Richard J Kinch wrote:
Don Foreman writes:


At 240
volts and 10KW, the avg (DC) current is about 41.6 amps, while the AC
ripple current from rectified single-phase AC will probably be well
under 1 amp. The difference between this and "pure" DC excitation
would not be noticable.


The original post question betrays too much ignorance of what is going on.
10 kW is just to much heat to dissipate on any kind of duty cycle. He may
be trying to duplicate a poorly cobbled prior repair or design.

It's always dangerous to accuse someone of ignorance.

A little research on the web shows that a 10KW/240VDC
magnet is actually a small one. Here's a company that
sells used round lifting magnets:

http://www.caseyusa.com/resultspho2....d=DC%20MAGNETS

Here's the specifications on the biggest one I could
find:

http://www.caseyusa.com/resultselec4...t2=94&cat3=231

That's about 46KW. I guess there must be a way for
the heat to dissipate.

I have an old Audel book with a chapter on lifting
magnets. It describes the controllers and how the
high induced voltage is handled. There's no mention
of current or voltage ratings, but the relative size
of the controller components and connections would
indicate something in the 50-100 amp range.

Besides, that many amps into that big an inductor has gotta produce one
wollop of a surge when you switch it off. Saying you can absorb that with
a reversed diode is like saying you can brake a big diesel engine to a dead
stop by putting your hand on the flywheel. I suspect part of the reason
for the DC generator is that you can clutch out the prime mover, and the
generator becomes a motor to slowly dissipate the stored energy in the
inductor.

The Audel book does not speak of any use of the prime
mover to control the current, just a controller with
contactors and resistors.