Problem with contactors wired in parallel

[hmmmnz]

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.

Any clues on what to try.


Cheers

 

[Lucien Nunes]

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?

 

[James]

Any pictures or wiring diagrams?

 

[snowhead]

How does the resistance of each coil compare?
What's the source of the control voltage to the coils?

 

[PEG]

Yep,more info needed,could be as simple as it's coil feed,going through it's NC switch?

 

[littlespark]

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.

 

[James]

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

 

[Pete999]

maybe the first coil contactor is providing current to a latching contact which brings in the second coil has developed a fault

 

[James]

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.

 

[Lucien Nunes]

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.

 

[hmmmnz]

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


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)

 

[WallaceP]

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.

 

[James]

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.

 

[hmmmnz]

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.

 

[Lucien Nunes]

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?