My story is about aircraft equipment using 400Hz three-phase power. A piece of equipment was returned because it wasnt 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.