Running in the Service Factor 4

[fastball]

Hello, I need some help in debating what the "service factor" for a motor should be used for. We (a semiconductor plant) purchased 3 rotary screw compressors from a local manufacturer. We failed to catch in the submittal documents that they were utilizing the S.F. of the motor to achieve the rated HP of the blower. The unit requires 398 HP and they submitted a 350 HP motor with a 1.25 SF. The units arrived with 350 HP 1.15 SF 4160V motors. We set the overload protection based on the nameplate and the starters begin to time out on overcurrent as soon as the unit loads.
We believe that the service factor should not be utilized for normal operation and the equipment should have 400 HP motors. The manufacturer of the motor, after being prompted by the compressor manufacturer, upgraded the SF to 1.25. The compressor manufacturer said, "It is very common in our industry to design a unit to run at 80% of the SF".
What are your thoughts on this?
Thanks,
Bill

 

[redtrumpet]

If the compressor manufacturer says it is common to design a unit to run at 80% of the SF, why aren't they doing it? 80% of the service factor of a motor with 1.25 SF is rated (nameplate) power, which would be acceptable. A 350 hp motor with 1.25 SF has a service factor rating of 437.5 hp. With 398 hp load, this motor is at 91% of its service factor rating, not 80%.

I would look at all the documents related to this purchase and see if you can find a way out. It looks like you are getting the shaft. Also very fishy that a motor manufacturer would nameplate a motor at 1.15 SF, then magically say it is good for 1.25 SF as well. If it's not stamped on the nameplate, I wouldn't accept it as a 1.25 SF motor.

If you search back in this forum, you will find several threads on service factor, temperature rise, insulation classes, etc. and the impact these will have on motor life.

 

[busbar]

There might be some recourse in the NEC:

“99NEC430-32. Continuous-Duty Motors.
(a) More than 1 Horsepower. Each continuous-duty motor rated more than 1 hp shall be protected against overload by one of the following means.
(1) A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating:

Motors with a marked service factor not less than 1.15 125%

Motors with a marked temperature rise not over 40C 125%

All other motors 115%”

(There may be exceptions to this.)

 

[jbartos]

Suggestions:
1. The motor with higher service factor may operate with the higher efficiency and power factor. The manufacturer may have more detail information about this.
2. There are other factors involved, e.g. motor weight, physical dimensions, etc.
3. It should not be overlooked that the manufacturers are in very competitive business. Therefore, any jumping to conclusions without a proper product evaluation is not prudent.

 

[cuky2000]

The Service Factor (SF) is one of the most debate parameters in the motor rating. I would like you allow me to present several quote from manufacturer, engineers and researchers associated with the use and understanding of service factor:

1) DEFINITION: Service factor is a multiplier that, when applied to the rated HP, indicates a continuous HP that the motor MAY deliver at rated voltage and rated frequency specified in the motor nameplate.

2) Continuous operation at the SF load at max. ambient temp. may halve the insulation life.

3) The SF=15% is standard for NEMA motor 200 HP and below, having Class B insulation. Higher temp. insulation would be used either above 200 HP or to provide SF > 15%

4) SF is primarily use for thermal margins and not necessarily permits a motor to do better job or handling intermittent high-peak load.

5) SF does not add in general any accelerating torque and cannot solve a starting problem. If more torque is require, higher inrush will be expected and the system voltage drop could preclude the torque advantage.

6) Rated-load temp. is related to actual temp. at SF. Even thought NEMA standard omits the rate load temp. for motor having SF rating, the general consensus is to be viewed as a percentage temperature difference applicable to different type of insulation classes as follow:

- Class A: 10 o C difference of 40-oC rise represent 25% added temp. for 15% higher load. Thus 80% rise motor should be expected to run at least 20 oC hotter at the same overload.
- Class B: 80 oC machine lacking of SF rating is necessary run well above the NEMA std. 90 oC limit at SF=115% load.
- Class F: 105 oC machine lacking of SF rating is necessary run well above the NEMA std. 115 oC limit at SF=115% load.
- Class H: 105 oC machine lacking of SF rating is necessary run well above the NEMA std. 135 oC limit at SF = 115% load.

 

[fastball]

Cuky,
Thanks for all of the info! Where would I find you point # 2?

All,
Thanks for your comments, they will help in the debate. I also spoke with another semiconductor plant yesterday that uses the same manufacture of compressors. They have 3 150 HP, 480V, 1.25 SF units that run at 123% FLA. They have had 3 motor failures since installation which was 1995. One from windings and two from bearings.
Thanks again.
Bill

 

[cuky2000]

Regarding your request on item # 2 you may find some wording of the EPRI Power Plant Reference Book Vol 6 (Motors).

The failure data presented justify the extra effort that you are investing. Be advise that the failure of motors are for different causes that not necesary directly associated with the SF.

If possible, let us know the final outcome of this investigation.

Good luck.

 

[jbartos]

Suggestion: Referring to the questioned point 2 regarding a half of the motor lifecycle at SF=1.15 or SF=1.25 is reasonably possible, since it is very difficult or impossible to preserve 60.0000Hz power supply of 60Hz rated motor, and rated motor terminal voltage at XXXX.000000V to have the motor life time as engineered and predicted for the SF=1.15.

 

[MotorMan1K]

Get the manufacturer to double the motors warranty or your gonna send it back. Then after he increases it put some fans on the motor to lower their temperatures. That's all that is going to happen as an earlier post stated ... the motor will run hotter and the insulation's life will be decreased. Then, years down the line have it rewound with higher temperature insulation that will last even longer.

 

[jbartos]

Comment: The previous posting nicely reveals the bitter truth about the motor SF.

 

[electricpete]

There needs to be a FAQ on service factor. If someone has the time they should round up all the old threads and summarize them.

My favorite aspect to emphasize is that thermal factors are not the only things affected by service factor load. NEMA MG-1-1998 makes it clear that mechanical factors are also affected:

Specifically:MG-1-1998R1 para 14.37 states: "A motor operating continuously at any service factor greater than 1 will have a reduced life expectancy compared to a motor operated at its rated load. Insulation life AND BEARING LIFE are reduced by the service factor load".

 

[fastball]

UPDATE! UPDATE!

We had another meeting and the compressor manufacturer brought a EE consultant and the motor rep. They concur that running "this" particular motor into its SF will not cause any damage or reduce its life. Here are the numbers they gave to support that statement:

The motor is a design code D, KVA code G,, Eff of 95.0, 40C max amb cont. They produced motor performance data that showed dyno testing on a like motor. The temp rise @ rated load was 35 degrees......low! They also calculated that @ 1.15 it would be 46 degress and @ 1.25 it would be 52 degrees.

They calculated the hightest temp the motor would see as: max amb (40) + rise @ 1.25 (52) + hot spot (15) =107. This is well below the 155 degrees C rating of class F insulation.

I questioned the calculated numbers for temp rise because on the same document they show that the motor was taken to 1.5 SF load and list the Power factor=89, Eff= 94.3, speed = 3562, and amps = 64 but did not show any temps for the 1.15 or 1.25 runs.

The 1.0 SF numbers are as follows: PF=90, Eff=95, speed=3580, amps=43.

Due to time constraints we can not afford to pull the motors or refuse the order. The motor and compressor manufacturers will offer an extended warranty to remedy the situation.

It is my feeling that the motors will last based on the temp rise data they provided and the fact that the units are in a clean building with temp control running at 21 C.

One added note that you may find interesting. The motor manufacturer brought a book that was written by one of the companies retired P.E.'s. I was able to talk him out of it and began reading it last night. I quote from his writings.

Definition of SF. "............, Simularly, a motor with a 1.15 SF can be expected to safely handle INTERMITTENT loads amounting to 15% beyond its nameplate HP."

SF info under "Types of Motors"
"......., service factor should not be used for basic load calculations"

Thanks again for all of the info!

 

[electricpete]

Here comes my broken record speech on the general subject that it is not wise from a life-cycle cost standpoint to operate at service factor load even though thermal considerations prove it to be ok.

#1 - NEMA tells you there are mechanical considerations (bearings) and that bearings and windings may not last as long if you operate at service factor load.
#2 - Bearings and windings account for probably 90% of motor failures.
#3 - THERMAL failure of winding is only one of many ways that a winding can fail. It can also fail by end turn vibration (depends on load) and slot vibration (depends on load), as well as some other factors that don't depend on load (contamination, defective design, voltage surge).
#4 - THERMAL failure of winding is probably one of the least common in a properly designed, maintained, and monitored motor. We can calculate the temperature and for large motors we can monitor/alarm the temperature to foresee problems and take preventive action. The other failure factors are much harder to predict monitor and those are the worst ones that will sneak up and get you without warning.
#5 - If your winding does fail, the manufacturer has the right to say "You operated at service factor load and NEMA warned you that this results in reduced winding life". Regardless of the true reason for failure, the burden of proof would be on you to prove that it was a non-load related failure.
#6 - The cost of motor failure is NOT limited to the cost of a rewind. There is manpower involved. There can be signficiant plant disruption. (I apologize... that is probably obvious to most plant folks, but I sometimes feel the need to remind design folks of those facts).

OK, off of my soapbox. My emotion/disagreement is not directed specifically at you and your situation... you have your own time constrains and weigh your own risk etc. I have become a little frustrated over time that most folks are so comfortable sharpening their pencil on the thermal aspects of service factor loading as if that is all that matters.

 

[jbartos]

Suggestion to the previous posting pertaining to NEMA marked ///\\\:
#1 - NEMA tells you there are mechanical considerations (bearings) and that bearings and windings may not last as long if you operate at service factor load.
///NEMA is covering itself since, generally, when there is a shaft load greater than rated shaft load, the bearing experience heavier wear. Also, some quantitative info appears to missing, e.g. graphs or tables indicating how much of the wear is involved.\\#2 - Bearings and windings account for probably 90% of motor failures.
///This is self-evident since they are most used. Obviously, the eyelet accounts for almost zero percent failure.\\#5 - If your winding does fail, the manufacturer has the right to say "You operated at service factor load and NEMA warned you that this results in reduced winding life". Regardless of the true reason for failure, the burden of proof would be on you to prove that it was a non-load related failure.
///It appears that the NEMA does not show any quantitative indication of the reduced winding life, which defeats the meaning of SF use.\\\

 

[electricpete]

jbartos
From my viewpoint, we are in hearty agreement that NEMA provides an UNQUANTIFIED warning of reduced life of windings and bearings when loading above service factor.

Although certain aspects of load-related winding degradation are quantifiable (thermal), not all load-related winding degradation is quantifiable (slot and end-turn vibration). To the best of my knowledge, NEMA does NOT limit their warning on reduced winding life expectancy to thermal effects (it certainly isn't addressed in section 14.37 where the warning appears)

It may be obvious to you that windings and bearings comprise 90% of the expected failures. The reason I point it out is that it makes the NEMA warning all the more troublesome.

All of this puts the user in a precarious position if he elects to load above rated IMHO.