Build Log - Herbert

[m_c]

You could use a booster, however 12VDC versions of those relays should be available.
I only gave the 24VDC part numbers, as those are what I use, and had the numbers handy.

12V version of 3PDT is 536-900, and DPDT is 488-2604

What is best to do depends on the machine, and how you implement safety features.
Going by the components you have listed, I'm going to guess this is a reasonably sized router?
Personally, I wouldn't worry too much about limit switches, as provided the design is good, should you run into a hard limit, the motor will stall and simply sit stay stalled. With dual Y axis, I'd be more concerned about racking, but no limit switch is going to prevent that.
If I was to fit limit switches, I'd hard wire them into the E-stop circuit, as you want things to stop as quickly as possible should you hit one, but then that means you can't use the limit switches for homing.

In a commercial setting, safety relays are good, however for smaller home machines I personally think they are overkill. The major benefit of them is they monitor the contacts, so should a set of contacts stick/weld shut, they'll prevent the machine from starting. If you were to use standard relays, should contacts stick shut, you would be none the wiser unless you noticed power was still on when it shouldn't be.
There are also time delayed safety relays, which purpose is to maintain power to certain circuits for a short period. A good application of them is servo controlled machines, where when the E-stop is triggered, it subsequently triggers the servo drives to carry out a fast stop, before then killing power to the drives after a few seconds. This allows the machine to be stop safely under power, before all power is removed, as otherwise the servos risk freewheeling to a stop without power.

As for contactors. They are essentially just a big relay, however they're contact design means they're less likely to fail/weld shut. Whereas a typical relay has a single contact that opens/closes, a contactor works by bridging two contacts with a bar/plate. Should a contactor weld shut, it's unlikely both ends of the bar will weld, so when it releases the opposite end should still break the circuit. Off course, unless you have monitored contactors, you'd be none the wiser unless the contactor failed completely.

The big thing though, is regardless of relays or contactors, is provided they're sized/rated/fused correctly, you should not have any problems with contacts welding or burning out.
My concern would be inrush currents as things initially get powered up. Even though the VFD/Spindle maybe only needs 8A under full load, it could have a power on surge of 20A as the internal capacitors charge up. The same for the switch mode power supplies. Any good technical data/specification sheet will state surge currents.