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  #1  
Old Mon 26 February 2007, 11:55
Gerald_D
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Mains power wiring for discussion

Here is a prelim. schematic for router users on 110V/220V split phase. Does it make sense? More importantly, does it look safe?

I know there are no fuses in there. We have circuit breakers on the incoming supply line.

(The wiring around the contactor was later proved to be faulty - see further in this thread jump)



This post will be edited often. The headings are placed here for notes.......

E-Stops on Gantry & Y-Car
WEG 22mm Non-metallic, Single Contact Block, 1 N.C. Price: $2.00 each (needs "operator" and enclosures with other "buttons" & switches)
Spec FactoryMation page



On / Off Pushbuttons
WEG 22mm Non-metallic, Double Pushbutton Operator w/ center illumination lens , IP40, "I / O", Price: $7.00 Requires Contact Blocks (1ea BC10 and BC01 sold seperately) (Lamp block optional)
Spec FactoryMation page



Disconnect
ENSTO Rotary Load Break Disconnect Switch, 3 Pole, 600V, 40A Price: $16.50 (needs handle & accesories)
Spec FactoryMation page



Contactor
TECO IEC Contactor - 16A, 120VAC / 60Hz Coil, 3 Pole 600V, 1 N.O. Aux Price: $12.00
Specs FactoryMation


Router Relay
Switched directly by 5V TTL from PMDX-122 breakout card. Do not connect relay "coil" or input to the mains circuit!
Magnecraft Solid State Relay, DIN Rail Mount with Heatsink, 25A SPST, Zero Cross NO, 280VAC max, 3-32VDC Coil, with LED, Price: $39.00
Spec FactoryMation page

110V Toroid Transformer
For the recommended motors (1.5Amp max), a 300VA transformer with 50V AC output. link. About 125mm (5") outside diameter. Price about $70.
Mouser has a Hammond 182S24 which is 500VA 48V @ $96 - they don't seem to have 300VA at that voltage. Mouser Page This will work with slight space & cost penalties - the heat will be the same, and bigger motors can be handled in future.


Rectifier
100V 25 Amp rectifier (total overkill, but no real size or price penalty) is being used because it has a metal case and pluggable connectors. International Rectifier 26MB20A D-34 package About $3 - $7 Mouser Page


Capacitor/s
Cornell Dubilier DCMC103U100AD2B Computer Grade Electrolytic Capacitors 100 Volts 10000uF 1.375"Dx4.625"H Screw Terminals Price $25.34
Spec Mouser Page


110V Cooling Fan
EBM-Pabst, AC Fan 92mm 115VAC 52.4CFM, Price $46.06
Spec Mouser page


110V/9V Transformer
A more friendly mounting system is needed, but this is the equivalent of what I used:
Transera/Pulse BV030-7305.0, Encapsulated Transformer 2 VA 9V@222mA Price $5.62
Spec Mouser Page
The ncPod will probably eventually replace the PMDX-122 - it requires 5 vdc regulated at about 250 ma. The PMDX-122 can also live off 5V regulated 200mA now. Small DIN rail 5V supplies are expensive! $80 from Mouser
Surface mounted 5V supplies are cheaper, but bigger. $37 Mouser page


110V Router
The parts above assume the router draws no more than 15 Amps constant and 25 Amp on starts
  #2  
Old Tue 27 February 2007, 07:09
Hugo Carradini
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Gerald thanks for your time. This is a big help. No more guessing.
  #3  
Old Tue 27 February 2007, 08:08
Mike Richards
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Gerald,
I should have waited one more day before rewiring my test bench. Yesterday, I spent the better part of the day drawing up schematic after schematic as I tried different possibilities. What I ended up with is very similar to what you've posted.

The only practical difference is that I don't run the A.C. Neutral line through the contacts of any switch. It is hard wired all the way from the service panel to all parts of the control box.

My Alpha came from the factory with the neutral line running through switch contacts just like your design - so I don't know whether the way I wire is dictated by code or by convention.

The way that it was explained to me is that any line that is a voltage source is run through a switch and is fused at the service panel and, if necessary, in the circuits where it is used. That would mean that in North America where we split a single-phase 220/240 line into two 110/120 lines, both L1 and L2 would run through switches. If the circuit were three-phase, all lines L1, L2 and L3 would run though switches. But with a single-phase supply the neutral is not switched so that if a switch contact on L1 or L2 ever welded itself shut there would be no possibility that the neutral could be open so that the person operating the machine could become the path of least resistance in the electrical circuit.
  #4  
Old Tue 27 February 2007, 09:18
Gerald_D
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Not so fast Hugo, this stuff here is only for discussion. It could still be modified a lot. However, your comment "No more guessing" is exactly the reason why I ventured into this dangerous territory. It is better for me, and the rest of us, to learn about the correct way, instead of just guessing....

Mike, the breaking/disconnecting of the neutral line seems to have different approaches on the 2 sides of the Atlantic. Those "disconnects" from FactoryMation are only available as 3-pole over there - that tells me in 3-phase circuits the neutral is never disconnected. Same with motor contactors, practically all 3-pole.

Out here, (basically Euro standards), we break the Neutral at nearly every opportunity. For example, in domestic wiring, the supply to a cooker/stove must (mandatory) have 2-pole breaker which breaks the neutral as well as the live. In industry, a star-wired motor will have a 4-pole contactor. Feeds from main boards to sub-boards are isolated all lives plus neutal.

Our "ground fault" protection may be a bit different to yours.... If I work with a screwdriver inside an isolated sub-board or control panel, I can touch (short) neutral to ground/earth without problems because the neutral is disconnected. Were I to leave the neutral connected, touching it to ground/earth will plunge the whole building into darkness. We have one ground fault device where the feed enters the building. (this device must break neutral as well). If neutral touches ground/earth after this device it trips. You discover this when you put a knife into a switched off toaster to remove a crust.....
Do your ground fault devices only break "hot" (live) and leave the neutral connected as per your note above?

Where can we get some authoritive answers? In May I am taking an 220V electric motor to Atlanta. Guys out there are warned to stock up on candles...
  #5  
Old Tue 27 February 2007, 11:17
Gerald_D
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I find lots of references to "....shall have a single means for disconnecting all ungrounded main power supply conductors...."

Suppose the question now is whether the neutral is grounded or ungrounded. Before our ground fault protection device, the neutral is grounded. But after our ground fault protection device, the neutral is not grounded - I know this because accidental grounding of the neutral causes a fault trip.
  #6  
Old Tue 27 February 2007, 13:16
Mike Richards
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Gerald,
It's a can of worms (at least here in North America). I've just spent several hours on the Internet trying to find a definitive answer that would clear up the question once and for all time, but there doesn't seem to be that kind of answer. So, I went to a sub-panel that was installed by a licensed electrician to measure the resistance between earth ground and neutral. In the subpanel there was no resistance between the two wires. That matches MOST of the 'advice' on the Internet.

A ground fault interrupter (GFI) seems to be a special case. Here in North America they are required in bathrooms (and in kitchens if an outlet is near the sink). They do have an isolation between neutral and earth ground so that 'any' rise in voltage between earth ground and neutral will cause the GFI to activate and cut off or open the Line or Hot wire.

My 'impression', after reading the various opinions is that NOT switching the neutral wire is safer than switching the neutral wire IF and ONLY IF something out of the ordinary happened that caused the Line or Hot line to stay connected when the neutral line opened.

Sometimes I'm a lot more conservative in my opinions about AC wiring than others. In my past life as a process control computer designer, I've been severely shocked more than once when I've had to work on 'standard' wiring in film processors. Because of the alkaline nature of the developer and the acid nature of the bleach and fixer, there is often a lot of rust that eventually forms, even in switches. After a period of time, things start to fail and people like me are called in to sacrafice themselves for the good of society (at least it seems like that).

Personally, I've rewired my Alpha so that it is wired 'correctly'. Unfortunately, even with the door switch turned OFF, there is still lethal voltage inside the control box. The 240AC line that runs the spindle connects directly from the subpanel to a contactor inside the control box. The 120AC line that runs everything else also connects directly from the subpanel to a the door switch and then to a contactor inside the control box. (That's the way it should be.) If I were being prudent, anytime I opened the controller's door, I would first flip the breakers at the subpanel. But, being extremely wise, I put the subpanel where I can't get to it without crawling over the Shopbot. So, when I open the controller to check something, I place a strip of red electrical tape where ever there is 'live' power present. The tape won't protect me, but it acts as a flag to remind me to keep my fingers and screwdrivers away from those areas of the box.

I'm hoping that a licensed electrician will post something that gives some references from the code book that will authoritatively answer the question.
  #7  
Old Tue 27 February 2007, 14:41
James Webster
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Here in the USA the ground and neutral are run to the same bar in the breaker box.

http://hyperphysics.phy-astr.gsu.edu...ic/bregnd.html
  #8  
Old Tue 27 February 2007, 15:37
Hugo Carradini
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Gerald ¿What is the name of the terminal blocks that you show in the tread of correct wiring?
I don't handle correctly the technical names and there are so much different types in Mouser page that is frustrating.
Thanks
  #9  
Old Tue 27 February 2007, 16:09
Mike Richards
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James,
That's a good reference that's similar to many others that I browsed today. After a little searching on the link that you referenced, I found a page under AC circuits and then household wiring that shows the use of switches for L1 and L2 with Neutral being unswitched. (Sorry, but when I tried to copy the exact link to that page, it didn't work - instead it sent me to the home page.) However, even though I'm totally in agreement with it, I can't say for certain that it meets code. (And I have no reason to say that it doesn't meet code.)
  #10  
Old Tue 27 February 2007, 18:13
James Webster
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In the USA, if you are switching a 220v device (like a big hot tub), you disconnect both hot legs, leaving the neutral and ground connected.

They call this switch a "double pole".
  #11  
Old Tue 27 February 2007, 22:44
Gerald_D
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Very interesting discussion!

Here in South Africa, we also talk of having a "double pole" switch/isolator for critical equipment, but then it always means live and neutral. We don't ever have the 2-phase with 2 hots that you guys have.

Every mini-factory (built in the last 30years) that have seen here has a single ground fault protector adjacent to (or serving as) the main incoming breaker. Every single device inside that factory is then ground fault protected. We tend to treat Live and Neutral with equal respect. Look at the Euro "Schuko" plug on cords, they are not "polarity" protected.

Would your code prevent the Neutral from being broken like in my sketch above, or as ShopBot does their panels?
  #12  
Old Tue 27 February 2007, 22:51
Gerald_D
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Hugo, do you mean these? From that company I would probably use the ER4 size (4mm wide). With Mouser, look near page 1263 - check current ratings, also don't go too small if you have big fingers.
  #13  
Old Tue 27 February 2007, 23:33
Gerald_D
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Mike & James, I can see your safety concern as a reason to keep the neutral connected, but what happens if someone "upstream" of you disconnects the neutral.....? Your disconnected portion of the neutral becomes hot if there is a load connected somewhere between you and the monkey that lifted the neutral.
  #14  
Old Wed 28 February 2007, 01:10
James Webster
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As crazy as this is going to sound, devices on 220v circuits tend to be the only device on a breaker. An electric stove, hot tub, whole house air conditioner, kiln, or tanning bed would never share the circuit with any other device.

Mini factories would step up to 3 phase service (usually not available to residential customers), this runs motors very efficiently. All three legs are hot, need overcurrent protection, and should be switched. 440v is popular, although 220v is also seen in 3 phase.
  #15  
Old Wed 28 February 2007, 04:11
Mike Richards
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This has become an interesting topic. Who would ever think that it would be almost impossible to find a definitive answer for such a simple question? Yesterday I spent a few minutes at the local library reading an outdated copy of The National Electrical Code handbook. The library had the 2002 edition. The 2005 edition, which is the latest edition, was not available. (A few minutes was not enough to find the answer. The index in that book is a total joke, as is the table of contents. Oh well, what should we expect from anything that has "National" as part of its title? Hopefully the book wasn't written by Republicans.)

I did find some interesting material on the APC website, particularly White Paper #21. (Edited: The link is not working properly, so you'll have to google APC White Papers, select White Papers from the APC support page, and then select White Paper #21 manually.) A copy also here. Of particular interest to me was this paragraph:

"Electrical power flows in the form of current, which must pass through the equipment and then return to the
power source. Therefore, it is convenient to think of one wire to the load being the "source" wire and one
being the "return" wire. This simple model is appropriate for DC systems but does not work for an AC
system because the flow of the power is continually reversing direction with a frequency of 50 or 60 times
per second. From the point of view of the equipment or the power source, the source and return wires are
constantly being interchanged. In fact, no equipment can tell which wire is which! It is easily demonstrated
that the two power wires to any piece of AC equipment can be interchanged without any affect on function.
In fact, in Europe, unlike North America, the plug on a piece of equipment can be plugged in either way!"

As usual, I found something with which I disagree, which is the part that I marked in Red. It is my understanding that the "polarity" of the Line/Hot leads, i.e. L1, L2, L3 changes as the voltage level of each lead fluctuates along its sine way from zero volts to positive voltage to zero volts to negative voltage and then back to zero volts (where the amplitude of the positive and negative voltages depend on the type of service provided by the power company).

But, going back to the white paper, it seems that the reason that both the Line/Hot lead and the Neutral lead are switched in circuits using "European" type connections is because the "European" connection allows the leads to be reversed by rotating the plug in relation to its receptacle. On the other hand, in North America, with its polarized plug and receptacle, accidental reversing of Line and Neutral is not possible (although it can easily be done by buying a 99-cent "cheater" plug at any hardware store).

I "think" that since the Neutral lead and the Ground/Earth lead are connected in the power panel (again, I didn't find a reference that stated the code reference that required neutral and ground to be connected: I only found diagrams showing that type of connection), that switching or disconnecting the neutral wire via a switch would allow the Line/Hot lead(s) to use the Ground/Earth lead as a return path if the Line/Hot lead(s)'s contact did not open when the neutral contact opened. Since all exposed metal on a machine "should" be connected to Ground/Earth, any person touching the machine "could" be shocked with Line/Hot voltage.

(How's that for a long, convoluted answer? Sometimes I feel like I'm being a lawyer, or even worse, a politician.)
  #16  
Old Wed 28 February 2007, 05:10
Gerald_D
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What if the "MechMate Company" insisted that "their" control box be supplied from "15Amp circuit breakers with Ground Fault protection"?

(The organisers at the Atlanta show say little of their requirements in terms of electrical equipment. Once before I took equipment to a show in Munich Germany where they insisted on ground fault protection on my equipment, and that I must provide a 2-pin socket for them to plug in their tester. They allowed 30mA fault current - I passed at 20)
  #17  
Old Wed 28 February 2007, 07:19
Mike Richards
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Gerald,
Your Ground Fault question sparked an idea (no pun intended). I googled ground fault protection which led to OSHA which led to these OSHA Documents. (NOTE: The title for Part 1926 is "PART 1926 Safety and Health Regulations for Construction"):

1926.405(a)(2)(ii)(D)

Disconnecting switches or plug connectors shall be installed to permit the disconnection of all ungrounded conductors of each temporary circuit.

Then, I revisited the APC support site, re-read White Paper #21, particularly the part where "grounding" and "grounded" are defined:

"Ed" and "Ing"
In North America, much of the confusion regarding grounding has its roots in the National Electrical Code of
the United States. In the Code, the receptacle ground connection is referred to as the "Grounding"
conductor while the neutral connection is referred to as the "Grounded" conductor. The subtle difference
between "ing" and "ed" has led many to erroneously interchange these terms in the literature.

Further on in the OSHA document I found this:

1926.405(j)(4)(ii)(B)

The disconnecting means shall disconnect the motor and the controller from all ungrounded supply conductors and shall be so designed that no pole can be operated independently.

So, if we use the APC document that describes the neutral line as being "grounded", then, the neutral line is not required to be switched. However, if the neutral line is switched, then it must be switched with a multi-pole switch exactly as you drew in your schematic.

Reading further I found this reference about door switches:

1926.408(a)(3)(ii)

Guarding live parts. All energized switching and control parts shall be enclosed in effectively grounded metal cabinets or enclosures. Circuit breakers and protective equipment shall have the operating means projecting through the metal cabinet or enclosure so these units can be reset without locked doors being opened. Enclosures and metal cabinets shall be locked so that only authorized qualified persons have access and shall be marked with a sign warning of the presence of energized parts. Collector ring assemblies on revolving-type machines (shovels, draglines, etc.) shall be guarded.

I've marked in red the sentence which lists a requirement to have the electrical cabinet locked. Does that mean that screws are acceptable or must one use an actual keyed lock? Because this paragraph is related to portable equipment, perhaps the "locked" reference does not apply to "fixed" or "stationary" equipment.

Now, back to your question about GFI protection. I found this reference that seems to apply only to construction sites:

§1926.404 Wiring design and protection.

(b) Branch circuits-(1) Ground-fault protection-(i) General.

The employer shall use either ground-fault circuit interrupters as specified in paragraph (b)(l)(ii) of this section or an assured equipment grounding conductor program as specified in paragraph (b)(l)(iii) of this section to protect employees on construction sites. These requirements are in addition to any other requirements for equipment grounding conductors.

(ii) Ground-fault circuit interrupters. All 120-volt, single-phase, 15- and 20-ampere receptacle outlets on construction sites, which are not a part of the permanent wiring of the building or structure and which are in use by employees, shall have approved ground-fault circuit interrupters for personnel protection.

In my opinion - which counts for nothing - adding or requiring a GFI would give additional protection against electric shock and would be a very good idea.

After reading the OSHA documents (which may not even apply to an electrical enclosure used with a stationary piece of equipment), I'm inferring the following:

1. Because the neutral line is a "grounded" conductor, it is not required to be switched.

2. If the neutral conductor is switched, it must not be switched independently of the Line conductor(s) - i.e., a multi-pole switch must be used.

3. The neutral conductor may be switched unless other documentation is found that prohibits neutral conductor switching.
  #18  
Old Thu 01 March 2007, 06:11
Gerald_D
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Thanks for all the research and guarded opinions Mike. I think the really key point that you make is that; if you are providing switches to disconnect Lives/Hots and or Neutral, these switched must work simultaneously and in unison - ie. a single multi-pole switch.

My inclination towards disconnecting the Neutral as well, is probably because I can never be sure who has done what in the circuit before it reaches the box with my fingers inside. Remember we are a single-phase 220V country - Live & Neutral could be transposed in the box before ours and we would be none the wiser.

I havn't found a solid objection to disconnecting the Neutral together with the Lives/Hots, although I can understand the preference for a permanently continuous neutral in a well-managed and maintained system. I will stick with the above schematic for the time being.

Snag now is....where does one find 4-pole disconnects in the USA for 3-phase systems (for the spindle users)?
  #19  
Old Thu 01 March 2007, 11:08
Mike Richards
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On the AutomationDirect website, the spec. sheet for the SD-1 series of non-fused disconnects, shows an SD1-4P add-on module, which is described as an add-on 4th pole, rated at 40-amps.

There is neither a part number nor a price listing for that module. Perhaps you could email AutomationDirect to see if they carry that item.
  #20  
Old Thu 01 March 2007, 11:22
Gerald_D
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I know that Ensto makes the pole add-on module as well (because I use one here!), but FactoryMation doesn't seem to carry them "on-line".

AutomationDirect list their SD1 auxiliary poles here. There is a neutral pole module as well.
  #21  
Old Thu 01 March 2007, 13:02
Mike Richards
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I didn't look hard enough on the AutomationDirect web site. When I clicked on a 25A disconnect, the listing only showed the SD1-AUX module, which I knew was too small to carry its share of the load. I didn't think to search for the SD1-4P.

(I'm fortunate in only having 1-phase 240VAC power. Although it limits the size of equipment that I can run, everthing can be handled with 'standard' disconnects and contactors. The door disconnect for the electronics box that I'm beginning to design will only control a 1-phase 120VAC line. That 120V line will furnish power for the control circuitry inside the box, including coil voltage for the contactors through momentary normally closed pushbutton switches and manual over-ride switches. The main contactor will have emergency push-button switch(es) in series with its coil so that if an emergency switch is pressed, all contactors will turn off.

The contactor furnishing power to the toroid transformer that supplies the Geckos will have mechanical limit switches in series with its coil - and a manual over-ride switch - so that if a limit switch is tripped, power will be cut to the steppers. The manual over-ride switch will allow power to be restored so that the gantry can be jogged to a safe position. Proximity sensors will be completely separate from the limit switches. They will be used to determine an axis's home position and the limit switch will 'limit' the travel of an axis.

The contactor for the spindle or router will allow power to the spindle or router to be completely disconnected so that the cutters can be changed safely.

The auxiliary contactor(s) will be used to control dust collection, vacuum hold-down and possibly other auxiliary equipment.

Using contactors rather than manual switches will allow either manual switches or computer I/O to control the coils to the various contactors. Each contactor can be ordered with a coil suitable for its function and suitable to the device that controls it - switch or computer. The 30A contactors that are available at the local QED store seem to be excellent quality and only cost $30, including the extra side circuit, so, as long as everything fits inside a reasonably sized enclosure, I would rather have an electronics box that controls everything rather than running around the shop turning things on and off.)
  #22  
Old Thu 01 March 2007, 20:22
Gerald_D
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I am battling to come to grips with the terminology on your continent....when you say "I'm fortunate in only having 1-phase 240VAC power" don't you mean 2-phase or split-phase? It does have 2 Lives (Hots), doesn't it? Talking in the same way that we all talk of 3-phase when there are 3 Lives.

My approach with the circuit above is minimalist - mainly to reduce chance of interference inside the control box housing the tiny wiggly signals from the computer. I believe that the dust collector should have a local contactor and pushbuttons, and only get a control signal from the main box.

The router "safety disconnect" should be right at the router - too tempting to change a bit without walking to the control panel. (simple mechanical switch)
  #23  
Old Thu 01 March 2007, 21:43
Mike Richards
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Gerald, I agree that the term 1-phase makes no sense when referring to our 240VAC power. It is exactly as you describe: two Lines (L1 and L2), a Neutral and Ground/Earth, but that's the terminology that the power company uses. Your schematic at the beginning of this thread shows our '1-phase' power perfectly. L1 is a conductor with black insulation, L2 is a conductor with red insulation (or black insulation that is marked red with tape, shrink tubing or paint), Neutral is a conductor with white insulation and Ground/Earth is either bare (copper) wire or wire with green or green/yellow insulation. Personally, I think the name '1-phase' came from the fact that the '1-phase' 240VAC line is almost always split into two '1-phase' 120VAC lines inside the electrical panel in most homes and businesses. Only the 'big' appliances such as a clothes dryer, cooking stove or air conditioner would require both sides of the '1-phase' power line.

Sometimes I get carried away with my process control designs - not because going complicated is any better than a less complicated design, but simply because having a new challenge helps keep me interested in the project. All of us who have operated CNC routers for awhile know how frustrating electro-mechanical noise is. It literally wreaks havoc with control computers. When we slam contactors open and closed, with their contacts arcing and voltages spiking at many multiples of line voltage, as induction in some of the circuits causes voltage to pile up just as if it were a tidal wave rushing towards shore, the task of controlling everything can become daunting. But that's where the fun comes in. Adding snubber circuits, suppression circuits, and EFI filter circuits adds the spice that makes circuit design tasty.

The more practical reason for stuffing everything inside the box is that the sparking contacts can be placed in their own area, away from the control circuits. The weak I/O signals can be optically isolated from the high voltage circuits. That opto-isolation will also help to keep the small gauge I/O conductors from acting as antennas. (Edited: With opto-isolation, it's easy to use 12V or 24V I/O signals that can handle a lot more 'noise' than the 5V signals that are normally used.)

I'm still studying the router/spindle circuit disconnect. Handling a router disconnect is simple. All that is required is to add a mechanical switch into that part of the Hot side of the AC line that goes between the Solid State Relay and the router (for instance, the On/Off switch on the router itself). Then, even if some unanticipated event turns on the SSR, the mechanical switch will keep the power off.

A spindle is more complicated. The simple approach is to use a three-pole contactor and simply open the lines between the VFD and the spindle, but that could cause some expensive problems. At the very least, shutting the cooling fan off frequently while cutters are being changed could cause the temperature to rise too high. Cutting the power between the VFD and the spindle could also damage either or both devices. But, I prefer having a self-latching mechanical relay or contactor in the circuit that requires a button to be pushed to activate the circuit over a simple mechanical switch. (It's kind of like having a magnetic switch on the table saw. In the even of power failure, we want all equipment to stay off until we turn them back on. Having a table saw's blade suddenly start spinning just because someone at the power station restored power is not acceptable. Using a self-latching relay or contactor assures that power can only be turned on by a deliberate act on the part of the operator.)

But, the design has to be practical as well as safe. Your schematic at the top of this thread would handle almost any 'standard' design. It is robust, safe and elegant. There is nothing about it that needs changing. (I've even come to accept the fact that the Neutral line can be switched if it is switched by a multi-pole switch/relay/contactor.)
  #24  
Old Thu 01 March 2007, 22:19
Gerald_D
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My thinking is that a woodwork spindle, normally under the control of a computer, should have a mechanical disconnect switch for the guy changing the cutter, even if it may damage the VFD. An PE55-like switch should be sitting directly above the cutter (say within 0.5m [2ft] and in view of the operator while the cutter is changed).
The only chance of this switch damaging the VFD is if the switch is turned off while the spindle is under load, which would be very unlikely. There is also a chance that the switch is turned on while the VFD is already on, but then you have probably saved a finger.
With a local mechanical switch, the guarded fan can stay running.
  #25  
Old Sat 03 March 2007, 19:34
reza forushani
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Can somebody explain the function of the contactor and how it works., I just got mine and need some help. thanks
  #26  
Old Sat 03 March 2007, 20:42
Gerald_D
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A contactor is simply a big relay. Does the sketch at the top of this thread help you at all? What relay did you get (photo or name&number)?
  #27  
Old Wed 07 March 2007, 10:31
Mike Richards
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This post was first made in this thread

Here's a little circuit that shows James's circuit schematically as well as the reasoning behind using a bleeder resistor - even with stepper motors.


The schematic posted here previously has been deleted as per Mike's request lower down in this thread (Friday, March 09, 2007 - 02:28 am) This schematic has replaced it.


In the middle of the schematic is the 70VDC power supply. The + side of the supply connects to the DIN rail terminal blocks through the 'A' contacts on the Contactor.

At the bottom of the schematic are the limit switches, 2 per axis. These are true limit switches in that they will remove power to the G202 modules if any one of them is opened. Notice the Limit Override Switch on the bottom right of the schematic. Its use is to temporarily 'override' a limit switch. It is turned on to supply power to the G202 modules, even if a limit switch is open. The purpose for the override circuit is to let the operator jog the axis that hit a limit switch back to the working area. The Limit Override Switch is a 2-pole switch. One pole bypasses the limit switches which allows power to be fed to the coil of the contactor. The other pole turns on a warning light (probably red) to warn the operator that the Limit Override Switch has been turned on. (Edited: When the Limit Switches are part of the Coil circuit, they only have to handle a small current, so they can be physically smaller than if they had to handle all of the power required by the controller.)

Just above the Limit Switches is the latching circuit that will turn on the Contactor. Two power supply lines, labeled Latch + and Latch - feed power to the coil of the Contactor. Depending on the voltage requirements of the Contactor's coil, the voltage might be 12VDC, 24VDC, 120VAC or 220VAC. Personally, I like to use the lowest possible voltage to avoid running high voltage through switches that are in close proximity to the metal frame of the machine - but that is just my personal preference.

In this particular latching circuit, when the Enable Push Button is pressed, the Contactor's coil is turned on. When the Contactor's coil is turned on, all of the contacts close, including the 'S' contacts. The 'S' contacts act as a bypass circuit that will furnish power to the coil when the Enable Push Button is released - which is why we call it a latching circuit. Once the coil is turned on, it will stay on until either a limit switch is opened or until the control circuit that furnishes the latching voltage is turned off. (As a side note, I always add a 'Stop' push button to every circuit that has an 'Enable' push button. The Stop push button, which would be a normally closed momentary switch, would function exactly as a 7th limit switch.)

After all of that background, we finally come to the purpose of having a Bleeder Resistor (R1) in the circuit. The whole purpose of having the limit switch/contactor circuit is to cut power immediately (20mS or less) when a limit switch is opened. No gradual power down is allowed - which could be the case with a large power supply and a single active motor. When the coil of the contactor looses power, all of the contacts are opened and all power to the G202 modules is cut off. With power cut off between the output side of the power supply and the G202 modules, the stepper motors cannot drain the power from the capacitors. (Remember, when power to the Contactor's coil is turned off, the G202 modules will be isolated from the power supply. That's a condition that will happen every time the controller is turned off.) However, with a bleeder resistor in the circuit, power will be drained from the capacitors. The time it takes to drain the capacitors depends on the resistance of the resistor. A low resistance will drain the capacitors more quickly than a high resistance. In this particular circuit, as Gerald suggested, a 22,000 ohm resistor would be a good compromise between too much heat and too slow bleeding; however, even though the math specifies 1/4-watt, I would use a 1-watt wire-wound resistor. A 1-watt resistor will still get hot, but not nearly as hot as a 1/4-watt resistor.
  #28  
Old Wed 07 March 2007, 11:05
Gerald_D
Just call me:
 
Mike, I thought it was bad practice to diconnect a G202's power supply while the motor is moving? (the motor needs a place to dump its energy?) Have you seen anyone else put a contactor/relay between the gecko and its supply?
  #29  
Old Wed 07 March 2007, 11:51
Mike Richards
Just call me:
 
Gerald, that's an interesting question and a question for which I don't have a definitive answer.

It is generally bad practice to manually turn a motor by hand (or manually move an axis) when the power if off, because, in that condition, the motor becomes a generator and generates power.

It is also a fact that a gantry will 'coast' to a stop, hopefully in a fraction of an inch, but that could represent hundreds or even thousands of steps.

I believe that the G202 is well protected against the voltage spike that shutting off the power would cause. The G203 is supposed to be 'unkillable'; so I believe that the G203 would not be damaged in any way.

On the yahoo groups Gecko forum, there is a paper that makes me think that nothing would be harmed. Here is the link to that paper. (Non-members of that group may not be allowed to access the PDF file. I know that you're a member, but some readers of this post may not be.)

I will forward the schematic to Mariss and ask for his advice.
  #30  
Old Wed 07 March 2007, 12:37
Gerald_D
Just call me:
 
My approach with E-stops is to use a pair of NC contacts under each E-stop button. One contact unlatches the main power contactor, cutting the input to the Gecko's supply...still leaving the issue of the capacitor being charged..

But, the second contact is wired to the PMDX-122 and its function is to stop the pulse stream instantly.
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