430.32 (A)(1) Separate Overload Device Setting Question

[xptpcrewx]

Per 430.32 (A)(1), why and/or how is a motor with a rating of 1.15 service factor allowed to have a maximum overload trip set-point of 125% of motor nameplate current? Wouldn't 115% of motor nameplate current make more sense for protecting the motor from overheating if in fact that is the intent?

 

[ActionDave]

The higher service factor means you can push the motor a little harder.

 

[xptpcrewx]

The higher service factor means you can push the motor a little harder.

Yes, I understand that; but why are you allowed to push a motor only rated for 15% more HP/current up to 25% more HP/current?

 

[jumper]

Yes, I understand that; but why are you allowed to push a motor only rated for 15% more HP/current up to 25% more HP/current?

The higher thermal overload of 25% is what is allowed for the heat produced when the motor is run at 115%.
It is not a 1:1 ratio.

See if this chart helps.

http://www.avonmore-electrical.com/contentfiles/Service Factor - What is it and What does it do.pdf

 

[xptpcrewx]

The higher thermal overload of 25% is what is allowed for the heat produced when the motor is run at 115%.
It is not a 1:1 ratio.

See if this chart helps.

http://www.avonmore-electrical.com/contentfiles/Service Factor - What is it and What does it do.pdf

Thanks for your response. I think what you are saying and what the examples demonstrate in the reference document you provided are two different things. Table 1 is titled, "Temperature Rise vs Percent Loading" and the document further suggests loading is proportional to SF ratings (current and HP are approximately linear). Note: for motors rated 460V, they are very near a 1:1 ratio. Lower voltage motors would be closer to a 4:1 ratio (16 times Watt-loss for same rated power as compared to 460V motors).

As I understand it, 1.15 SF loading corresponds to temperature rises in the 115% column of Table 1, while 1.25 SF loading would essentially correspond to the 125% column which is what the NEC is allowing for some reason. There must be a better explanation as to why the NEC is allowing this general rule; especially when its very different (worse) for lower voltage motors.

 

[xptpcrewx]

flogging this dead horse

flogging this dead horse

Lets try this again. I need a better explanation for why/how an overload setting of 1.25 is acceptable protection for a 115% SF motor. I would like a reference or logical reasoning. See equation below... Thanks in advance.

 

[Jraef]

It's actually a complex and dynamic system that is boiled down into what can be thought of as a "compromise". Motor FLA ratings are actually a way of saying "At this much continuous current draw, this motor will not cease to function in a reasonable amount of time." The actual amount of current that a motor DOES draw is based on load; so if you continue to increase the load, the motor will draw more and more current until the internal heat produced by that current causes winding damage. The point at which that takes place is called the Thermal Damage Curve of the motor and it is a curve because it changes due to the motor's ability to dissipate this internal heat over time on its own and as the motor temperature increases, the resistance of the windings also increases which means the motor tries to draw MORE current to maintain the load speed, setting up the possibility for a thermal runaway. In our part of the world these damage curves are set down in NEMA guidelines, elsewhere they have IEC guidelines (which is why you see different OL protection requirements for IEC motors), both are mostly based on empirical data, meaning people have studied and observed where the damage occurs over this moving target of time.

Thermal Overload Protection curves are all based on these Thermal Damage Curves. Thermal OL relays then are designed to trip at points "under the curve" in that they are designed to take the motor off line BEFORE the thermal damage curve is reached. This OL protection curve can be highly complex and is not going to be the same for any two motors (unless identical in all ways). In a perfect world we would all buy and use highly expensive and complex protection devices called a "Motor Protection Relay" (MPR) that allows you to design a customized trip curve for any given motor, based on what the motor manufacturer tells you the thermal damage curve points are. A typical MPR capable of that will cost over $5000, so it's not in the least bit practical to put one of those on a $500 motor (unless the down time of that machine is worth a lot more, separate issue).

So the compromise is that NEMA (and IEC) have boiled down those complexities into a more simple concept based on an "I squared t" parabolic curve; current (squared) x time. This allows for the use of inexpensive thermal OL protective devices that will do an adequate job of protecting the motors, erring on the side of being conservative, without creating nuisance tripping that costs money in terms of lost productivity.

Then to have a "curve", you must define a starting point. In motor protection curves, that is called the "pick-up point", the place at which the I^2t curve starts. That pick-up point is where the motor is getting dangerously close to the Thermal Damage Curve to where it is approaching that thermal runaway point. Below that, the motor MIGHT recover if given enough time. So the pickup point is designed to minimize nuisance tripping since there is still a CHANCE that the conditions will rectify in time to avoid that.

For motors with NO Service factor (which also includes all IEC motors), that pick-up point is at 115-117%, meaning that if the motor current stays at 115% continuously, defined as 3 hours or more, the OL curve will begin to time down. The higher you go over 115%, the less time it takes to trip. For motors with the 1.15 SF, that pick-up point is 125% because the motor is DESIGNED to handle a little more abuse in that the additional mass of the metal in it has a certain amount of thermal hysteresis (the time it takes to change temperature). Think in terms of putting a cold aluminum pot on the stove burner, vs a cast iron pot on the same burner. You can keep your hand on the cast iron pot for a LOT longer before the temperature becomes unbearable. So were you to design a system to prevent that cast iron pot from melting, you could go for a lot longer before having to turn the burner off.

Lastly, you also have to understand that the NEC is not actually all that concerned about the life of your motor, it is about not starting a FIRE. So that 125% for a 1.15SF motor is a MAXIMUM value, not a minimum, based on their knowing that the risk of a thermal runaway and subsequent fire increases exponentially above that point, and the NEC is all about risk mitigation.

 

[iceworm]

.... So that 125% for a 1.15SF motor is a MAXIMUM value, not a minimum, based on their knowing that the risk of a thermal runaway and subsequent fire increases exponentially above that point, and the NEC is all about risk mitigation.

Tell me about "thermal runaway" in motors. I'm not familiar with that.

 

[iceworm]

You guys and girls are addictive. I'm supposed to be working.

Lets try this again. I need a better explanation for why/how an overload setting of 1.25 is acceptable protection for a 115% SF motor. ....

... Lastly, you also have to understand that the NEC is not actually all that concerned about the life of your motor, it is about not starting a FIRE. So that 125% for a 1.15SF motor is a MAXIMUM value, not a minimum, based on their knowing that the risk of a thermal runaway and subsequent fire increases exponentially above that point, and the NEC is all about risk mitigation.

Jraef pretty well got it. System failure should not start a fire. Let me refer you to 90.1.A, 90.1.B (I'm getting repetitive again and again ....)

Just curious - What do you think about setting the overloads at 140%? Personally, I have never had a motor fail from setting the overloads up. I have seen one fail from setting the overloads down*.

The Worm's Law on motor feeders: The load is not limited by the overloads. The load is limited by design. If the motor is running hot because the current is high, setting the overloads down does not fix a thing. The only time the overloads should come in to play is if the motor bearings or driven equipment bearings start dragging - something broke.

But wait - We have a system where the operators can affecting the loading. If they push too hard, we have to shut it down. That is a design problem. One I saw/worked, 4160V, 900hp, ID fan, had imbedded RTDs. Operators were provided with a readout. When the motor temp got too high, operators twitched the knob and slightly lowered load, temp came down.

If that kind of money is too much for a 15hp silo auger, install a big enough motor that they can't overload it. And if they don't want to pay for the bigger motor, set the overloads to NEC max. Eventually the overloads will get reset four time in an hour, and the smoke will come out - but it won't catch fire. Owner buys new motor. NEC is happy.

MG-1 has quite a bit of information on insulation temperatures, service factors, overloads. Maybe there is some indication of the history on how 125% - 140% were selected.

*(And this is a true story) System was a 100hp, 1.15sf motor on a centrifugal pump. Operators could line up the hydraulics and slightly overload the system. Overloads were set up to 140%. I don't see any fix except to install a bigger motor or trim the impellor. Nobody likes that - it's cubic money/downtime. Motor eventually burns up. Been in service maybe five years - could have been as much as eight years. Uh guys, now is the time for a bigger motor. Nope. Money/time to change the base, and we got the exact replacement on-site.

Motor runs for about a year - occasional trips, maybe every couple of months.

I get a call in the dead of the night. Motor tripped four times in an hour and the smoke came out on the fourth start. Oh - oh

Night electrician and I check the starter, wiring, while the mill-wrights are changing the motor. Overloads had been change to 125%. Uh, David? Yep changed them last night. Wanted to stop the operators from overloading the system and burning up the motor.

 

[xptpcrewx]

Jraef,

Thanks for taking the time to reply. I appreciate your response. I am only responding to statements I disagree with. Please don't take it personally:

It's actually a complex and dynamic system

No its actually not. Look at the equation in post #5. This is only a FLC and logical O/L settings correlation. If you want to talk about heat generation then that's based on empirical data with safety margins (damage curve standards). No need to complicate matters with dynamic anything or heat transfer mechanics.

The point at which that takes place is called the Thermal Damage Curve of the motor and it is a curve because it changes due to the motor's ability to dissipate this internal heat over time on its own

To be clear its the "damage curve" not "thermal damage curve" and there are an infinite amount of points along this curve where damage takes place. The reason its an inverse-time curve is because of the limitations of the materials involved. Its not just thermal damage but can be mechanical damage as well. Theoretically, for thermal damage, this is right after the long-time pick-up (set at the onset of abnormal current) all the way up to where the curve changes shape.

...and as the motor temperature increases, the resistance of the windings also increases which means the motor tries to draw MORE current to maintain the load speed, setting up the possibility for a thermal runaway.

Your motor mechanics and electrical theory are basically very wrong. Increasing winding resistance due to heat generated is pretty negligible. Increasing resistance (however small it may be) does not have the effect of drawing more current. There is no thermal runaway... maybe/eventually a fire at locked rotor if protection does not operate due to insulation failure and short circuit.

So the compromise is that NEMA (and IEC) have boiled down those complexities into a more simple concept based on an "I squared t" parabolic curve; current (squared) x time. This allows for the use of inexpensive thermal OL protective devices that will do an adequate job of protecting the motors, erring on the side of being conservative, without creating nuisance tripping that costs money in terms of lost productivity.

I^2t = k just means constant energy.

Then to have a "curve", you must define a starting point. In motor protection curves, that is called the "pick-up point", the place at which the I^2t curve starts. That pick-up point is where the motor is getting dangerously close to the Thermal Damage Curve to where it is approaching that thermal runaway point.

You actually don't need a starting point for a curve. The long-time pick-up is highly unreliable because on this point of the curve the value is undefined (infinite slope). If you tried to test the timing at/near long-time pick-up you would get very imprecise results every-time.

For motors with NO Service factor (which also includes all IEC motors), that pick-up point is at 115-117%, meaning that if the motor current stays at 115% continuously, defined as 3 hours or more, the OL curve will begin to time down. The higher you go over 115%, the less time it takes to trip. For motors with the 1.15 SF, that pick-up point is 125% because the motor is DESIGNED to handle a little more abuse in that the additional mass of the metal in it has a certain amount of thermal hysteresis (the time it takes to change temperature). Think in terms of putting a cold aluminum pot on the stove burner, vs a cast iron pot on the same burner. You can keep your hand on the cast iron pot for a LOT longer before the temperature becomes unbearable. So were you to design a system to prevent that cast iron pot from melting, you could go for a lot longer before having to turn the burner off.

Look at the equation in post #5. Explain that one... Also thermal hysteresis isn't what you are describing. I'm not a heat-transfer guy, but I think you are talking about thermal conductivity and some sort of time constant.

Lastly, you also have to understand that the NEC is not actually all that concerned about the life of your motor, it is about not starting a FIRE. So that 125% for a 1.15SF motor is a MAXIMUM value, not a minimum, based on their knowing that the risk of a thermal runaway and subsequent fire increases exponentially above that point, and the NEC is all about risk mitigation.

Yes I get that. The NEC doesn't care about the life of your motor. Neither do I, but consider that your motor is long protected before fire even takes place (assuming no defects). I would say the maximum value is actually 130-140% based on 430.32(C) so that sort of invalidates your other point. Also, I don't see the correlation between exponentially increased fire risk for values above 125%. Sound like this is made up. Please stick to FLC and O/L setting correlation unless you can give me solid quantitative reasoning/correlation for the amount of heat generated and increased fire risk. Thanks.

 

[xptpcrewx]

You guys and girls are addictive. I'm supposed to be working.

Jraef pretty well got it. System failure should not start a fire. Let me refer you to 90.1.A, 90.1.B (I'm getting repetitive again and again ....)

Just curious - What do you think about setting the overloads at 140%? Personally, I have never had a motor fail from setting the overloads up. I have seen one fail from setting the overloads down*.

The Worm's Law on motor feeders: The load is not limited by the overloads. The load is limited by design. If the motor is running hot because the current is high, setting the overloads down does not fix a thing. The only time the overloads should come in to play is if the motor bearings or driven equipment bearings start dragging - something broke.

But wait - We have a system where the operators can affecting the loading. If they push too hard, we have to shut it down. That is a design problem. One I saw/worked, 4160V, 900hp, ID fan, had imbedded RTDs. Operators were provided with a readout. When the motor temp got too high, operators twitched the knob and slightly lowered load, temp came down.

If that kind of money is too much for a 15hp silo auger, install a big enough motor that they can't overload it. And if they don't want to pay for the bigger motor, set the overloads to NEC max. Eventually the overloads will get reset four time in an hour, and the smoke will come out - but it won't catch fire. Owner buys new motor. NEC is happy.

MG-1 has quite a bit of information on insulation temperatures, service factors, overloads. Maybe there is some indication of the history on how 125% - 140% were selected.

*(And this is a true story) System was a 100hp, 1.15sf motor on a centrifugal pump. Operators could line up the hydraulics and slightly overload the system. Overloads were set up to 140%. I don't see any fix except to install a bigger motor or trim the impellor. Nobody likes that - it's cubic money/downtime. Motor eventually burns up. Been in service maybe five years - could have been as much as eight years. Uh guys, now is the time for a bigger motor. Nope. Money/time to change the base, and we got the exact replacement on-site.

Motor runs for about a year - occasional trips, maybe every couple of months.

I get a call in the dead of the night. Motor tripped four times in an hour and the smoke came out on the fourth start. Oh - oh

Night electrician and I check the starter, wiring, while the mill-wrights are changing the motor. Overloads had been change to 125%. Uh, David? Yep changed them last night. Wanted to stop the operators from overloading the system and burning up the motor.

Iceworm... You are next...

 

[kwired]

You guys and girls are addictive. I'm supposed to be working.





Jraef pretty well got it. System failure should not start a fire. Let me refer you to 90.1.A, 90.1.B (I'm getting repetitive again and again ....)

Just curious - What do you think about setting the overloads at 140%? Personally, I have never had a motor fail from setting the overloads up. I have seen one fail from setting the overloads down*.

The Worm's Law on motor feeders: The load is not limited by the overloads. The load is limited by design. If the motor is running hot because the current is high, setting the overloads down does not fix a thing. The only time the overloads should come in to play is if the motor bearings or driven equipment bearings start dragging - something broke.

But wait - We have a system where the operators can affecting the loading. If they push too hard, we have to shut it down. That is a design problem. One I saw/worked, 4160V, 900hp, ID fan, had imbedded RTDs. Operators were provided with a readout. When the motor temp got too high, operators twitched the knob and slightly lowered load, temp came down.

If that kind of money is too much for a 15hp silo auger, install a big enough motor that they can't overload it. And if they don't want to pay for the bigger motor, set the overloads to NEC max. Eventually the overloads will get reset four time in an hour, and the smoke will come out - but it won't catch fire. Owner buys new motor. NEC is happy.

MG-1 has quite a bit of information on insulation temperatures, service factors, overloads. Maybe there is some indication of the history on how 125% - 140% were selected.

*(And this is a true story) System was a 100hp, 1.15sf motor on a centrifugal pump. Operators could line up the hydraulics and slightly overload the system. Overloads were set up to 140%. I don't see any fix except to install a bigger motor or trim the impellor. Nobody likes that - it's cubic money/downtime. Motor eventually burns up. Been in service maybe five years - could have been as much as eight years. Uh guys, now is the time for a bigger motor. Nope. Money/time to change the base, and we got the exact replacement on-site.

Motor runs for about a year - occasional trips, maybe every couple of months.

I get a call in the dead of the night. Motor tripped four times in an hour and the smoke came out on the fourth start. Oh - oh

Night electrician and I check the starter, wiring, while the mill-wrights are changing the motor. Overloads had been change to 125%. Uh, David? Yep changed them last night. Wanted to stop the operators from overloading the system and burning up the motor.

With the 15 HP silo auger, it can be common to place an ammeter where user can see what it is drawing and an instruction not to load more than a certain level. (they may not know all the details but understand if they run too high on that meter it may not be good), then they control load by opening or closing gates that feed the auger enough to get to desired loading. This usually more common on a vertical leg than a horizontal auger. You don't really want a loaded leg to shut down, yet they always want to move material as fast as possible, this gives somewhat inexpensive way to monitor loading.