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I've read elsewhere about people using lamps rather than resistors as a "visual" indicator, is this the same kind of thing? I'm intrigued if a lamp connected to a normally closed relay would work, though I have no intention of trying it out...Quote:
Originally Posted by JAZZCNC
Don't see why not, but bear in mind a lamp can blow/fail - so even if it's not illuminated you don't know for sure that there's no voltage across it.Quote:
Originally Posted by Rogue
Took the words right out my finger tips.! . . Resistor is a safer bet, could always use a resistor and a bulb but why complicate things.Quote:
Originally Posted by Jonathan
Again, its good practice to have 'voltage present' indicators, especially if you have a secondary fuse. (plus I like lots of lights :) ).
For the primary side a panel mounted neon indicator wired directly across the transformer primary.
For the secondary side a 5mm LED mounted in a panel mount with a 3k9 2W resistor wired across the output.
After a cup of tea and a bit of a read, I have a quick question in relation to 0.5CV^2. Where you give 0.5 * 0.005 * 70^2 = 6, I end up with 12.2. In fact using the time honoured technique of "randomly changing numbers until they worked", I only got 6j by changing the 70v to 16v?Quote:
Originally Posted by irving2008
Trying to follow your calculation and sticking with the 5 second discharge, I ended up with this:
0.5 * 0.005 * 70^2 = 12.25j
12.25 / 5 = 2.45W
70^2/2.45 = 2000ohm, so I would need a 2k ohm @ 3W resistor to discharge over 5 seconds.
I'm rather hoping you made a typo because the alternative is that I'm more stupid than I thought!
Quote:
Originally Posted by Rogue
Errrmmm... looks like I made a boo boo... :hororr: might have multiplied by 0.5 twice in error... your calculation is correct... now I'll go and edit my post..
BTW 2k2 @ 3w will be fine
Then I shall take off my dunce's cap and come out of the corner. For now, at least :whistle:
Okay,
I have been reading and talking to folks (Thanks Jazz) on the power supply issue. This is what I understand and what I want to make sure of.
- Mains power (fused at plug) To
- Fused Switch (main Switch as will wire in a secondary for the motors) This is feed to a + rail - rail and GND
- Positive rail Feeds:
- A Circuit Breaker going to the Switch for the Soft start for the transformer (will be asking on how to build one of those)
- The PMDX-126 BOB (Which has built in Circuit Breaker with auto reset)
- The on board relay (on the PMDX-126) for the spindle (30A relay) Currently this feeds a Kress 1050 FME. Does this need a Circuit Breaker as well?
- Negative Rail Feeds same as listed above.
- GND includes all of above as well as grounding the shielding on the stepper cables, the computer frames (control box and computer), Machine frame, and the unshielded side of the PMDX board.
Okay that is so far, now onto the power supply that is going to be going in shortly. The way I am looking at setting it up is as follows:
- From Circuit breaker in number 3.1 to Switch for motor power (both +/- plus GND if that seems best)
- From there Negative and GND go to the Soft start for the transformer (need help on designing one of these so that I can build it in)
- Positive goes to a Relay (12V) attached to the PMDX in the set up in NO position so that the motors cannot be powered unless the board has power.
- From Soft start to 750VA 50+50 Standard Range Toroidal Transformers: CM0750250: 750VA 230v to 2x50v This is to power four 4.2A peak current Nema23 steppers and by what I have read, should meet even heavy running requirements.
- I am going with the 7.5A 70V in Parallel as that gives my 15A output in theory minus any loss and should more then cover the 12.3A needs to drive the four stepper motors (I recall from Physics that current is on demand and Voltage you get what is there, a 3A item only draws 3A but will take all the Voltage that comes with those 3A).
- This is where I am a bit confused. should I put another fuse here between the Transformer and the Rectifier or should I run directly to them?
- The Rectifier is a 35A 200V Metal casing rectifier and will be mount to a ali' plate to help with heat transfer and I have thermal paste (does the bolt/post need to be isolated?).
- Next is The Capacitors and I am looking at 5 4700 uF units (23.5 uF to meet a need for 21.4uF)
- Okay I am looking at this and trying to figure out how to wire in the the resistor to drain off power when the system shuts off. The idea I have is to put a relay in place that is in the NC position with the resistor (50W as they are not expensive and give some added safety margin) set to drain off the power when the switch closes. Now I could power this relay from the PMDX as with the other relay in place the motors should not normally be able to be powered if the board power is off. Other then that I am at a dead end on this one. Also the resistor should go to ground when it is activated correct? Also could use a double check on the Ohms I need for that Wattage.
- After the Capacitors the power is then run to a common rail from which the fuses (5A) for each Driver is in line with the + and the _ runs back to a common.
If that all makes sense and anyone wishes to help with the Soft start modules and other questions I have posted here in, please do. Also hopefully this will help some other folks with design and structure building. I plan on putting this all on a board and Then mounting on standoffs in the control box. Yes it is bit of over kill but then again I make a part of my living from this machine and it needs to be safe and run well.
For those who want to get a bigger picture of the electronics of this System I am using a AMD based PC running XP Pro SP3, using Mach3, That goes to a Smooth Steeper Ethernet card and then to the PMDX-126. Which then passes the data onto AM882's Digital drivers. This drives 2 motors on X, and one each on Y and Z axis. Spindle is currently a Kress 1050 FME. I have Jazz, James, and a few others to thank for aid in the development of this machine and in the new thread it passed the test you can see a picture of the detail it can do. The reason for the new power supply is that I owe the one I am using back to the person who has so kindly let me use it this long.
Michael
edited to correct a typo
Soft start for that size of transformer isn't really needed. Just use a zero volt switching SSR (most AC SSR's are), and you shouldn't have any issues.
One thing to note though, is if the SSR fails, it'll likely fail short circuit, so oversize and fuse it to minimise the risk. If you're really concerned, put a mechanical relay in circuit before it, incase anything does happen.
Generally you only fuse the transformer primary side. Any overload on the secondary side should blow the fuse on the primary side. If you oversize the rectifier enough, then any overload on the DC side should also blow the primary side fuse.
Fusing individual drives isn't usually recommended, as if it fails for any reason, then you risk the drive being fried due it having no where to dump power from a moving motor to.
The drain resistor should simply connect across the capacitor terminals. Unless you have some reason to quickly dump power, you don't need to have it switched. The motor drivers themselves will quickly drain the capacitors with power removed.
Mike,
Can I suggest you get some terminology correct, it might save you some pain (physically and metaphorically) later
When referring to AC (mains) its Live, Neutral, Ground The colours being Blue, Brown, Yellow/Green
When referring to AC (transformer secondary) its AC Secondary and general practice to use Brown
When referring to DC then its +/positive, -/negative, chassis ground and colours are generally Red, Black, Green
Here's a sample circuit with estop and interlocks. Turning on the main switch powers up the +24 volt safety circuit. Push to start closes relay 1 (AC side) and powers up the +70v and aux supplies. Relay 2 (LV side) also closes holding in the relays unless the eStop is triggered. As an example I've shown one set of Realy 2 contacts switching the bleed resisors though I personally don't bother and have them permanently connected.. Other arrangements are possible...
Attachment 6881
Irving my apologies for not using the correct terms. I know the AC usage and colors (Though I still prefer the older colors as they made much more sense to me).
Michael
Just out of interest, if you wanted to get 5V out of that system as well, would you run a 7805 off the +24v circuit, or should that circuit be kept purely for the relays/e-stop?
Depends. The 24v is present as soon as the primary switch is turned on. If its OK for the 5v to be present too, and the current drawn is small, say < 0.1A, then yes. Otherwise I'd run a small 5v supply off the aux AC.Quote:
Originally Posted by Rogue
Incidentally +24 is nominal. A 15v transformer would give 21v, an 18v transformer would give 25v. A 3VA transformer is more than enough (200mA output). The smoothing capacitor need only be 470uF @ 40v rating and needs no bleed resistor. The relays are 24v, 600ohm coil, needing 40mA each, with 7A 4PDT contacts.
Intriguing, and hopefully stripboard friendly! I think I will start here before attacking the bigger stuff.
Here we are talking about mA: would the same 35A bridge resistor still work? Or would it be too "robust" (for lack of knowing what the appropriate term may be) in this situation? Does there come a stage when the safety margin becomes so excessive that it degrades performance at the lower end of the operating range?
Quote:
Originally Posted by Rogue
You mean 35A bridge rectifier. It would but one that size is needless. This one is more than sufficient and cheaper.
Here's the plan - listing is indicative of order, should roughly correlate to this:
Attachment 6926
It's a little rough, I rushed it for the post. You might need to pretend there is a relay-controlled bleeder. And you might need to pretend there are any outputs to the rest of the world, but hey, that's the power of i-m-a-g-i-n-a-t-i-o-n!
Fused IEC Chassis Plug (23-0360) with 5A slow-blow fuse (20-1072)
High Current Toggle Switch DPST (on/off) rated 250VAC@10A/12VDC@20A (75-0253)
Splits out to
- 12mm 240V Green Neon indicator (42-0330)
- 24v system
- 70V system (via relay)
- potentially other systems, ie BoB etc.
24VDC System (well, 23-ishV)
Transformer: PCB transformer 230V 12VA 2x9V (88-3877)
Bridge Rectifier: Woo5 1.5A 50V Bridge Rectifier (47-3190)
Indicator: LED to be determined (probably have something suitable upstairs)
Smoothing Cap(s): 2000uF according to my magic spreadsheet, so 2 x 1000uf 63v Caps
Bleed Resistor: Not needed, but otherwise I worked it out as 3.5kOhms 1W
Toggle switch 24V for turning on 70V system
70VDC System
Relay: 24VDC 4PDT, rated 240VC@7A (60-1310)
Transformer: Well this is annoying, see below.
Fuse: Slowblow 15A in fuse holder
Bridge Rectifier: 35A 600V Bridge Rectifier (42-3228)
Indicator: LED to be determined (probably have something suitable upstairs)
Smoothing Cap(s): 4 x 4,700uF 100V caps (11-2912)
Bleed Resistor: I work this out as a 5 second bleed = 2 kOhm 3W, controlled by a relay wired NC.
Transformer for 70V system - Rapid has 500VA 2x50V transformers which should give me 50VDC@10A in parallel. I'd prefer 625VA but they don't carry 2x25 and the 2x50 has a 1-2 month wait from order. As I'll be sourcing this component last I have time to hunt around. A few other links to sources were posted in the thread; I'll check up on them later.
The wires to the relay that turns on the 70V will pass through another, normally open relay. This second relay is powered
Various crimps, wire and connectors should be easily sourceable at the local shop.
If I don't post for a month, my widow will probably be selling my stuff on ebay so keep an eye out for some bargains - she might be selling it for what I told her it cost before I got turned into crispy bacon...
For safety reasons don't use a toggle switch for the 'start' switch on the 24v supply to relay 1. It must be a momentary push to make to enable both relays and use a contact on relay 2 to lock it in, as I drew it.
Why? your machine hits a limit switch, triggering estop (or you hit estop). everything turns off. You turn off the power at the main isolator but in your desire to see what happened you forget to turn off the 'start' switch. You reset the estop, or move the machine off the limit switch and turn on the main isolator... but the start switch is still closed so the 70v rail comes up and the machine starts moving again because you forgot to stop the PC...
Its a safety interlock, it mustn't remain in an ON position. Also you don't need both contact on Relay 2 to drive Relay 1, only one side of the coil needs to be switched.
Using my imagination I've imagined some of that same 24v running thru E-stop going thru limit switch's as well and the RL2 relay having another contact for the 5v signal from BOB.!!
Edit: Oh and you'll need an limit override momentry button in there as well to allow reversing off the switch's.
Ahhh. The reason why I basically re-drew your drawing was to get my head around what was happening. Now you put it like that I can see what is happening there. I guess I should have asked. It's not like it's the first daft question I've had :)Quote:
Originally Posted by irving2008
I know, but I wasn't using those poles for anything else at the time and it looked untidy. I was envisaging the second poles driving a relay that disconnects the bleed resistor when the system is running.Quote:
Originally Posted by irving2008
Running the limit switches off the E-stop wire sounds like a good idea - they are effectively doing the same thing as an E-stop, after all. However, I've come across a few discussions about the BoB, with several people saying that it would be better not to cut power to it.Quote:
Originally Posted by JAZZCNC
I guess that might help... :cool:Quote:
Originally Posted by JAZZCNC
An easy way to provide the limit override (non-locking) and start (locking) logic is with a diode:
Attachment 6929
An alternative would be to use a seperate contact on Relay 2 to enable Relay 1, but I prefer to keep contacts free for use.
Attachment 6930
Just out of interest, can multiple contacts be wired together, ie a 4 pole with contacts rated for 10VDC@10A wired in series to make effectively a single contact of 40VDC@10A? Or would teeny tiny difference in switching speeds cause one set of contacts to close first and take the full brunt before the other contacts caught up, leading to melting, fusing and the opening of portals to alien dimensions?
I ask because one of the plans had a relay for the bleed resistor on the 70V circuit, but I've not seen many (well, any) relays with 70VDC contacts.
Edit: Thinking about it, voltage increases in series - if it's in series then it doesn't matter if they all close at exactly the same time or not.
Voltage on relay contacts is generally not important, its the current carrying capacity. If you need more current then you wire the contacts in parallel. The reason why contacts have a lower DC voltage rating is to do with the arcing that happens when the contact opens. Generally the contact gap is very small, maybe <0.5mm or so. With a DC current the arc ionises the air in the gap and causes a plasma to develop. If the voltage is too high this plasma will continue to pass current until the contacts have opened sufficiently that the voltage cant sustain the plasma, which seriously shortens the contact life.
One solution to the arcing is to put a snubber across the relay contacts - a capacitor and resistor in series - the idea being that as the contact opens the capacitor charges up thus drawing current off the contacts and stopping the arc forming until the contacts are far enough apart to prevent the arc being sustained. This alows the contacts to be used at a higher voltage at the rated current.
This isnt a problem with AC contacts as the voltage returns to zero every cycle thus quenching the arc, thus they can be rated at much higher voltages.
Now, in your situation, using it for the bleed resistor isn't a problem. The current passed is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated.
Incidentally, where did you get the 10v from? If you look at graph H55 on the spec for those relays, you will see that the 4PCO version is rated at 30v 6A. At 70v its derated to 0.5A... but thats only to meet the contact lifetime figures at 1800 cycles/hour. If you switch a dozen times a day it'll be a lot!!! On the AC side graph F55/1 shows that at 900VA the contact life is 200,000 cycles at 1800 cycles/hour so its not going to fail in your lifetime (about 45years at 12 cycles/day) lol.
While I wanted to keep everything from the same source for convenience, it looks like RS was my next stop.
24VDC DPDT, contacts rated to 250VAC @ 7A / 125 VDC @ 7A (RS Online 699-6869)
As far as I can see, this relay should work in any of the proposed situations that involves relays powered from the 24VDC system. As DPDT it is straightforward to wire it as normally open/closed as required.
As far as mounting goes... I could hot glue the relay somewhere and fix wires directly to the contacts. This would mean glueing it upsidedown or on its side. For various reasons I don't really like this idea. There are are DIN sockets available (699-6881) that should be much easier to fix somewhere, and would give me screw terminals to work with. While it adds on another £3 per relay I think that it might save me a bit of worry.
Now I'm up to (in effect) £10 per relay. Hmm.
Let's look back at Irving2008's previous post with drawings. In the second drawing, Relay 1 is handling the AC current and needs to be suitably higher spec. Relay 2, however, is only handling 24V. A 24VDC DPDT relay rated to 220VDC @ 2A (619-3013) is a much more comfortable £1.78 + VAT. Seeing as the transformer isn't even supplying 2A in the first place, presumably that would be sufficient?
Since I started to write this, I note that Irving2008 has posted again. If "..the current passed [for the bleed resistor] is tiny (30mA) so the voltage rating of the contacts isn't an issue as very little arc will be generated", then would the 2A relay above also work in that situation?
Yes. The originally spec'd relays (Rapid 60-1310) were fine for both positions.. I would socket mount them.
The RS699-6869 is overkill with the inbuild LED etc.
The RS619-3013 would be fine for Relay 2, but as a PCB mounted device its hard to use.
If price is critical then consider using Rapid 60-1667 in sockets. While these are only rated at 240vAC/5A and 600VA for the switching rate you will use them for they'll be fine and if you're paranoid you could wire the contacts in pairs.
The 10V was pulled out of the air for simplicity and example, I wasn't referring to any specific bit of kit.Quote:
Originally Posted by irving2008
As for the graphs, the issue for me is translating what I can see into usable information and that is where my lack of knowledge shines through. For example, you turn the supply on and run it for an hour. For that hour you're drawing a fairly constant 5A. My understanding was that, while the contact only switches once, it still has to carry 5A for that hour. My choices were made on that basis. Clearly that basis was wrong which is leading me to make some less-efficient choices.
I'm glad to hear it, as keeping everything to one supplier makes life easier for me. However, I only found reference to it supporting DC coil, not DC contact, which is why I went looking elsewhere.Quote:
Originally Posted by irving2008
True, but the lower end ones are out of stock until January. I'm not bothered by LEDs and test buttons as I don't anticipate seeing it once the cover is closed! As the Rapid version is fine then it's a problem solved (or at least avoided) for today.Quote:
Originally Posted by irving2008
Fair enough. At this stage I'm keenly aware of wiring convenience.Quote:
Originally Posted by irving2008
Price is critical over all. I don't want to cut corners but I lack the experience to know which corners can be rounded gently and which can be no more than gently tickled with a chamois cloth.Quote:
Originally Posted by irving2008
Never being one to rush things, I've now got a toroidal transformer sitting on the bench ready to get wired up!
Specifically, this: Toroidal Transformer 625va 0-50v 0-50v --- link to Data Sheet PDF
I understand from the Data Sheet that:
- on the Primary side, I want to wire the Grey and Violet wires together, thereby wiring it in series to handle 230Vac from UK mains,
- the Blue wire is now 0V and the Brown wire is now 230Vac, then
- on the Secondary side, I want to wire the Orange and Black together, then the Yellow and Red together, and
- the Orange/Black is now 0V and the Yellow/Red is now 50Vac.
Wiring it in parallel will give me 50Vac @ 12.5A.
After being rectified this will give me 70V @ 12.5A (less voltage drop across the bridge rectifier)
Is anyone happy to cast their eye over the datasheet and confirm that I'm connecting up the right wires before this goes anywhere near the mains? The instructions look straightforward, but I've said that about lots of things that later went horribly wrong :joker:
The original drivers I was looking at from Zapp are now discontinued. The alternative (DM856) has a max supply voltage of +80, which I understand to be a comfortable margin for this PSU. The closest driver current options, however, are slightly different - 3.8A or 4.3A, whereas my steppers are 4.2A. The old PM752's would provide 4.02A as the closest fit. Is there a significant enough difference in the current provided that would make it worthwhile hunting for different drivers, or is 3.8A fine? These will now be driving an X2, which I'll be using as a stepping stone to my initial DIY design.
The good news is that I should be finished by 2016, possibly late 2015 if I pull my finger out...
Spend little more and Get the AM882 with lead and you can program them to exact amps and much more, they are great drives.Quote:
Originally Posted by Rogue
A small problem with wiring the secondary in parallel is that if the amount of turns on them is not exactly the same the transformer is not as efficient.Quote:
Originally Posted by Rogue
I got around this by rectifying the two windings separately then combining them at the DC side.
Just a thought. ..Clive
Intriguing. I've seen a bit posted about the AM882 drivers but a lot of it didn't seem positive. On some further reading, most of the issues seem to be down to user error and misconfiguration. The price difference at this stage is minimal as well.Quote:
Originally Posted by JAZZCNC
Is it better to match up the driver output with the motor rating exactly, or leave a bit of a margin?
Also intriguing, though how would you establish that? Would it be as straightforward as checking the voltage from each output separately? I suspect I will take the easy way out and just use the one rectifier, but I'm intrigued by the idea.Quote:
Originally Posted by Clive S
If they're mismatched that configuration will still draw less current from the winding with a lower emf.Quote:
Originally Posted by Clive S
The following quick simulations I just did demonstrates this:
Attachment 9507
Attachment 9508
The first graph shows the current drawn from each winding for the aforementioned mismatched transformer with a light load and the second graph shows the same supply with a greater load. This shows that the effect if greatest when the load current is low, so although the currents are mismatched they will both be within the rating for a light load. Similarly, the mismatch is smaller when the load is greater, so if the rating of one winding is exceeded it is unlikely to be by enough to cause a problem. Also, there are a lot of other factors besides load current involved here, so the above simulation is by no means comprehensive.
In short it's extremely unlikely for the windings to be mismatched enough for either method to cause problems, so although I'd advise buying a transformer rated for half the voltage so you can put the secondarys in series, if you find one for the right price which has to be wired in parallel I wouldn't worry.
I have just joined the two outputs from the caps together with no problems at all. ..CliveQuote:
Originally Posted by Rogue
In theory you should be safe to run any driver up to its rating, since that's the whole point of a rating, if you can trust it. In reality however it's always best to overrate the drivers if you can, since then they will run at a lower temperature which prolongs the life of the components.Quote:
Originally Posted by Rogue
Yes - the difficult bit is determining how much of a mismatch would pose a problem.Quote:
Originally Posted by Rogue
Agree here and I've built both ways and never seen any difference to machine which is what matters at end of day. I buy what's cheapest or available and don't care which.Quote:
Originally Posted by Jonathan
Yes there have been but that's mostly been because they have little experience or never used digital drives before but trust me they are fantastic drives and give super smooth performance with great resonance handling. The Fact you can configure them thru software helps if you have problems with resonance or just want to get the best performance you can.Quote:
Originally Posted by Rogue
Yes it's always better to match current to motor ratings but not go higher. The only margin you want to leave is on the voltage but it's also always better if the drives are not max'd out on current IE max current setting = motor current so the drives not working flat out.!
Hi Jonathan, thanks for chipping in again. I'm still aiming to finish this before you finish at Uni so I'll need you to work towards a PhD, should give me enough time.Quote:
Originally Posted by Jonathan
I was thinking about this from the point of view of the stepper motors (4.2A) rather than the drivers. Either of the two drivers discussed are rated higher than the stepper (up to 5.2A for the DM856, up to 8.2A for the AM882) so there is plenty of margin. My question was whether it's better to run the steppers at their max or give them a bit of a margin. If your answer still applies (which logic suggests it does) then it seems reasonable to set the drivers for a little under 4.2A.
That answers that question, I should have waited another minute or two before replying! There's approximately 10V difference between the transformer output and the driver max input which should hopefully be fine.Quote:
Originally Posted by JAZZCNC
Yes 10V will be fine and the fact your using toroidal transformer helps further still because the caps will absorb back EMF better.Quote:
Originally Posted by Rogue
That's the plan, however at work they seem to want me to do the PhD whilst working them, in which case you've only got a year left!Quote:
Originally Posted by Rogue
Yes my answer does still apply. As you increase the current in the motor windings, the copper losses increase by the square of the current, so the motor will get hotter. The degradation of the insulation on the motor windings is dependent on the temperature, as with most chemical reactions, so according to the Arrhenius equation we can say (very approximately) that if the motors run 10° hotter, their lifetime will be halved (assuming they fail due to insulation breakdown). This sounds bad, but we have to put it in perspective - if the motors last 20 years then halving this to 10 years probably isn't a big deal.Quote:
Originally Posted by Rogue
I'd set the driver current as near as you can to the motor rating without exceeding it. You might be able to go lower, but I think it's better to ensure the machine will not stall by having a good safety margin, rather than worrying about the lifetime of the motors.
Going back to me previous point about the power supply, I just ran the simulation again but this time with both configurations. The first graph is with the windings connected in parallel on the DC side and the second graph has them connected in parallel before the rectifier:
Attachment 9509
I realise that there's not much point discussing this further, but it's interesting to note that the waveforms do look slightly better for the DC coupled version. There's still not much in it.
AM882's are an excellent bit of kit and really are easy to use once you get around the chinese strange way of setting up the software for programing the drives. I use them and they really do make things run very very smoothly. -Michael
Glad to hear it, it's not that common these days to get your foot in the door of a good job straight out of Uni. Make the most of it and don't forget us whilst you're busy making money!Quote:
Originally Posted by Jonathan
From reading around a bit more I think I'm sold on the AM882 drivers. Currently got 3 of them (+ a lead) sitting in my basket at Zapp. Wisdom dictates that I don't pull the trigger until the end of the month, but my trigger finger is getting fairly itchy at this stage :whistle:Quote:
Originally Posted by m.marino