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OK I've got 2 contactors where the coils are wired in parallel.
They are sitting right next to each other,
One pulls in and holds perfectly well. The other pulls in and drops out. Pulls in again drops out. Ect. Ect. For as long as you are trying to keep the contacts closed.

I've tried swapping out contactors, I've tried swapping the wiring around,

Same thing.

This is an established system that's been working fine for probably as long as I've been alive. :D

Any clues on what to try.


Cheers
 
Have you proven that the coils are indeed wired in parallel and that the voltage at the coil terminals is what you expect it to be, correct for the coil and constantly energised?

What is the voltage? AC or DC? What type of contactor? How rapidly does it operate & release?
 
Any pictures or wiring diagrams?
 
Does it work ok with the other one out of the circuit? If not, I say a dodgy contractor.
Replace both with new, identical units.
 
I have seen contactors that were originally 2 no + 2nc
changed for 3no +1nc or 4no etc, you get the idea.
can have some very strange effects on machine opperation
 
maybe the first coil contactor is providing current to a latching contact which brings in the second coil has developed a fault
 
there are so many ways the 2 contactors could interact with each other it can only be speculation or suggestions of what "may be wrong" some more info from the op would be great.
 
The OP does say specifically at the beginning that the coils are wired in parallel, i.e they should both energise and release at the same time regardless of anything else. This then suggests one coil is faulty or the wrong type, but he claims to have substituted them.

I recently had a head-scratcher where I had made a similar assumption that two relay coils were paralleled. They looked like it and tested like it, but when I picked the wiring loom apart it turned out they were not, there was a contact elsewhere in series with one of them that sometimes opened when I was not looking.
 
OK,
Its 110volt dc coils on the contactors switching a 110v dc load.

The coils are literally wired in parallel.
There isn't any hiding it in the loom or anything like that.

I've attached a diagram.
S1 and s2 are the coils fed from wires 231 (+) and 232(-)

S1 closes the contacts 212 to 222
S2 closes the contact 229 to 213
Problem with contactors wired in parallel IMG_20191030_201756 - EletriciansForums.net

the s2 contactor is the one thats dropping out, it happens in a very even pulse like way , maybe drop out and pull in every second. (half a second each)
 

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If you have checked the basics like connections and nothing else has changed before it caused a problem I would say you need a new contactor with the sam specs. Could be something has broken of inside and stopping the contactor to fully pull in.
 
All I can suggest is that you re check the voltage going to the contactor coil that is turning on and off.
is it changing from 110v to 0v as the contactor goes through its off stage?
if the power to the coil is always there, then you have a faulty contactor with a fault that I have never seen before.
my suspicion is that there have been modifications to the circuit and you might not be able to trust the wire numbers. check physically the connections between both a a1 and a2 of the contactors and ensure they are DEFINATLEY IN PARALLEL.
 
both contactors have been changed, (this happens every 5 years or so)
thats where i came in because it was happening, i managed to find the old contactors that were removed, and tried them, problem was still there (im not sure if the problem was there before my work mate changed them originally) ive tried another set, ive tried swapping them side to side. doesnt make a blind bit of difference.
 
OK, so when start is enabled, SCC1 and SCC2 energise. Their contacts complete a circuit for the solenoid pull-in coils in series with the motors and the hold coils simply in series. When both pinions are in mesh the solenoid contacts energise S1 and S2. These apply battery to the motors and bypass the pull-in coils. All pretty standard stuff.

Is one of the starter solenoids engaging and disengaging in sync with S2? Anything else moving? There is a sneaky interaction that could take place if there is an earth fault partway through the coil of S2 (although substitution should have located that).

Defo need to put a voltmeter on S2 coil before making any more guesses!
BTW, what engine is it?

Waaaait. Did this problem begin once the contactors were changed for some other reason (e.g. worn out contacts?). Has it been put back together right? Could there be a lead that looks like it's on the coil terminal but is actually in contact with the frame? So when you subbed in the other contactors, you re-instated the fault?
 
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Could you swap over S1 and S2 and tell me if the problem moves position?

I suspect the hold-on functionality internal to S2 is defective which means only the pull on coil is doing any work to operate S2 - but the pull on coil becomes de-energised when the power contacts S2 close - so to keep the power contacts closed the hold-on coil must be energised. Check the coil resistance of S2's hold on coil. Check it has 110V to energise it - if it does not then trace the control wiring to it backwards to find out where a Normally Open contact is not being closed.

Or something like this - off to watch the apprentice now. Will look again afterwards.
 
Two good posts,from Lucien,and Marconi :) Having seen the type of contactors,i would have suggested testing both the pulling and holding resistances.

It is the same type of fault,seen on many engine control solenoids.

....I too,would like to know,what the equipment is ;)
 
Although, it's not the solenoids that are misbehaving, only the motor contactors.

interestingly, if one contactor fails to operate or make effective contact, the corresponding solenoid's pull-in coil gets over-volted because its opposite number is bypassed, and it gets held lilke that until the start attempt is abandoned.

As described, the fault defies logic, which usually indicates a false assumption or mistaken observation. We must wait to hear what is going on electrically at S2's coil leadouts.
 
Could temporarily link the coils locally to prove if it's a coil fault or not....with loads disconnected or isolated. Seems like a wrong connection wiring fault.
 
Could temporarily link the coils locally to prove if it's a coil fault or not....with loads disconnected or isolated. Seems like a wrong connection wiring fault.

The coils are linked, I added an extra set of wires across thinking it may have been a problem with the with the wiring being broken inside the insulation

Could you swap over S1 and S2 and tell me if the problem moves position?

I've done that, it stays with wiring not the contactor,
Did this problem begin once the contactors were changed for some other reason (e.g. worn out contacts?). Has it been put back together right? Could there be a lead that looks like it's on the coil terminal but is actually in contact with the frame? So when you subbed in the other contactors, you re-instated the fault?

I have know idea if it was happening before they were changed, they get changed as part of "preventive" maintenance :D

I'll check the wiring to the solenoids and see if something is dodgy there.

The engine is a MTU 16V4000
A quad turbo v16

I'll get some more photos and a video later this morning
[automerge]1572504139[/automerge]
The contactors are albright sw200a-33
 

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