Eric said:
Greetings John,
I built a power supply for a CNC machine retrofit using the
original 3 phase xmfr and 5 full wave rectifiers. I used 4 1/2 of the
5 because the xmfr was actually 9 windings wound on 3 cores.
Without drawing pictures, I am having a little difficulty
imaging that schematic. Is the secondary array essentially
3 Ys or 3 Deltas that all feed one DC supply? If this is
the case, and you use separate rectifiers for each
secondary, then the impedance of those secondaries acts as
the current balancing resistance (and inductance), to help
force all the rectifiers to share the load current.
Anyway,
the machine is only using the original contactors, spindle motor,
servos and the xmfr that the machine came with new . All the other
electric and electronic parts are new. I did not use any resistance
with the rectifiers to force the diodes to share the current more
equally because I didn't know I should. The machine runs fine and the
conversion has hundreds of hours on it. I think the rectifiers are
rated at 20 amps. The servos draw 15 amps max before the control shots
down. This has only happened once when I jammed one of the axes.
So the 20 amp rated rectifier is usually running well below
its rating. That's good.
The
xmfr is wired to the rectifiers with 10 gauge wires a little over 3
feet long each.
Some questions: How does the resistance help spread the load across
all the diodes?
It just adds additional voltage drop to he rectification
that is proportional to the instantaneous current, unlike
the very nonlinear and negative tempco drop of the diode
junctions. It helps to make the overall drop more
proportional to current and with a less negative tempco.
It doesn't force perfect sharing, but only makes the sharing
less unequal.
Why should the resistance be located between the xmfr and the
diodes?
I was afraid you would ask me that. Ideally, you would add
a bit of linear (and optimally, with a positive tempco)
resistance in series with each diode junction. I was
picturing the parallel rectifiers being fed from a single
secondary, and powering a single storage capacitor. So it
may work as well in series with any of the 4 legs of the
bridge, since each leg carries either all the AC current, or
all the DC current.
I was not taking the time to visualize all the permutations
and their relative performance, but just told you about the
first one I thought about. I stand by the requirement that
if there is one AC source, and one DC load, the sharing
resistors all need to be in the same led of the bridge.
Why haven't the diodes in my setup been destroyed? Is this because
of the long 10 gauge wires?
Perhaps it is because, most of the time, the average current
is pretty low, and the diodes have a fair thermal mass to
absorb the big thumps. Most rectifier diodes have
impressive surge current ratings.
And, yes, the wires are low value resistors with positive
temperature coefficient.