I have been sourcing parts for a CNC conversion build, and as part of that process involves learning how to properly switch and control higher power loads.
It’s been an interesting but very slow process.
Shown above is a 3-wire control circuit, provided in reference materials by Schneider Electric (PDF). The diagram is easy to understand, but it took a very long time to find.
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I also found one YouTube video particularly helpful:
Before getting there, I found so many bad videos and countless websites that seem to have copy/pasted the same information from each other.
Here’s how each research step has gone:
Step 1: Figure out what I need to learn – usually by figuratively stumbling over an obstacle.
Step 2: Find an example of how it’s done.
Step 3: Find explanations of why things are done that way.
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Step 4: Determine what I will need for my build and future projects.
Teknic, the company whose motors I purchased for my build, provided an example of a safety stop control solution:
The parts list they provided has specific recommendations, and I’ll be straying from it a little bit.
I don’t really need to completely design something new for my CNC control panel build, as there are a couple of other examples I could also copy and adapt without too much difficulty.
But I’m the type that feels compelled to understand why certain components are chosen, and why things are done in a certain way.
Is it crucial for my current build? Probably not. But I have others in mind where there could be greater consequences if I get something wrong.
Every step turns into a huge rabbit hole.
Shown here is an example of how an Omron safety relay can be put to use with E-stops and interlock switches.
Others are a little easier to read.
Allen-Bradley provides another example of how start/stop buttons can be used with contactor state monitoring and automatic reset.
Once I find a good explanation, everything falls into place.
For my build:
Basic: 2 momentary switches and E-stop(s) can control a contactor for powering a load on and off.
Better: 3 momentary switches, E-Stop(s), and a safety relay can read and control a contactor for powering a load.
Best: 3 momentary switches, E-Stop(s), safety relay, and (2) contactors can power a load.
Safety contactors are ideal, but could be overdoing it. Even a safety relay might be overdoing it, but seems to be the proper way to do things.
The costs quickly balloon out of control, and so it’s also a matter of understanding and assessing risks.
A safety contactor has linked switches so that if a normally-open switch fails in the closed state (welds shut), a normally-closed switch cannot close. That could be a bit much for my purposes.
My plan will be for push button start and push button stop. E-stops will trigger a safety relay that requires a reset button to re-energize before the main start button can work. Build-in LEDs will indicate the state of the system – green for active, red for off, blue for standby and ready for reset.
If something goes wrong with any of the E-stops, the safety relay will trip. If something goes wrong with the contactor, the safety relay won’t allow it to be re-energized.
If (when) I build an enclosure, or create other safety interlocks, they can be routed to the same relay.
It definitely won’t pass industrial standards, but covers multiple bases and should give some protection to both myself and the machine. A little more would be needed if anyone else were to ever use the machine, but I don’t plan on that ever happening.
jim mc
If you want to use a safety relay that looks like a good solution – other safety items – light curtains and the like can be wired in as well depending on the application – do the motors require the brand name solution ot can you go with other brands. A simple e-stop (twist to release) could easily be added to the start/run circuit without a safety contactor – happy to help if you need some assistance – have quite a bit of experience in machine automation
Stuart
Yes, but what if down the road there’s a fault in the E-stop? Or contactor? I’m exploring what a safety relay adds to the control.
My current potential plan involves a safety relay, as well as an E-stop in the control panel and another E-Stop placed in another external spot, with a bypass switch in case it needs to be temporarily unplugged or reworked. I plan to add an enclosure with door down the line, but that will be a future project.
Teknic suggested Omron, but I’m looking at Dold (via Automation Direct) and also Phoenix Contact.
It looks like proper redundancy calls for a safety relay and two [safety?] contactors, but I’m not going that far.
Stephen Wilson
I’ve spent almost 30 years working in automation, and Allen Bradley (Rockwell) dominates the industry in the US. They make a good solid product but they tend to cost more.
As for redundancy, most of your safety relays are designed to handle redundant inputs as well as outputs, so you only need one relay really. Some produce a distinct signal on each channel and have to see that same signal back on the right inputs before they can be reset. Others are much simpler. Allen Bradley’s Guardmaster safety relays and Guardlogix PLCs are great but I can imagine Omron’s equivalents would be substantially cheaper.
Anson M
I’ll second Allen Bradley. Where I work we doesn’t do the panel builds but we purchase from different vendors. Allen Bradley and Phoenix Contact are the two we always see.
I’ll also second installing any/all recommended safeties. A contractor for work had a new portable band saw so I stopped to a lookie loo. It was new to them so there was a couple complaints about having to brace the work instead of just holding it and one-handing the saw.
https://www.rosendin.com/news/rosendin-electric-and-dewalt-partner-for-safe-dual-trigger-band-saw/
jim mc
I guess the question becomes what level of guarding and risk is there – if this is something where an untrained person might operate it then you get into keyed safety interlocks on guards and the like –
And then the question becomes how much automation are ypu doing – is the a plc/drive/controller you are using – sensors for missing material etc good luck
TonyT
I took a very quick look at AD and Banner.
The AD safety contactors similar in price to what I’ve used (Lovato, Siemens), and look OK except:
1) Coil power consumption is high, at >5W, so the contactor will get very warm or even hot, and you’d need ~250mA at 24VDC.
2) I couldn’t find any DC ratings for the main contacts. DC ratings are not the same as AC ratings (in fact, DC is harder to switch), so if you’re going to be switching DC make sure you find the DC ratings.
I recommend downloading some manuals and looking at the recommended schematics. Banner’s manuals have these; I’ve used them as a guide.
Stuart
The contactor I went with specifies 5 to 7W for the closed circuit. That’s the manufacturer datasheet, AD specified it at 5.8W.
I checked the one that Teknic specifies (Sprecher + Schuh CA8-12C-M31), and it specifies 2.6W warm and 3.0W cold.
I checked a similar Siemens (3RT2024-1BB40), and can see why they charge more. They have the same rating at AC-3, but so much higher ratings at more demanding AC and DC utilization. The Siemens has 5.9W closing and holding power.
I checked a Schneider Electric (DPE12BL), and it’s rated at 2.4W at 68°.
I picked up a Weg CWB12-11-30C03 (24V 12A). Teknic shows an example where the AC input and DC output passes through the coil, which I suppose adds redundancy, but I’m not sure I’ll do that.
Teknic specifies a different brand of contactor in their example, with 24V coil and 12A AC-3 rating. Their power supply can peak at 900W, and they confirmed a 10A type C breaker should suffice on the AC side (I figured 900/125 x 1.25 and sized up before I asked). I was worried if I should size-up the contactor, but their specified contactor is rated at 12A for AC-3 (as is the one I picked).
AD’s spec sheet doesn’t mention AC or DC for the main contact voltages, and the main datasheet specifies a “rated utilization current” for different DC categories. The DC-5 ratings are a little lower than the AC-1 rating (25A), and drop dramatically after 60V where I wouldn’t/couldn’t use this for 75V DC.
(Utilization categories are very new to me, and a completely different layer of current rating than I’m used to.)
I know switches are often derated for DC, but didn’t think to check this out yet for the contactor. Your input reinforces my reluctance to pass DC through the contacts; I’ll only use this one for AC, and am unlikely to seek out a DC-rated alternative.
Here’s Centroid’s sample schematic for multiple E-stops and a motor drive contractor – , and here’s an example with one E-stop and the spindle controlled by the contactor as well.
I want to be able to control motor drive power separately. Each E-stop would have two contact blocks, one with a the circuit started and completed with the Acorn controller, and the other separate but also controlled with an Acorn relay block that sends a closed NoFaultOut signal when everything is good. This allows the controller to de-energize the contactor and motor drives in lieu of a manual activation.
My intent is to wire manual controls to the E-stop coil power loop, with latching start via N.O. auxiliary coil. Push for momentary on, push for momentary off. Red light through N.C. aux contact for “off,” green light through N.O. aux contact for “on.”
If a safety relay is added in, the Acorn E-stop circuit connects through one of its N.O. load contacts, and the contactor power circuit connects through another N.O. load contact.
Should an E-stop be triggered, the safety relay de-energizes and the Acorn sees an open circuit as if an E-stop circuit is opened.
Let’s say I build an enclosure into the door as planned. As I understand it, if the door is open and then closed, but one of the switches failed and remained closed rather then opening with the door, that safety relay will sense that and won’t reset until the fault is fixed.
It’s not crucial to this build, but could be to the next one, which is why I’m determined to understand as much as possible.
Daniel L
Definitely look into DC ratings. Often times you’re looking at anything from 1/4 to 1/10th of the AC rating.
As to a safety relay: for my money, I would think that a safety relay is a bit overkill for the application here.
That all depends on a few factors here, though. Generally speaking, is very unlikely for N/O contact switch of sufficient rating to weld closed on a control circuit like this. That’s not to say that it couldn’t happen: that’s only to say that industrial standards assume that the probability that the switch will be used enough that the probability of this occuring somewhere, sometime is 1:1. Not so much because anybody has any real concern for the potential harm to human life, granted: but because potential liability is so expensive. Call me a cynic.
Home, personal use by a single operator makes the probability of failure far lower, but there are still cheaper ways to account for this risk.
A simple way to account for a welded momentary switch would be to use a current switch on the conductor between “start” and point #3 on the first diagram there.
Use the N/O contacts from that current switch to power the coil of a time delay relay. Set that timer to something like 5 seconds.
The current switch will be set to close when the “start” button is depressed. Once the button is released, the conductor the CS is “sensing” will no longer be carrying current, as the current that is holding (M) in a powered state will be bypassed through the |M| normally open auxillary contacts.
IF that conductor carries current for more than five seconds, the time delay relay could be configured to cut power to the circuit.
That wouldn’t account for the actual contractor’s contacts welding shut. But that’s where a master E-stop switch would come in.
Scott
I learned ladder logic while I was a co-op for Burroughs welcome 30+ years ago and that brings it all back. I always thought PLC were really cool. Working pneumatic control logic and switches was the most fun. BW had a lot of it since in the clean room we had pvc walls that were washed down with low pressure steam so electronics were a no no.
Stuart
Ladder diagrams have been making some of this much easier for me to digest.
I’ve been reading basic schematics ever since I was a teenager, but relays and contactors are completely new to me.
Bob
This hit home. parents both worked at Burroughs then Unisys old Man almost made it to retirement until he was one of the last remaining when they let him go in the mid 2000’s 30 years. This is where learned loyalty is a myth, Always do you.
MM
As you said, this rabbit hole can get deep and very expensive very quickly. Unless I have specialized requirements my go-to for this sort of thing is as follows:
I wire up a DPST relay or contactor with the appropriate ratings to switch both the line and neutral. The relay is wired such that it energizes its own coil, which is bypassed with a momentary NO switch to turn the device “on”. So you press the switch, the relay holds itself on. Thus if there is a power failure and the power comes back on again the device will not power itself back up automatically, you have to press the “on” switch to reset. That same coil circuit is also wired through a NC “off” switch. Emergency stop switch is redundant and separate, and if possible both disconnects Line and Neutral and interrupts the main relay as well. Depending on the machine you might wire other things to emergency stop switch. I like to use EAO modular style control panels switches, that way you can combine whatever contact blocks are necessary to make the E-stop function how you want. For example, a piece of test equipment I built a few years ago contained a 5kV DC power supply. I wanted to make sure that if someone hit the E-stop then it would also crowbar the capacitor in that power supply so there was no way anyone could be shocked if they started touching things they shouldn’t so I wired up an extra set of contacts on the E-stop to do that. I’ve also done similar on pneumatic equipment where the E-stop not only shuts down the electric power but also shuts off the incoming air line and vents residual pressure in the system.
One thing I will suggest is to not skimp out on the quality of switches, especially limit switches and your emergency stop switche(s). They are safety critical and not somewhere to try and save a few bucks.
Stuart
Initial Plan: Button controls and E-stops to control contactor that energizes motor power switches. A future build can include spindle control too.
The controller I’ll be using has an example for E-stop wiring.
https://www.centroidcnc.com/dealersupport/schematics/uploads/s15190.r1.pdf
One contact block sends an E-stop signal to the controller, and the other cuts power to the motor drive contactor while. A controller relay is in series and allows for software-controlled de-energizing of the motors as well.
I’m not familiar with EAO, but ordered a couple of clearanced switches (I love Newark for things like that) to play around with. For actual project use, I’m using Fuji Electric from Automation Direct. The AD stuff is good too, but the Fuji have a nicer feel and are fairly affordable.
The harder part has been in determining the differences between brands. Is an Omron safety relay, priced at $240, better than the Dold Automation Direct carries at $165? Do *I* need a safety relay at all? Is there value in my getting a safety contactor over just a contactor? Does this project call for redundant contactors? Lots of questions.
I bought an anti-restart E-stop box 10 years ago (https://14cyiuhvcgv.com/lovegreen-power-control-box-review/%3C/a%3E%29 and feel like I’m finally understanding how to build similar functionality into other equipment.
MM
If it matters, I’ve built dozens of custom pieces of test equipment & machinery and I’ve only ever used a “safety relay” twice, and that was because the customer specifically requested them, along with specific light curtains, as part of the project. I use contactors for larger equipment and ice cube relays for smaller, being very conservative with the ratings, and let the project itself determine the need for redundancy.
These are the control panel switches I like, the Eaton 04 series.
https://us.rs-online.com/switches/?a10=EAO&searchWithin=04%20series
They are modular, so you’d combine an actuator (pushbutton, rotary lever, E-stop, etc), and one or more contact blocks to form a complete switch. The contact blocks snap together so you can stack them up to make whatever configuration you want. There are LED or bulb holders you can snap between the contact blocks and the switch to illuminate them. The illuminated switches are available with a variety of colored lenses, etc.
This image shows the general idea; from left to right, you can see the switch itself (flush mount, in this case), a bulb holder for illumination (the dark gray part), and then three contact blocks stacked behind (yellow).
https://us.rs-online.com/product/eao/704-032-618/70029703/
There are other types of switches which work in a similar manner, those just happen to be my favorite.
TonyT
I recommend first that you download some manuals (such as safety controller manuals) and skim them – they often have very good advice in them.
Unless you are using a safety controller that does external device monitoring (EDM), using safety relays or contactors does not make a difference. Safety relays are typically limited to 6A. If you want to use EDM with contactors, you don’t need a special “safety contactor”, instead you need mechanically linked contacts or better (there are two applicable European standards which I can’t remember right now), which most quality contactors (e.g. Siemens, Lovato, Schneider) meet.
You can’t just throw a light curtain on a system, and expect any added safety – for example, the light curtain must be far enough away from the dangerous operation that the safety system can stop the machine in time.
In general, I’m not an Omron fan. I wouldn’t recommend Phoenix, either, because you’re just paying more for relabeled gear (e.g. the Phoenxix safety PLC is clearly a green Leuze). In the past couple decades, I’ve primarily used Banner and Leuze for safety light curtains, safety controllers, and safety PLCs, and Lovato and Siemens for contactors. I’m sure there are other good brands.
We typically double up (e.g. use two E-STOP contacts in series, use two contactors in series) for redundancy.
I’ll try to add some more when I get time.
Stuart
I intend for my build to progress in phases, with things like interlocked doors, air blowers, and such, but don’t plan to mess with light curtains or similar.
My controller does monitor for E-stop status.
The relay seems unnecessary for this build, but could provide a more elegant means of separating and checking E-stop circuits separate from power off controls.
Brand availability is a tough consideration. I have a single Lovato E-stop (ordered with the controller), but no clue where else to buy more of the same.
A lot of distributors (e.g. Mouser, Digikey) make it easy to get exactly what you need. I sometimes look at datasheets, pick a part that looks perfect, and find that it has to be special-ordered. I’ve been liking Automation Direct because they give me a running start on figuring out what I need. They’re kind of like McMaster, where there’s little overlap, whereas Mouser might have many versions of the same Siemens product just with different terminal types.
TonyT
AD is kind of like Costco: they only sell a couple, high volume flavors, which is fine if those meet your needs.
At work, I prefer to work with local distributors; typically, pricing AND service are both better than Mouser/DK, but they’re not easy to work with as an individual (I’ve done it a couple times). There’s also been a lot of consolidation, with the result of huge firms with hundreds of product lines.
On the good side, there’s more industrial automation available online. As you’ve noticed, Digikey, Mouser, and Newark have added a lot in the past few years. Allied Electronics, at least in the past, has had a better selection. There’s also dedicated sites like Automation24, Wolf Automation, Motion USA, MotionPlus, C3 Controls, Industrial Control Direct, IMO Automation, and Standard Electric Supply), but I haven’t used any of them.
MM
Allied, now RS, has fantastic customer support, I can highly recommend them.
Stuart
Allied Electronics (now RS-only) really soured me on their customer service practices a few years ago. I’m not likely to shop there anytime soon.
I’ve heard of some of those other distributors (such as C3 Controls), and their line cards tend to vary wildly.
If I can’t find what I need at Mouser, Digikey, Newark, or Automation Direct, I usually reevaluate what I need.
MM
That’s surprising to hear, I’ve always had awesome service from them.
Stuart
There were a couple of very different issues in a relatively short span, giving me enough headaches that I actively avoided ordering from there since then.
Badger12345
Unless you are going to build more than one or are just eager to learn more as part of a hobby, then purchasing a programmable premade unit is likely easier and safer IMO.
Stuart
Agreed. But for me, there are cross-project benefits.
In theory, something like this might be enough: https://www.amazon.com/s?k=paddle+router+stop+switch&tag=toolguyd-20
But I have at least one project where I need more customized solutions.
James
I second the local supplier avenue if you’re not too rural, particularly if you’re buying through a business. As you’re building a panel, some will give you OEM pricing which will knock about 90% off your $235 relay.
Additionally, although they have a ton of product lines to sort through, find one with a counter guy or inside sales person who is knowledgeable. My supplier up here has saved me on a few occasions, because like you, although I wire in relays and components all the time, I am by no means an electronics technician,
MM
One of the reasons why I had recommended Allied earlier was that their reps have given me far better service than the local shops. I can’t say I’ve deal much with them in the past year or so, but I’ve been dealing with the same rep at Allied / RS for more than 10 years and the dude is a badass when it came to tracking down items I needed but none of the big places like Mouser, Digi-Key, Newark, etc, had. Maybe I got handed off to an unusually good rep because at a time I was buying probably 50k a year of parts from Allied but they have been fantastic.
James
I wasn’t knocking Allies for sure and I’m not sure what the landscape is up here in Canada vs USA. And to be fair I’ve never bought stuff like this online.
Typically I will buy over email but if I’m stuck between a couple of possible routes I’ll go into the shop and talk to someone who can usually be very insightful. The guys I use up here are called Futech and I’ve been very happy.
MM
I think the real take-home here is that a good sales rep is a fantastic thing, wherever you might find them!
James
💯. People with knowledge who are eager to share make our days.
TonyT
Yes! Typically, the sales staff turnover at local automation distributors is pretty low: there are a number of sales staff I’ve dealt with for > 20 years.
cball
I am not an electrician nor do I play one on tv but I do own a lot of older (WW2 era and on) single and three phase bits of equipment. All the fancy name brand stuff fails eventually too (although it might take 30+ years), the design philosophy for most of this stuff is made to “fail safe” (E-stop should break contact rather than make because it is hard to make a switch not break but a bit of dirt can make it not make) So appropriately sized, I wouldn’t sweat too much about having a contactor weld on or what have you.
One thing that I might recommend is if it is something for personal use and lowish cycle rates, look at the Chinese clones of these parts, you can get all the buttons, contactors, overloads and disconnects for a fraction of the Telemechanique or Allen-Bradley version, and realistically, light and medium industry is probably using this version of these parts in about 3/4s of the world. If for no other reason than you can mock up what you want to do with fully functioning parts at a very low entry cost to play with.
Here are a couple random examples of things I bought recently that would have been at least 4 times the price had from my local wholesaler:
https://www.amazon.ca/gp/product/B07QQZ4M1D/
https://www.amazon.ca/gp/product/B00RVAJZVC/
Stuart
In reading about how E-stops could fail, contact blocks could short or fall off the button block, in which case pressing it might not break the circuit.
As for cheap switches, there are going to be a total of 3 for this machine – green, red, and blue. Going with a name brand, even a value-priced one, won’t add considerably to the price of the build.
Automation Direct pushbuttons start at $5, and the Fuji Electric they carry are $10. I like the feel of the Fuji a lot better, but the AD ones are decent for the money.
It’s also worth a little more to be able to get replacement or add-on contact blocks and similar.
TonyT
BTW, IDEC has E-Stops that they claim have fewer failure modes. I’ve mostly used IDEC estops in the past so I can recommend them but they’re probably more than you want to spend (around $50 IIRC).
Stuart
I’ve been looking at Idec too. For whatever reason, the E-stop that says “EMO” on it is $12 less.
TonyT
EMO is a SEMI standard, meaning ElectroMotive Off. When you hit the EMO button, all power except for safety systems and (I love this one) computers has to be turned off. Yes, that includes monitors and non-safety PLCs.
IDEC EMOs are very nice, and you don’t have to follow SEMI standards. For Estops we used lighted with 4 NC contacts.
I also love the IDEC LW7L flush flat illuminated switches, but they’re over $30 now I think.
Stuart
Thanks! That makes sense.
Still, it struck me odd that the same exact switch just with lettering would be less expensive at distributors.
cball
Fair enough. The switches I bought in the example above were drop in replacements for an NLA part from one of the big names and, at least in my case, the action was identical, with off brand stuff there is definitely a potential gamble but so far I have been lucky or at least not buying the stuff that doesn’t have a track record of positive reviews.
I’ll just throw out there that ultimately the best redundancy is physical redundancy that isn’t sharing an enclosure with the potential failure point, all bets are off if your control box is full of fire or fire ants for that matter. You are probably not hard wiring this thing in place so the plug counts as a mechanical disconnect (assuming you have set things up so it is within easy reach) and if for some reason it needs to be hard wired, a physical disconnect might be the right choice.
This might be a good time to also make sure the main breaker panel and any sub-panels are unobstructed and the breakers are clearly labeled. For example if my cabinet saw entered some sort of runaway state, there are two e-stops, then a disconnect, then a plug, then a three phase breaker, then the stop on the rotary phase converter, then the single phase breaker for the phase converter then the main breaker for the building. This is of course the most over the top example but does show that even if the fire or fire ants have forced me well away from the device, I still have options to turn it off and so should you.
TonyT
If you really want to have fun, go look up SCCR (short circuit current ratings), where the concern isn’t your machine going up in smoke, but what’s providing power to your machine shorting and then making your machine go up in smoke. IIRC, we were typically asked for 5,000 or 10,000 Amp SCCRs.
cball
Commercial electrical is really a wormhole. My copy of the CEC is a couple volumes and the commercial/Industrial part is around 1100 pages. Open to a random page and then ask yourself, are pipe organs so electrically unique that they need special handling in the code, should I use a type 2 enclosure if there is a chance the machine operator is going to spill his coffee, oops sawdust, perhaps type 5. Why does every explanation of calculating out conduit bends restrict itself to 2d space when all the hard bends are 3d, that high school trig was a long time ago.
Its almost like one can build an entire career and have a couple years of specialized education and perhaps even an apprenticeship to get it straight, even then you probably wouldn’t let the residential guy near a sub-station or the commercial guy near an auger bit (Ok perhaps not so much the second one..)
Or if you will, If you are going to go whole hog and be worried about getting the details right, buy your own copy of the CEC or NEC or whatever your nation uses, and do what is being done, ask questions and get somebody to check your work because if you are just trying to build a thing it is entirely possible to not even know there are questions that need answering. Or keep it simple enough that you can just unplug it… Up until ten minutes ago, I didn’t even know that as long as the draw was under 1500W and plugs in, you do not need any power control on the device at all (14-602).
And that is just the rules to get up to some device, UL/CSA testing is a whole different world of rules for what is going on inside, do you need to physically route out part of the PCB to keep the mains voltage section from the low voltage section, maybe. You bet there is a guideline for it. Of course for personal use, put a wall wart in a box, connected to a couple arduinos held in place by the wires, controlling some H bridge stepper controllers and go to town, or step it up a notch and use a bunch of din rail jellybean parts to get to the same place. Only the user can figure out what they want the neat/cheap/reliable triangle to look like.
Like anything, the trickiest bit is knowing enough to know you don’t necessarily know enough, if you know what I mean. Or at least that is my perspective from a guy who is definitely not an electrician, or a roofer or a mechanic or a welder or a machinist or…
Stuart
@cball,
Code is definitely weird at times.
SawStop’s PCS and ICS both have 220V 3HP motors, but the electronics are different.
The PCS has a “portable cord,” with prewired plug, and the ICS requires permanent building wiring and has a motor shutoff switch.
I spoke with the company, and they said that they have to say building hardwiring is required to comply with code.
They suggested that building a properly rated cord for the ICS is fine, but because of the usage environments it’s designed for, they had to build it to different codes and installation requirements.
The way I approach my own projects, I like to learn proper methodologies, not just for that specific project, but for all future ones.
It also helps me understand other equipment.
I have to swap a power switch in my new benchtop lathe, and learning how to build proper controls for my CNC panel project helped me understand how the lathe works.
I heavily overthink things, but with purpose.
There’s also a LOT of bad advice out there, especially when it comes to hobby electronics, and so I’d like to ensure I’m not just copying bad and potentially dangerous practices.
Stuart
Thanks – that’s good advice.
TonyT
There are plenty of quality brands that are a lot cheaper than AB or Schneider (Telemechanique). For example, C3 controls, Fuji Electric, and LSIS (LG automation).
For example, in power supplies, I might use Meanwell, but even for personal use I’m not using some random brand off Amazon or AliExpress.
Nathan
So you said your controller has a recommended diagram and setup. I would use that to start with if for nothing more than liability
Now the odd questions. Do you want to just cut power or do you have motor brakes to engage for a more sudden stop? If brakes does the controller also control them.
Last machine I played with was 25 years ago and the controller would send a brake signal and the pull all power after throwing a single stop button.
Without the brake signal the spindle motors would take minutes to stop if at full speed this was a high speed CNC mill.
Stuart
The sample diagrams are basic and focus on individual subsystems, such as how to connect one or multiple E-Stops, how to wire proximity sensors for home switches and/or limit switches all inline – things like that.
I bought a brake for the z-axis drive. The others should stop when power is removed, except for the weight of the z-axis potentially sliding down the ballscrew.
The brake receives 24V power to disengage, and will be controlled via controller relay output. I’ll need to test things on the bench first, but I would think the brake output gets cut when the controller receives an E-Stop signal. No power means the brake coil is released and spring brake activates.
The others should stop near-immediately.
The spindle is on another circuit, but a later step will be to rewire it for automatic control. In that case, the speed should drop to zero or get cut entirely. Safety interlocks should do the same. All that will be a later phase.
Nathan
So does your controller need to stay powered while the e:stop activated for safety? Sounds like it. Does it happen to have it’s own safety system?
Otherwise of the options I like your first idea as a starting point
Stuart
It should – seems like good practice.
110V feeds 2x 24V power supplies and a 75V power supply for motor power. One 24V PS will feed logic, and the other will power various coils and relays. The contractor for engaging the 75V power supply will be on the AC side, with its coil powered by 24V.
So essentially, the on/off and E-stop will be for motor control.
The controller, which should require less than 1A at 24V (it ships with an external dual voltage power supply with 1A on the 24V rail), will be controlled with an on/off selector switch. It’s not really essential, but figured a switch for all of the logic could help with power cycling/reset if needed.
David
Next, jump on relay logic. Then, ladder logic.
Peter Fox
If you ever want an extensive but still ineligible resource for learning about anything even remotely related to industrial or process control I highly recomend checking out the open source free textbook “Lessons in industrial instrumentation”
https://www.ibiblio.org/kuphaldt/socratic/sinst/book/liii.pdf
Check out chapters 9, 10, 11, and 12 for content related to basic control logic
The author also has some really good material to teach electronics
http://ibiblio.org/kuphaldt/socratic/model/index.html
dstblj 52
Honestly you can be a lot better of simplifying a lot of this by using something like a banner sc2 which handles a lot of this for you and is relatively affordable by automation standards.
TonyT
I think a programmable safety controller like the Banner SC26 or XS 26 is overkill (Sick/Mitsubishi and Leuze have comparable units).
Unless you really need the flexibility I’d recommend something simpler, probably ES/GM monitoring safety relays, or equivalent from Automation Direct, etc. (I’ve used MMD series, but they are too pricey now, SC10 is cheaper).
JoeM
I feel you on this one, Stuart. I understand the diagrams, I can read requirements, I can even do final assembly on most things… But the actual research into the almighty “Why?” Seems to lead me into rabbit holes and blackouts. The actual answers as to Why seem to only come when anything I’m putting together is actually done, and I can physically see the Why in action. Otherwise, I really don’t get it, and it hurts my head. Everything is logical, until you ask why. Then things get convoluted.
Stuart
The “why” is usually logical too, it just tends to be buried unless you’re already in-the-know.