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Motor overload setting
9

Motor overload setting

Motor overload setting

(OP)
Hello,

The existing motor is 3-phase, 208V, 3.82A FLA. It is controlled via an Eaton motor starter rated for 18A and an overload setting of 4.56A. The motor is damaged and as a stop gap measure, an available 1-phase, 208V, 5.68A FLA motor could be installed. I've confirmed with the manufacturer that the exisiting motor starter and overload could be re-used for 1-phase motor, however the max setting of the existing motor overload is 5.23A.

Could the existing motor starter and overload be re-used for this application. I understand, that the max. overload setting should be 125% of the motor FLA, but in this case the max. overload setting is less than the motor FLA. To avoid any starting or overload issues, would it be best to install a new motor starter and overload with higher amperage setting? OR could the existing overload max. setting be acceptable?

I'd appreciate your comments on the above.

Thank you.

RE: Motor overload setting

Try it and see. If your motor is loaded to maximum capability then it might trip, if not then it will probably work OK. You should wire the three overload elements in series for single phase duty.

RE: Motor overload setting

@katwalatapan,
Your original motor is rated 3/4 HP (3.2A, 208V, three-phase) and you are going to replace it with a smaller capacity 1/2 HP (5.56A, 208V, single-phase).! Assuming both motors have the same full-load speeds, if the motor actual load is really around 3/4 HP, you'll overload your 1/2 HP for sure. IDK if you could change pulley sheeve or what, so you can make use of that single-phase motor.

RE: Motor overload setting

You can set an OL relay for any value you like that is under the FLA, the issue is the load on the motor. If the load requires more current than the setting allows, it trips. If not, you're OK. I would hazard a guess that you are going to get nuisance tripping.

By the way, you assumption about the 125% is likely what killed your old motor. The Eaton OL relay has ALREADY factored in the 125% in the pick-up point of the OL trip curve, so by you adding an ADDITIONAL 25%, your OL was not even beginning to count down until the current reached 156% of the motor FLA!

Common mistake. You assumed that the NEC allowance is an instruction sheet, but that's not the case. It is a MAXIMUM in order to prevent fires. Setting of OL protection is done by a process called "RTFM"; Read The "Factory" Manual. The manual for that OL relay would have told you to set it for 100% of the FLA.

(The F usually doesn't mean Factory in the more vernacular version)


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Motor overload setting

Quote (jraef)

The Eaton OL relay has ALREADY factored in the 125% in the pick-up point of the OL trip curve, so by you adding an ADDITIONAL 25%, your OL was not even beginning to count down until the current reached 156% of the motor FLA!

Common mistake. You assumed that the NEC allowance is an instruction sheet, but that's not the case. It is a MAXIMUM in order to prevent fires. Setting of OL protection is done by a process called "RTFM"; Read The "Factory" Manual. The manual for that OL relay would have told you to set it for 100% of the FLA.
I have installed many overload devices that required you to add a safety factor of up to 125%.
I have also installed many overload devices that had the factor built in and were set at the motor FLA.
This takes us back to RTFM. You can't guess. For proper protection and avoidance of nuisance tripping you must "READ THE FLIPPIN' MANUAL".

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

(OP)
Thank you very much for all the response.

RE: Motor overload setting

Quote (jraef)

By the way, you assumption about the 125% is likely what killed your old motor
katwalatapan -
I don't know how jraef knew that likely was the cause of the motor failure. I didn't see anything in your post that even alluded to that being the cause. And anyway, even if the overloads were inadvertantly set too high - that is not what damaged the motor. The motor was damaged by poor design resulting in overload, bad bearings, poor operator control allowing process to overload, coil winding shorted, something else of these natures. The best the overloads can do is shut off the motor when there is a failure, before it catches fire, and maybe catch dragging bearings before the windings get taken out. The overloads don't come into play until something broke.

Some think motor overload can be cured by changing the overload setting. That seems to be a "common" mistake.upsidedown However, You can't. The motor load is set by design. Setting the overload up or down won't change that. So, if the motor burned up because it was overloaded, fix the design - cause setting the overloads down don't fix a thing.

Yes, you do have to read the particular manual for the overloads. And my recomendation is to also check the running current right at first commissioning. If the design is crowding the motor loading - then fix that. There is no reason to be afraid of setting up the overloads - where ever they need to be. They are a protective relay, not a process control.

Curiousity questions:
Q1: As you said, the motor is 208V. That is a bit odd, most motors for 208V systems are 200V nameplate. However, sometimes motors are 208V - 230V nameplate. Is this what you have?

Q2: 208V system, likely 60Hz. Is this a NEMA motor? If so what is the sf?

I ask because these will matter if the loading is crowding the motor. And if they are IEC motors, that will matter as well

Overload philosophy per
The Worm

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

I see both 200 Volt rated motors and 208 Volt rated motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

While I agree the improper setting of the OL relay didn't cause the overload, if there was an overload, the improper setting allows the motor to be damaged before it acts. If the OL relay is set properly, AND something in the load causes a temporary overload condition, the OL relay trips and saves everyone a lot of headaches while you investigate the cause of the overloading condition. By the time an OL set to not pick up until current gets to 156% of FLA before it even begins to take action, the windings are going to be damaged by the time it finally trips.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Motor overload setting

Interesting - post went away. maybe someone is telling me to try a different tact. okay
Glad to see you understand the overload setting did not cause an overload.

Quote (jraef)

You can set an OL relay for any value you like that is under the FLA, the issue is the load on the motor
And this one is just wrong. Contains assumptions that are not in evidence concerning the ability of the one doing the settings to calculate the actual setting.

Quote (jraef)

By the time an OL set to not pick up until current gets to 156% of FLA before it even begins to take action[/jraef]
This and several other comments contain assumptions that the op mis set the OL. There is no evidence in the op this is true.

[quote jraef]If the OL relay is set properly, AND something in the load causes a temporary overload condition, the OL relay trips and saves everyone a lot of headaches while you investigate the cause of the overloading condition
We have different philosophy concerning the purpose of an overload relay.
To my thinking, the primary purpose, in the event of a failure that overloads the motor, is to shut off the motor before it catches fire. If the failure is internal to the motor, there is no motor to save - it failed. If the failure is external to the motor, then the secondary purpose is to save the motor. And sometimes that happens - and that is good when it does.

Got to go to work - got someone paying my time. later

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

ice,
While you downplay TOL function as merely a fire protection requirement, most in the manufacturing industry see it as a "profit/loss" item! When motor thermal protections are set to closely mimic the motor thermal capability, you'll prevent motor burnouts-->lesser rewinding/dismounting/installation costs. Why design for destruction by fire, when you can save the equipment?

RE: Motor overload setting

Parchie -
You certainly have some excellent points. However, I don't think I am down playing the importance of motor overload protective functions.

When you say "most in the manufacturing industry see it as a "profit/loss" item", I'm curious as to exactly which industries. Cause, I don't see that much, especially for smaller motors - say under 200 hp.

Caution {rant-on} suggest burn before reading
I've been involved in Oil and Gas refinery and pipeline, petroleum bulk plants, pulp industry for around 50 years (welders helper, 1965). Engineering since 1973. The majority of my clients (not so much for bulk plants) are concerned with high reliability, running flat out, 24/7. They would insist on 365 days per year, if maintenance shutdowns weren't mandatory. Starting from that mindset, the concept of a dead fractional horse power motor showing up on the P/L statement is completely alien. Now consider that some processes are conceptually a series string of 50 pumps/motors with maybe as many valve actuators. One miserable 3/4hp motor can take down the whole process. Common practice is to have install spare pumps/motors for every critical piece, yep, right up to multi-thousand horsepower.

Considering this mindset, is it suprising that unless specials are involved, fractional horsepower may not be repaired at all, under 10hp may not be considered for rewind. Generally there is no squawking if a motor trips on OL. Things do break - manmade, it will break - that is a fact. However, generally, if the customer could remember the last time, they are telling me they want to investigate increasing the reliability.

Consider the customers are not whining about the motor costs, rather they are majorly whiny about the downtime to repair. Okay, you say, keep the overloads set down and they won't burn up and require replacement. Possibly. Maybe in the world of IEC motors, it is necessary to design for 90% loading and set the overloads at 100%. I couldn't tell you - no experience. Nema motors, the design will be as close to 100% loading as possible. Most clients will opt for 1.15sf to increase MTBF. And the OLs will be set 125% - 140%. And truely, I've never had a motor burn up (winding failure) because the overloads were set up to NEC max. They burn up because:
  • The windings shook a hole in the insulation;
  • 5 years of high ambient, heavy load, 24/7 finally took its toll. When one considers that is close to 50Khrs - that's not bad;
  • Bearings failed and put the rotor into the stator;
  • Cleanup crew using a fire hose (pulp mill - amazing how much water one can get past the shaft on a TEFC with a fire hose);
  • Operators can adjust process to overload motor and trip. Reset repeatedly until the smoke comes out. (Pulp Mill - wood room) OLs with thermal model not commonly available.
  • HVAC air handler designers spec/buy 1.0SF 208-230 motors, load them right to 100%, locate in high ambient areas, and then are suprised when they burn up frequently. Fortunately this one is easily fixed. Replace with 200V, 1.15sf motor - often same footprint, just twice the money. Yes, on 208V systems, heavily loaded 208-230V motors run hotter than 200V motors. Twice the money pales next to repair/replace every few years
  • And one that maybe could have been saved by overloads. Definitely could have been saved by a modern electronic overload/motor protection relay. 4160V, 900hp, 900rpm to a hydraulic speed reducer, driving an ID fan. Fused contactor, no overloads, operator set the load by adjusting the speed reducer output RPM, relying on a winding RTD readout. One fuse opened, motor single-phased for maybe 30 seconds. Ruined it beyond rewindable. Don't know why the fuse opened. Electronic motor protection not commonly available - not uncommonly available either. We put in a phase loss relay. Don't know why they did not have one before.
So where am I headed:
  • Yes, saving the motor or limiting the damage to the motor is important.
  • Putting out the fire is top of the list. Especially since there may not be a motor to save anyway.
  • Fractional hp rarely (like never) make the list of financial concerns.
  • Advising to set OL at 100% sounds wrong on so many levels.
  • Yes, plenty of us can read.
  • Automatic assumption that the OP had not bothered to correctly set the OL and that caused the original motor failure does not sit well.
  • Up to a few hundred horsepower, replacing a motor is just a cost of doing business.
  • The more expensive the motor, the more extensive the protection (Really? Well thank you Captain Obvious)
  • I have not been around any industry where the cost of a fractional horsepower motor could ever possibly matter. The down time will, but not the motor.
  • Yes, it is always about the money
  • Example: Say you have an installation where a fractional/integral hp motor experiences 3 trips/ year resulting in 1 hour downtime and a plant startup each trip. One sets the overloads up to max and gets no trips for two years, then motor burns up. Takes 10 hours to replace motor, maybe a problem. Takes 1 hour to change a motor - not a problem.
  • I highly suspect we live/work in different worlds - and that is okay. We all have to make a living. However, the rules for one likely do not fit the other
  • Yes, it is always about the money.
if you got this far, thanks for listening {/rant-off}

continued overload philosophy per
the worm

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

Pumping applications are not the only use of industrial motors. Sawmills, rock quarries and gravel pits, conveyor drives and many other applications do not have the perdictable loading of pumps. Overloading motors is more of an issue and most of the folks working in those industries will remember quite a few overloaded motors that were saved by properly set Overload Devices.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Quote:

Pumping applications are not the only use of industrial motors. ....

Quote:

... Sawmills, rock quarries and gravel pits, conveyor drives and many other applications do not have the perdictable loading of pumps. ....
Both of these are true. I don't see how they apply to anything I said.

Quote:

... Overloading motors is more of an issue ....
I don't agree. The issues in these industries are no worse than the ones I highlihgted..

Quote:

... most of the folks working in those industries will remember quite a few overloaded motors that were saved by properly set Overload Devices.
I never suggested anything other than "properly setting the overloads". And still don't.

Having been in a few sawmills associated with pulp mills, a prevalent issue is:

Quote:

Operators can adjust process to overload motor and trip. Reset repeatedly until the smoke comes out. ...
That is a problem in any industry where the operator can adjust the process to overload. And it generally shows up when the management rates performance by the output ton. "Properly set overloads" won't fix this. Improperly set overloads won't fix this. Low set overloads won't fix this. .... .....

From what I hae seen of rock pits and gravel quarries, the same applies here as well. If the operator can get at the process vaialbles, they can take out the motor - no way out of that.

The Fix:
Construct such that the motors are not overloaded by design - if possible. Yes, it is always about the money.
Where the operator can adjust process, consider thermal modeling electronic overlaods
Yes the overloads must be properly set. "Properly set" does not mean that the motor will never fry from overload.
"Properly set" does mean trip inside of the cable damage curve. And for the first and maybe even the second trip per hour, inside of the motor damage curve. No promises after that.
"Properly set"/extent of protective relaying, is different for fractional hp than 200hp. Blanket statements covering both are about silly. Yes, there is no surprise there.
Its okay to actually measure the motor currents during commissioning - even for fractional hp. I'm suprised how rarely that happens. It appears that plenty operate on the principal, "Hey, we set the overloads in spec. It started and stays running. let's move on."

I'll still assert:

Quote:

And the OLs will be set 125% - 140%. And truely, I've never had a motor burn up (winding failure) because the overloads were set up to NEC max
I don't see any evidence this does not apply equally well the the industries you mentioned.

and still further delving into the philosophy of overloads by:
the worm

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

Quote:

And truely, I've never had a motor burn up (winding failure) because the overloads were set up to NEC max."

Not once in 43 years of work? Companies will be hiring you just for the good mojo you must be spreading.

RE: Motor overload setting

I agree! I keep telling them I need a good karma bonus. But I can't get anyone to pony-up. Bummer.

Nope I don't recall even one. Of course that mostly means I could always find another cause besides the overloads were set uo to NEC max.

Anecdotal information suggests the most likely cause of premature motor failure is "PM unto death" - cleaning, greasing right at the top, followed by vibration, cheap bearings, followed by mis-application (such as thin tin, 1.0sf, 100% loaded, high end of ambient spec - add 230V-208V nameplate on a 208 system, and they don't last long at all)

Also anecdotal information:
  • The techs like high end American made bearings. As far as I can tell, high end bearings from any country are equally good. Low end bearings from any country are equally poor. Which is not strange, they all use the same machines, just a difference in QC and quality of finish between the high end and low end.
  • Get/specify the right grease. One customer was taking the new bearings out of the package, cleaning and repacking with an ngli2 -60F spec grease. Found out that they ordered enough bearings that if they grouped up the orders, they could specify exactly what they wanted for grease. Took an extra four weeks for delivery. Good deal for all.
  • Close tolerance balanced rotors/pump shafts/impellors seem to aid longevity. Suggestion from an old millwright/rotating equipment mechanic. Seemed to increase bearing/seal life.
  • Careful alignment adds to bearing/seal life. Assisted with specing/buying highend optical alignment equipment/training. Were definitely able to affect a couple of problem children.
Just random thoughts. But I got someone wanting to pay me and this is not billable time - enjoyable, but not billable.

Later guys and girls

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

Quote:

Last year this bearing tripped a properly set overload,...
That motor on our largest compressor is still running now thanks to the overload ...
That's great. Good job. Excellent choice on overload setting. You are to be commended.

Just curious: Why would you ever consider NOT 'properly set overload'? I certainly would not. That doesn't make much sense.

Back to work for me. This is interesting, but isn't billable.

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

I know this is an existing plant. But in my line of work, commissioning equipment, if I don't set the overloads correctly (fla name plate motor = overload setting), I will not see all the mechanical mistakes of misalignments or just sloppy mechanical adjustments.

RE: Motor overload setting

2
Sorry, but saying that the motor being overloaded by the process is what caused it to fail and not the improper overload is just using silly semantics to justify your overload settings. Not having a proper overload or not setting it correctly will both fail to protect the motor, and then the motor will fail.

When your philosophy is to set the overloads so high that they will only prevent a fire then you're obviously only installing the overloads to pacify the electrical code, with no plans to use them for their intended purpose.

RE: Motor overload setting

Only have a few minutes --

Quote (LionelHutz )

Sorry, but saying that the motor being overloaded by the process is what caused it to fail and not the improper overload is just using silly semantics to justify your overload settings. ...
This has the appearance of an arrogant demeaning response as opposed to a professional opinion. I don't agree, so automatically, I obviously don't understand. That came through clear. Not sure how to respond - I'll go with a professional opinion.

Quote (LionelHutz )

... Not having a proper overload or not setting it correctly will both fail to protect the motor, and then the motor will fail. ....
I think every one here knew that - yes including me However, you group all failures together. You don't treat separately, several common failure modes where the overload cannot, will not save the motor. For those, the best the overload can do is prevent the fire.

Quote (LionelHutz )

... When your philosophy is to set the overloads so high that they will only prevent a fire then you're obviously only installing the overloads to pacify the electrical code, with no plans to use them for their intended purpose.
Let's look at this statement. Are you suggesting that the maximum 140% allowed by the NEC for a 1.15sf motor is not to be used? Are you suggesting that OL should be set at 100% (unless they won't start - of course)? Cause if you are - I don't agree with that. In industry - at least the ones I have seen/worked, 24/7, flatout production, 1.15sf, 140% overload settings are quite common.

My whole point - the one that seems to cause severe hate and discontent: If the process is overloading the motor, fix the process (or the motor) Setting the overloads down won't help - in fact might even burn the motor up sooner. Seen that happen once.

Note to jaerf: The "140%" and "100%" refer to actual load currents, not some mindless table lookup with unknown bias.

It's only "silly semantics" if you choose to not understand. Notice I didn't say you didn't understand - cause I think you do.

Back to work for me -

ice

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

It was a professional opinion. A proper overload will prevent the thermal failure of a motor regardless of the load the process applies to the motor.

You have completely failed to understand the comments about other applications. In a quarry, tripping the motor overload typically means the operator has to shovel conveyors off to clear the equipment. So, the operators quickly learn to not overload the equipment until it trips. Failing to properly set the overloads means the motor also gets replaced while the operators are shoveling, which means the overload has failed to do it's job.

Setting the overload so it only prevents a fire and setting the overload at 140% as allowed by the NEC are 2 exclusive things. I'm not sure why you are equating one with the other. But, since you brought it up, on a 1.15% SF motors the NEC ONLY allows using a 1.40% overload setting when the motor won't start when using the maximum NEC limit of 1.25% for the overload setting. So, if you are preventively setting the overloads to 140% then you are not following the NEC rules. My company sells 1000's of motor starters a year and every one has the overload factory set to 1.15% with a class 10 overload. We get maybe 1 call a year where the customer has issues with the overload tripping on motor start-up. So, I find your claims that a 140% overload setting is common to be rather odd since my experience shows that almost every application can easily start with the overload set to 115%. And when almost every motor starts with the overload set to 115%, there are very few applications where the NEC would allow 140% as the setting.

Your continued posting against what Jeff (jraef) posted is rather silly. He posted that Eaton overload, which would be part of the Eaton motor starter, has a built-in 125% factor on the overload setting. So, setting the dial to 100% means the overload is already set to 125% which is the maximum allowed by the NEC unless the motor doesn't start without tripping the overload. So yes, I 100% agree that this Eaton overload should first be set to 100%.

As for your fire comments. The short circuit protection is there to prevent fires, not the overload.

Your constant claims of only having a little time to post because you are working seems to imply that we are quite privileged to have you take time out to post on this forum. So, what is your point of doing that exactly?

RE: Motor overload setting

The other point I think iceworm is missing is that the allowances stated in the NEC are actual current values, NOT the settings allowed. So again, it boils down to RTFM. Some mfrs of OL relays will tell you to set the dial, or select the heater element using the FLA itself, because they have ALREADY factored in the 125% current into the telay pick-up point. THAT is the big mistake that people continually make. They read the NEC and do NOT read the instructions for the relay. So by selecting for 125% as a knee jerk response and then the mfr has ALREADY factored that in, effectively the IL relay will not even begin the i2t curve response until the current is at 156% of FLA, so by the time it finally trips, you are well into the thermal damage curve. If you set/select at 140%, you may as well buy your replacement motor now, because the first time something causes an overload condition, you wlill have degraded the motor insulation to the point of early failure.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: Motor overload setting

Failure to read and follow the manufacturer's instructions is, in and of itself, an NEC violation.

RE: Motor overload setting

Quote (LionelHutz )

... A proper overload will prevent the thermal failure of a motor regardless of the load the process applies to the motor.
And this is where we disagree. One does not fix the process overloading a motor by setting down the overloads. That just makes the overloads trip more often.

You really only asked one question - so I'll attempt to answer that.

Quote (LionelHutz )

... Your constant claims of only having a little time to post because you are working seems to imply that we are quite privileged to have you take time out to post on this forum. So, what is your point of doing that exactly?
Your reasoning on my implication is in error. I do my damndest to not be arrogant or demeaning. I do try to give reasoned responses. I don't have the kind of time that I am willing to devote that you do. My point? When I can't get back I'm not ignoring you, nor am I hiding. It is unfortunate that I could not offer better responses. Although I suspect it would not matter what I had to say after I picked on your buddy jraef.

This has deteriorated to throwing rocks. I see no further point.

By the way I am quite impressed with you owning a business the sells 1000's of motor starters per year. That's a pretty big deal.

Harmless flakes working together can unleash an avalanche of destruction

RE: Motor overload setting

Jeff - I did point that out in my 4th paragraph.

iceworm - Do you not understand the section in the NEC on overload settings, or do you simple chose to ignore it? Also, from your list of 7 reasons why motors fail, 4 of those motor failures would have been prevented with a proper overload setting.

And I do understand what you are basically posting. Your posts basically say that it is better to ignore the NEC and crank the overloads which allows the motor to fail if that means more uptime.

RE: Motor overload setting

{Anecdote on}
Early in my career I worked for a supervisor who was a fanatic about Over Load relays No relay/heater combination was put into service until its tripping curve had been verified by testing with the Multi-Amp. He had the lowest burnout rate in the industry.

Quote:

Just curious: Why would you ever consider NOT 'properly set overload'? I certainly would not. That doesn't make much sense.
Well that would be at an unheated plant, tying to start the motors at minus 30. The early crew arrived at midnight and started trying to start the equipment. The grease in the gear reducers was almost solid. Round and round the plant they went, resetting the overloads. At about 6:00 AM the early millwright shift arrived and started helping. At 7:00 AM the production shift and the shift electrician arrived. At about 8:00 AM, with the production shift standing idle and one part of the production line only one motor away from operating, the O/Ls were bypassed and some production was started.
Two extremes, both accepted by management.{Anecdote off}
A few things that have not yet been mentioned, ambient temperatures and RMS loading.
Not all O/Ls are temperature compensated and not all temperature compensation is accurate.
The motor and the O/L may not be at the same ambient temperature.
Theoretically an over-load relay should provide protection for RMS loading, but in practice the O/L may have to be set a little higher to avoid nuisance trips.
A note to my friends, David, Jeff, Lionel, Controlsdude, I checked iceworm's profile (click on his name). He has been given 10 votes or stars by his peers. Are we missing something here? Should we be accepting every word and comma?ponder

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Bill, the NEC also has a section on overload settings depending on the ambient temperature of the motor. The correction factor is 1.0 at 40*C and then rises to 1.36 at 0*C. So, a motor in a 0*C environment could have the overloads set to 1.25 x 1.36 = 1.70 while still following the NEC. So your anecdote about bypassing the overloads possibly could have been covered by a properly set overload for the ambient temperature.

RE: Motor overload setting

Bill

That anecdote is against all common sense, let alone regs. Thank God, it just remained as an anecdote and not some beancounter enforced 'policy'.

iceworm

As a rewinder, I am all for your theory of scorched earth maintenance management.

Muthu
www.edison.co.in

RE: Motor overload setting

Hi Edison
That was over 40 years ago. I have never since seen motors systematically abused that badly. All the O/L heaters in the plant were at least 2 sizes oversized. The amazing thing was how many motors survived the abuse.
Given the cost of idle production crews compared to the cost including labour to replace a motor, Management was quite willing to take a chance on a motor burning out. The crew could swap out a motor in about 15 or 20 minutes. We lost more motors than most plants, but management was happy with the bottom line. The trades foremen got less than $4 per hour. Plant downtime was stated at $50 per minute but we were allowed 20 minutes a shift grace period before the $50 per minute clock started..
Ideally almost all of the motors in the mill should have been replaced with larger motors and gear reducers. This was discussed several times. The issue was not so much the cost of the motors and reducers, it was more an issue of space and time. There were only a few millwrights on staff who had the training and experience to install and align new drives. They were very busy with work that was considered more important.
At times I would see a motor being abused so badly that I would locate a spare motor and have it nearby in the event that we had to do a fast change.
In one instance I was able to have a badly overloaded belt drive 20 HP switched out for a 25 HP spare. The boys had the change completed and running inside of a 30 minute lunch break.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Bill

The motors were much better built those days. Today's motors, whoever is the OEM, would just flip out over such abuses. :)

Muthu
www.edison.co.in

RE: Motor overload setting

Yes Edison
Those were the old "U" frame motors. We were horrified at the specs of the "T" frame motors when they appeared in the market. I remember hearing a specification of 14 seconds locked rotor to burn-out. I'm not sure if that was accurate, it was a long time ago. It was a very short time.
The "T" frame re-rate developed more HP from a given frame size.
I watched a millwright clear an overload on a waste conveyor;
The motor would start and run and drag the conveyor about three feet before stalling. The millwright would hit the stop button and then spin the coupling between the motor and the gear reducer with his foot. The coupling guard was easily removed by lifting off.
When all the slack in the drive train was taken up in reverse, he would hit the start button. The motor would start and advance the conveyor another three feet before stalling. He did this about 8 or 10 times until the overload was cleared. The only thing that I could do was bring out the spare motor and have it handy. Incredibly, it wasn't needed. The original motor remained in service.
Another time I stuck an air hose into an Open Drip Proof motor to help cool it until the abuse stopped.
Head office made the decision to abuse motors to destruction rather than upgrade to larger motors and reducers.
All we could do on the mill floor was to get very good at changing motors on short notice.
I don't advocate setting O/Ls above the proper levels, but after over 50 years in the field, I don't have a lot of patience with a rigid, my-way or the highway, approach to overload settings. There are too many special circumstances that may suggest other than the first textbook setting of overload relays.
My first setting of an overload relay is based on the rated FLA, and the special instructions for the specific relay that I am using.
I reserve the option to change the setting for unique issues.

Interesting:- This discussion doesn't have a lot of connection to the OP's original question.
The Original Poster said "Thank you" to Scotty, Parchie, Jeff and myself back on 26 January, and left the building.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Bill

I wish I was a vendor for your old company. :)

I agree there might be some rare cases where you go by the actual field conditions. But in all other cases, it makes sense to stick with regs.

Also, I don't see how a very low ambient temperature lets one to reset the O/L to a much higher value. After all, isn't it all about I2t, which could burn out the windings regardless of how low is the ambient?

Muthu
www.edison.co.in

RE: Motor overload setting

Often the electrical room is heated and the motors are outside. If the motor is in the same ambient as the overload, then use the recommended setting.
At low ambient temperatures, the motor rejects the I2R heat much more efficiently. The O/L relay may be in an ambient of +21 Deg. C, while the motor is in an ambient of - 10 Deg. C. At those temps. the motor can withstand more of an overload without overheating.
Actually we bought few motors. We did keep a rewind shop busy. Whenever possible we used rewound motors in problem applications. The rewind shop did a much better job than the manufacturer.
The loading on many motors in a sawmill is highly variable. Most of the motors ran well within their capability most of the time. However when a pile up or jam occurred, time was money, much more money than than the cost of a new or rebuilt motor.
We also had the occasional failure with motors that were properly protected and were never overloaded.

Quote (Muthu)

I agree there might be some rare cases where you go by the actual field conditions. But in all other cases, it makes sense to stick with regs.
Exactly!

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Bill,

The fact that the site could get more throughput bypassing or upsizing the overloads basically says that the overloads weren't matched to the motor. The overload trip curves were likely well below the damage curves of the motors, likely you had class 20 overloads on a motor that could handle class 50 or class 60 overloads. I could say that a properly setup/matched overload would have done much better, but you were stuck with bimetallic overloads back then. These days, you could find electronic overloads that would help the plant keep production up without allowing motors to be damaged so regularly.

The other fact is that the management pushing the "drive it to failure" philosophy was false economics no matter how they tried to justify it. Saying that clearing a jam was worth the motor just shows that the operators were trying to brute force more product through the equipment than it was designed for and basically playing a "can I shove this much through without a trip" game all day.

RE: Motor overload setting

Bill

I was referring to I2t, not I2R. Also, the machines are derated for higher altitudes where the ambient temps are cooler but air is thinner.

Muthu
www.edison.co.in

RE: Motor overload setting

Hi Lionel. Actually we had melting alloy O/Ls. I should mention that when a motor did burn out, the oversized Over-Load relay (often two sizes above recommended) had probably already saved the motor two or three or more times.
The large motors were well sized and not subject to abuse. It was the 10 HP, 15 HP, and 20 HP conveyor motors that were most abused.
Our management was actually quite astute. Changing to a bigger size motor and drive was considered for a few locations. Management compared the cost to date of motor replacement with the cost of alterations and did not like the ROI. If it had taken an hour instead of 20 minutes to change a motor, we may have been able to justify the cost of upgrading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Motor overload setting

Primary issue is that whether it is adviceable to use three phase motor starter to single phase motor. I dont think it is right. You should not use 3 phase motor starter to single phase motor. Now coming to rating setting, if the setting is less than 125% no damage will be done to motor but it will trip when motor is over loaded or even refust to start at the first place if the starting current is more than the current setting. Plz go through the sarting current rating of the new motor and set the current setting on the starter accordingly.

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