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paulengr

AC vs. DC for control

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I was reading up on some of the articles. I'm confused by one point: switching from AC to DC for control wiring because of capacitance issues?? Can anyone explain this? I can't really use "what someone said on the internet" as a justification for switching to DC in new systems. Is there any other compelling reason to switch to DC? I work in an all-AC plant (AC inputs, AC outputs). The only place I use DC now is when something has to be done in DC...which is then followed by a relay between AC and DC. And please don't repeat that same old tired electrocution hazard myth. This is one that should be on Mythbusters. 10 mA of strategically placed current kills you dead no matter what the voltage is. In the medium/high voltage world (600+ Volts), arc safety distances increase. But the arc safety distances in 24 V vs. 110 V are academic. PAIN does increase as well with voltage, but the possibility of turning a nice shade of blue is still the same.

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Hope you don't consider this the same old tired story. The time was 1998 and in a now closed Plant in NY state we were using AC control for our conveyor start stop circuits. That particular night an operator got his shirt sleve caught on the conveyor and his partner pressed the Stop and E-Stop buttons frantically hoping to save his friend. The conveyor kept running, until an alert mechanic jammed a crowbar into the conveyor and tripped the motor overlaods. Minor cuts and bruises and one pair of scared operators. How did this happen and why didn't the conveyor Stop or E-stop. To learn this we have to examine the controls and building layout. This particular conveyor was 3/4 of a mile long with one Start/Stop/E-Stop loop around it from the MCP to all work stations and back. The Run Relay and the E-Stop relay were Omron G2432AN ice cube relays with a holding current under 15 ma. A recent plant expansion had run a 480 feeder parallel to the conveyor for about 1/4 mile. Result was a transformer effect between the 480 feeder and the Start/Stop/E-Stop circuits which produced about 40 ma of current into the otherwise open circuit. This is what had kept the conveyor running and almost cost an operator his arm or worse. Yes, it is possible and we did re-design the system to overcome this issue, but DC control has less of a problem with induced noise and capictance as such. Just IMHO.

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For what it's worth, I have been told that DC controls provide faster response time when used with standard PLC's. The internal circuitry of the I/O modules are simplified thus providing the faster response. Milliseconds of scan time add up and will make a difference. This may have just been some hype provided by a PLC manufacturer's sales person, but he made it sound convincing.

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As to safety, there are several common sense issues. I have not studied the relative safety issues of AC versus DC control. I have seen the information on what current will do to you we were given before we were allowed in the labs at school, but that has been a little while. First, 10mA of strategically placed current (for the right length of time) will kill you, it is true, but it is easier to get 10mA of current through human skin from 110VAC than from 24VDC. Would you rather put your tongue on a 9V battery or stick your finger on an 110VAC outlet? Keep in mind the resistance of your tongue is significantly less than that of your finger. Even if you don't believe Ohm's Law, there is the issue of the reversing current causing a "sticking" phenomenon, and the fact that the built-in current limiting of a DC power supply is much faster than a fuse or circuit breaker. Just my two cents on the safety aspect. I would wait for someone with more expertise to give a more detailed answer. Another reason to use DC instead of AC is speed. AC signals introduce up to a half-cycle (8ms) delay on the actuation of a device. This is also totally dependent on the timing of the signal relative to the supply cycle, so is unpredictable and not repeatable. DC signals do not have this issue, so when precise timing is required, DC should be used.

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Many of the regulations kick in at the 50-60volt threshold, by using 24VDC, you can avoid a lot of compliance issues.

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BAM, that's the reason for me, under 50V. I also had a cap. issue once with a multiconductor tray-cable, only 150 feet long was enough to induce a current from a 120V circuit to hold a solenoid on.

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We pretty much standardise on 24VDC for all but the largest of systems. The main reasons include: We use lots of PLCs, most of which have 24V inputs. Ease of which to interface with hardware such as variable-speed drives, sensors, small solenoid valves, LEDs, and other electrical/electronic devices, most of which again standardise on 24V. From a safety point of view - a requirement to keep voltage levels below 50V. Relatively low cost of DC power supplies nowadays. The only reason we would use AC for control would be when dealing with higher current devices where the cost of contactors and other control gear becomes expensive. In this case we use a mix of AC for control and DC for I/O devices.

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This is of concern on long cable runs controlling light loads. The distributed capacitance between two conductors over a long length can add up to a significant value. In an AC circuit, a normally closed contact at the far end of a long cable run can appear to never open, since the impedance of the distributed capacitance is low enough to keep the load energised. Using DC avoids this.

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I really can't see using AC sensors these days. For one thing, DC sensors are almost always cheaper, and the selection is greater. (it is becoming pretty common for new sensor styles to be offered in DC only) Also, sensor cables always seem to be "in the line of fire" and get cut/shorted/etc. Finally, sensors use so darn little power that you can operate a great many of them off a very small supply. In my opinion, output devices can be a different story. I'll tend to order valve packs with AC coils most often. There's very little harm to personnel, the power requirements are non-trivial (especially with hydraulic valves!), and the cost is roughly the same for AC or DC solenoids. Now I haven't even mentioned safety or compliance issues.. I know of at least one situation where a maintenance man was slightly injured while replacing an AC prox.. We were all in the habit of leaving the power on while replacing quick-disconnect sensors, and this machine happened to have AC switches. A maint. man started unscrewing the cable connector (which was damaged), and in the process became a path from line to ground. As always happens, the electricity itself didn't cause harm, but the scare of being shocked caused him to jerk away quickly and he cut the back of his hand on a rough edge of the tooling. As far as OSHA compliance though, I wouldn't be too worried. I've never been visited by an inspector that was knowledgable enough to get that in-depth into the workings of a machine. They probably never have to.. there's always a wealth of violations that the inspector can't help but trip over or slip in :)

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i agree, it's been very long time since i had an ac sensor in my hands. i have seen few on some old conveyors but they seam to be so rare nowdays. it all depends on situation (load size, distances, portability). dc is easier to work with, safer to touch (i've seen technicians making shorts on purpose, just to identify wires, it works because psu folds back on short and restart automatically), safer when something goes wrong (pinched cable etc.), much faster response (with DC you can easily find sensor that can switch output 1000 times per second - or more), no problemes with compliance, no problem with capacitance or leakage, sensors are cheaper, there is normaly less problems if DC wiring is installed near or along communication signals etc. using DC also means everything is 24V while on AC it depends (110V in America, 220-250V in Europe etc.), just make sure to think about it before shiping machines internationaly. when it comes to outputs, thing could look diferent, specially if the load and distance to load are bigger. in my case this wasn't the issue in at least 6-7 years so all outputs i use these days are DC. if there is still one odd ball that needs AC, usually cheap relay does the job. solenoids on penumatic valves are tipically tiny loads, on hydraulic they can be bigger (last one i used on 24V was drawing some 1.15A). this should be considered when designing system (psu size, return wire ampacity, voltage drop etc.) so if there is more than 2-3 valves, AC solenoids would quickly become more attractive. also you would want to keep things common because it could happen that someone replaces AC valve with DC look a like.

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