results in overload then the motors should fail within a year.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: AHU motors running at FLC

A clogged filter reduces the motor effort. That's why the air handler will list the minimum back pressure required. No back pressure - the motor will be over the FLA.

I'd rather have a little head room on the motor loading but running them right up against their FLAs is done all the time without obvious issues. They are typically in a fast moving air stream.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AHU motors running at FLC

Thanks a lot guys.

It seems that was seeing the issue of the clogged filters all wrong.

ScottyUK made a very good point on the motor efficiency and I also had thought about it, but I was looking at our design requirements and it does refer that the motors have to be NEMA Premium efficient as per NEMA MG1 and that the motor size has to be large enough to not require a service factor above 1.0. So, I don't think I have a leg to stand regarding to motor's "sweet spot"...

RE: AHU motors running at FLC

Yes, this is a mass-flow issue so the motor's work is to move air mass. If you restrict the amount of mass that can move then you reduce the motor's work load. It's rather counter-intuitive until you recognize the mass-flow aspect then it will square with your intuition better. People have a lot of problems with pumps in this same area. Throttling most centrifugal pumps reduces the load because again, the mass-flow is reduced.

Almost everyone, at first, thinks a vacuum cleaner with the nozzle stuck to something is working extra-super hard and the nozzle has to be unstuck immediately or the howling beast might explode. In reality it's howling because you've completely unloaded it and it speeds up because there's no work to do. Same deal.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AHU motors running at FLC

Hi MedicineEng,

All motors have a 'sweet spot', and it's almost always about 80% load. Choosing a premium efficiency type (the IE3 category in the IEC world) just shuffles the whole efficiency curve upward one or two percent, but it retains the same basic shape. A larger motor running at 80% load will generally use less energy than a smaller one running at 100% load, regardless of whether it is a premium or standard efficiency class.

The manufacturers publish efficiency curves for their motors. Over here the bigger manufacturers make them available online; I imagine it's much the same in your patch of the world too. If you can't argue the case for a no-cost change of motor based on the contract, you may be able to make an economic case for an at-cost variation to contract and know you'll recoup your costs within a fairly short period. How big are the motors in question?

RE: AHU motors running at FLC

Thanks for the hint. I'll try to get some motor curves to see what's the payback if we change for larger motors.
From what I saw, I think we're good on the cables, I need to check the rest of the switchgear.
Motors range from 3kW up to 50kW.

RE: AHU motors running at FLC

I just looked and any motors over 10HP (7kW) tend to have efficencies in the low to mid 90% at FLA. I believe running them below FLA is more of a waste. Whereas motors under that power... just suck all-around in efficiency (FLA efficiencies of mid %60 to %75) and Scotty is probably correct.

Keith Cress
kcress - http://www.flaminsystems.com

RE: AHU motors running at FLC

Hi Keith,

You need to factor in things like cost of energy, annual running hours, overhaul costs etc. to get an overall life-cycle cost.

I'm currently replacing a lot of older 30kW motors which run flat out 24/7/365 because the differential in cost between overhaul and buying new ones gives me a return in less than a year. The motors eat nearly £20k worth of energy per annum, and that's at a relatively favourable rate because we take our supply at high voltage. For us 30kW seems to be near the sweet spot for economic payback; for any general-purpose motor under 5.5kW the economics work out that it's cheaper to replace the motor than overhaul it. For the big HV machines the economic payback on efficiency is too long to justify replacement, although if we ever bought a new one for other reasons then efficiency would be an key consideration.

Everyone will have a different set of circumstances and should do their own calculations: what pays back at my site won't pay back at many others.

RE: AHU motors running at FLC

You're the Engineer: if you don't want the motors to run at FLA, increase the motor size.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close