SF 1.15 Induction Motor on Overload 13

[kjb]

Gentlemen
I am new to eng-tips and am fascinated by the enlightening discussions going on.
I have a question about a 3 Ph induction motor 2000HP, 4.16kV, SF 1.15, Max Amb Temp. 40C, Temp Rise for Design: RTD 85C at SF = 1, Insulation Class: F, EEMAC Design B. The motor is currently running at near FL. Is it possible to increase the continuous load on this motor to 107% of FL without any adverse effect on insulation life?
Your input would be highly appreciated.

 

[THooper]

Suggestion:

I have always felt that service factors of motors are for intermittent overloading. NEMA standards do permit continuous running at the service factor overload. But the result will be shortened insulation life.

Page 13, In Let's Solve your problem says: NEMA temperature ratings make clear that continuous operation at service factor load can mean winding temperature 10C above insulation system limits. That cuts winding life in half. If the motor runs only two shifts a day, or ambient temperature never exceeds 25-30C, that may be no problem.

Kind Regards,
motorhead1

 

[kjb]

Many thanks for the quick response motorhead1!

You are absolutely right that the continuous operation of the motor at SF=1.15 load would cut the winding life to half.

However, in view of the following, I would be inclined to think that it should be safe to run this particular motor on SF=1.15 load without expecting any degradation of windings:

The motor is designed for 40C ambient temp. Max historical ambient temp. on site is 35C.

Insulation is Class F. As per NEMA MG 1-1993 tables 20.40.1 and 2, max temp rise for class F insulation is 110C at SF=1 and 120C at SF=1.15. Since the name plate temp rise is 85C at SF=1, I would expect a temperature rise of 95C at SF=1.15 load. Permissible temperature rise as per NEMA is more than actual temp rise (110C vs 95C). Thus it should be safe to run the motor at SF=1.15 continuous load. Am I right in making this assumption or some other factors need to be considered?

Thank you for your valued opinion.

 

[electricpete]

I gave motorhead1 a star and I agree with his answer.

It sounds like you have seen previous discussions on service factor in this forum before (if not, do a search and you'll see quite a few). I'm going to think a little bit about the factors you you mentioned for your particular motor to see what the winding temperature predications tells us.

I would add that in real life there is no line-in-the-sand rating of a motor. Motors operated within their ratings often fail for various reasons and motors operated beyond their ratings may live long lives. In the face of such considerations and other slightly complex factors associated with evaluating a motor rating, perhaps we should fall back on commonly-accepted practice, right? That's the funny part... there is no such thing. You'll find a wide variety of opinions and interpretations as has been shown on this board.

More later.

 

[THooper]

KJB,

I beleive that your assumption is correct to a point. Two things need to be considered, efficiency and power factor. The power factor may drop as motor output rises from the rated load to the service factor load. Also as you stated make sure that the temperature rise allowed by NEMA at the service factor horsepower does not exceed normal insulation limits.

I believe that a 10 C rise in temperature is expected with a 15% increase in horsepower. However the increased resistance in the copper when temperature goes up could be greater in a class F system by 30%.

Depending on the motor and by using only part of the service factor the life expectancy will be much longer than that of a motor with a 1.0 SF especially with the Class F insluation. The lower ambient temperature would actually serve similar to a cooling system and will help the motor with the increase in temperature that is created from running at the higher horsepower. There is only a 5 C difference in the lower ambient and you could be looking at a 10 C rise in temperature. The rise could be slightly higher and cause some thermal degradation to the insulation.

Kind Regards,
motorhead1

 

[THooper]

electricpete is 100% correct, there is no line in the sand rating of a motor. I will check the search also for more info.

 

[jbartos]

Suggestions:
1. Many postings assume that the increased temperature at 1.15 SF motors is detrimental to their insulation and that the insulation is or has to be the same as for 1.0 SF motor. The motor manufacturer can build the 1.15 SF motor with insulation that outlasts the 1.0 SF motors. However, this is not mentioned in current NEMA standard. The past editions of NEMA standards apparently may have considered this fact. The problem with the 1.15 SF motor is the requirement to keep the motor terminal voltage at its rated value and the frequency at its rated value. Since both variables can fluctuate, the 107% load is very risky on continuous basis since chances are that there will be voltage sags or swells, and frequency deviations that will be detrimental to your motor running at 107% of shaft load (HP). To stabilize frequency at 60HZ ±0.1% or so is very costly and the same holds for the motor terminal voltage 4.0kV.
2. Perhaps, there may be some other solution than to run this motor at 107%, e.g. trade the motor for larger one, if it is still relatively new, or run it at 107% till it exhausts its lifecycle, and then buy a bigger one.

 

[electricpete]

Here's a general discussion of service factor. At the end I add specific comments for your situation.

There is a statement within MG-1-1998R1 para 14.37 which 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". I think that's a clear signal from the motor manufactrer's that you're on your own if you operate at service factor load.

The way I see it, operating a motor above rated load will reduce the life expectancy. The service factor is an attempt to quantify the effects of a specified level of overload (1.15) and limit the temperature rise to only 10 degrees more than would be seen at rated. As mentioned that corresponds to halving of insulation life, but probably acceptable from a pure thermal aging standpoint if motor operates less than half the time.

The real problem is that thermal aging is not the only effect of operating at higher load. As mentioned above the bearings will see higher load. Likewise the vibrating force on coils within the slot increases with square of load and will take a greater toll over the life of the motor. And the same thing applies to end turn vibration. So if that motor operated at service factor load should fail 1 year from now and comes to a court of law, they would look at NEMA MG-1 and conclude the manufacturer warned the user about possible early failure. Who would the company point the finger at?.... the designer who spec'd the motor to operate at it's service factor load.
(I'll admit that's a pessimistic scenario, but why would any designer take that chance?)

For your specific situation I suspect that the thermal margin you have gained from reduced ambient combined with over-designed insulation will limit the THERMAL aging of your motor at service factor load to less than or equal to a normal motor operated at rated load. But as discussed above, thermal considerations are not the only ones present.

 

[rhatcher]

Unfortunately, there is a lot of ambiguity in motor ratings and the standards. Take the following facts:

- delta T from 1.0 -1.15 sf should be around 10C
- 10C rise in insulation temperature halves insulation life
- motor operating at sf>1 has reduced service life
- insulation temperature rating is the maximum temperature that the insulation can operate at without a reduction in service life.

Applying this to the motor in question, the ambiguity arises because of the difference in temperature rise and insulation class. This motor is rated for a temperature rise at sf=1 of approximately Class B (80C), yet the insulation Class F allows for a rise of 115C for a total temperature of 155C without a reduction in the life of the insulation.

So, from an insulation standpoint, you could say that the motor will operate at sf=1.15 without a reduction in the "normal" life of the Class F insulation. This does not contradict the listed facts as you will reduce the life of the insulation as Class F insulation operated at a Class B rise would have a life expectancy greater than "normal".

The other case for service factor is a motor whose temerature rise at sf=1 is equal to the insulation temperature class. In this case, the above facts will be true, but the insulation life will be reduced by half from the "normal" life when operated at sf=1.15.

 

[kjb]

From the views expressed by the knowledgable participants, I derive the confidence about the correctness of my initial assumption. The conclusion is as follows:

Running the motor at 107% of the rated load will cause a temperature rise of approximately 92C.

Expected temperature rise is well below the max permissible temp rise for class F insulation.

Historical ambient of 35C provides an extra cushion of 5C.

It seems that the motor may be run at 107% load without expecting any detrimental effect on normal life.

We shall consult the manufacturer about the validity of above and also seek their advice regarding any mechanical considerations including bearing life.

I will post the final conclusion on the forum.

Many thanks once again for your valued suggestions!

 

[kjb]

We discussed the issue in detail with the motor manufacturer. I am glad to note that the manufacturer agrees that this motor may be run at 107% of SF=1 load without seeing any detrimental effect on the winding or bearing life.

Thank you all once again for participation.

KJB

 

[electricpete]

KJB - good discussion.

Was this an OEM or a repair/rewind shop? Did they offer any other insight on the general questions discussed about service factor. In particular whether they consider there would in general be possible adverse mechanical effects from operating above nameplate as discussed in MG1?

 

[kjb]

Electricpete

This motor is driving a pump at one of our client's petrochem site. An increase in the load is anticipated due to modifications to the piping system.

The manufacture basically agreed to our assumption that we could overload this motor as long as the expected temperature rise is below that for class F insulation. The motor is equipped with sleeve bearings with more than adequate safety margin for increased load.

 

[THooper]

Just a thought,

Babbitt bearings or sleeve bearings operation is 130 degrees F to 160 degrees F max optimum operation, as per EASA. In the machinist handbook 24th edition page 2139 to 2142, A babbitt bearing melting point is 466 degrees F and maximum operation is 300 degrees F. The problem is most turbine oil required for lubrication of a sleeve bearing will start to breakdown around 200 degrees F with rapid temperature heating reaching danger melting points and causing premature bearing failure if exceeding oil breakdown temps.

Just something to think about, I would check your lubrication specs for breakdown temperature along with proper bearing rtd settings etc..

 

[jbartos]

Suggestion: The manufacturer as well as many others may be aware the fact that the ambient temperature plays a significant role in the motor aging. E.g. if you cool this motor by locating it in the walk in freezer, the motor will have the temperature(s) significantly lower than indicated in the tables. This means that the lifetime would not be affected in comparison to the 1.0SF motor. The standards are stating boundary conditions (worst case condition) to be accurate and preserve equipment and personnel safety including lifetime/lifecycle. Also, the voltage and frequency have to be kept constant at their rated values for the 1.15SF not to cause the motor to exceed the temperature limits.

 

[motorman]

Motor designers will tell you that if the nameplate states that it is a service factor machine (design) then it can be run, continuously, at the service factor load. Of course, the life will be shorter at this higher load than if you only ran at the lower load BUT, the 1.15 service factor design has extra capability (which means it also cost more) over the same machine with only a 1.0 service factor. (If you would like to test me on that, weigh two identical motors, same manufacture, with only difference being one has a higher service factor. Guess which will weigh more?)

I think that a big part of the quandary with service factors is that there is no definition on what is the design life supposed to be? 10, 15, 20, 30 years? It is different from one manufacture to another and even between one designer and another inside the same company! (I once knew a designer that "will never design a machine with a rise more than 62 deg C; AT service factor; no matter what insulation class is used. Well, I can tell you, there are no motor manufactures today that can afford this kind of designer!) So the life is a function of: service factor use, designed temperature rise (see motor nameplate) AND what the temperature class is of the insulation that is actually used in the machine. You will most likely find that the motor has a design rise of only class B (80 deg C) but the temperature capability of the insulation is rated class F. this machine will have mucho life!

Bottom line, you paid extra for a service factor motor, you might as well use it! For more on this from our friends faced with the same questions in Europe, go to:

http://www.bevi.dk/t_topics/tmp_insl.htm

 

[jbartos]

Suggestion to the previous posting: The ambient temperature is a crucial factor that determines the motor aging. E.g.
1. Supposing that the motor runs at 40°C maximum rated ambient temperature. One adds the temperature rise, Tr, per the motor insulation class appearing on its nameplate. This is the maximum temperature the motor can operate without any damage and the baseline temperature limit for its rated lifetime.
2. Supposing that the ambient temperature decreases to 25°C. This condition will decrease aging of the motor of the same temperature rise, Tr. This will increase the motor lifetime.
3. Supposing that the motor operates at 0°C (32°F) and the motor Tr stays the same. This ambient temperature will again increase the motor lifetime with respect to 1. and 2. above, and decrease its aging.

 

guys, the real answer is another question. What is the current Rise by Resistance for the load, and what is the rise by hot spot RTD? If these values are nearly equal and indicate less than a B rise one can calculate the new rise at the 107% load. If the insulation system is really F rise and properly executed there will be no problems. You may be taking a 50 year winding life changing it to 35 years.
If the RbyR is 79 c and hot spot is 94 c you could be taking a 25 year winding life that would never give more than 10 years actual because of poor execution down to 2.5 years.

 

[electricpete]

One more hop onto my soapbox.

You've already heard my opinion... which is that any loading above nameplate will forfeit your ability to hold the manufacturer responsible in the event of a failure during the warranty period unless you can very clearly show that the failure was solely due to thermal factors... a pretty tall order.

I have been involved in one situation where our company had to "eat" a rewind cost of $250k for failure during warranty period... facing another case where possibly will eat a rewind cost of > $500k on large motor where cause is unknown but failed immediately after wokred by repair shop.

From first-hand experience, I can tell you it is not a good feeling to have to tell your management that you don't have much ability to hold the manufacturer accountable in court of law due to some aspect of your specifications or the way you operated your motor. That is exactly the trap you will fall in if you choose to operate your motors above nameplate. All of the opinions in the world (even well-supported opinions based on meticulous analysis of margins using Arhenius aging law) don't amount to squat when NEMA says in black and white that service factor loading reduces life expectancy.

Just my opinion. Everyone's entitled to one!

 

[THooper]

electricpete is speaking words of wisdom!! which I agree 100%...