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Service factor duration

Service factor duration

Service factor duration

How long can motor stay in full Service factor?

Everybody says for shirt time ..
But short time means ? How many minutes or seconds?

If you motor 40kw and service factor 1.15
How do you set the thermal overload without RTD

Thanks in advance

RE: Service factor duration

SF rating is continuous with the caveat that the motor will run hotter due to higher load loss with consequent reduction in winding life. You set the thermal overload protection for the SF current.


RE: Service factor duration

A five year motor might run 4 years if you run it at the full service factor. That's allowed. You decide the trade-offs.

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

RE: Service factor duration

As I recall, years ago it used to be defined in NEMA MG-1, I seem to remember it being 15 minutes per hour, i.e. a 25% duty cycle. But I have tried to find that in older copies of MG-1 that I could find and it wasn’t there.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Service factor duration

NEMA MG 1-2014, 14.37.1.

When the motor is operated at any service factor greater than 1, it may have efficiency, power factor, and speed different from those at rated load, but the locked rotor torque and current and breakdown torque will remain unchanged. A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared to operating at rated nameplate horsepower. Insulation life and bearing life are reduced by the service factor load.

As Keith says, it's a trade-off. I prefer to go in opposite direction and overspec the motor slightly to ensure a long life.

A great FAQ at NEMA aka the horse's mouth



RE: Service factor duration


Standards do not state a duration time to operate the motor under Service Factor condition. Also, Note that accordingly with NEMA MG 1 Clause 14.2, SF only applies to usual service conditions.

In real life there are some OEM that prefers to use motors with SF in order to avoid to install a larger motor, handelling the "overload percent peak" with the SF percent (compressors and pumps). Also, this is directly related with the energy savings.

However, if you have to operate a motor at its SF during long periods of time of very frequently my recomendation is to buy a larger motor.


RE: Service factor duration

If you are looking for a precise overload protection you need to have the overload relay´s curve and then you can set the max amp for your SF

1,15 SF means that the motor is capable to draw 1,15 of FLA if voltage, amb temp, freq etc. are kept within nominal values

RE: Service factor duration

Probably goes without saying, but operating motors above 1.0 SF in electrically hazardous locations is not a good idea.


RE: Service factor duration

Dear Mr Alms88
This is my personal opinion and accept NO legal responsibility whatsoever. Caveat emptor.
1. I am of the opinion that " Service factor " is a sham, created by a group of motor manufacturers in the US. It is NOT an international Standard such as IEC or ISO. BTW: SF does NOT appear in any IEC publications.
2.1 Neither NEMA nor any US motor manufacturers would make a declaration that their products are designed for running continuously on SF 1.15, ... or even 2 without any detrimental effect.
2.2 Neither NEMA nor NEMA members would give you a definite answer the duration of "short time".
3. In general, ALL motors can run at 1.15 times the rating for say 10s in say 24h.
4.1 Thermal over-load Class 10A, 10 20..30 having different tripping times.
4.2 Select the correct Class and set it to the rated current per the motor name-plate. Ignore the SF totally.
Che Kuan Yau (Singapore)

RE: Service factor duration

The approach to service factor in the design process NEMA world vs IEC world is sufficiently different that the result gets to essentially the same place.
See the attached presentation, pages 6, 7, and 8 for NEMA's point of view on how service factor compares with the similar concept in the IEC world.

I am aware of one US compressor manufacturer that always sets up centrifugal compressors with max amps = 1.15 FLA, unless the customer specifies otherwise. The max amps is active control of compressor massflow, so the motor current stays at 1.15 FLA as long as the compressor is "Fully Loaded", this can be hours or weeks.

With large motors 400 HP and above this seems to work OK.

RE: Service factor duration

I was always told operating any motor @ > 1.0 SF will shorten its life from design, and of course it will. It is also my understanding motor manufacturers take this into account, and that the purchaser can expect to achieve the full shortened operating life provide there is no exceedance of load beyond SF. My employer built coal-fired generating stations that loaded most of the auxiliary motors right to the edge of their 1.15 SF when the unit was at full load, which it could and often enough did maintain 24/7 for weeks at a time.

My utility first began constructing these plants shortly after WW2, and their expected operating life was often chosen to be no more than 35 years, often with a view to their being replaced with newer technology and higher efficiency power plants in the future. "Life extension" projects were undertaken at a few of the more strategically located ones . . . but very near the turn of the millennium the last of them was decommissioned, and to the best of my knowledge only the motors of a few of the electrically driven boiler feed pumps ever failed during all those years.

So on that basis my utility would have answered the OP's question of "How long can motor stay in full Service factor?" with the word "Continuously."

Nowadays my utility specifies that trafos are not to be deliberately loaded above their continuous ratings, except when one of a pair of them is out of service, in which case it is permissible to load the transformer to what we call the "10-day limited time rating." I also recall seeing a transformer station where the trafos were loaded to 1.5 x their continuous rating . . . for 20 seconds, followed by an almost-no-load-at-all period of forty seconds or so [ the load was some manner of steel mill process ]. The trafos operated for decades without sign of injury.

Analogously, if one wishes to exceed the loading of a motor beyond, say, 1.15 x continuous, some manner of limited time rating would have to be developed and rigorously adhered to.

Hope this helps.

RE: Service factor duration

One thing I'd add is that the predicted "life" of a motor is generally an elusive unknown quantity.
There are a diverse number of failure mechanisms, not all of them load related or age related.
ambient temperature, moisture, contaminants, loading patterns, supplier quality control, rodents, vibration, voltage, voltage spikes, lots of things can play a role.
Sometimes when a motor fails even after post-mortem there is no obvious cause (we humorously chalk it up to rumoured ancient indian burial grounds under our plant).
Modern thermosetting insulation systems with epoxy and polyester resins don't particularly obey Arehennius equations, so even insulation aging is not an easily quantifiable thing these days.
Nevertheless we know that the more you push the loading the more you are pushing your luck. How much, you can make an educated guess for a specific motor but it's not necessarily straightforward.
The older motors tended to have more margin designed in, the newer motors not as much.

(2B)+(2B)' ?

RE: Service factor duration

To electricpete's point, I don't thing the over heating problem with this motor had anything to do with the service factor...

That's ground corn by the way.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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