On Tue, 22 Jun 2004 22:46:36 -0500, 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.

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.

Dump current can be no greater than excitation current, so diodes
than can deliver excitation can handle it no problem. 35 amp 600
volt diode bridges cost less than $2. The energy from the
collapsing field is mostly dissipated in the resistance of the load
just as excitation power is. Diode dissipation is fwd drop * current
whether exciting or dumping.

10 KW is about 13.4 HP. A 25% efficient 5 HP lawnmower engine
dissipates more heat than that, with far less surface area. Gae
thee to a scrapyard, behold the size of the electromagnet....

The original post asked for a replacement generator of specified power
for a system that has worked in service. Declaring him ignorant is
neither helpful nor defensible by technical premise you present.