A couple of you were essentially correct: It is a resistor, or a current-limiting ‘shunt’, part of the protection network of my multi-thousand horsepower variable frequency drive.
Variable frequency? Yeah, three-phase AC motors are just about the simplest things imaginable: Iron, wire, ONE moving part. However, the simple ones have a single speed, fixed by the requency of the power system. My 7000 and 9000 horsepower units run at 1785 RPM, essentially 1800 RPM, which you can see is tied as a multiple to the 60 Hertz (used to be 60 CYCLES) frequency of the American power grid. If I want to vary the speed of that motor (and I do) then I need to be able to feed it something other than 60 Hertz.
We do this by turning the AC to DC, not a particularly difficult thing, then we use some transistors the size of paperback books to chop that DC up ind make the motor think it’s AC again. How fast we do the chopping controls what the motor thinks the frequency is, and it spins itself accordingly. That’s normal operation. The resistor int he picture has nothing to do with normal operation.
If the smart stuff in my drive sees a problem, it stops turning those transistors on. That takes care of stopping the motor. However, then I have all this DC electricity sitting there that I need to ditch. That’s where this resistor comes in. A protection circuit called a ‘crowbar’ (because it fakes the action of tossing a physical crowbar across two conductors, one negative, one positive) fires as an electronic switch and puts this resistor from the positive conductor to ground and from the negative conductor to ground. Bingo! The DC is gone!
Since these two conductors are 2000 volts to ground and they have some capacitors connected to them storing a charge, it’s a pretty good chunk of electricity. In practice, this resistor is in an enclosed cabinet, safe from being poked and prodded by curious dolts, but it still manages to gather a certain amount of dust. When the crowbar fires, that resistor gets hot really fast. Dust? Nope! Now it’s smoke.
The actual firing and discharge sequence takes a matter of a few thousandths of a second. It’s quite impressive.
And that’s just some of the fun stuff I get to play with.
Back in the late 50’s early 60’s an electrical engineer at Clyde Iron Works in Duluth Mn designed and patented a control system to vary the speed of ac motors so we could use ac on our swing drive for the cranes we built instead of dc. I was chief estimator and had GE Westinghouse and Reliance out bidding each other to get the control system design. In fact Reliance was the last one to win a bid and was $50,000 less than GE and Westinghouse. I had used Westinghouse in my estimate because the salesman said the customer wanted them. We found out after the order they didn’t care who we used so we gave it to reliance and pocketed an extra $50,000 per crane on a 3 crane order. Also you may have seen Clyde cranes as we sold the 500 ton to 1000 ton revolving barge mounted cranes to brown & root and ingram to use for offshore work in the gulf.
It’s descriptions of stuff like that, that do nothing to dispel my overly healthy respect for electricity.