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Diodes are rated at average current. The peak that
they can handle is many times the average.
This is the result of the original usage of diodes,
as in power supplies, where rectifying current is a "part time" usage and they
are rated as such.
The average current in any single leg of a 3-phase
line is 1/sqr3 times the average current in a single phase line, including
rectified to DC.
Taking the 50 amp diodes as a unit in a 6
diode 3-phase bridge rectifier, the current through any pair of diodes is the DC
bridge output / sqr3.
The alternator windings are good for 60 amps DC
output so the diodes need to handle 60 / sqr3 = 34.6 amps.
Wolfgang
----- Original Message -----
Sent: Friday, October 19, 2012 8:28
AM
Subject: Re: TCM 60A
alternator--diodes
I wouldn't feel quite that confident in the replacement diodes. The
stator current flows in series through 2 diodes at any given time and the
current cycles between the diodes as the alternator rotates. So for a
portion of the time a given diode will be passing 100% of the current - but
not all the time since they "take turns." I don't know how the diode
package was rated, but I think the 50-amp rating is marginal for a 60-amp
alternator. Given that, the alternator will rarely, if ever, be asked to
deliver a full 60 amps, and there is certainly some margin built into the
diode rating. And the rating is a function of operating temperature, so
if the alternator stays cool, it's better. If you could find a diode
assembly rated at 60 amps or more it would be a good thing. The 50 amp
assembly will probably work okay, though. It's just not 50A X 6 = 300A.
And if you routinely fly IFR at night with everything turned on and lit
up....well, that's up to you.
Gary Casey
The diode assembly was sourced at my local shop
with a spare set of brushes for $20 total. These are 50A diodes (50A x 6
diodes) which is a robust replacement satisfactory for even the 100A
alternator. I had to remove the B+ stud from the old diode assembly and
press it into replacement diode package. The re-assembly took about 20
minutes and it fired up with 10A charge on low engine idle. This is much
higher charge at low idle than I can ever recall.
Paul
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