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.



It's a bit dangerous to accuse another poster of too much
ignorance when you are not too sure of your own facts.

Jim Stewart has already dealt with your comment on power
rating. Your comment on reverse diodes is equally misleading.

In an earlier post I recommended that the reverse diode be
rated for at least the nominal coil voltage and current.

This is because, at the instant of disconnection, the current
that was flowing in the inductor is diverted to and flows through the
forward direction of the reverse diode. This current then decays at a
rate determined by L/R of the magnet coil. The stored energy (1/2 L x
I squared) is dissipated in the resistance of the magnet coil - not
the diode.

The reverse rating is determined by the voltage that appears
across the magnet in normal operation.

Jim