[WesM]

So I am trying to figure out what size breaker will be needed to feed to the CNC cabinet I'm putting together for a knee mill I am setting up. I am a complete novice when it comes to electronics, so feel free to tell me when/where I'm off track.

Right now I have the 3HP, 220v, 8.6a induction spindle motor, which I plan to power with a DURApulse GS21-23P0 or a delta MS300 (VFD11AMS21ANSAA), which I think might be an identical drive with with different branding. Reading through the manual it appears they recommend either 50a fuse or a 70a breaker. For the breaker the delta manual says the breaker should be 1.6-2.6 the rated input current of the drive (which is 27.5a). I'm thinking of using this, would that be sufficient at 2.3x the rated current?

https://www.automationdirect.com/adc...ors/gmcb-2d-63


For the servos I purchased 3 of the stepperonline T6 750w servos and drives. Rated current is 4.1a peak is 13.4, looking through the manual I am not seeing a recommended breaker size. Should I just select something around the peak current? 15a and 13a seem to be common sizes available for DIN mount breakers. Something like this maybe?

https://www.automationdirect.com/adc...ors/gmcb-2d-15


The breakers are available in B, C, and D curves. Is there a particular curve I should select for the VFD and servos?

Finally If I have a 63a breaker and three 15a breakers, totaled up thats a pretty high mains amperage requirement. Similar builds on CNCzone using DMM or other brands are only feeding their cabinets with 40a circuits. What size mains power circuit would you recommend for this kind of build?

[RaderSidetrack]

Post a link to your choice of VFD spec sheet. For sizing the "mains power" breakers, start with the "input current" amps shown on the VFD spec sheet. Add in amps that your power supply that drives the servos/steppers and other devices.

Amps drawn by the steppers/servos themselves come from the power supply driving them. So the amps used by the steppers/servos are not the numbers to be looking at, its the input amps to to the power supply(s) and VFDs that are relevant.

Post the spec sheets of the VFDs & power supplies.

[WesM]

Post a link to your choice of VFD spec sheet. For sizing the "mains power" breakers, start with the "input current" amps shown on the VFD spec sheet. Add in amps that your power supply that drives the servos/steppers and other devices.

Amps drawn by the steppers/servos themselves come from the power supply driving them. So the amps used by the steppers/servos are not the numbers to be looking at, its the input amps to to the power supply(s) and VFDs that are relevant.

Post the spec sheets of the VFDs & power supplies.

Here is the specification for the VFD I am planning to use. I am using the GS21-23P0, which has a rated current of 27.5a variable and 24.2a continuous.



The 750w T6 servos have a continuous current rating of 5.2 (arms) each. with a peak of 18.4 according to the spec sheet (which is different from what they have on their website).

[RaderSidetrack]

I couldn't find a "Rated Input Current" in T6 VFD documents. But, assuming you are planning 3 T6 drives, 27.6/4*3= 20.7 amps is a reasonable estimate for the combined 3 T6 VFDs. So add the 20.7+27.6 amps together. Add to that whatever ancillary power supply devices (fans?, control systems?, coolant pumps? etc) draw in amps. And it would be smart to add say 15% just in case.

Note its really not just a "breaker" issue, the conductors in the wall/conduit to wherever that mains breaker is located need to be rated for the amps the breaker is rated for, (as well as the conductors from the breaker to your machine).

[WesM]

Just to be clear;

• The T6 is the servos and drive.
• I think?? 5.2a is the rated input for the T6 servo drives from what I can tell.
• The GS21-23P0 is the VFD.
• 27.5a is the rated input for the VFD.

Note its really not just a "breaker" issue, the conductors in the wall/conduit to wherever that mains breaker is located need to be rated for the amps the breaker is rated for, (as well as the conductors from the breaker to your machine).

Right, I am trying to figure out my total machine amperage draw before I have the wiring/ mains breaker put in my garage panel for this machine.

[joeavaerage]

Hi,

Right, I am trying to figure out my total machine amperage draw before I have the wiring/ mains breaker put in my garage panel for this machine.

Well you'll need 25A for the VFD alone....so get as big a circuit as you can. I'd suggest 32A minimum, 40A preferably and 50A if the supply to the garage can tolerate it.
Its not that you'll draw even a small fraction of that current 99.9% of the time, its just in those few brief instants of overload where you need it all. A large, even over large
circuit to feed your machine just means that there is a vanishingly small probability that the circuit will overheat or catch fire. The main breaker is to protect the wiring from
damaging overload....not the machine.

The over current protection for the individual components in the machine should be fused/breakered within the machine.

I had a current monitor on my 750W servos when I first started using them, but the average and normal current is almost to small to measure, except very occasionally when it hits the roof.
I soon realised there is no point in trying to monitor the current of the servos. Spindles can and do however draw plenty of current for hours at a time.

Both VFDs and servos require D curve breakers, they are very inclined to draw large currents for a few ten of milliseconds and ordinary breakers tend to open on those short duration
loads.

I use 6A D curve breakers for each of my servo drives, they are cheap and readily available. It is true that the servos can draw much more than that but its so brief and so seldom that I've
never had a 6A breaker open. My spindle is only 800W so I have a 10A D curve breaker for it, and I've never had a trip. A 2.2kW spindle will require more, probably 25A or 32A is required but
whats the bet that if you had a 20A D curve you'd hardly, if ever, trip it.

The important point to realise is that we tend to over build our AC supplies when 99% of the time they are loafing along at 1% of rated load. Only in brief periods does the power level go up.
That's where you need to make sure your connections and terminations are good, because even brief over current events can cause them to open circuit spectacularly, arcing and smoking.
Look at your wiring, terminations, and so on, less with a view to how much that it can sustain over long periods but can it handle 300% -500% for brief periods of time?.

Craig

[WesM]

Hi,



Well you'll need 25A for the VFD alone....so get as big a circuit as you can. I'd suggest 32A minimum, 40A preferably and 50A if the supply to the garage can tolerate it.
Its not that you'll draw even a small fraction of that current 99.9% of the time, its just in those few brief instants of overload where you need it all. A large, even over large
circuit to feed your machine just means that there is a vanishingly small probability that the circuit will overheat or catch fire. The main breaker is to protect the wiring from
damaging overload....not the machine.

The over current protection for the individual components in the machine should be fused/breakered within the machine.

I had a current monitor on my 750W servos when I first started using them, but the average and normal current is almost to small to measure, except very occasionally when it hits the roof.
I soon realised there is no point in trying to monitor the current of the servos. Spindles can and do however draw plenty of current for hours at a time.

Both VFDs and servos require D curve breakers, they are very inclined to draw large currents for a few ten of milliseconds and ordinary breakers tend to open on those short duration
loads.

I use 6A D curve breakers for each of my servo drives, they are cheap and readily available. It is true that the servos can draw much more than that but its so brief and so seldom that I've
never had a 6A breaker open. My spindle is only 800W so I have a 10A D curve breaker for it, and I've never had a trip. A 2.2kW spindle will require more, probably 25A or 32A is required but
whats the bet that if you had a 20A D curve you'd hardly, if ever, trip it.

The important point to realise is that we tend to over build our AC supplies when 99% of the time they are loafing along at 1% of rated load. Only in brief periods does the power level go up.
That's where you need to make sure your connections and terminations are good, because even brief over current events can cause them to open circuit spectacularly, arcing and smoking.
Look at your wiring, terminations, and so on, less with a view to how much that it can sustain over long periods but can it handle 300% -500% for brief periods of time?.

Craig

Hey Craig, thanks for the info. I can get a 50a circuit to the garage fairly easily.

So would you suggest just going with 6a D curve breakers inside the cabinet for the 750w servos I have? The leadshine/SO servos/drives seem to have a fairly similar, if slightly lower power draw than the DMMtech 750w servo specs.

For the VFD I was thinking a 40a D curve breaker, its at the lower end of the recommended spec in the manual, but would still be in spec. Does that seem reasonable or would you suggest sizing the breaker up a bit?

[joeavaerage]

Hi,
the reason I chose 6A breakers is because they are a standard size therefore readily and cheaply available.

You can get 5A, 4A etc breakers but they are harder to find and more expensive.

https://nz.element14.com/eaton-cutle...tic/dp/1735725

https://nz.element14.com/eaton-heine...tic/dp/1704961

For the VFD I was thinking a 40a D curve breaker, its at the lower end of the recommended spec in the manual, but would still be in spec

Well if 40A is the spec then it makes sense to use it. Just remember that 99% of the time it will draw just a few amps. I would suspect that a D curve breaker of 20A would be enough.
The breaker inside the cabinet is to protect the device, in this case a VFD. We do not want it to trip unnecessarily, but we do need it to trip to save the device....hopefully.
I think I would try a 20A or 25A D curve to start with. If you get no nuisance trips, all well and good, but if you do get nuisance trips then up the breaker until they stop, say 32A or 40A.

You should reserve some budget for line filters. VFD's and servo drives are renowned for being electrically noisy, and you should use either line reactors or at least two stage line filters
to try to prevent electrical noise from propagating from the VFD (servo drives) to other sensitive 230VAC powered devices like the PC. This sort of thing would be suitable for the VFD:

https://nz.element14.com/united-auto...36a/dp/3618172

Having one filter per servo drive would be nice, but gets expensive, so you'd probably get one 16A-20A line filter to do all three:

https://nz.element14.com/corcom-te-c...ase/dp/9586490

Craig

[WesM]

Hi,
the reason I chose 6A breakers is because they are a standard size therefore readily and cheaply available.

You can get 5A, 4A etc breakers but they are harder to find and more expensive.

https://nz.element14.com/eaton-cutle...tic/dp/1735725

https://nz.element14.com/eaton-heine...tic/dp/1704961



Well if 40A is the spec then it makes sense to use it. Just remember that 99% of the time it will draw just a few amps. I would suspect that a D curve breaker of 20A would be enough.
The breaker inside the cabinet is to protect the device, in this case a VFD. We do not want it to trip unnecessarily, but we do need it to trip to save the device....hopefully.
I think I would try a 20A or 25A D curve to start with. If you get no nuisance trips, all well and good, but if you do get nuisance trips then up the breaker until they stop, say 32A or 40A.

You should reserve some budget for line filters. VFD's and servo drives are renowned for being electrically noisy, and you should use either line reactors or at least two stage line filters
to try to prevent electrical noise from propagating from the VFD (servo drives) to other sensitive 230VAC powered devices like the PC. This sort of thing would be suitable for the VFD:

https://nz.element14.com/united-auto...36a/dp/3618172

Having one filter per servo drive would be nice, but gets expensive, so you'd probably get one 16A-20A line filter to do all three:

https://nz.element14.com/corcom-te-c...ase/dp/9586490

Craig

The VFD manual says 1.6-2.6 x the input current for breaker sizing. I'm unclear on what the input current will actually be. The VFD manual says it pulls up to 24.2a input in continuous torque mode (which is what I'm reading I should use), with an output of up to 11a to the motor.

Would it be reasonable to say that since my motor is going to draw 78% of this drives output current, its only going to be pulling up to 78% of the drives rated input (ignoring inrush during initial startup) under normal operation? (.78*24.2)/1.6 ends up being around 30a.

I hear what you are saying about line filters, I plan on using an EMI filter for the VFD and servo drives. I will look at the dual stage ones though, I was initially only planning on using single stage EMI filters.

Below is the spindle motor spec plate for my machine.

[joeavaerage]

Hi,

Would it be reasonable to say that since my motor is going to draw 78% of this drives output current, its only going to be pulling up to 78% of the drives rated input (ignoring inrush during initial startup) under normal operation? (.78*24.2)/1.6 ends up being around 30a.

You are trying to estimate the power factor...don't go there. Use the rated current of the VFD....that is what you are trying to protect. If the rated input current of the VFD is 24A, then a 30A breaker would be fine.

Craig

[WesM]

Hi,



You are trying to estimate the power factor...don't go there. Use the rated current of the VFD....that is what you are trying to protect. If the rated input current of the VFD is 24A, then a 30A breaker would be fine.

Craig

Hah, fair enough, 30a it is. I'm planning on using a 24vdc contactor for VFD and I will have the 3 servo drives on another contactor. Do you generally size the contactors at the same amperage as the breakers? I was looking at these from Automationdirect

https://www.automationdirect.com/adc...mc-32a30-22-ds

[joeavaerage]

Hi,
do you need contactors? The only real reason that I can see for having contactors is if you require that the machine de-power in response to an emergency stop.
My machine very seldom Estops for any reason, but when it does all the enable signals to the servos and VFD go low and they all stop. Maybe for a commercial machine
you are legally required to de-power the machine....but my machine does not and I have no intention of making it do so. I've never needed it in ten years I've been CNCing
and I can't see any need for it now.

My servos and VFD are connected to the main power feed via a breaker. I can use the breaker to isolate any individual component if I need.....but seldom, if ever do.
If I need to work on the machine, say adding a new servo drive or similar then I depower the whole machine and remove the plug from the wall. Haven't electrocuted my self yet.

It's your money, so if you think you need contactors then go to it, but I determined that I don't need contactors and can use the money elsewhere.

Craig

[WesM]

oh, well I suppose there was a reason all the other builds I was seeing on cnczone used one. All of the centroid schematics for the acorn use one too. I definitely do want the E-stop to power down all of the motive power. I like the idea of having a big red button that will kill power to my spindle and servo drives if something goes wrong. Its something I have had on every machine I programed and ran, from lowly Haas' to high end 5 axis Mikrons and would like it on this one.

[joeavaerage]

Hi,
yes just about every machine does that....but is it necessary? I suppose if the computer throws a complete crank the servos and spindle could go wild.......I'm still
waiting for anything like that to happen. If I hit EStop, my machine crash stops, ie the servos and spindle stop and the Z axis brake comes on......and has done every time
I've ever used it for ten years.

Craig

[WesM]

Right, that would have to be answered by someone with a lot more experience than me. I can tell you how industrial equipment operates, but not how (well, maybe a little of the how, but not much) or why it operates that way.

I am kinda basing my cabinet design off of LtDans build from several years ago. He made use of a 4th servo for his spindle and used a lot of fuses, but otherwise the basic setup is similar to what I am trying to do.

https://www.cnczone.com/forums/dmm-t...23202-cnc.html

I am doing a full assembly in SolidWorks for my cabinet once I get the components figured out. I will be making a build thread for my machine once I get all of that figured out and post the CAD models for you guys to check out/critique once its ready.

[WesM]

Got slightly distracted with this project with a major home remodel project... Figure I will continue using this post to keep this project together.

Currently looking at EMI filters for the spindle VFD and servo drives. two stage has been recommended, but beyond that I know nothing about EMI filters. Would either of these be suitable for filtering servo drives/VFD? They list specs like leakage current (assuming you want as low as possible?) and capacitance (do I want more or less?), but I'm not sure really how these relate to EMI filtering.

https://www.digikey.com/en/products/...DPGS5C/3830600

https://www.mouser.com/ProductDetail...8Bk2mxYQ%3D%3D

[joeavaerage]

Hi,
either would be fine.

Low leakage currents are the norm and desired, but any line filter on the market will be of that type.

The capacitance is related to the inductance, it is when all said and done, a low pass Chebyshev filter, so the capacitance/inductance choice is constrained by the pole placement of the filter,
and the poles are placed optimally for a given filter response. Don't sweat it, any good two stage filter will be fine.

Craig

[WesM]

Hi,
either would be fine.

Low leakage currents are the norm and desired, but any line filter on the market will be of that type.

The capacitance is related to the inductance, it is when all said and done, a low pass Chebyshev filter, so the capacitance/inductance choice is constrained by the pole placement of the filter,
and the poles are placed optimally for a given filter response. Don't sweat it, any good two stage filter will be fine.

Craig

Does it matter where the EMI filter is in the circuit? Say would it be ok to have something like this for a wiring layout?

240v AC outlet -> main Cabinet switch -> Contactor -> 30a EMI filter -> 3, 10a circuit breakers -> 3 servo drives.

[joeavaerage]

Hi,
yes that arrangement would be fine.

Craig

[joeavaerage]

Hi,
maxspongebob is correct, ideally you'd have separate breakers for each item....but where do you stop? I have five servo drives, one VFD, one servo drive as a spindle, one coolant pump and one
PC/power supply/electronics outlet....does that mean I should have nine circuit breakers? I don't have that sort of money to waste. I have the VFD/servo spindle on one breaker, all the servos and coolant pump
on another, and one for the PC/power supplies/electronics. Its nice to have a dedicated breaker for each circuit, but its not strictly necessary either.

Craig