Running at FLA
Running at FLA
(OP)
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
Thanks for you input in advance!
Doug





RE: Running at FLA
You could use a larger motor, but not too much larger, or PF and efficiency will be a problem.
RE: Running at FLA
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.
RE: Running at FLA
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.
RE: Running at FLA
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
RE: Running at FLA
The reliability mostly depends on how hot it gets over a period of time. Sizing up might be beneficial
RE: Running at FLA
Quando Omni Flunkus Moritati
RE: Running at FLA
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.
RE: Running at FLA
You might consider some economical monitoring equipment that could tell you when there is a problem with operating parameters if you have not already.
RE: Running at FLA
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
RE: Running at FLA
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
RE: Running at FLA
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.
RE: Running at FLA
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
RE: Running at FLA
Thanks Again,
Doug
RE: Running at FLA
0.9 x 0.9 = 0.81, which is within the ballpark of those above postings calling for 80% motor shaft load sizing.
RE: Running at FLA
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