# Contactor Coil



## dronai (Apr 11, 2011)

Chris1971 said:


> I have a customer that is constantly replacing starter coils on 4-5 three phase contactors. after losing a phase the coils go bad and we have to replace them. The coils are 120 volt rated. Any suggestions to reduce the failure rate?


If the coils are 120V and he loses a phase they either drop out or still work ? The only thing single phasing is the motors :blink: I don't see why the coils would be failing for that reason.


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## buddhakii (Jan 13, 2011)

Do the coils share a neutral? Sounds like something is not quite right.


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## Chris1971 (Dec 27, 2010)

dronai said:


> If the coils are 120V and he loses a phase they either drop out or still work ? The only thing single phasing is the motors :blink: I don't see why the coils would be failing for that reason.


Not sure? Every time they lose a phase a coil or two go bad and needs to be replaced.


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## HARRY304E (Sep 15, 2010)

buddhakii said:


> Do the coils share a neutral? Sounds like something is not quite right.


That's what it sounds like to me,I've seen them set up with a bootleg neutral by using the EGC ,,,,,crazy bastards :no:


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## dronai (Apr 11, 2011)

Maybe the single phasing is causing a high voltage on the control wiring from the imbalance of a dropped phase ? All that stuff is fused though


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## wendon (Sep 27, 2010)

Chris1971 said:


> I have a customer that is constantly replacing starter coils on 4-5 three phase contactors. after losing a phase the coils go bad and we have to replace them. The coils are 120 volt rated. Any suggestions to reduce the failure rate?


Install phase protectors. Then the coils will drop out when you lose a phase.


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## kbsparky (Sep 20, 2007)

Another reason to use an isolation control transformer, instead of tapping onto one of the phases for control purposes ....


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## Wirenuting (Sep 12, 2010)

Install a snubber across each coil and insure your control transformers are properly fused.


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## denny3992 (Jul 12, 2010)

wendon said:


> Install phase protectors. Then the coils will drop out when you lose a phase.


Heres ur answer


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## Safari (Jul 9, 2013)

Chris1971 said:


> I have a customer that is constantly replacing starter coils on 4-5 three phase contactors. after losing a phase the coils go bad and we have to replace them. The coils are 120 volt rated. Any suggestions to reduce the failure rate?


sounds strange phase loss affecting the coils.but hey its electrical and strange things are bound to happen.phase protectors i guess will do. is there a possibilty for him to change then to 240v coil contactors and see.where i work most of the contactor coils are 240v.and i will tell u i dont know when was last i changed one.


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## JRaef (Mar 23, 2009)

nickson said:


> sounds strange phase loss affecting the coils.but hey its electrical and strange things are bound to happen.phase protectors i guess will do. is there a possibilty for him to change then to 240v coil contactors and see.where i work most of the contactor coils are 240v.and i will tell u i dont know when was last i changed one.


Just FYI, 240V coils work in your part of the world because you have 240V from phase to neutral on a 3 phase system. We do not. On a 3 phase system, we either have 120V phase to neutral or 277V phase to neutral, or no neutral at all. But most oftem we use a control power transformer to get 120V controls. If we want 240V coils, we can order them, but we have to wait to get them.


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## JRaef (Mar 23, 2009)

buddhakii said:


> Do the coils share a neutral? Sounds like something is not quite right.





HARRY304E said:


> That's what it sounds like to me,I've seen them set up with a bootleg neutral by using the EGC ,,,,,crazy bastards :no:


I'm in this camp. Most likely it's some kind of funky ground connection issue in your control circuit, or it's not connected to ground at all (floating). I had a Navy electrician who worked for me for a while, he was used to not grounding control transformer neutrals all the time, because that's how they do it in the Navy on ships. But in the real world, people have ungrounded delta power systems and when you do that, the lack of a grounded neutral on a 120V CPT causes a high voltage transient in the circuit if one power phase goes to ground.


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## denny3992 (Jul 12, 2010)

denny3992 said:


> Heres ur answer


I say this cuz ive seen it first hand... We uave a building not on generation( all others have xfer sw with ph loss) and when we get single phased by utility we loose contactor coils on starters in mccs they burn up due to low voltage


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## Safari (Jul 9, 2013)

that explains it thanks


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## IslandWire (Aug 3, 2013)

*Bonded Neutral?*

If you have a 120V control and 480V motors, then you must have a control power transformer. It is likely to be Primary480V L-L:Secondary 120VL-N You must NOT connect the 120V Neutral to your Primary Neutral. This 120V System is a separately derived system If necessary it would be OK to connect the 120V Neutral to your Ground. 

All that being said, it looks like if you loose one of the phases that powers your CPT while a motor is running, you would get the regenerative phase back to your CPT from the motor and it would be a low voltage on the CPT primary. That would give you a low voltage on the CPT secondary and this might raise your coil current enough to cause it to heat up and with enough time the coil would burn open. 

So, I would use a 480V phase monitor and use it to open a contactor on your 120V secondary of the CPT. Use a momentary switch and a latch on that contactor for the reset and your problem should go away and you will protect all connected motors as well.


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## xlink (Mar 12, 2012)

If your controls transformers aren't all on the same phases, doesn't the loss of a phase put some in series on the remaining two phases?


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## IslandWire (Aug 3, 2013)

xlink said:


> If your controls transformers aren't all on the same phases, doesn't the loss of a phase put some in series on the remaining two phases?


Yes, they will be in series when there are no 3 phase loads connected BUT, particularly not in series if the 3 phase load is a running motor with 1 phase dropped. Now, the motor will not, or should not, run long. The overloads should be set to trip on the increased current when the motor runs on 2 out of 3 phases. But during that time is when you run the risk of burning the starter coils. Likely, the overloads are set too high and that allows the motor to run far too long with a phase loss and increases the low voltage time on the starter coils. 

I say this is likely because if they are having the problem they complain about and have not diagnosed it yet, there is a good chance the overloads are not set properly either. Often there are several problems that all need to be fixed at the same time.


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