Design Engineering Detectives

Having cut my teeth on pulp detective fiction, there's nothing I love better than a good mystery. But even Poirot couldn't hold a candle to the sleuthing abilities of engineers, who are frequently confronted by devilish design challenges and mysterious phenomenon of the most inexplicable sort.

To celebrate the most famous investigations into engineering's most diabolical real-world cases, we're introducing a new section in this issue called Sherlock Ohms — see page 22. Trust me, these stories are not only extremely entertaining, they may even teach you a thing or two, as many of the tips and tricks of the trade are learned and passed down in just this way.

Take the Case of the Shorted Rotor Turns, recounted by design engineer Kunal Ghosh:

"Early in my career, I worked for a government organization. I shared my room with a wiring technician, Mr. Singh. He was resourceful and taught me a lot. I did not know the extent of his fame until one day I came to work and found the room full of government officials. They had brought a small rotor from a dc motor that was part of a politically prestigious project. It had a shorted winding they wanted repaired with as little damage as possible.

Mr. Singh coolly informed them it would take two weeks to repair. They demanded he do it sooner and left. I was concerned, but the gleam in Mr. Singh's eye told me he was in command.

As usual he was in no hurry. He lit up a cigarette and smoked away in silence. I was growing impatient, but then he told me I was about to see a secret and he preferred I keep it that way. I agreed.

He took out half of a rusty hacksaw blade and a transformer and plugged it into the main ac power. He placed the rotor on the transformer and the blade on the uppermost pole. Nothing happened, so he turned the rotor to the next pole. This went on, pole by pole, until the blade started vibrating violently. He marked the pole with chalk of the shorted winding and verified the health of the remaining windings.

After 30 minutes, we had rewired the coil with new wire and the rotor was good as new. I asked Singh how it worked, but he had no technical explanation. It left me to unravel the mystery. An examination revealed that the inverted L-laminations facing each other that made up the top half of the transformer had been cut so that a V-groove was formed. In essence, this alteration allowed part of the magnetic flux to leak from the two faces of the groove.

When the windings were placed in the alternating magnetic field, nothing happened to the normal windings. But a shorted winding had an induced alternating current flowing through itself, which in turn created a second local alternating magnetic field just above the shorted coil. This alternating magnetic field caused the steel hacksaw blade to vibrate."

If you have a similar tale, tell us about it and you could get $100 and your Andy Warhol 15 minutes.

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