Re: [CAD_CAM_EDM_DRO] Re: Mariss' power supply circuit

jmkasunich wrote:

> Shorting the armature of a motor while it is spinning at high
> speed will also cause 50-100 times rated current to flow. And
> with a short, there is no current limit. I am personally aware
> of one case where the semiconductors in a DC drive failed,
> shorting the armature of a 25 HP motor while it was spinning
> a roller on a paper machine (high inertia load). The fault
> current generated enough braking torque to snap the shaft of
> the motor (around 2" in diameter)!

Due to poor installation of bus bars in a large coal-fired generating
station near here (Labadie plant near at Labadie, MO) two
sets of bus bars carrying the output of two 840 MVA alternator sets
at 14 KV flew off their insulators and shorted the two alternators
out. The alternators were ripped partly out of the floor (they weigh
hundreds of tons each) and they whole plant was severely wrecked.
Two men died, I think, one immediately and one later. The plant
was down for about 2 years, and the direct damage alone was
hundreds of millions of Dollars! There should have been
current limiting fuses installed right at the alternators, the two
bus bar sets should have been separated, and, of course, properly
sized supports for the bus bars should have been used.

When this thing popped, all the lights in town went down to a DIM glow
for about 10 seconds, and then came back up slowly. that slow voltage
rise clued me in that there had been a MASSIVE failure somewhere.
I didn't know what happened until the paper came the next day.

The rotors of these alternators are HUGE, of course, a solid billet
of steel about 18" diameter and 6 to 10 feet long! They mill a squared
spiral groove on the surface of the rotor, and wedge massive solid
copper bar into the groove for the rotating field winding. They run
something close to 1000 A at about 100 V on those rotor fields!

(Nice to see you here, John K.)

Jon