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Paralleled Relays for load sharing

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Mosaic

Well-Known Member
Hi all:
I have an app. that runs pulsed power thru an automotive 12VDC coil relay when it is not switching (64mS debounce buffer). The relay is rated @ 80A and contains a single SPDT contactor. I have seen SPDT contact temps up to 100C.

I am considering a 100A relay that carries a pair of contactors as well as paralleling the existing 80A relay.
The pulsed currents average about 7.2A max, but they can approach 800 A with a very low DC%, which is why the contact is heating.
Heat seems to ramp 15°C per RMS ampere.

After some months service, the SPDT contact starts to 'stick' sometimes. Cleaning the contact solves the problem for a while.
Paralleling relays is not normally suggested, but with no load switching and the positive thermal coeff. of the contacts, I see an opportunity for load sharing.

Any thoughts?
 

camerart

Active Member
Hi M,
Are there capacitors across the contacts? I seem to remember that this helps the points avoid sparking and stick. (60s motorbike memories:))
Camerart.
 

Cicero

Active Member
I'd say if its not switching any kind of current, and only taking those larger currents when closed then load sharing like that will help work to delay the onset of the sticking somewhat.

Why does it reach those kinds of huge currents in the first place?
 

Mosaic

Well-Known Member
I'd say if its not switching any kind of current, and only taking those larger currents when closed then load sharing like that will help work to delay the onset of the sticking somewhat.

Why does it reach those kinds of huge currents in the first place?
That's my reasoning as well.
The currents occur as a byproduct of a potential difference used to punch thru metal oxidation. Something like a spot weld effect, once the plasma gets going the currents spike.
 

crutschow

Well-Known Member
Most Helpful Member
What about using a larger relay such as a vehicle, continuous-duty contactor.
Some on this page are rated for up to 500A continuous, 1500A intermittent duty.
 

dr pepper

Well-Known Member
Most Helpful Member
Paralleling contacts isnt generally a good idea, however if one contact is capable of withstanding the load current using 2 contacts wouldnt be a fire risk.
If you look at Fork lift parts suppliers you can get some really hefty 12v contactors, 1000amp is no problem for a lot of mobile plant, they'll cost you more but your better using the right parts for the job.
 

Mosaic

Well-Known Member
I did some analysis.... turns out the relay is not being overloaded, but it's at the max.
At 2 mΩ (I measured it) and 80A continuous load = 12.8W dissipation.
With the 800A triangle pulse we have 800 A at a 2% DC =800*800*.002*.02 =25.6W, averaged to a square wave = 12.8W. Exactly 100% loading....what a coincidence.

So at about 100% loading the relay only lasts a few months before contact failure/sticking.
I'll test the paralleling idea this weekend and do some thermal analysis to assess the load sharing. If I can get the Relay peak loading down to below 70% I suspect the lifetime will jump significantly.
 

Mosaic

Well-Known Member
Well paralleling did help somewhat but the load sharing is more like 1/3 and 2/3 between the relays.
Overall I decided to clock back the RMS load to about 82% of max which translates to a steady state of 75°C peak contact temperature @ 31°C ambient. This is with dual relays, the cooler relay runs around 60°C
 
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