[WesM]

I am still figuring out the BOM for my build. Right now I have the servo drives and VFD on their own breakers. Lube pump and coolant pump will probably get a breaker. There is an air actuated spindle brake as well I need to figure out how to operate, I think those will all be wired to the centroid acorn relays. I will have to see what centroid recommends for circuit protection in those situations. I have very little experience with all of this, so I'm taking things slow reading a lot and asking lots of questions!

[joeavaerage]

Hi,
I think you are over doing it. Its not that I would advocate reducing either interlocks or circuit breakers or contactors to the extent that the machine and/or personnel are at risk.

For a commercial machine to get an Electrical Safety Certificate, which is required if you want to sell the machine then you must follow certain rules. For instance it seems that
an Estop must cut the power supply to the entire machine, despite the fact that all machine controllers shut down movement, spindles, pumps etc WITHOUT interrupting the power.
I'm of the opinion that the regulations are lagging the technology in use. What might have been appropriate in the 80's is likely to be outdated in the 2020's.

If you are selling your machine then you must comply with the regulations even if they don't make sense anymore....but if its your machine, then who cares? Its up to you to decide
on the level of safety that you require.

For instance if you have separate breakers for each servo, lets say five of them, then you've spent how many hundreds of dollars?. Whereas if you had one breaker for all five servos
you still get the short circuit or catastrophic fault protection that you need, but without having to spend as much money.

I have limited sums of money to throw at CNC. I don't want a dangerous machine....but neither do I want to waste money trying to make it fool proof.

My recommendation is spend the effort protecting those devices that are most likely to fail, VFD's particularly. When VFD's fail they can really 'go South' in a big way.
Coolant pumps can and sometimes do fail as well and usually a short to earth type fault, so a fuse or circuit breaker is advised for then also.

Craig

[WesM]

You mentioned hundreds of dollars on breakers... maybe I am looking at the wrong things, but these seem fairly reasonably priced.

I plan on using these for the servos:
https://www.automationdirect.com/adc...ors/gmcb-2d-10

and this for the VFD
https://www.automationdirect.com/adc...ors/gmcb-2d-32


Like you I am on a pretty limited budget, but I do want this to look "professional" if that makes sense. I may also end up selling this in the distant future too, so that is a concern. All in I'm trying to stay under $2k for the whole retrofit.

Right now this is what I have:

Centroid Acorn Board $329
Centroid mill pro $159
3, Leadshine EL6/T6 750w AC servos $600
GS20 3hp VFD $253

That leaves me about $700 for all the breakers, contactors, switches etc. I'm still building the BOM for this, so will see how it comes out.

[joeavaerage]

Hi,
yes they both look fine....so how many do you want to buy? If you start buying lots of them, thats where the cost comes.

This is the type that I want:

https://nz.element14.com/potter-brum...tic/dp/1566490

If you bought ten of those the costs start going up!!

The truth is that the servo drives are rated for 6A each, so if you have five servos as I do then you'd need a 30A breaker to do all five....right? But a 30A breaker will not really
give as much protection to a single 6A drive will it?

As it turn out I have one 10A D curve breaker do all servos. You'd think that the breaker would pop all the time....but it hasn't in 2.5 years. The fact is that the servos
run at something like 1% of their rated current 99.999% of the time, so having just one 10A breaker handily powers all five servos but without popping even once in 2.5 years.
A 10A breaker will however pop no trouble if one or more of the servo drives blow up. My aim here is to protect the equipment from a catastrophic fault like a short circuit, not to try to limit
the current to any one servo....they do that themselves.

I may also end up selling this in the distant future too, so that is a concern.

Even if you do go to sell this it will still never have a Certificate....so don't get too hung up about it.

Craig

[WesM]

Wow thats kinda crazy that 5 on a 10A breaker are not tripping it. I doubt my Z axis will ever pull much current, since its just driving the quill, in fact 750w for that was probably insanely overkill, but it cost pretty much the same as the 400w. I have a feeling the X and Y axis will pull some load when I'm cutting stainless or carbon steels, but it will be interesting to see how much.

At least for now I will plan on having 6-7 breakers, one for each servo, the spindle/VFD and the coolant pump and lube pumps. At $16 each I'm fine with $100 for a little peace of mind, even if I know its overkill.

I do have a question about contactors, looking at the various ones available it looks like the coils can be energized by voltages varying quite a bit. If I already have 240/120 at the cabinet, is there any reason not to get contactors with 240v/120v coils?

[joeavaerage]

Hi,
I had three 6A breakers, one for each linear axis, of the same type I linked to, but later I replaced with just one 10A breaker. I sold the 6A breakers to a friend whom was doing a retrofit,
so I could get my money back. Got lucky. When I finished my fourth axis, I just hooked that servo to the existing single 10A breaker, and it worked fine. Now that my fifth axis is finished
I hooked that to the same single 10A breaker, and it seems to be fine.

I'm thinking that I might get another breaker so that I can separate the fourth and fifth axis servos, ie I can turn them off independently, while leaving the X,Y,Z servos on.
I have just finished a trunnion fifth axis, and I'm still trying to work out whether I need to remove it from my machine (and require the servos be unpowered)
or whether I can put my three axis vise on to of the fifth axis platter. Leaving it in place would save a lot of messing around...it weighs 85kg so removing it and later refitting it
is a PITA.

Craig

[joeavaerage]

Hi,

do have a question about contactors, looking at the various ones available it looks like the coils can be energized by voltages varying quite a bit. If I already have 240/120 at the cabinet, is there any reason not to get contactors with 240v/120v coils?

A great deal of industrial equipment has control circuits based on 24VDC. Have a look at the selection of relays alone and you'll see just how common 24VDC is.

Imagine now you have an EStop circuit, and that circuit has to carry the contactor coil current. If its a 240VAC coil then all your Estop switches and limit switches or indeed any other
device which can Estop your machine needs to be rated at 240VAC. Do you want a limit switch swilling in coolant and chips carrying 240VAC?

Craig

[Titaniumboy]

Wow thats kinda crazy that 5 on a 10A breaker are not tripping it.

Time Current Coordination curves for thermal magnetic breakers behave differently than many people think they should.

For example, your Gladiator breaker will take between 8 seconds and 2 minutes to trip at 20 Amps. At 15 Amps it will take between 30 seconds and 10 minutes to trip.

It is much the same for Joe’s Potter Brumfield circuit breaker. At 20 Amps it will take between 2.2 seconds and 20 seconds to trip. At 15 Amps it will take between 4.5 seconds and 40 seconds to trip.

[joeavaerage]

Hi,
that is exactly why I like these magnetic/hydraulic breakers. They open very promptly with high fault currents, but with ordinary loads they open slowly.
Given that servos draw large currents, but only intermittently, they are well suited.

Craig

[pippin88]

Time Current Coordination curves for thermal magnetic breakers behave differently than many people think they should.

For example, your Gladiator breaker will take between 8 seconds and 2 minutes to trip at 20 Amps. At 15 Amps it will take between 30 seconds and 10 minutes to trip.

It is much the same for Joe’s Potter Brumfield circuit breaker. At 20 Amps it will take between 2.2 seconds and 20 seconds to trip. At 15 Amps it will take between 4.5 seconds and 40 seconds to trip.

That's because a lot of people don't understand that circuit breakers are to protect the wire (not the component / appliance) and stop excessive current draw resulting in fires.