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We've had odd shorts on occasion. Don't know if this is what your problem is. But, this is what I would check.
Shorts from the Victor to case (in this case the robot). We've had shorts from the Victor mounting screws to the M+ lead. We now use plastic mounting screws.
We've had shorts from the drill motor drive wires to our cooling shroud and thence to our case.
So, I would recommend to perform an insulation resistance test between M+ and robot case and M- and robot case. I would do this for all speed controllers and relays, just in case you have a really weird ground path.
If you're getting shorted windings in your motors, you can check the no-load current. If that has gotten really big (10 Amps), I would replace the motors. It could be that your motors are drawing a lot more current than they did at the beginning of the season. I doubt if this would cause the problem you're observing. But, it might aggravate another problem which has been present all along but hasn't manifested til now.
You should be able to check for shorts between the ports on the 40A breaker.
Since you've already blown a bunch of speed controllers, I would recommend against swapping parts out before finding the problem. It might be worthwhile to pull the offending motors, speed controllers, etc off the robot and try only those bits. This would remove the possibilty of sneaky ground paths.
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