I have several three pole relays that I want to monitor.
Randy
Posts: 79
I have several three pole relays that I want to monitor. The relays have 120 volt coils and supply 208 volts three phase to mostly motors. I want to have a signal that will verify that the relay is energized, no fuses have blown and there are no other opens in the three phase feed to the motors. My idea is to put a resistor and optoisolator from each hot line to ground and feed the on or off output signal to an AND gate that will then inform a Stamp of the condition. An RC filter between the optoisolator and the gate would eliminate the flicker of the AC signal. I do not need instant response. This is only for monitoring and has nothing to do with operation. I am assuming that as long as the resister is the correct value the optoisolator can be connected to 120 volts. Am I right? Is it safe? Better idea? This needs to be cheap as there are thirty plus 120 volt lines to monitor, some three phase some single phase. I've never used an optoisolator above 12 volts and only for DC. I realize that an answer on this forum is not from some safety commision but I trust a few of you are knowledgable about such things.
Thanks
Randy
[noparse][[/noparse]Subject Updated]
Post Edited By Moderator (Chris Savage (Parallax)) : 7/27/2007 12:57:40 AM GMT
Thanks
Randy
[noparse][[/noparse]Subject Updated]
Post Edited By Moderator (Chris Savage (Parallax)) : 7/27/2007 12:57:40 AM GMT
Comments
http://www.veris.com/subcategory.asp?idMainCategory=42&idCategory=42
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Have Fun
TR
Post Edited (TechnoRobbo) : 7/27/2007 3:50:01 AM GMT
For an optoisolator, you'll need current limiting resistance and that will help a lot. If you use two 10K resistors, one to each side of the line, that will help a lot to protect the optoisolator. They'll need to be 2W rating each which will make them physically large enough to help protect the optoisolator.
Post Edited (Mike Green) : 7/27/2007 4:35:52 AM GMT
Attached to this a pdf file .. how to connect an opto to 240VAC..
Be carefull .. all pins and components are at 240VAC.
Simple low watt resistors do the job.
Cheers
Ronald Nollet·· Australia
First, are these contactors (straight relays w/o overload coils) or motor starters?
Second, are you looking to monitor the device itself (contactor or starter) or are you looking to monitor each one of the three three phase legs?
Third, do you have a neutral (ground) available or is this straight 208 at the contacts?
Regards,
Bruce Bates
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Mike your comments are much of my concern. This is for monitoring a roof top mounted chiller that sees temperature extreams as well as weather, rain and snow. It has had a twenty year history of blowing 15 amp fuses for the fan motors and more often than not there is no fault found. Megering, replacment, rewiring, new fuse holders have not changed its desire to periodicly fail for no apparent reason.
I do have a neutral Bruce, wrong of me to suggest ground. They are striaght contactors and I am thinking that by testing phase leg to phase leg only when the contactor is energized that the AND gate will be true only if all is well. Most of the relays are 208 coils and three poles. There are some circuits that have 120 volt coils and there are switches and other circuits that are single phase. Still haven't figured out how I am going to do all of the logic as there are many more things to monitor than Stamps have pins, thus the logic gates. Good chance this will be a 2p40 but I haven't got that far yet.
Any particular specs I need to watch for on the optoisolators? Would a very low current fuse be a good idea?
Thanks again,
Randy
Grainger has current sensors around $82 for 10A with 300A in rush
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Have Fun
TR
Post Edited (TechnoRobbo) : 7/27/2007 3:47:31 PM GMT
The condenser fans have simple three pole relays with FRN 12 fuses, no delays, they are staged by pressure controls. The motors pull 8 or 9 amps when running. I don't know off hand what the inrush is but those fuses should be fine. This machine has a twenty year history off intermittent problems that a hundred tecks have looked at but no one has been watching it when it fails. They have put voltage and current loggers on it for 90 days and on the 91st day it will fail. We all know about intermittent.
I don't want this forum to get off track. I am not seeking answers to fix the machine although I do appreciate it and you could be given the status of a god if you had the answer. I am more looking at making something that will notify me when it has failed. At the moment an allready warm building is our notification device. I also need to make finding the failed component a little simpler. Some of the people that must respond to a failure are not familiar with it and troubleshooting can be time consuming. It has relays, pressure switches, temperature switches, a few time delays and no intelligence, it is all mechanical. As it has been modified over the yeears we don't even have a correct schematic, something I will have to make before I can progress too far with this monitor idea rattling around my head.
Thanks
Randy
Several years ago I had to monitor the condition of 32 switches, high or low only. But, I only had a Stamp-1 with 8 ports. So, I used two CD4067 CMOS switches and I only needed six ports of the eight the stamp had. It worked great, and still is.
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D. A. Wreski
For what its worth, the circuit posted by OzStamp is the one used by most PLC manufacturers on their 120/240 VAC input cards. Works well without generating much heat.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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Thank for your Link to http://www.veris.com/subcategory.asp?idMainCategory=42&idCategory=42
That help me in a project that i am working on·
These are prices at $80.00 For a part #·722LC
But this is what i was looking for
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Sam
In the circuit that OzStamp has does the capacitor keep the LED lit so it does not pulse? I was thinking that I would have to extend the pulse after the optoisolator. This is too simple, it can't be that easy.
Randy
Or you could put a few turns of insulated wire around one leg of the power to the motors. Then a smiple diode with a small cap to the + side of an Op-Amp. This way there would not be any dangerous voltage anywhere.
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D. A. Wreski
Have a good look at the link ..
It explains what a cap in an AC circuit does.
Capacitive reactance..
The resistor across the cap discharges it .slowly but surely..
As Mr Tom Sisk mentioned this circuit or very similar to it
is widely used in PLC type controllers..
However PLC's in most cases have tyically 24VDC type inputs.
Cheers
Ronald Nollet·· Australia
I don't think I ever did thank you for the link. Thanks.
Capacitive reactance, current lead - lag, etc always has confused me. My BC wired brain has trouble enough with DC. I have done a days worth of searches and get formulas and graphs but no explaination that I understand of what it does for a circuit. I understand the basic concept (barely) but just don't get the whole picture. I can do the math but don't know what to do with it. If I understand it, and that is a big if, the capacitive reactance in this circuit tends to tame the voltage and current signwaves to more of a flat line somewhat like DC so the LED brightness is more even. It also gets combined in parallel with R2 and in series with R1 to limit the current to the LED. If that is close to a correct picture the negative half of the cycle runs an eraser accross the blackboard in my mind's classroom and the chalk dust makes my brain sneeze.
While I assume the values in this diagram will work with just about any LED or optoisolator I don't know how to·verify that. If I were to have this diagram with no component values other than the LED, I would have no idea how to compute them. My first thought is if this should work just connect it, use it and be happy. I don't think any of us·who dabble with electronics is happy with not understanding and I am reluctant to connect anything to more than a few volts or fractions of an amp without an expectation of what is going to happen.
Any help with clairity is appriciated.
Thanks
Randy
PS It has been several years but by far the best beer I have ever had was down under. I doubt I will ever meet you but buy yourself a beer on my thanks for your help.
Follow this link http://www.wisc-online.com/objects/index_tj.asp?objID=ACE602
A primer on Capacitive reactance... basically the cap is a resistor ..
It limits the current...
The circuit that I emailed in my org post would work on 240VAC .
Probably enough current thru it as well for 120VAC.
The resistor in series with the cap is purely an inrush limiter ..its value is not critical
220 R or 330 R and even 470 R will work 1 WATT type..
The current transfer ration of most opto are enough to switch inputs on micro's.
Cheers Ronald Nollet Australia
When i followed the link my browser came up blank. I tried the same with "misc" thinking maybe a typo but that took me to Japan, in Japanese. Might there be a typo?
Randy
I read this post a few day ago and i re read just now let·ask you something
This is just some
When you say that it blowing fuses is that all three fuses are blowen or are just talking·about·one fuse blowen if only one is blowen what may be happing is that you are losing voltage (dip)·on one phase just long enough to blow a fuse
Or check the contactors that they work freely and are not binding up
because if not all poles are going in at the same time this may cause the same
type of problem
This happens to some of our A\C units and chillers· fans
If this is the case you can buy Phase Loss Controlers just like Time Delay for
A\C·units they control the Contactors for the Compressor
And for you it would be control your fans
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Sam
Post Edited (sam_sam_sam) : 8/3/2007 9:13:09 PM GMT
·I re read your post and what OzStamp is talking about will work it was in
Nuts And Volts about·two year ago and if i can find what month it was in i will
let you know an email it to you if you want me to
I made a Night Light for the ·bed room· nice and bright with this same
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Sam
Thank you. I will look for the Nuts and Volts article. If you happen to find it let me know what month/year it was.
The fuses always go in pairs. I would suspect that one blows and as soon as the motor has lost one leg the current goes up and takes the second fuse. The third fuse is fine because there was no short. When it did this twice a couple of weeks ago it was the first time in about a year it blew the fan fuses. It has always been an inconsistant problem which is why it is hard to pin point. Everything from the fuse holders, the wires, the motors have been replaced, meggered, checked with an ohm meter, monitored for current, phase loss, you name it. Never a problem when it is being monitored. We have never put monitors on it permanently. It is like a sneaky little kid that knows when it is being watched. That is why I want to put something on this that will watch and log all of its functions. Mostly I just want an alarm to know when it acts up. It is not just fuses, it sometimes trips on low or high pressure or oil, what ever and almost always no problem is found. It may well go another year or it might not be running right now. The boss is trying to get budget approval to replace it so this all may be for naught. I am waiting to get a better feel for if we are going to have this chiller for a while or if a new unit is in the works before I procede.
If it stays I am leaning towards using MID400's as they interface with AC by design and are inexpensive. I have a good idea of what all I want to monitor and how to work out the logic and multiplexing.·I will probably split this into three or four boards with just a terminal block, the isolators and current limiters and then a separate board for the low voltage logic and communication.·Still placement is a problem for the high voltage stuff. There is just not a lot of room and I am trying to avoid running twenty some 12 gauge wires several meters to another enclosure.
Anyway for now I just get to think about it as it will be two more months before we get an answer on the budget. If I end up doing this I will post what I have done or more likely some other question I have when what I build doesn't work.
Randy
I have been keeping an eye on this thread, and I have an idea what might be going on here. I have a sneaking suspicion you may be looking at the WRONG MOTOR in troubleshooting this problem, and that's why no one has been able to find the problem. See what you think of this long distance diagnosis.
1. You've never mentioned anything about problems with this motor intermittantly reversing. That immediately removes the possibility of a phase reversal.
2. If something external to the motor was causing the high current (intermittantly binding drive components) the mechanical part should have shown enough wear to be apparent by now, or even have broken. You've said nothing about anything like that.
3. If the problem was internal to the motor, such as an intermittantly bad winding, or an intermittant short in a winding, then you would blow ONE fuse (the leg feeding the bad winding) and that seems not to be the case.
4. If the problem was mechanical and internal to the motor (bad bearing, binding shaft) that too should have become so bad as to be noticible after 10+ years.
5. My sense from what you've said is that it isn't any sort of ordinarily overload (bound drive train) and also because two fuses are popping each time there is a failure.
6. My guess is that the incoming line (power company) has been monitored for voltage over an extended period of time. That would eliminate line sags or brownouts at the source.
7. There are apparently no shorts in the wiring, or general wiring problems such as worn insulation.
Those are probably most of the common reasons for the overcurrent protection becoming involved. Here are two thoughts which may or may not bear some fruit.
1. If the motors which are popping these fuses, are not the same motors over and over again, see if you can determine in some way (trouble reports, maintenance logs) either how old the fuses are that have most recently popped, or if they'd ever popped before. In other words, I would look for fuses which had long service life and which had a lot of years on them, and had never been swapped out.
Although it's not a very common occurance, the fuse metal (tin-lead) inside the fuses can crystallize over time and will fail (safely) by blowing at an abnormally low amperage level. This is most common in areas with large temperature gradients. Typical scenarios might be smelting areas, near ovens or kilns, or even geographically intemperate areas with sub-zero winters and very hot summers where the environment is open-air.
2. My bet is that although voltage levels have been checked upstream of the motors, they might not have been checked downstream. Are all these motors fed separately, and is there only one motor per feed? If not, what I'd be looking for here is a 3-phase motor being fed in a normal manner with a 2-line tap-off (at the motor) going further downstream feeding a smaller, single phase 220 volt motor. I'm not sure what this "pony" motor might be doing, as I'm not familiar with your exact application. That "pony" motor might be the culprit, and all the while its big brother has been the one under investigation. Speaking of "little kids"
Last but not least, you hadn't mentioned what type of power distribution you have for the 3-phase at the head end. Is it open delta, closed delta, or something else?
Regards,
Bruce Bates
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Anyways, since they're staged condensor fans possibly a hysterysis has been dialed down or a start up delay has been bypassed.· Some of the toughest problems were caused by well-intending technician (even the ones with good reputations) changing setpoints not looking at the delicate dance of co-ordination that control sequences must follow. Often the machines are improperly operated then the manufacturer is blamed.
So, if any, I mean any, control modifications have been made to the controls system or settings - bring it back to factory specs. I garauntee the design engineers knew what they were doing.
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Have Fun
TR
Thank you. I will look for the Nuts and Volts article. If you happen to find it let me know what month/year it was.
I found the Article..................................... August 2004 on page 57 to 61
Name of the article is................................ A White LED Night Light
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Sam
Thanks for the info. I will look up that article.
Bruce,
I think the things you mentioned have probably all been looked at if only quickly. The first thing you mentioned, the motor running backwards due to phase reversal did give me something to think about. It can get quite windy here for days at a time. I am wondering if the wind could cause a fan to freewheel strong enough to cause an overload when it starts. If the weather is cool and the wind blowing I can see where head pressure would remain close to normal and not cause a shutdown until some time later when no one thinks about the wind gusts of 80mph two weeks ago.
Anyway as I said before this machine is close enough to the end of it's expected life that fixing it has become secondary. Getting a Stamp to notify us that it is down or is about to go down and what has failed is my goal until a crane shows up in a year or two.
Thanks,
Randy