# Restting overload on a PLC controlled motor starter



## rlj213 (Dec 9, 2010)

Many tmes have seen a PLC or some other type of controller automatically starting and stopping a motor. One day, on a machine where the PLC was controlling the starting and stopping of a three pahse motor, the electrician said that he had reset the overload, and when he did, the motor started, which he said is not permitted by code. He said the motor must be restarted by some other means after resetting the overload. I remember he even showed me the reference, but I can't remember where he found it. Is he correct, and should the PLC have been programmed to force some other function to restart the motor after resetting the overload? I can't seem to find any reference to this in the NEC Article 430 section and I don't currently have an NFPA 79.


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## McClary’s Electrical (Feb 21, 2009)

rlj213 said:


> Many tmes have seen a PLC or some other type of controller automatically starting and stopping a motor. One day, on a machine where the PLC was controlling the starting and stopping of a three pahse motor, the electrician said that he had reset the overload, and when he did, the motor started, which he said is not permitted by code. He said the motor must be restarted by some other means after resetting the overload. I remember he even showed me the reference, but I can't remember where he found it. Is he correct, and should the PLC have been programmed to force some other function to restart the motor after resetting the overload? I can't seem to find any reference to this in the NEC Article 430 section and I don't currently have an NFPA 79.


 
It can automatically start, but needs to be labeled with warning label. It would be legal to set the overload to automatic reset.


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## Hunter1151 (Nov 4, 2010)

mcclary's electrical said:


> It can automatically start, but needs to be labeled with warning label. It would be legal to set the overload to automatic reset.


It is a personal preference, I would make a manual restart, the plc is not dropping it's latch, 3 wire or a set, most likely the contact from the ol is not a input.


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## busymnky (Feb 16, 2009)

*learing again...*

is it normal to have to manually reset the heaters too? the contact points in the starter were burned pretty badly.


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## Wirenuting (Sep 12, 2010)

busymnky said:


> is it normal to have to manually reset the heaters too? the contact points in the starter were burned pretty badly.


The o/s's trip when there is a problem. If the points are truly burned, replace them and at the same time check the motors for damage or an overload condition.


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## Jmohl (Apr 26, 2011)

Use another set of o/l relay contacts to an input to plc. Plc programmed to drop output when o/l occurs and will not restart without operator input.


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## busymnky (Feb 16, 2009)

we make concrete railroad ties, ... we overload everything. All the time. No kidding we break sledgehammers! I manually bypassed the starter and the heaters and we ran production, then I tore down the starter and filed the "points", and had a new starter delivered on a Sunday. smile, +35k production but... 
I appreciate the input guys!


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## JRaef (Mar 23, 2009)

busymnky said:


> we make concrete railroad ties, ... we overload everything. All the time. No kidding we break sledgehammers! I manually bypassed the starter and the heaters and we ran production, then I tore down the starter and filed the "points", and had a new starter delivered on a Sunday. smile, +35k production but...
> I appreciate the input guys!


What you are describing is "2 wire control" vs "3 wire control" schemes. Both are "legal" but you always need to understand consequences of any control decision, OL resetting being one of them. 2 wire control means there is just a circuit that closes to run, opens to stop. most PLC controls are 2 wire, but you can emulate a 3 wire scheme inside the PLC with a feedback of the motor starter condition to the PLC. Without that, you have a potentially hazardous situation, as it appears is the case here. This is a fault in the PLC program design and unless you want to assume responsibility for the entire machine, I'd steer clear and direct the machine OEM to fix it if it. If it's old and you own it outright anyway, you will need the PLC program and you must know what you are doing with it before making changes.

In case you don't know, filing the "points" on a motor starter will hasten the next failure and should be avoided in all but the most desperate of circumstances. Sounds like you knew that because you ordered a replacement starter. But I'd also order the replacement contacts and have them ready for the next failure or better yet, a PM schedule. Better to replace them now than suffer down time again, because Murphy's Law says the next failure will happen at the worst possible time.

That said, if it is not very old the contacts should not be burning and the OL should not be tripping. If it is, something is under sized for the application. If the system is being abused, the components should be designed to survive the abuse. You may want to consider a bigger motor and at least a bigger contactor.


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## oliquir (Jan 13, 2011)

most of the time when i programmed plc i monitor o/l so when they activate i just make a stop and make a light flash or any indicator then it needs to be restarted manually.


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