Running at FLA 1

[dougjl]

We have an application that is requiring a 3H.P. motor to run at its FLA of 3.95A constantly for extended periods of time (2-3 days). The motor has a service factor of 1.15. Would I buy anything by going to a larger motor from a reliability standpoint.

Thanks for you input in advance!

Doug

 

[DanDel]

If it is a continuous duty rated motor, it should be able to be run at its rated FLA without any problem. However, I would be concerned that if its running at FLA, it may dip into overload under some conditions during the 3 day run. You could check the running temperature at the end of its cycle.
You could use a larger motor, but not too much larger, or PF and efficiency will be a problem.

 

[busbar]

In a basic sense, running at the motor’s full-load current is OK, but realize that some headroom may be useful for deviations from nameplate voltage, frequency, voltage imbalance, and ambient temperature.

 

[electricpete]

It is a judgement call but I don't think you buy much solely by increasing the horsepower for this application which is not significantly challenged. You have margin in the 1.15SF that is not being used AND you are not even operating continuously since you shut it down every few days. Many motors operate for months and months without shutdown.

IF you are serious about improving the reliability of this application, I would recommend that you look at the possiblity of buying an off-the-shelf IEEE841 severe-duty motor (in addition to possible upgrade of horsepower). It incorporates many specifications designed to produce max reliability, particularly for (but not limited to) motors exposed to moisture or chemical attack.

 

[PUMPDESIGNER]

Tough question even if you did give us more information, but here are items to note:

Because of their small diameter small motors are less vulnerable to runaway heating than large motors - winding heat escapes more easily.

Site conditions could be excellent, that can happen (when is it my turn?). Perhaps the room is cool, motor in the shade, voltage is balanced, loads are not highly variable, who knows?

As previous posts have stated it is wiser to use the 3 hp and IF you have a problem there are excellent 3 hp options such as higher service factor, severe duty, etc. Richard Neff
Irrigation Craft

 

[lectrik]

It may also depend on the environment in which you are running. (i.e. Temperature)
The reliability mostly depends on how hot it gets over a period of time. Sizing up might be beneficial

 

[jraef]

As electricpete said, you have some "wiggle room" in the 1.15SF rating to cover minor contingencies. I agree with pumpdesigner. I would not run out and buy a new motor without proving first that there was a problem. Unless of course it is a critical process machine but has the cheapest motor available to the OEM who was only concerned that it outlast the warranty. OEMs will rarely supply a good reliable motor, as electricpete recommended, without specific requirements being demanded by the user. They must always consider what their competitors are doing, and that sometimes involves compromises with consequences that are suffered by the end user.

Quando Omni Flunkus Moritati

 

[jbartos]

Suggestion: The motor sized right on the dot should better have the nameplate data satisfied, namely, rated terminal voltage, rated frequency, rated load (without spikes), rated ambient temperature, rated altitude, etc.
If some of the above nameplate requirements tend to be somewhat off, e.g. motor rated terminal voltage is lower from the power distribution system, then the next motor HP size would provide the more reliable solution in terms of lower motor overheating since the motor current would be relatively less increased at this higher HP rated motor for the same motor load.

 

[buzzp]

I would stay with the same motor unless measurements prove otherwise (as previously suggested). It sounds like your motor is operating in its designed range for best efficiency.
You might consider some economical monitoring equipment that could tell you when there is a problem with operating parameters if you have not already.

 

[coingy]

dougjl,

Before the days of the "bean counters" an old engineering rule of thumb was to size motors so they ran at 80% of the duty thus giving the spare capacity required. I still apply this rule but just don't advertise it too much.

Cheers, Coingy

 

[PUMPDESIGNER]

I like coingys method.
We do about the same.
Much ado about nothing this idea of closely matching the horsepower to the load coming from theorists crunching numbers for the short haul but never called to account for the actual outcome over the next five years as they brag about their finely tuned analyses. PUMPDESIGNER

 

[electricpete]

good comments coingy and pumpdesigner, I am pretty sure anyone here would agree it is good to provide some margin in a new installation.

I am likewise hopeful that you will agree that the criteria for sizing a new motor is a little different than the criteria for ripping an installed motor out.... which is the subject of this thread.

 

[PUMPDESIGNER]

electricpete
I think you make an excellent point about replacement versus new design, new construction. Please correct me if I am wrong about how I took your last comment.

In existing installations, if one is knoweldegeable and obtains all the required data, it is possible to match the motor to the load more closely and obtain that efficiency the desk jockey thought about.

However in new construction there are way too many variables to finely tune that puppy so closely.

Is that where you were headed electricpete?

I really like to work on existing systems if they are willing to pay for the up front costs of site analyses, or obtain the data for me to work with. PUMPDESIGNER

 

[dougjl]

I would like to thank you all for your comments. It has been very helpful. To be honest the motor that was specified for the task was actually larger than what should have been required, due to some recipie changes, the stress on the motor increased causeing it to run near full load. So it sounds like we did as most were suggesting and were fortunate that we had excess capacity available.

Thanks Again,
Doug

 

[jbartos]

Suggestion: Perhaps, the motor "80% of rated load sizing" might be based on 90% of motor terminal voltage (continuous voltage drop) and 90% of the efficiency (due to motor-load efficiency deterioration in time). Then,
0.9 x 0.9 = 0.81, which is within the ballpark of those above postings calling for 80% motor shaft load sizing.

 

[Marke]

Oversizing the motor a little will result in:
1) Reduced motor heating and therefore longer life
2) Possible increased motor efficiency
3) Higher starting currents if conventional starter is used
4) Higher price

If the motor is operated at it's ratings, then it should survive for the design life of the motor, but for every ten degrees C you reduce the temperature rise, expect to double the life of the insulation.

Best regards, Mark Empson

http://www.lmphotonics.com