Posted on 02/01/2013 7:21:28 PM PST by fatima
Four people were hospitalized with minor injuries Friday night after an explosion on a bridge over the train tracks at 30th and Cecil B. Moore in Brewerytown.
The explosion was originally thought to have been an explosive device, but after an extensive investigation, police are calling it a "thermic event."
(Excerpt) Read more at nbcphiladelphia.com ...
Yeah that’s mostly my understanding (see previous post.) With AC you could mitigate the huge transmission loss you get with direct current, but as you increased the frequency you start to have higher and higher inductance losses in the windings of a DC motor. 60 Hz was too high for those motors. 25 Hz was a trade-off point between internal losses and line losses.
Thank you gentlemen.
I assumed that there would be a FReeper who actually knew the answer.
Two is an unexpected bonus.
Oh Dear ! All this thermic stuff beats the Hertz out of me.
“17,000 volts AC at 16 hertz can cause a lot of damage if bridged by frozen water. I wouldn’t want to be nearby.”
Amtrak catenary on the New York to Washington segment is energized at 11,000 volts 25hz.
Going under an overhead bridge, there’s not a great distance between the bridge structure and the wire itself — the insulators are often short there to maximize clearances between the wire and the rails. What happens is that water which under warmer conditions might drip bridge-to-wire freezes and then extends downward until it may touch the wires, then — zap!
You gave me a good laugh with that one.
25Hz power came from the design RPM of the Westinghouse Niagra Falls turbines, which were designed before alternating current became the standard. They could not change the hydraulics, so they had to chose between 16-2/3, 25, and 50Hz. 16-2/3 caused flicker, and 50 didn’t work for the motors of the day, so they settled on 25.
To #19. Overhead caternary?
Well, put up chicken wire to keep them darn cats and canaries off the electrical wires.
By the way, he you entered that word in the New Dictionary of corrupted American words?
Icce therms!
Ugh, make that icky therms!
Thermic Event = Massive Tig welder.
Probably not. There was no report of a seismic event at the same time.
Actually you have a point. If the DC motor is not a permanent magnet motor, then yes it will run on AC. But then, having the AC be low frequency helps mitigate inductive loss as you mention. AC would also avoid asymmetric corrosion issues that would make apparatus need servicing sooner than otherwise.
I could envision a kind of rotational rectifier apparatus, a synchronous motor driving a commutator, to get pulsating DC.
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