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Baldor Motors/SFA

Baldor Motors/SFA

Baldor Motors/SFA

(OP)
We need to tabulate Service Factor Amps for various motors we are considering matching with drive.  Can't find the SFA published in any materials either in Baldor Motors catalog or on their website.  Does anyone know if this information is published anywhere?  I'm still waiting for Baldor Motors to contact me back.

RE: Baldor Motors/SFA


Maybe I have misunderstood the need, but I can’t find the term “service factor amps” or “service factor current” in 1999 NEC Article 430 or 240.  There are a number of references to “motor nameplate full-load current”.  I think “service factor amps” could be a phrase cooked up as a marketing term by the low-budget motor trade, and subject to possibly some significant abuse and maybe borderline shady huckstering.  By Article 430, one does not size motor-overload protection based on “service factor amps”—but uses “motor nameplate full-load current.”  This is possibly why a manufacturer may understandably hesitate using or publishing data based on the term.
   

RE: Baldor Motors/SFA

The "service factor" on motor nameplates is for HP, not as many assume, Amps.  This is per NEMA standards.

There is a rough parallel between service factor and additional amperage draw above FLA but no motor MFG will want to go there.

RE: Baldor Motors/SFA

(OP)
The reason I ask, is that Baldor lists the Amps at service factor on the nameplate of the motor.  But they don't seem to publish it.  We're in the practice of sizing to run at as much amps as possible without causing damage to the motor.  We use the SFA to gauge about how much we can do this safely, so it's of reasonable importance.

It just seems odd that they'd list something on a nameplate, but not publish it anywhere.  I tried to formulate how SFA would related to FLA, but so far haven't managed to arrive at any plated values.

RE: Baldor Motors/SFA

"I tried to formulate how SFA would related to FLA, but so far haven't managed to arrive at any plated values"

I would think that to a close approximation:

SFA ~ FLA *SF

There is some error in the above due to the fact that the load-component increases by SF but the magnetizing current does not increase.  So SFA will be slightly less than FLA*SF.

If no-load current were available, a better approximation could be made.

NLA = No-load amps
FLA = Full load amps
LA= sqrt(FLA^2-NLA^2) = load component of the current.

Your approximation for SFA will be
SFA = sqrt(NLA^2+SF^2*LA^2)

If you don't have NLA, try using 20%*FLA for 2-pole motors, 25% for 4-pole, 30% for 6-pole.

RE: Baldor Motors/SFA

Suggestion: The usage of SFA would be a deviation from industry standards since empirical approximations are applied. A question is how to justify usage of SFA, if the motor or equipment experiences a short life-cycle in comparison to the motor applications that use normally applied design margins.

RE: Baldor Motors/SFA

I was thinking it could be possible for the motor manufacturer to work backwards, having access to motor design information, and calculate the amperes at rated conditions of voltage and ambient, that would produce the temperature rise of the stated SF. On the other hand, it could be a sales dept. calculation.

RE: Baldor Motors/SFA

jbartos - NEMA MG-1 defines SFA in section 1.78.  Further more section 10.41 for medium polyphase motors requires that SFA be marked on the nameplate (in addition to FLA) when SF exceeds 1.15

RE: Baldor Motors/SFA

The condensed version of MG1-2002 references “service factor amps” as a label requirement in §7.10.  I stand corrected.  It is worth noting that the same document also states:  A motor operating continuously at any service factor greater than 1.0 will have a reduced life expectancy compared to operating at its rated nameplate horsepower…  In those applications requiring an overload capacity, the use of a higher horsepower rating is recommended to avoid exceeding the temperature rises for the class of insulation system used… [§9.15.1]
  

RE: Baldor Motors/SFA

ElectricPete,

People have been burning up motors for years because they have mistakenly

multiplied the motor FLA by the SF number and operated them in an overloaded state. While it is rare to see such a failure in the first thirty days of operation...failure does occur over time as the motor life is consequentially foreshortened.

The amps do not track the power because of the power factor; which

increases when the motor is loaded above rated. As a consequence, the

increased power (rated power x PF)....occurs at an amperage value that is

less than FLA x PF number).  So, as any motor manufacturer will tell you,

operating at the SF x FLA will result in an overloaded motor.... and one

which will void the warranty.

I would be remiss if I did not also address the issue of efficiency as the motor is operated into the service factor. Efficiency does drop off some as the load is increased above 100%.  So, the the increase in current due to loss of efficiency is somewhat  offset by the increase in power factor.  I leave it to others to decide the resultant consequence. But from tests made on a dynamometer at a major motor manufacturer, I can say that you get to the SF  power at a value less than the SF x the rated amps.

Now then, to get back to tmo42's original post, operating the motor at the SF

on inverter power can cause the motor to overheat and/or operate

overloaded as it negates any margin for harmonic heating in the motor. Many

motor manufacturers who rate their motors for definitive use on inverter

power, will rate them as follows:

1.0 SF Class B rise on sinewave power
1.15 SF Class F rise on Sinewave power
1.0 SF Class F rise on inverter power

Such motors typically have class 'F' insulation.


For tvo42 I would suggest he have a conversation with Baldor to see

what FLA they sanction for the particular motors when operated on inverter

power. You may find that you cannot use the Service Factor while operating on inverter power or you may not be able to use the full measure.

And lets not lose sight of the fact that not all inverters are created equal.

Differences in modulation schema do have considerable impact upon the

harmonic content in the consequent output waveform. Tests run on various

manufactures that employ sine-coded or star modulation show that the rms

fundamental output voltage can be down by as much as 82% from the rms

fundamental of the applied mains voltage. Consequence is that the motor

operates voltage starved, having to slip more to produce a given value of

output torque. Thus, less torque per ampere means that more amperes are

required to produce the same value of torque.  

Oh, and using an ammeter to observe the motor amps when on inverter power, can lead you astray too. The ammeter, even the true RMS types, will show you the total current; that is, the fundamental + the harmonic currents.
Only the fundamental produces useful torque.... some of the harmonic currents/torques actually subtract from the delivered shaft torque.

Interesting, isn't it, that what you see, is not necessarily what you get.

So the decision to operate into the service factor of a given motor should not be made lightly. There are many factors to consider before taking the leap.

RE: Baldor Motors/SFA

Hi jomega - Did you read my whole message?  I gave SF x FLA as a first approximation.  I clearly stated SFA will be a little less than computed this way and the refinement is to calculate SFA = sqrt(NLA^2+SF^2*LA^2) which gets us closer

RE: Baldor Motors/SFA

Suggestion to electricpete (Electrical) May 3, 2003 ///\\\
jbartos - NEMA MG-1 defines SFA in section 1.78.  Further more section 10.41 for medium polyphase motors requires that SFA be marked on the nameplate (in addition to FLA) when SF exceeds 1.15
///Thank you. It is good to know. I actually meant "The usage of SFA would be a deviation from industry standards when empirical approximations are applied." This implies when the SFA is not on the motor nameplate. Even, when it is on the nameplate and properly used, the motor may experience a shorter life expectancy, which is hard to justify; especially, if it results in a higher cost of motor procuring. Obviously, if one is irreplaceable or indispensable, there is nothing to be worry about.\\\

RE: Baldor Motors/SFA

I will mention that the method I suggested is not empirical but based upon the equivalent circuit, with simplifying assumptions (neglect leakage reactances, neglect change in efficiency).

An example calculation of the above method using a motor test data from the Reliance catalog:

http://www.reliance.com/pdf/pdf/aced/W01004-A-A001.pdf

50hp 2-pole 460v motor.  FLA = 55.5A  125% current is 68.7A

Since current at 125% load is given,  we will for purposes of this excercize assume that this is a 1.25 SF motor.  (the objective of the method is to calculate current at some load above full load given FLA... it doesn't matter whether it is the actual motor SF or not).

> FLA:=55.5:
> NLA:=0.2*FLA; # For 2-pole motor
                             NLA := 11.10

> LA:=sqrt(FLA^2-NLA^2);
                             LA := 54.38

> SF:=1.25:
> SFA:=sqrt(NLA^2+SF^2*LA^2);

                             SFA := 68.88

> # actual amps at 125% load is 68.7 Remarkable agreement.
>
> # Note that actual NLA is higher than the estimate used. If actual NLA were used the SFA estimate in this case would not be as close to actual!...  would be around 68.0A. Still very close to actual.   The error in estimating NLA improved the accuracy (by luck).

RE: Baldor Motors/SFA

I believe the error of neglecting change in efficiency in my method above is usually conservative (since usually efficiency decreases above full load), and will lead toward a low estimate of SFA. Not sure about the effect of neglecting leakage reactance.

Perhaps worth mentioning that if the above example were calculated using the simple SF*FLA = 55.5*1.25 = 69.375, the result is within 1% of the actual 125% current (68.7).

I agree with the other folks who have suggested that sharpening your pencil to get the last few percent out of a  motor is usually penny-wise and pound foolish, in the long-term.

RE: Baldor Motors/SFA

tmo42, SF is simply a multipier of horse power. It is the acceptable limit that a motor can be loaded to without damage. I have been a technician for 20 years and I have never sized a motor to a load using it's SF or SFA. A motor with a 1.15 SF is a better motor than a 1.00 SF. It's just a higher duty motor. FLA is always used to size OL protection.

RE: Baldor Motors/SFA

Typically the OL can be set higher for SF=1.15 than for SF=1.0.

RE: Baldor Motors/SFA

electricpete is correct but it is not a common practice. but is acceptable

RE: Baldor Motors/SFA

Many times I have come onto the project after the mechanical engineer has purchased the package, which for example, may be load equal to 98% or 101% of motor HP, with a 1.15 SF. What do you do then? Probably set the motor relay at 1.15 pickup (for example with a motor protection relay).

Another case is the process control people, since current is available at the PLC, to adjust the process to maximize motor amperes, which they choose as either FLA or SF*FLA (regardless of voltage also). They measure amps, then adjust the process.

These are the real things I see.

RE: Baldor Motors/SFA

Suggestion: The SF over 1.0 rated motors are often used for special loads, e.g. Motor Operated Valves (MOV) to provide an extra HP margin without using too large HP motor rating. Also, some MOVs function several times per year only.

RE: Baldor Motors/SFA

Hi Tmo42

I have just finish load  tetsing 4 - Baldor motors and the 4 VFDs that will eventually drive it.The unit has a Single front end and 4 inverters.One of the Inverters is water cooled ,the other 3 are air blower cooled.
We ordered these motors from Baldor for a specially project.1- 200 hp for a VIB Screen,1-600 for an Impactor /crusher and 2 -600 hp for Slurry Pumps.
The windings are Inverter duty of course,with Class F
insulation with Class B Temp rise.
I am somewhat confused by the talk of SFAs and what it suppose to mean.
The motor nameplate shows the motor FLAs.It does not care if the S. factor is 1/1.15/1.25 or whathave you.
Overloads are set normally at a max of 1.25 of motor full load current(FLC).Service Factors are related to H.P not to nameplate FLAmps.
I don't know how large your motors are,but regardless of the size,electric motors should not be operated at their service factors on a continuous bases.The Factor is in place to allow for momentary process changes,voltage variations,or amb.temp changes.If you operate your motors at SFactor you lost the ability to deal with these variations and run the chance of severelly overheating your motor.For every 10 Dec inc. above TRise your motor life is reduced by a half.(I guess you all know that)
TMO42-I don't know where you are located,but I do know the name and PH # of a Baldor Rep in our area that was most helpful with all the info we needed during the tests.If it Ok with him I'll give you the Ph #.
I asked this fellow to get me the Data sheets for the windings,showing Stator slots /Rotor slots  and I had it in my hands the following day.I don't recall many of our Suppliers being that forthcoming.

Hope it does not confuse the issue even more.  

GusD

RE: Baldor Motors/SFA

(OP)
Hmm.  The nameplate for Baldor Catalog #VM3550 lists FLA as 2.3 and SFA 2.5 with a SF of 1.15...

As I was explained it, SFA was the maximum continuous amps without damaging the motor.  Looked at as sort of a 'danger' value.  But it's desired to know just how much we can squeeze out of the motor.  I suppose it's more likely the penny pinchers up higher that cause this to be a concern, but even still.

The formula listed earlier... while it does seem to be accurate in about half the cases I tested.. in others it seems to be a bit... wildly high.  Especially in low horse motors with high service factors.

RE: Baldor Motors/SFA

tmo42 & ElectricPete,

Appreciate the data on the Baldor motor. That's why in my original post to EP I cautioned about using the SF as a multiplier of the motor N/P FLA.... and yes, EP, I did read your May 02 post and did note your comment about the "first approximation" and "SFA" ... However, years of experience teach us that the uninitiated will see that and think it acceptable to go with SF x FLA ...mostly because they don't know any better or haven't experienced motors burning up or failing prematurely as a consequence of such action.  

I think that as the data tmo42 just provided shows,  that such approximation only leads to trouble... and should not be considered as you cannot operate the motor long term at the value of SF x FLA.... without overloading the motor.

In fact, the data shows that 2.5/2.3 is a 1.08695 multiplier of the motors FLA.... and is 94.5% or the current you'd be running at if you used the 1.15 x FLA = 2.645 amps ...  or another way of seeing this would be to say thatif you operated at SF x FLA, you'd be operating at almost a 6% current overload.

So, I would ask.... what's the benefit of even considering it.... as it has no useful purpose and cannot be used. May be fine for esoteric discussions and limited thereto; but for general comsumtion, I find that sticking to the facts based in reality (as opposed to generalizations)  and playing to the conservative side keeps ya out of trouble.

"Ini facilius facias quam feras."

RE: Baldor Motors/SFA



The original poster expressed interest in calculating SFA from FLA and I proposed a method based on the equivalent circuit.  All equations were identified as approximations.   I see no problem with that.

I have a quiz for you:  What happens to power factor when load component increases but magnetizing current does not increase?  Now,  re-read my 5/2 message and your 5/3 response.

RE: Baldor Motors/SFA

EP :

Excellent Question.

Need some time to research/model/analyze/and respond.


RE: Baldor Motors/SFA

I don't know exactly what the explanation is for the small motors.  It may be that the error  of neglecting series leakage reactance is more pronounced. One other thing is that Baldor only provides this info to one decimal point.... the rounding error might make a difference for small motors.

What if fla = 2.6A actually represented FLA = 2.251 rounded ot nearest 0.1A?

FLA:=2.251;:
                            FLA := 2.251

> NLA:=0.2*FLA; # For 2-pole motor
                             NLA := .4502

> LA:=sqrt(FLA^2-NLA^2);
                             LA := 2.205

> SF:=1.15:
> SFA:=sqrt(NLA^2+SF^2*LA^2);
                             SFA := 2.576
Actual SFA reported as 2.5 could have been as high as 2.549
Now only 1% error.  Just a thought. If the small motors are CONSISTENTLY off, there is probably something else going on.

RE: Baldor Motors/SFA

Small motors have higher leakage reactances and lower magnetizing reactances on per-unit basis than comparable speed large motors.  So not only does the error in neglecting leakage reactance increase, but the NLA is likely a larger fraction of FLA.  Higher NLA estimate will bring the SFA estimate down slightly.  

RE: Baldor Motors/SFA

(OP)
Hmm.. my comments about the low horse, high SF motors was based on me testing the data for some of the Franklin Motors...  it seems that for the Balder motors, the SFA calculation is pretty close; in many cases, if I round down to the nearest .1A it's almost dead on.

RE: Baldor Motors/SFA

(OP)
I also want to note that I actually have the NLA data for the Balder motors, wheras I was estimating for the Franklins.  I suspect that, as you just said, I was using an estimate for NLA that was too small, and that's where my error was originating.

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