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?
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
You identified repetitive failures, but I would recommend to keep an open mind to the cause – may have nothing to do with loading.
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
RE: 75 HP or 100 HP
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
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
Rafiq Bulsara
http://www.srengineersct.com
RE: 75 HP or 100 HP
Rafiq Bulsara
http://www.srengineersct.com
RE: 75 HP or 100 HP
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
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.
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RE: 75 HP or 100 HP
I would definitely go with a larger motor.
David Castor
www.cvoes.com
RE: 75 HP or 100 HP
RE: 75 HP or 100 HP
RE: 75 HP or 100 HP
Dan T
RE: 75 HP or 100 HP
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
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.
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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
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If we learn from our mistakes I'm getting a great education!
RE: 75 HP or 100 HP
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
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...?
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If we learn from our mistakes I'm getting a great education!
RE: 75 HP or 100 HP
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.
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
Sorry for repeating myself and Dick. I'm done now...
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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
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.
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
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.
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If we learn from our mistakes I'm getting a great education!
RE: 75 HP or 100 HP
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
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(2B)+(2B)' ?
RE: 75 HP or 100 HP
If he only needs, say 84hp, then we have an easier time with the redesign.