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75 HP or 100 HP

75 HP or 100 HP

75 HP or 100 HP

(OP)
I have a vacuum system with a 75 HP pump running on 480 with a SF of 1.15.  FLA is 86.

The motor is running overloaded at/above the service factor load (~99-100+).  We had a contractor look at the pump and give us three recommendations:

Rewind the motor to a 1.25 SF
Buy a new motor with a 1.25 SF
Buy a 100 HP motor to handle the load

We have had the motor fail many times and recent testing resulted having the largest difference in resistance between T leads be about 0.72% (readings in mOhms: 83.2, 82.6, 82.8).  Resistance to ground is over 200Mohms for each T lead.  The motor is only a few years old and has run for 12 hours every 36 hours.  Is 0.72% a big enough difference to be concerned?

I am weary about buying a new motor/rewinding a motor for a higher SF just to run it above the FLA rating.  Would this cause potential problems such as the current ones?
 

RE: 75 HP or 100 HP

The resistive imbalance that you identified is not cause for concern imo.  

You identified repetitive failures, but I would recommend to keep an open mind to the cause – may have nothing to do with loading.
 

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

Vacuum systems often see highly variable loads. Have you considered that a simple process change may solve the overloading problem?

RE: 75 HP or 100 HP

The service factor is a measure of periodical overload capacity. Your motor is not intended to be run higher than 86A continuously or for extended periods of time.  A motor running continuously at a service factor greater than 1.0 will have reduced life.  In your case, you're running at a service factor greater than 1.15 at times.

In my opinion, the proper solution to your problem would be to replace the existing motor with a 100hp motor.  This may mean replacing the existing starter and cable, and is the most expensive solution, but should greatly extend the life of your motor (assuming thermal issues are your only problem).

RE: 75 HP or 100 HP

(OP)
Thanks for the responses.  In our application the pump runs continuously at a fixed speed and goes into a bypass when the vacuum supply is met (which is not often).  I do not believe we have much, if any variation in the load.  We have gone through several ideas to reduce the load/operation, just not possible in our application.


I hear contradictory statements with regards to the SF - and I do not have a lot of knowledge when it comes to motor design.  Are there motors that are designed for better cooling so that they can handle operation at the service factor?  i.e. if they wind it in such a way to have a 1.25 SF, are they going to design it to have better cooling than a motor designed to have a 1.15 SF?

X49 - thanks for the recommendation - At this point that does sound like the best approach.  I will have to check on the thermals of the motor and see if I am exceeding rated rise temp.  What type of thermal impact would I have say I am running 10°C below the rating versus 20°C below - would that impact life expectancy? Or should all temperatures below the rating keep the life expectancy in the same general range?


Sorry for all the questions! Thanks again.

 

RE: 75 HP or 100 HP

I would however look at adequacy of the cooling or even enhancing it. If the failures were due to overload/heat.

Rafiq Bulsara
http://www.srengineersct.com

RE: 75 HP or 100 HP

The life will greatly change over a 10C difference.  Chemical reactions typically double their rates for every 10 degrees. Your failures are ultimately due to chemical changes in the insulation.

Is the motor in a hot poorly ventilated space?
Does direct sunlight ever fall on it?
Is the motor's case clean?

The voltage running to it will be a huge determining factor of its running temperature.   You do NOT want the voltage to be low as that will will greatly increase I2R losses which will contribute directly to the heating problem.  If the voltage is more than a few volts low from the nameplate you should probably get that corrected. If your facility is like a lot of places, during the hottest part of the day the voltage is the lowest which compounds the heating issues.

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

RE: 75 HP or 100 HP

The term "Service Factor" was for years very loosely defined and only recently was it semi-officially defined in the NEMA MG-1 design specifications to which North American motors are made. But outside of the US, motors don't even have any Service Factor.

That semi official spec now allows for the motor to operate at a 10deg C additional rise when running into the SF. The generally accepted rule-of-thumb is that for every 10deg C increase in rise, insulation life is cut in half (probably based on what Keith posted above. So although the specs do not specifically state a finite value for decreased motor life when running into the SF, it implies that it can be expected to be lower. It also mentions that efficiency, power factor, torque and speed ratings are to be expected to be other than as stated as well.

In other words, use SF at your own peril. It is meant to allow for occasional overloading conditions or temporary changes in voltage/frequency outside of excepted limits. But because the definition does not specifically state a time limit, many OEMs tend to use it and allow the end user to suffer the consequences, knowing they won't occur until the warranty expires.

By the way, I am trying to imagine how a motor can be "rewound" for a higher SF. They can use a higher insulation rise on the winding material, but I'm not convinced that necessarily increases the SF of the motor. You are not changing the amount of iron or the cooling method, all you are doing is allowing everything to run hotter and survive a little longer under more adverse conditions. But your efficiency and power factor may go so far out that it would be cheaper in the long run to use the 100HP motor in terms of operating costs.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: 75 HP or 100 HP

I agree with jraef - the intent of the service factor is to cover temporary overloads and other variables.  It's still a 75 hp motor.  Also, the efficiency and power factor will not be at nameplate values at the SF amps.  

I would definitely go with a larger motor.   

David Castor
www.cvoes.com

RE: 75 HP or 100 HP

(OP)
Thanks for all of this input.  All of it is very helpful.

RE: 75 HP or 100 HP

It should be a 100hp motor based on your description of the operation.

RE: 75 HP or 100 HP

I'm interested in hearing about the "failures".  

Dan T

RE: 75 HP or 100 HP

SF is a marketing hogwash, imo, practiced only in US. IEC motors don't have them.

If your motor winding is failing is due to high temp (typically winding burn-out), time to replace it with right HP motor.

Muthu
www.edison.co.in

RE: 75 HP or 100 HP

S.F. is defined in NEMA MG-1, so it is not purely marketing.

I'll agree the interpretation of exactly what it buys you is not straightforward.

The op situation is not well defined but if there were a concern for loading I would be inclined to go with 100hp if it was available in the correct frame size.    

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

If the larger frame for the 100hp won't fit, a good motor shop just might be able to wind 100hp into the 75hp frame using Class H insulation or some other such trick.

A good motor shop can often work outside of NEMA limitations to get you what you need, in a pinch.

No, you won't be able to buy a commodity replacement motor so you will have to think thru what level of spares you require.

RE: 75 HP or 100 HP

It's one thing taking care of the insulation temperature class but squeezing more magnetic flux into a core is another thing entirely. If this is a fairly modern motor I'd be surprised if there's enough margin between operating flux and saturation to squeeze the extra power out. An old motor will almost certainly be more generous with the iron. If you're operating voltage is toward the bottom end of the range that works in your favour too.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 75 HP or 100 HP

Skogs, I'm sure you are aware that a typical NEMA Design B motor has about 220% short-term peak overload torque available.  That tells me that the iron is capable of at least that level of flux.

Converting some of that short-term capacity into continuous capacity is what I've seen motor shops do with custom designs.  Usually, this is accomplished by increasing the thermal capacity of the motor and by reducing losses which result in heat.

As always, there are limits to how far this can go.

RE: 75 HP or 100 HP

Dick, it's me not Skogs!

I'm not intimately familiar with NEMA motors, being in IEC land, but generally I'd say that NEMA designs generally have more 'fat' built in than IEC ones. Perhaps it would work with a NEMA motor. With an IEC motor I'd expect that the core would be well on the way to saturation with the currents associated with peak torque and the iron would get damned hot quite quickly. I'm sure someone must have done a more in-depth study of this - maybe they're reading this...?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 75 HP or 100 HP

I think everyone here is correct. As Dick mentioned, the main concern related to magnetics in increasing load beyond nameplate is whether you have margin to breakdown torque.

As Scotty alluded, there are small local saturation of leakage reactances (primarily the teeth) as current goes up.  I don't think this is a huge factor because:
1 - the magnetizing reactnace does not go further into saturation... it goes further out of saturation because of increased voltage drop accross the magnetizing reactance.
2 - if magnetizing reactance does not go into saturation, we are not going to draw more magnetizing current.  This is one of the big factors which normally causes heating due to saturation. Without this, all that is left is hysteresis and eddy losses.  I'd say the eddy losses increase proportionally to the square of flux...(so 10% increas in flux and 20% increase in heating), while the hysteresis loss get bounded at some point (the area inside the b-h curve gets pinched off).   
3 - Whatever increased heating occurs in the teeth is localized and can be transferred to cooler parts of the core by conduction which is a very good heat transfer mechanism.  
4 - Temperature of degradation of core laminations is ridiculously higher than temperature of damage of electrical insulation on the windings.
5 - Putting it all together, there is not a concern for localized heating, just a concern for the contribution of that localized heating to the overall temperature of the machine.  If there is any concern for global temperautre increase associated with an uprate, it is probably more associated with the bearings than the core.

At least that's my opinion thinking it through for the first time.  But I'm not claiming to be any expert.

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

Also I was making assumption as Dick said that there is still substantial margin between steady state operation and peak torque... bounce the 33% mentioned upgrade against typical 250% peak torque.  The peak torque concerns generally relate to momentary overload rather than steady state thermal performance.

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

One more thing to add - being near breakdown torque does not imply being near saturation (other than localized tooth /leakage reactance saturation associated with high currents alluded above). The reason that the ratio of current to torque increases as we increase load toward breakdown torque is that the power factor is decreasing due to increasing rotor frequency resulting in increasing rotor reactance.

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

The last comment - I was not sure why saturation was being tied to breakdown torque.  Now I think I see the point was that if you try to increase flux level during rewind to increase breakdown torque, that will push you toward saturation.  That is true... the options are to push the core to slightly higher flux levels (use up some of existing magnetic margin) or more likely to use up some of the substantial margin between steady state operation and breakdown torque which exists in original design... as long as we don't expect the motor to be exposed to temporary overloads that will exceed the new breakdown torque.

Sorry for repeating myself and Dick. I'm done now...

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

Sorry, Scotty.  Indeed, it is you!

And, electricpete, I am fascinated by your more detailed analysis of my comment about increasing motor capacity.  I've watched motor shop people do this but have precious little understanding of exactly what's happening.

You, of course, have the details and I defer to your view of this.  It sounds like you believe this type of redesign is possible.

RE: 75 HP or 100 HP

Thanks. I probably made it more complicated than needed with my rambling, but I agree with everything you said. I think 133% uprate (75hp->100hp)is a distinct possibility for a small motor like this, but a competent shop (like edison and others) would have to look at what they have to work with in that specific motor to see what can be done... and if there was an off-the-shelf that would definitely be the preferred option.

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

I agree with scotty on the new motors having not much margin for an upgrades since everything has already been "optimized" already. I have done many uprates of older motors (typically pre 70's) when the designers were very conservative and allowed liberal margins, both in the current density and insulation thickness. For LV motors, I do only class H since I have found the class H insulation is not only thermally superior but also electrically. Typical uprates vary from 10 to 25% and in some pre 50's machines, it was as high as 50% (the conductors were individually cotton/empire cloth covered).

As for magnetic loading, nothing much to play around since almost all motors are designed around the knee of the saturation curve and any steep increase in flux density (like over 10%) will cause excessive no-load currents, which the clients don't like.

Muthu
www.edison.co.in

RE: 75 HP or 100 HP

It sounds like you are saying there is not a lot of thermal or magnetic margin in today's design.  That is a little different twist than what I understood Scotty to say... it sounded to me that he suggested the core flux density would necessarily be challenged through an uprate.  I don't see that to be the case....if thermal margin were available there is no need to increase the flux density (provided that margin between breakdown and momentary overload is maintained).

But it is a minor distinction.  The more relevant question is whether it can realistically be done and I'll defer to your opinion on that.

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

Hi ePete,

I think I had in my mind that the motor would be operating with a higher flux density to achieve the higher output, but in retrospect I think others are considering continuous operation much higher up the slip curve than would normally be permissible and maintaining stator flux density somewhere near nominal.

I am not 100% certain how well the rotor would cope with the higher power given its limited ability to reject heat to the outside world. The first thing to experience problems might be the bearings as heat is conducted out along the shaft, although the rotor would obviously contribute considerable heat to the stator through the shared cooling air circuit.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: 75 HP or 100 HP

Thanks - that makes sense.

NEMA MG 1-12.39.1 specifies minimum breakdown torque of 200% FLT for motors in this size range, and typically motors exceed the required minimum by quite a bit (the example 60HP motor I just posted in the other thread on power factor wiggles has breakdown torque around 350% of full load torque).  So I suspect that from among the multiple constraints, breakdown torque is not a limiting constraint in the design... for example if you tried to lower breakdown torque to the minimum you might bust an efficiency target. For pump and fan loads of this size in NEMA world I don't think there would be a problem uprating the motor 33% without changing breakdown torque if there is thermal margin.

Good points about the rotor etc.  It is not a simple review and usually ends up getting a lot less attention/expertise than the OEM's would throw at a new motor.  And for a small motor like this, I imagine the economics of large engineering review become unattractive pretty quick

=====================================
(2B)+(2B)'  ?

RE: 75 HP or 100 HP

Guys, I don't know if the application needs all 100hp.  We were talking 100hp because that's the next size up.

If he only needs, say 84hp, then we have an easier time with the redesign.

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