# How long can a motor ride overload?



## joebanana (Dec 21, 2010)

They'll run until they go into spontainous, self induced, rapid self disassembly.


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## gpop (May 14, 2018)

Keep the motor clean with good airflow and it will last longer than one buried under rock dust at half load. Overloads are a calculation based on winding temperature.


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## JRaef (Mar 23, 2009)

The motor damage and the overload trip curve basically follow what's called an "I2t curve", meaning the higher the current is over the rating, the faster the damage happens and therefore, the faster the OL is designed to trip. Here is what those curves generally look like:










So at 160A on a motor rated for 115A, that is 139% of FLA. So if you extrapolate a point on the bottom axis at 139, then go straight up, the motor damage will happen at about 350 seconds (almost 6 minutes), but the OL relay would trip (if set for 115A) in 300 seconds (5 minutes).


A note about the SF settings: you CANNOT assume (ASS-U-Me) that your OL relay must be set at 125%, that's one of the biggest mistakes people make that leads to motor failure. The reason is that the brand of OL relay MIGHT ALREADY DO THAT. Some OL relay mfrs build in a "pick-up point" in their trip curve that is ALREADY based on the rules allowed in the NEC (and other codes around the world). In the INSTRUCTIONS for that mfr, they tell you how to ADJUST the setting or selection based on Service Factor. So for example if it is an IEC style OL relay, they generally ALREADY build-in a 115% pick-up point, which is the value used for motors with a 1.0 SF. So if you set it for 125% of the FLA, you are actually setting it for 125% of 115%, which is closer to 144%, which may allow motor damage. The ONLY valid setting if you insist on using the SF is per the instructions of your SPECIFIC brand of OL relay.


Also, using the SF on a motor is acknowledging that you understand that the motor life and performance will be affected. On something like a crusher, I would not use it. SF is INTENDED to allow for uncontrollable variables such as supply voltage swings without as must risk to motor design life and performance. If you run the motor into the SF continuously, you have consumed that "fudge factor" it gives you.


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## paulengr (Oct 8, 2017)

The overload relay standard NEMA ICS 2 Part 4 can be downloaded for free off nema.org. The overload relay rating is ALREADY 115% of FLA plus and minus 15%. This is done because for instance a bimetallic relay is not very accurate so if they said say 2% most would never pass ULC or CSA testing.

If you set it to 100% it would nuisance trip up to 15% below FLA. If you set it to 125% chances are better than even it wont give good protection. And MANY plants learn the hard way that yes you can run at say 120% continuously and the relay won't trip but the motor will burn up instead. You can't fix stupid.

To be fair microprocessor relays are much more accurate. Like say 1% error. But imagine what happens in an application where they are used to the way the old bimetallic relay worked, the one that was 115% plus or minus 15%. So the electronic relay can certainly trip at 101% but now it seems like it is nuisance tripping so electronic relay manufacturers typically run higher than the bare minimum.

The big thing with crushers is high inertia starting especially if the load can't be cleared and stalling or jamming. If you really want to protect the motor get a good quality microprocessor based relay. Two I can recommend are the Motortronics RX (cheap) and the SEL 749M. These give you jam and stall, protection independent of overloads. So you can safely open up the overload protection a bit during starting since it has stall protection which is what destroys rotors. Overload protection is more about the stator. Then clamp down to safe limits during running.

Also motors have drastically changed in the last 20 years. It used to be that motors were designed on a calculator and tables then tested for performance. Of necessity the tolerances were loose and there was always a safety margin. You could literally do burnout with a torch, something that would destroy the core steel of a modern motor. Energy efficiency now is so tight they are designed on a computer then built. There is NO margin at all. You simply cannot run above rated values without damage for any time at all.

As to how it affects it, heat is proportional to the square of the current, what we call I-squared-R because it shows up all over in electrical design. A 10% increase in current is a 21% heat Increase. And a 37% current increase is an i88% increase in heat over name plate. Every 10 C increase in temperature cuts insulation life in half. To be fair though it depends on ambient conditions. If say Florida Rock goes even a little past name plate on a 40 C day, the motor will burn up in a few hours. If an oil sands operation abuses a motor severely on a balmy -40 C winter day in Fort MacMurray, well, at least they won't have to beat the ice off of it.

Granted crushers are very hard on motors. Often the bearings fail way before the motor insulation does. But if you can have the rebuild shop at least look at it when they take the rotor out. They should easily be able to tell you if it was overheated. If it is then your argument is costs and lost production. If it's only cooked a little by the time the bearings fail, don't worry about the overloadx. Just keep in mind too as a maintenance person your job is to maintain the equipment as long as possible. A production guy is supposed to produce as much as possible. So naturally you are on opposite sides. BUT the trick is to convince them that slow and steady wins the race..It is better to run the equipment at maximum RATED design as long as possible instead of jamming as much through as possible until it fails. The biggest mistake production supervisors make is letting guys sit around while maintenance is working on it. If they are doing cleanup the whole time, the nastier and wetter and the more back breaking the better, there is an incentive not to break the equipment.

And the counter to this is crushers work best if they are choke fed. You don't want to let a crushed go empty if at all possjble.

Sent from my SM-T350 using Tapatalk


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