Tuesday, August 2, 2022


Thank you Cornell Digital Library for preserving this image!

What's going on here? I can speak with some authority, because using this equipment was part of the mandatory and much-feared "Superlab" junior-year electrical engineering course. This is a control board for three-phase power machinery. Adjacent to the board is a three--phase motor hooked to the power grid, mechanically driving a three-phase generator as a small simulacra of a power generation station driven by a turbine. The task was to connect the generator to deliver power back to the grid! If the generator phases were synchronized with the grid, the voltages on each phase would be the same and you would just close a giant 3-bladed knife switch to connect it. If it was not synchronized, there was likely hundreds of volts difference on each phase, and closing the switch would result in sparksapoppin and fuzenblowen -- if not wires melting and arcing. Exciting!

So the task was to synchronize the phases by changing the speed of the generator to match the voltage, frequency and phase of the grid. This was done, if memory serves, by a giant rheostat (variable resistor) on one of the motor windings -- possibly what the model in the picture is controlling with their left hand.

Wikipedia has the details on what happens next (HT @SwiftOnSecurity):

So that is what the trio of lightbulbs shows: each one is connected across the corresponding phase of the generator and the grid. When it is dark, there is no voltage difference and the generator can be connected.

So the process is to carefully tune the generator speed to slow the beat frequency to nearly zero, displayed by a rotating sequence on the bulbs, then wait for the phase to match, indicated by dark bulbs. Then you could throw the knife switch and cackle like Dr. Frankenstein.

By the time I arrived at college, this equipment had been moved out of the lovely Rand Hall into the dank windowless basement of the cinderblock horror that was the engineering building, giving it even more of an evil mad science vibe.

(Note that the kid in the picture is doing it quite wrong: we were under strict instructions to put our non-dominant hand in our back pocket. Given the large number of non-insulated contacts at line voltages, this was a safety precaution: should we inadvertently complete a circuit, the current would be less likely to go through our heart and stop it prematurely.)

Here's another fun image, with the "circuit board" in the background. Note the short sleeves: I will bet that was a requirement to prevent catching in rotating machinery!

Postscript: after a pretty thorough web search I do believe the "superlab" designation has been retired at Cornell. Like Saturday morning lab sessions, this is probably a casualty of time and probably should not be mourned.

Tagged: vintage



RSSicon.png  RSS feed