Tell us more about this relay that fails. Is it a control relay or power relay (do all three phases run through it)?
What fails? does the coil burn up or does the contacts burn up?
What does the relay do exactly? Is it ac or dc voltage?
That will help to figure out how to answer your question.
First, I want to be clear here, I'm not looking for solutions for the relays that burn up. I know why they burn up. They're cheap Fotek relays that are notoriously unreliable...but I have a stack of them I need to run through before I can move on to better stuff.
What I want is help sorting out a three phase brake.
As to your questions, they're Fotek SSR's that go between the batteries and inverters...and as for why they burn up, well they're Fotek SSR's, it's kind of their thing
And I want to be clear that I do not want to give advice and have it burn someone's stuff down. I mean if a 15 amp fuse is blowing, I am not going to tell someone to put in a 100 amp fuse even though it has #14 wire.
So knowing why the relay is failing is kind of important.
My guess is you are using to small of a relay. IDK what kind of current you are pulling between battery and inverter, but my guess is that it is more than the relay can handle. SO if the system works otherwise, except when the relay fails, why add anything? Why not just put the right relay/contactor in?
You see why I asked those questions now?
Now that we can skip does this brake system need to be tied into this failing relay (kind of important to know).
Yes, get a adjustable dc voltage relay, sometimes called and over/under voltage relay. Hook it up to the place you want to monitor the voltage (battery voltage). Have it fire a 3 pole relay/contactor when it hits whatever voltage your heart desires. Tap all three phases from the mill, maybe where they connect to the rectifier. run each phase to individual poles of the relay, on the other side of the relay/contactor hook them all together.
Once your battery voltage hits the set point, the battery power will fire the small voltage sensing relay. When it fires it will fire the larger 3 pole relay/contactor and since the other side of those poles are all connected together it will short the mill out. YES you will have a small drain on your batteries to keep said relays fired. Which is why a over/under (or 2 separate voltage sensing relays wired together) is nice, because something has to break the power to the relay/contactor that is now braking the mill, so that the mill can restart and charge the batteries again. AN over/under relay switches according to voltage. So you only need one relay. At the high set point it fires the 3 pole relay/contactor and stays fired until the voltage drops to the lower set point and then it cuts power to the 3 pole relay/contactor and lets the mill restart/
Sorry if I am crabby this morning, sleep has been hard to come by lately. Good luck with your project.