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SB-9; Adding Contactor; 115VAC vs. 24V Control Circuit

EPAIII

Diamond
Joined
Nov 23, 2003
Location
Beaumont, TX, USA
I have been using my SB-9 with just a drum switch for some years now. It is a single phase, 115V, 1/3 HP motor and the drum switch allows me to run it forward and reverse as well as having the center Off position. So it is working.

But for some time I have wanted to add an Emergency Stop switch. And after moving it into my present garage shop, I have noticed that the drum switch is in a position where it can be easily activated by someone just walking past it. So I really do need a separate power cutoff at this point before an accident happens. I have been digging into the world of contactors. At first I was going to go for a solid state relay/contactor. They are relatively inexpensive on Amazon. But they also go fast: I had a name brand one selected and was filling up the rest of the order and when I checked back; they were out of stock. And I could not find another name brand one to replace it.

I decided to switch to a mechanical design as they appear to be more numerous and I can easily get a two pole one to switch both sides of the power line. OK.

They are generally available with either 115V or 24V coils and here the question begins. Since I am going to be running a circuit around the lathe table for the E-Stop controls, I am thinking about using a 24V version for safety. I have seen and worked with a lot of low Voltage control circuits on professional electronic equipment; 24V, 12V, even 5V and they worked quite well. Never any problems due to the low control Voltage. But a lot of machinery does use the higher, 115V for the control system. So, my question is, do machine tools have any special considerations that make a lower Voltage control circuit undesirable? Any unique problems that can occur? Or am I safe going with the 24V controls?
 
Should be fine with a 24v setup. It allows lighter duty wire and it has no shock hazard. Just need a small 24v control transformer, but those are cheap and available anywhere that sells HVAC parts.
 
Wouldn't it be easier to just pull the plug when not in use? How about a disconnect switch that could be locked? IMHO.

Small 'rotary' lockable disconnects are dirt-cheap, and should 'be there' anyway, but that doesn't address the E-stop need.

Crydom and clone SSR's are not just on ebay. Mouser and others have tons of them. Integral opto-isolated control voltage is typically anything from 3 to 35 VDC, so not hard to supply.

CAVEAT - they publish a spec that sez they have leakage. Not much of it, never been a problem forty years of use and counting, but that's part of why you ALSO need a plug or disconnect.

Personally, I'd use a array of them to even replace the drum switch itself, do the whole job in one go, E-stop included, and enjoy 'zero crossing' switching for an electrically friendlier environment and no wearing parts, arcs, or sparks.

As I have done, early 1970's to present-day.
 
Well, pulling the plug what I am doing now. And I first thought about just adding a switch, but that does not add an Emergency Stop function, which I also want. I think I said that in my original post.



Wouldn't it be easier to just pull the plug when not in use? How about a disconnect switch that could be locked? IMHO.
 
Curse you Red Barron! Now you have me thinking about that. But I will have to trace the present circuit to see just how many of the SSRs I would need, probably about four.

OTOH, I like the look and feel of the drum switch. It just seems to "fit" with my 1400s South Bend. As for wear, it hasn't quit yet and I have been using it for about 13 years. Drum switches are well built and have VERY heavy contacts. At least the older ones do.



Small 'rotary' lockable disconnects are dirt-cheap, and should 'be there' anyway, but that doesn't address the E-stop need.

Crydom and clone SSR's are not just on ebay. Mouser and others have tons of them. Integral opto-isolated control voltage is typically anything from 3 to 35 VDC, so not hard to supply.

CAVEAT - they publish a spec that sez they have leakage. Not much of it, never been a problem forty years of use and counting, but that's part of why you ALSO need a plug or disconnect.

Personally, I'd use a array of them to even replace the drum switch itself, do the whole job in one go, E-stop included, and enjoy 'zero crossing' switching for an electrically friendlier environment and no wearing parts, arcs, or sparks.

As I have done, early 1970's to present-day.
 
I'd personally go with a 120 VAC contactor or starter so you can utilize your existing control power source without having to add a control power transformer or DC power supply. Regarding safety, if installed properly, the 120V circuit shouldn't be a problem.
 
What's the difference between an e-stop and just putting the drum switch in the off position?

It's not like the e-stop switch will have any electrical/mechanical braking action to it.

If you're going to go to the trouble of doing what you propose, why not move to a VFD setup with an electric brake? Then you could have a true e-stop.
 
Well, I guess I have decided as I ordered an old fashioned, mechanical contactor and a transformer for 24 Volt control wiring. They arrived today and now that I have the exact dimensions I am going to start looking for an enclosure (box) tonight. I bought a three pole contactor with two thoughts in mind: first I can switch both sides of the AC line with two of them and use the third one for the 24 V holding contacts. I believe this was less expensive than purchasing an auxiliary set of contacts for it. Second, somewhere way down my list of projects is doing some experiments with a three phase motor and I will probably get one that is about 1/2 HP for that. When those experiments are done, I may just put that motor on the SB. So this contactor will be able to handle the three phase if I so wish or the single phase input to the phase converter that I choose.

I am also going to be searching for a couple of PB switches for Start and Stop. My choice of 24 V control wiring will make that process easier and less expensive as low Voltage switches are less expensive than those made for higher Voltages.

As for the E-Stop without braking action, I still think it is desirable. As I explained in my first post, one of my objectives here is to provide a safer way to turn the power off as the drum switch is too susceptible to accidental activation. I also think that if I am caught up in the rotating work ANY E-Stop action, even without braking, would be of immense value. I could be stuck in a position where I could not reach the drum switch. As for braking, as my hand and arm starts to wrap around the rotating work or chuck, it should provide an effective braking action once power is cut. The trick is that the power has to be cut. Now I am sure that the present, 1/3 HP motor would stall in that process, but it would be nice to limit the damage as much as possible. In short, if there is an accident, I want to be able to turn the power off EASILY. If I install a bar across the front of the table to activate one or two of those E-Stop switches, that would almost be automatic as my body is dragged into it.

A true VFD? Perhaps later. Right now I just want to get the lathe back in operation. Besides, if my life and limb is at stake, I do not trust electronic brake controls. Too many parts that can go poof with too little smoke for evidence. I like consider a true EMERGENCY STOP to be one that is as simple as possible. With a 24 Volt control loop that passes through one or more normally closed PB switches HOLDING the power ON, if anything fails, then the power goes OFF almost certainly. Consider the possible failure modes:

24 Volt transformer fails: no 24 Volts to the contactor: POWER OFF

Wire is cut: no 24 Volts to the contactor: POWER OFF

A "Stop" switch goes bad: no 24 Volts to the contactor: POWER OFF

The contactor coil goes open: no 24 Volts to the contactor: POWER OFF

The contactor coil shorts: fewer windings to hold it on: Power likely goes never comes On

Almost any failure mode that I can think of will result in no power to the motor. And that is the purpose of an E-Stop circuit, to CUT power to the motor or machine. Not to apply brakes.

I have yet to see any published E-Stop circuitry that lies inside a VFD. That circuitry probably consists of some logic ICs and, YES, they can fail. There is no way of knowing what will happen if such a failure occurs. Will the motor stop? Will it run with no way of stopping it short of pulling the power plug? Or what? Only the engineer who designed the circuitry of the VFD could know and in many cases, I suspect even that person does not actually know as the failure analysis was not even done.

Often the external switches for the VFD's stop circuit are NO (normally open) style. So, if a switch fails or a wire is cut or a plug falls out, it will NOT activate the stop function, no matter how many times you push a Stop button or how many of those Stop buttons you can reach and press.

I am sorry, when you use the term "E-Stop" I read that as "EMERGENCY Stop" and as such it should be as reliable as it is possible to design and build it. Electronic controls, in my mind, are just not there: they have too many points of failure. They are inherently NOT the most reliable. And yes, I know that they are widely used. But I prefer a simple, electric loop.

After doing this, I will probably construct a similar circuit for my mill, but it will need to be designed for 230 VAC. Thinking about it, my intended circuit here with the three pole contactor, will probably work there too.

I will take pictures and post them as the project progresses.



What's the difference between an e-stop and just putting the drum switch in the off position?

It's not like the e-stop switch will have any electrical/mechanical braking action to it.

If you're going to go to the trouble of doing what you propose, why not move to a VFD setup with an electric brake? Then you could have a true e-stop.
 
And E stop switch on a VFD Is no less reliable than a E stop switch using your method. Most VFD's have a self-diagnostic system built-in. It can diagnose most any internal problem that could exist. When there is a problem it will shut the machine down automatically. 90% of your problems will come outside of the VFD itself. Bad connections, broken wires, or a faulty switch. And guess what all these things can occur in your system as well. A properly installed VFD will out perform anything that you can come up with. Note: I said properly installed. If you have to ask which was the best way to wire up your machine, you probably do not have the knowledge to properly install it, regardless of which method you use. When it comes to ectrical wiring and electronics, do your homework. make sure you follow the codes for your area.
Make sure you understand what wire you should use, what type connectors you need, and how to use them properly. Make sure you use proper grounds. I would be worried more about the safety of your electrical work than the reliability of the VFD. don't get me wrong, I'm not trying to say your work would be faulty in any way. The VFD is very reliable these days. If a VFD goes bad it would be more likely because of a Lightning strike or improperly installed. More than likely, the failure rate of A VFD will be less than .1%. And that would normally show up in the first week of installation. I know you want a safe system. We all do. But the safest system you have is the one on top of your shoulders. Following all the safety rules associated with your machine is much better than any E stop switch. Be aware of your environment, where your you are in relationship to your machine, especially your hands, Hair and clothing. avoid distractions from others. Concentrate on what you're doing. Don't continue working when you start getting tired. Set up rules in your shop, Make sure every one knows the rules and follows them. Having a safe shop and working safely is better than having a E stop switch. And note, a E stop switch is so someone else can turn off the machine quickly. And in an emergency, is perfectly acceptable to kill the power to a VFD as well. Although this will not do the VFD any good. if it comes down to you or the VFD, you know what I would choose. The answer is you. the VFD can be replaced.

stay safe and have fun.

Joe.
 
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And E stop switch on a VFD Is no less reliable than a E stop switch using your method. Most VFD's have us self-diagnostic system built-in. It can diagnose most any internal problem that could exist. When there is a problem it will shut the machine down automatically. 90% of your problems will come outside of the VFD itself. Bad connections, broken wires, or a faulty switch. And guess what all these things can occur in your system as well. A properly installed VFD will out perform anything that you can come up with. Note: I said properly installed. If you have to ask which was the best way to wire up your machine, you probably do not have the knowledge to properly install it, regardless of which method you use. When it comes to ectrical wiring and electronics, do your homework. make sure you follow the codes for your area.
Make sure you understand what wire you should use, what type connectors you need, and how to use them properly. Make sure you use proper grounds. I would be worried more about the safety of your electrical work than the reliability of the VFD. don't get me wrong, I'm not trying to say your work would be faulty in any way. The VFD is very reliable these days. If a VFD goes bad it would be more likely because of a Lightning strike or improperly installed. More than likely, the failure rate of A VFD will be less than .1%. And that would normally show up in the first week of installation. I know you want a safe system. We all do. But the safest system you have is the one on top of your shoulders. Following all the safety rules associated with your machine is much better than any E stop switch. Be aware of your environment, where your you are in relationship to your machine, especially your hands, Hair and clothing. avoid distractions from others. Concentrate on what you're doing. Don't continue working when you start getting tired. Set up rules in your shop, Make sure every one knows the rules and follows them. Having a safe shop and working safely is better than having a E stop switch. And note, a E stop switch is so someone else can turn off the machine quickly. And in an emergency, is perfectly acceptable to kill the power to a VFD as well. Although this will not do the VFD any good. if it comes down to you or the VFD, you know what I would choose. The answer is you. the VFD can be replaced.

stay safe and have fun.

Joe.

The VFD also has internal torque limitation capabilities. So if something was to accidentally go wrong and provide more load than you desire, the VFD will shut down the machine. Maybe the operator can catch this, maybe they can't.
 
Wow! What a declaration of faith in VFDs and their designers. Did you, do you design them?

I have 45 plus years of experience with the repair of electronic equipment and much of it used control circuits. I have had to diagnose and repair these circuits, usually at the component level. And I have seen problems that would really shake your BLIND faith in circuit designers. I have designed, built, used, and been responsible for the maintenance of systems that were far more complicated than this simple power control. All of this in an industry where seconds of down time cost real dollars. I have found and corrected more design errors than I can remember. Design engineers are usually smart, but not infallible.

I think I know a little about reliability. So pardon me if I think I can design a circuit that is more reliable than the average VFD. Perhaps if they published the schematics of those devices and I could examine them, I may have a little more faith in their reliability under emergency situations.



And E stop switch on a VFD Is no less reliable than a E stop switch using your method. Most VFD's have a self-diagnostic system built-in. It can diagnose most any internal problem that could exist. When there is a problem it will shut the machine down automatically. 90% of your problems will come outside of the VFD itself. Bad connections, broken wires, or a faulty switch. And guess what all these things can occur in your system as well. A properly installed VFD will out perform anything that you can come up with. Note: I said properly installed. If you have to ask which was the best way to wire up your machine, you probably do not have the knowledge to properly install it, regardless of which method you use. When it comes to ectrical wiring and electronics, do your homework. make sure you follow the codes for your area.
Make sure you understand what wire you should use, what type connectors you need, and how to use them properly. Make sure you use proper grounds. I would be worried more about the safety of your electrical work than the reliability of the VFD. don't get me wrong, I'm not trying to say your work would be faulty in any way. The VFD is very reliable these days. If a VFD goes bad it would be more likely because of a Lightning strike or improperly installed. More than likely, the failure rate of A VFD will be less than .1%. And that would normally show up in the first week of installation. I know you want a safe system. We all do. But the safest system you have is the one on top of your shoulders. Following all the safety rules associated with your machine is much better than any E stop switch. Be aware of your environment, where your you are in relationship to your machine, especially your hands, Hair and clothing. avoid distractions from others. Concentrate on what you're doing. Don't continue working when you start getting tired. Set up rules in your shop, Make sure every one knows the rules and follows them. Having a safe shop and working safely is better than having a E stop switch. And note, a E stop switch is so someone else can turn off the machine quickly. And in an emergency, is perfectly acceptable to kill the power to a VFD as well. Although this will not do the VFD any good. if it comes down to you or the VFD, you know what I would choose. The answer is you. the VFD can be replaced.

stay safe and have fun.

Joe.
 
with all this experience behind you, you just had to ask the original question. And you didn't know the answer already. Seems kind of strange.

I too have seen my share of bad electronic components and badly designed circuits. And yes I've told the engineers how to fix there problems, In one case, My Suggestion saved the company from losing approximately one month in sales. I was referring to the failure rate being close to equal in both applications. Do I trust all electronic engineers, No. I have seen some really stupid mistakes made by some really smart people. when it comes to a VFD, from a good manufacturer, There is a team of designers working on it, and may be 100 people critiquing and testing it. Then it must go through certification. Yes there will be failures. But this really is in every product. I'm just saying The failure rate is probably close to the same.

If you really want to see badly designed products, take a look at some of the hobby relay boards made for the Arduino and other platforms. One board had a trace rated for 250 V, 0.05mm Away from a signal trace. another high current high-voltage relay board was designed with the signal input on one side of the board and the High-voltage output on the other end. the problem was the relay Was turned 180° around, so the relay coil side was at the output side of the board and the relay contact was next to the signal side. and this was the third or fourth revision to the board.

Do I trust all service technicians No. I have seen my share of bad service technicians as well. Even highly trained and experienced service technicians make stupid mistakes. Everything from not properly securing a contactor, To not paying attention on how they crimp an electrical Terminal. I even caught one so-called technician just Twisting the wires together, with no insulation at all. Just tied them in a knot. Yes it was low-voltage, But that is not the proper way of connecting to wires.

no I do not blindly trust my VFD. or do I blindly trust a basic on-off switch. Even the simplest of circuits have problems. But the failure rates should be comparable. and I periodically inspect and test all my equipment.

And as far as the emergency stop switch, the best one is one that is never needed. Pay attention to what you are doing and how you are doing it. think about how you can accomplish the task safely, First. And don't let yourself get into that type of situation.

stay safe and have fun.

Joe.
 
Joe,

You talk about my asking the original question. That question was, "So, my question is, do machine tools have any special considerations that make a lower Voltage control circuit undesirable?"

My experience was not with the controls of shop machinery. It was with professional audio, video, and TV equipment. My purpose in asking the question was to find out if there was anything different in this particular environment that would dictate the use of higher control Voltages. I was wondering just why the designers of some of these circuits used the power line Voltage for these control circuits. I really do not see any good reason to run 115 VAC around the switches and various controls of the machines. In my mind, it is just asking for problems when a switch fails or is broken. I have seen switches that were so worn out that you could put your finger a half inch into them. Apparently there is no real reason for using line Voltage or at least no one here knows of one. 24 Volts is a lot safer and I am using that in my design.

In my mind, at least, asking that original question made a lot of sense. Yes, I could have just relied on my experience and judgment and it probably would have been OK. But I wanted to know. I wanted to enlarge my knowledge if possible.

As for those "well designed" VFDs, I have to wonder just how many of their E-Stop circuits would work if just one of the wires to a remote E-Stop button on them was accidentally cut. Any such device that uses N.O. (normally open) style switches for this function would not work if the wires to them were cut. And, just how many of them would have any kind of alarm or indication of the fact that those E-Stop buttons will not work. I am trying to include a circuit design which will provide an indication of at least some failure modes and do it at a very reasonable cost. I am still working on this detail.

Perhaps I am being a bit anal about this, but I have the time and inclination to do it this way. So I am. I am not going to put a VFD on the lathe at this time. I have other, preliminary plans for upgrading the motor as well as the control system at a later time. For now I just want a safer power control.
 
I have an old VFD that I plan to use with mine. It has no internal stop circuits, the manual shows it being fed from a contactor for emergency shutdown. I wired it that way, using the NC contacts on an e-stop button. If the wire breaks or I hit the button, the contactor drops and the vfd loses mains power input. At that point it has to stop the motor.

I ended up going with a 120v coil, but it was simply because I didn't feel like adding a 24vac transformer to the mix. No real reason otherwise. None of my e-stop wiring leaves the control cabinet though. If I were doing multiple stops I might go with 24 vac just because it can be run in lighter wiring that doesn't need to be in a conduit.
 
Joe,

You talk about my asking the original question. That question was, "So, my question is, do machine tools have any special considerations that make a lower Voltage control circuit undesirable?"

My experience was not with the controls of shop machinery. It was with professional audio, video, and TV equipment. My purpose in asking the question was to find out if there was anything different in this particular environment that would dictate the use of higher control Voltages. I was wondering just why the designers of some of these circuits used the power line Voltage for these control circuits. I really do not see any good reason to run 115 VAC around the switches and various controls of the machines. In my mind, it is just asking for problems when a switch fails or is broken. I have seen switches that were so worn out that you could put your finger a half inch into them. Apparently there is no real reason for using line Voltage or at least no one here knows of one. 24 Volts is a lot safer and I am using that in my design.

In my mind, at least, asking that original question made a lot of sense. Yes, I could have just relied on my experience and judgment and it probably would have been OK. But I wanted to know. I wanted to enlarge my knowledge if possible.

As for those "well designed" VFDs, I have to wonder just how many of their E-Stop circuits would work if just one of the wires to a remote E-Stop button on them was accidentally cut. Any such device that uses N.O. (normally open) style switches for this function would not work if the wires to them were cut. And, just how many of them would have any kind of alarm or indication of the fact that those E-Stop buttons will not work. I am trying to include a circuit design which will provide an indication of at least some failure modes and do it at a very reasonable cost. I am still working on this detail.

Perhaps I am being a bit anal about this, but I have the time and inclination to do it this way. So I am. I am not going to put a VFD on the lathe at this time. I have other, preliminary plans for upgrading the motor as well as the control system at a later time. For now I just want a safer power control.

There in in fact very good reasons why line voltage was selected for control circuits. RELIABILITY! In an industrial environment with silver faced contacts, it has been shown both by application and by testing that a minimum of 90 volts is required for circuit reliability.

In a personal case, I submitted a motor starter to Ford testing labs for qualification. The lab told us that the contact bounce was way too long. We, the GE lab manager and I went to the lab, examined the starter and asked how they tested it. 24 volts. I told them to use line voltage and it passed. The result was that Ford Motor Company changed their testing standard to require line voltage testing. If you want reliable operation at 24 volts, special contact construction is required. Bifurcation, gold plating, reed switches, that kind of thing. This is also the reason for 4-20 ma current loop.

Tom
 
Tom, thanks. I think this is the first response that actually gives a real world reason for the higher control Voltages. And it does appear that this is something that is unique to the machine tool world as I have seen many control circuits that function just fine for years and years with 24 and even 5 Volts.

Unfortunately, it comes a bit late as I have already purchased 24 Volt components: contactor and transformer. I will try them as my shop environment is probably a bit more relay friendly than most. We shall see. but I do appreciate the answer.

Thanks!



There in in fact very good reasons why line voltage was selected for control circuits. RELIABILITY! In an industrial environment with silver faced contacts, it has been shown both by application and by testing that a minimum of 90 volts is required for circuit reliability.

In a personal case, I submitted a motor starter to Ford testing labs for qualification. The lab told us that the contact bounce was way too long. We, the GE lab manager and I went to the lab, examined the starter and asked how they tested it. 24 volts. I told them to use line voltage and it passed. The result was that Ford Motor Company changed their testing standard to require line voltage testing. If you want reliable operation at 24 volts, special contact construction is required. Bifurcation, gold plating, reed switches, that kind of thing. This is also the reason for 4-20 ma current loop.

Tom
 
Tom, thanks. I think this is the first response that actually gives a real world reason for the higher control Voltages.
Unfortunately, also a seriously flawed reason.

:(

All a switch needs is 'wiping' contact form, no special alloys required. Those are for enhanced longevity, primarily at 'break', not because ignorant copper won't conduct at 'make'.

The physical contactor 'mechanism' needs a certain amount of energy delivered over a certain period of time, plus adequate holding power. It doesn't give a s**t HOW that energy is delivered, could be happy as a clam if it were hydraulic, steam, air, rope, or hand-lever.

His coil didn't have enough Ampere-Turns @ 24 V to deliver what was needed to do the job is all. Use an adequate coil for any given voltage and you are fine.

Automobiles worked fine for ages with SIX volt starter solenoids, relays, and switches throughout, after all.

Before that, some had a FOOT pedal and no solenoid at all on the starter motor. Pedal engaged the gearing, closed the contacts at the same time. No "110 volt" required.
 








 
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