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scsteve

Please grade me, update on first program.

6 posts in this topic

You guy's, who had constructive input on my original thread,"please grade me" thank you again. I comissioned my first program, mid november. The train crossing. Well, not so smooth. Get a call, it's not working. I'm thinking, it "bench tested great", it ran for three days on the bench. Using a ML 1000, 10 point. It started to fault out. A hard fault. Software error. I use the help menu, and it said to check grounding. I checked grounding, again and again. Sometimes it would fault out immediately, sometimes hours or days. The help menu also said to check surge suppresion. I had surge suppresion on the processor. I figured that was enough. So wrong I was. I used slave relays to control the lights, bell and gate. Four to be exact. They are general purpose 8 pin "ice cube relays", operating at 110 volts a.c. Installed surge supression on my relays, and all is good. No fault for thirty days. I guess I am asking how can a slave relay cause so much trouble? I'm sure you guys could of figured out my problem in no time,but, I was trying to diagnose the problem on my own.

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"Inductive kick" Not unlike the coil on your car. See links below for a better explanation. http://en.wikipedia.org/wiki/Snubber http://www.google.com/search?q=inductive+k...lient=firefox-a ML1000SurgeProtection.pdf Edited by Mickey

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Hi scsteve, Don't be too hard on yourself you did find the problem in the end and in the process have learn't some valuable information regarding control circuit design. Surge suppression is a very important consideration when designing control circuits a large contactor coil with no suppression ( or a dc coil with no free wheeling diode) being driven by a plc output will dramatically effect the life of the output. Also try googling "back emf" this should have some links to information that will help you understand what is happening. Cheers

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Next time, I recommend getting rid of "cheap" ice cube relays forever. They tend to be not so cheap in the long rung. Using them as control relays tends to be not so cheap in the long run due to their many issues: 1. They have inductive kick problems which can cause strange lock-up issues, ringing, etc. 2. They are slow to cycle (relatively speaking). 3. They can freeze up in winter from moisture or from sand/dust getting inside any time of the year. 4. They have limited mechanical lives. This is the #1 reason that PLC's are so much more reliable than control relays...reducing/eliminating control relays reduces down time. Solid state relays don't have any of those issues. An AC solid state relay is a triac (effectively, two SCR's wired back-to-back). They CAN have ringing issues but typically have built-in snubbers to take care of this problem. Older models were electrically noisy but newer ones are virtually trouble free. They integrate the contacts directly into them so you can use them directly without terminal strips. They last a heck of a long time (decades) as long as you fuse them to protect against burning them out. Some come with built-in fuses, and most have indicator lights for troubleshooting. They often (but not always) form the output modules of many PLC's or PLC I/O cards. The downsides of solid state relays are: 1. They are more expensive than ice cube relays, but they don't require terminal bases and often don't require surge suppressors or indicator lights so this is not a 1:1 comparison. 2. You change them out MUCH less often (almost never). Don't expect them to be a source of call outs. 3. They are solid state, so the open circuit resistance is still very high (commonly only a few megaohms) but not as high as an ice cube relay (several megaohms). If you have one without any load on it (disconnected) and you stick a meter on it, it will appear to have 120 VAC on the output terminal. This disappears with the load connected (acting as a voltage divider) but still fools neophytes that aren't experienced with this and mistakenly believe that the relay has shorted out when in fact it hasn't. 4. They generally tend to run a bit hotter and require more space in the panel for cooling. This is offset by the fact that you can get them in down to say a 4 mm DIN form factor. This allows you to fit a half dozen of them in the space of a single ice cube relay on a DIN mount terminal strip. 5. DC and AC are NOT interchangeable. Voltage ranges are not typically interchangeable either. If you attempt to use an AC output SSR with a DC circuit, it will switch on ONCE only and then "stick" on and not switch off! If you attempt to use AC on a DC SSR (a transistor instead of a triac), it will half-wave rectify the output. 6. The sources for them are usually a bit different. To get good pricing on SSR's, typically I have to buy them from electronics (instead of electrical) distributors. For instance, from Omega (a recent source of decent prices for a lot of electrical items), an ice cube relay plus a terminal block costs $21 list. From the same source, the cheapest SSR's are running around $50. A Magnecraft 861SSRA208-DC-1 which is an 8A relay with DC inputs and 24-240VAC outputs costs $18.65 list from Newark Electronics. A Durakool DP6-1P-24VAC/DC also from Newark for about $20 is only 4 mm wide and can be driven from 24 VDC or AC. The output is only 6A at 95-230 VAC. Both have indicator lights and both are less than the ice cube relay once you include the socket. 7. They have limited scalability. This used to be true but no more. They have gotten really, really large. A Crouzet GN350DLZ ($150) switches up to 600 VAC with a 50 A rating. It's a 3-phase relay. Spec-wise, it reads like a NEMA size 2 starter. There is also a reversing version available for a little more money. Teledyne makes a version up to 75A. Omron makes the old "hockey puck" style single phase relays up to 90 A. There's no technical limitation on the underlying technology (SCR's) but at these current levels, usually it is preferable to use a soft start anyways to reduce the thermal and mechanical shock to the motor, and a soft start is based on the exact same power electronics as a solid state relay but includes some extra bells and whistles. Above about 100 A if all you need is a switch, vaccuum contactors have come down in price to the point where they are less expensive than air contactors or solid state equipment, and they address most of the ugly mechanical issues with air contactors (immune to dirt/moisture, fast switching, small size, little arcing, very long mechanical life).

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Thanks, for the info guys. It was a great deal of help. I now understand inductive kick. And better options, besides ice cube relays. On another note. I have a Ml 1000 here at home, I use to practice with and learn new code. I have RS Logix Micro on my laptop. Right now I am trying to figure out the sqo programming. And, yes, I am having a little trouble. I am gonna give myself a little more time. And then Steve

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don't forget to check local electrical code when using solid state devices.

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