FLA Formal Definition 9

[PUMPDESIGNER]

I am stuck on FLA (Full Load Amps) for motors.
Where does the formal definition of FLA come from?
I suspect NEMA may provide that, but I cannot afford to purchase MG1 unless I know that contains the definition.

I find many code uses for FLA, but in NEC and in the Freeby Condensed MG1 I find no definition.

When I boldly try to define it myself, It gets sticky with Service Factors, Code Letters, etc.

How does a motor manufacturer determine FLA?

PUMPDESIGNER

 

[electricpete]

FLA is on the nameplate (I think you already knew that).

Roughly speaking it is the current the motor is expected to draw at rated voltage and rated load.

S.F. is a multiplier applied on top of FLA (ok, to be technical it is applied to horsepower.... I don't make a distinction but some in this forum will).

NEMA says FLA should be included in the nameplate. I don't think they come out and say it but I think my definition is good.


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[PUMPDESIGNER]

Thank you electricpete,
But how is FLA arrived at, how does the motor manufacturer determine an FLA?

I suppose what throws me is that FLA becomes mushy when you start to figure SF. I have used FLA for years, I know what NEC says to do with FLA, but I really do not know what FLA is. NEC does not define it, they just say what to do with it.

I try to define FLA as maximum rated current a motor can continuously draw without damage, but then service factor enters and now FLA does not really mean what I defined it as, because the SF means the motor can safely operate at higher current.

But then, NEMA MG1 9.15.1 states that the motor will have a shorter life if operated continuously in the SF.

So, my best guess now is this definition such as it is,
Ignoring all factors except winding insulation, FLA is the maximum rated current the motor can continuously draw to obtain “good” or “normal” life expectancy, but the motor can operate continuously in the Service Factor without short term failure, but life span will be reduced.

That seems like a mess to me, but that is how I see it at this moment.


PUMPDESIGNER

 

[dpc]

Look again at what electricpete said: "the current the motor is expected to draw at rated voltage and rated load."

This load is the rated horsepower on the nameplate.

When operating in the service factor, the motor is producing more horsepower (above rated load) and will draw current above full load amps.

 

[rbulsara]

Full Load = Rated kW/((SQRT 3)*Rated kV*PF*efficiency).
This equals what electricpete said.
All design values are known to the mfr by design and tests.

1HP=0.746kW

 

[aolalde]

NEMA MG1 does not have a definition for Full Load Current.

However it has:

1.40 RATING OF A MACHINE.
“The rating of a machine shall consist of the output power together with any other characteristics, such as speed, voltage, and current, assigned to it by the manufacturer.”

1.47 FULL-LOAD TORQUE
The full-load torque of a motor is the torque necessary to produce its rated horsepower at full load speed. In pounds at a foot radius, it is equal to the horse power times 5252 divided by the full-load speed.

In my opinion, the full-load current has to be that defined with the machine rating.

All the rating parameters are defined by the manufacturer by calculation and prototype tests.

 

[buzzp]

I would agree with your summation Pumpdesigner, any operation of above FLA will reduce the life below that of normal expectancy.
Must remember NEC cares less about motor protection than it does about safety, hence the high overload settings according to NEC compared with NEMA.

 

[jbartos]

Comment: IEEE Std 100-2000 "Dictionary" does not define "Full Load Amperes (FLA)" or "Full Load Current"

 

[jbartos]

Suggestion: Visit

http://www.reliance.com/prodserv/motgen/b9652new/b9652.htm

for a definition of Full Load Current, which is often used instead of Full Load Amperes (FLA)
Namely:
FULL-LOAD CURRENT
The current flowing through the line when the motor is operating at full-load torque and full-load speed with rated frequency and voltage applied to the motor terminals.

 

[PUMPDESIGNER]

I sorry for having to leave town, was anxious to read your posts.

OK, I have it now, just be nice to dense minds like mine and perhaps your patience will pay off.

FLA is the current drawn by the motor to produce the rated horsepower at specified conditions.

PUMPDESIGNER

 

[rmw]

At what power factor would the manufacturer have determined the FLA? Power factors in the real world vary all over the map, so to speak.

rmw

 

[busbar]

Haven't earlier posts in this forum mentioned that the “bonus 15%" can be limited by variation from nameplate voltage/frequency, and particularly by voltage imbalance?

 

[jbartos]

Comment: The NEMA standard nameplate shows Rated Motor Current, which is synonym for FLA or Full Load Current.
Visit

http://www.ab.com/manuals/dr/Motors/Motors9_2.pdf

for more info

 

[jraef]

PUMPDESIGNER,
I think you already have it, but this is how I teach it in training classes I put on and maybe this will help too.
It appears to me that you are looking at FLA as being a motor design criterium that has a definition. In actuallity, motor design criteria are speed, torque and to some extent efficiency and lifespan (lets leave out power fator for the moment, although rmw has a good point).

Once the motor has been designed and manufactured to perform to the first criteria, the FLA is simply a measurement made when they are met in the lab. So when you select a motor for your application, it is done by speed and torque. HP (or kW) is just the rating used by the manufacturer to describe these criteria in one term, and is the measurement unit the motor is sold by. FLA is the maximum that you can expect to see under those specific conditions, and is what your protction and control system must be designed around for safety and reliability.

Service factor is just a way for someone to stretch the limits of the criteria without running the risk of immediate catastrophy. You will however expect to sacrifice the efficiency and/or expected lifespan criteria in the long run. Some OEMs routinely utilize the service factor rating of the motor as a continuous output, but they are usually only concerned with a lifespan long enough to satisfy a warranty. Other applications, as typified by valve operators for instance, are short duty cycles anyway so lifespan and efficiency become irrelevant. Demanding more torque, and therefore running into the SF amps, is a way of keeping the frame size down that will not sacrifice anything really significant. With cyclicle loads like compressors, SF is often used to allow for the increased loading during compression (loaded) cycles without sacrificing efficiency that would be lost by running a larger motor during unloaded cycles. The same would be true for pumps and fans where viscosity or density may be variable. SF leaves some headway for occasional increases in motor load.

"Venditori de oleum-vipera non vigere excordis populi"

 

[rbulsara]

rmw:

A motor's power factor is fixed by design as stated on its name plate. What you are referring to is the system power factor. For example if you add PFC in parallel with the motor, the power factor upstream of the point of connection of the PFC only improves, the pf between the point of PFC connection to the motor remains unchanged.

 

[QUAVIET]

If you scroll down ten or fifteen threads you will find one named "Free corporate electrical courses". I urge you to take the one by Siemens. It only takes about 30 minutes or so. In the part about AC motors and drives it describes torque and FLA (with graphs) in very good layman's terms. It answered some questions I have had a long time as an electronic technician who bluffed his way in as an electrican so many years ago.

 

[PUMPDESIGNER]

Thank you all for good comments.
I have never wanted to be an electrical guy, but am forced to face the issue at times, no one else around that knows anything on the job sites, so I end up having to watch out for myself to get my pumps taken care of. You all have seen it out there, wire too small, circuit protectors too small, service too small, transformers too small, etc.

We established a company policy years ago that no pump would be used with an impeller than can load the pump into the service factor at every point on the pump curve. This is a wonderful rule, we can take a lot of electrical abuse and still keep on tickin. A whole lot of bad things are required to stop us.

Just for note, one of the largest pump manufacturers recently tried to sell us a pump that was listed in their books as requiring X horsepower, but when we tried to order it they wanted to provide it with 3/4 X horsepower, said that the motor manufacturer gave them permission to use that motor at the larger horsepower, even though they will not label the motor as such.

Another manufacturer caused us a problem, we had no accurate power data that we trusted on their pump, we were in a hurry so we just ordered the pump with the phrase, "Trim Impeller to Totally Non-Overloading".
In the field the pump was drawing exactly the entire service factor. We called them, they claim that Non-Overloading" means not above the Service Factor.

OK, I learned one more thing I wish I never had to learn, you have to define non-overloading to these buggers.

PUMPDESIGNER

 

[jbartos]

Comment on the previous posting marked ///\\We called them, they claim that Non-Overloading" means not above the Service Factor.
///This is technically and theoretically correct. However, it is practically difficult to maintain the motor running at S.F.=1.15, for example, and simultaneously, keep motor rated frequency and voltage.\\\

 

[PUMPDESIGNER]

jbartos,
Thank you for your comment, I see it this way.
SERVICE FACTOR may have a technical agreed upon definition, which if you know of a source for that definition I would appreciate knowing it.

But SERVICE FACTOR generally means a fudge factor to account for field things that happen like voltage sag, a little unbalance, things like that which cannot be predicted.

For a pump manufacturer just to walk in and use that service factor without asking the customer means that there is no longer a service factor available. Those suckers can't seem to resist the tempation to grab something that provides a little cushion and makes life a little easier, pushing us onto the edge, and shortening the life of the motor.

Which then leads to the question, why have a stinkin service factor?

PUMPDESIGNER

 

[CJFlatters]

Boy oh boy

what a lot of correspondence - i Think jbartos hit the nail on the head on his memo 26/3.

FLA = Mech kW/(Un x Sqrt3 x Cos Phi x Effy)

Of course, optimal PF & Eff'y are usually between 85%
& 95% of rated loads, thus it is actually possible to get a motor current higher than FLC when a lightly loaded machine is operating at a naff power factor and effeciency (ie an oversized pump motor running a long way down-curve).



_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.