ptonsparky
Tom
- Occupation
- EC - retired
What was the original setting of the breaker? If it was "A", the motor may not have reached peak long enough for the breaker to trip, until the load changed, be it bearings, windings or just to dang cold.
The time that residual magnetism exists is measured in cycles. It is a concern only when plugging and jogging a motor. Once an AC induction motor comes to rest residual magnetism is absolutely not a possibility.
A transformer core is the same as a motor, after several cycles the residual magnetism is gone. Unless the transformer is "immediately" re-energized it is not a factor.I see what you are saying. I guess I was getting this confused with a transformer core still having residual magnetism when turing off a transformer at zero voltage due to current being at peak at this time and
090105-0923 EST
It is clearly not a problem from bearings, belts, or load on the motor because you are correct that maximum starting current could exist forever and never trip a properly sized and correctly working instantaneous breaker.
For normal overload operation (below full rated fault currents) breakers do not get weaker each time they trip. The NEMA standard for mechanical and electrical full load operations of breakers is in the thousands of operations for breakers smaller than 225A.
The NEMA design test is for thousands of mechanical operations, so I would not consider that all breakers inherently get weaker as they are used.Could Shallo be refering to the physical tripping mechanism, in which the lever slides across a small ledge?
We continue to discuss this topic here and more and more scenarios come up.
One that got me thinking is what happens when a fan that is spinning backwards due to some draft caused elsewhere and then is given a start in the forward direction. Should this trip the breaker? The same question goes for any load spinning backwards.
My initial response it that it should not trip the breaker based on the same argument I've has all along in that the motor will never draw more than LRC so even if its spinning backwards it still will only draw LRC. The only thing I can see happening is that as the motor attemps to reverse the direction of the fan and turn it the right direction it will draw this LRC for a longer period of time. In this case it may either trip the overloads, or trip a time delay breaker but should not trip an instantaneous breaker due to what I have been saying about LRC.
Do you guys agree?
LRC is not the only cause of tripping of instantaneous trips on CBs. That is why the rules in 430.52 have exceptions and allowances for when it is proven to NOT work when a breaker is set per the chart. LRC can be defined as the maximum STARTING current, but not the maximum INRUSH current. True inrush is indeed essentially the same as a transformer and that can cause breakers to trip magnetically. Newer energy efficient motors are becoming notorious for that in fact.
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As to spinning backwards, my experience is that it depends. Some motor designs have higher residual magnetism than others and will in fact be generating a small amount of voltage when spinning on their own, such as a fan windmilling. I have measured it when troubleshooting repeated VFD failures and by implementing a braking cycle just prior to starting, we were able to eliminate the damage to the VFD transistors. So in theory, that out-of-phase power could indeed exacerbate any natural inrush current issues and cause higher than normal instantaneous currents when the motor is energized. I have seen it proven out by an engineer (who is much more willing than I to crunch numbers) as potentially being as much as 2000% of FLA if the contactor is closed in the right point of the phase angle.
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The issue is essentially the same as what happens when you transition a Y-Delta open transition starter from Y to Delta; the re-application of power while the motor is still in regen can cause massive spikes that can damage equipment or pop fuses and trip CBs. It won't happen every time, but under the right circumstances it can be really really bad. Somewhere I have a paper written by Richard Nailen about this which shows the mathematical probability of it, if I can find it or a link to it, I will post it here later.
090107-1407 EST
mull982:
If you reverse drive a motor then the current will be greater than locked rotor. This would be an extrapolation of the curve I mentioned in an earlier post:
On page 209 of "Alternating -Current Machinery", by Bailey and Gault, 1951, McGraw-Hill.
Our local engineering library, U of M, is one of about five that Google is scanning in its entirety. I believe 7 to 8 million volumes. Bailey and Gault has been scanned. In this case apparently from The University of Wisconsin, rather than Michigan. Other than for a few random parts of the book you might not find it useful. However, on the used market it is readily available. My original new cost was $6. One used book store on the Internet has it for $15. Should be in many engineering libraries.
However, using Google Books is not a useful experience for me. The snippets are too short, curves are not displayed when specifically selected. I can legally copy more with a Xerox machine than Google provides in their snippets. On the other hand very rare old books out of copyright are supposed to be fully available.
You can run your own line current test to some extent with maybe a 1 HP motor. Locked rotor is fairly easy. Next you could rotate forward, then immeadiately change to reverse. You might want a current recording instrument for the tests.
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The breaker has since been cranked all the way up and has not seemed to cause any problems. . .
With that being said, theoritically, a circuit breaker setting of "B" corresponds to 500A and is above the 352A LRC. Therefore with a setting of 500A this breaker should never trip due to any starting current or any associated load current. Technically with this 500A setting the conveyor could be loaded any given amount, and the breaker should not trip since its above 352A. Since this is an instantaneous only pickup this 352 could last any given amount of time or until the motor burned up.
The electricians have since turned the breaker setting all the way up which I have instructed them is not the ideal solution, and am trying to explain to them why a setting of "B" should work. The problem has seemed to has ceased at least for now but I'm still interested in providing them with the correct answer. . .