My story is about aircraft equipment using 400Hz three-phase power. A piece of equipment was returned because it wasn’t working. It had burn marks in strange places, even though there was no voltage higher than the usual +/-15V on the backplane. It was an absolute mystery, suggestive of misuse.
After a lot of thinking about the circuitry, it finally occurred to me that a simple fault could apply a feedback path to the main switch-on relay, making it chatter. That is, as soon as it was switched on, a signal turned the relay off, and when power was lost, the relay went on again.
Relay chatter can be fast, on the order of 1 ms to 10 ms, and produces a fast risetime, or arcing (if there is inductance in the circuit), as the contacts make and break. Relay chatter generally results in high RF content, which is sometimes used as a source of random RF interference.
I recollected that a few months earlier, a substation power transformer went faulty, with the ground open circuit. That doesn't sound like a lot, but since the transformer supplied several buildings, with each of three phases supplying random loads in each building, the missing ground meant that the center-tap of the transformer could swing anywhere in -- and even out of -- the delta of three phases. So, equipment could be over-voltaged from normal 240V to way over 440V. A lot of PCs and test gear were destroyed!
What was happening with the aircraft equipment was related. When the power-on relay chattered, each of the contacts was making and breaking at slightly different times. The L-C input filter was resonating all over the place, generating enormous voltages, which were breaking through where they should never have been, causing arc scorches. This had never happened in earlier equipment, but in the model I was analyzing, some engineering upgrade had added inductors to the input filter.
The cure was to revise the logic a little, so that the original fault would not cause relay chatter.
—This entry was submitted by Rod Dalitz and edited by Rob Spiegel.
Rod Dalitz studied physics at university in England. After graduating, he worked in electronics: in precision weighing, brewery automation, and for the last 25 years, in avionics. He is now retired.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
I remember as a kid pulling apart an old phone from the '20's (I assume, perhaps earlier). It had a relay that had it's coil connected thru the NC contacts. Figuring out that circuit was my first experience with Engineering (it impressed me at the time repurposing something by using it "wrong", and it's probably why I became an EE later). To spell it out, that relay was actually used as a buzzer (no bell on this phone). Provide DC and it closes, opening the circuit making it start all over (i.e. makes a buzzing noise).
Of course, what impressed me more was accidentally touching the input contacts while removing power on this low voltage DC circuit. The shock made me respect the physics behind inductors and their ability to do damage. Over the years I've seen similar circuits elsewhere, usually designed to boost voltage or generate AC (including HV for the spark plugs in older cars).
I've read several articles written by IT guys and it's amazing what we who don't know how to use our computers properly do to make them not work. I also find it a little frustrating when they tell me to shut it off and turn it back on before they will help me. I have also learned they have some interesting terms for people like me with problems I have. It's called a "PEBKAC" errlor. Problem Exists Between Keyboard And Chair. Or it's an "I" "D" "10" "T" error. you might need to write that one down on a piece of paper.
Also, I think it is smart to remember that if you are a hammer, everything looks like a nail. We need to be careful when trying to solve a problem not to play directly toward our strengths but let the problem lead us.
I'm intrgued by one particular line in the story: ..."some engineering upgrade had added inductors to the input filter." I'm always leery of upgrades. Seems like, more often than not, upgrades end up downgrading some aspect of a product's performance.
Chatter in any realy control circuit can cause all kinds of problems, that has been true for a long time. But the very most damaging occurrence that I am aware of was when my employer shipped a large hydraulic system with a 150HP motor to a customer in a "more primitive" country. The system used a two-speed starter system, which was intended to reduce the starting inrush. The first time that an attempt was made to start the motor at the customers site, the power supply line regulation was so poor that the starter went into a chattering mode and all of the contacts were destroyed. The customer was quite unhappy.
This happened a few weeks prior to my joining the firm, so I was one of few who were not stained by the terrible string of errors that led to that disaster. The working fix was to replace that starter with a "soft start" system from a company named "Safco", based in south Africa. The downside was that we ate the cost, and had to install the replacement system at our expense. From that point forard I have been careful about working with motor starting inrush currents.
That's funny, Jenn. I'll be you're glad you figured out it wasn't plugged in before IT showed up. It is the simple things. Over and over again we see that with the Sherlock Ohms blogs. One of my favorites is the guy who carried magnetic disks in his pocket that also held a magnetic screwdriver.
When trouble shooting I always remember these two mottos:
1 - If at first you don't succeed, ask yourself, "Is it plugged in?" People often get offended when I start with that but then they get kind of sheepish when I do plug it in.
2 - "The malfunction is at the junction'" covers about 90% of remaining electrical / elecgtronic problems.
Great advice, @gsmith120. More often than not, common sense flies out the window when trying to solve a technical problem. Is the thinking that if you have a highly technical problem, the answer must be highly techical? It's amazing the way the brain works for some people.
Just today, for instance, my printer would not work. I restarted it a couple of times, checked the paper tray, made sure the ink cartridges were in working order. It took 5 solid minutes before I picked up the phone to call IT. It was then I realized that the printer was not plugged into my laptop. Sometimes it IS the little things.
Naperlou, you are so right about the small things that cause problems. I have found more times than not very simple and often over looked things causes the biggest problems. In community college as a part of a troubleshooting exercise the teacher stated if something isn't working check the simple things first. If the equipment will not turn-on, make sure it is plugged in and power switch "on".
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