Alternator problem

Nathan Young writes:

Common culprits to the Cherokee electrical system include:
1. Field current breaker develops corrosion (it is probably 30 years
old, so not surprising), and begins to intermittently introduce a
resistance into the circuit. This allows a voltage drop across the
breaker, so the VR turns up the bus voltage until it sees 14V. The
problem here is that because there is voltage drop across the breaker,
the master bus might be sitting at 15,16,17V. Eventually the plane
hits some turbulence and the breaker gets moved a slight amount, the
resistance goes back to zero, and the overvoltage regulator sees 16V,
so it goes open circuit, which cuts the voltage to the VR, which then
has no power to give to the altenator field, so the alternator goes
offline. Solution: Replace the field breaker.

Does this alternator have auxiliary diodes? Automotive alternators
have two sets of diodes. The six main ones are in a three-phase
array that provides the output.

There are also 3 small {~5A} diodes to provide field excitation. At
startup, the battery will supply the needed field excitation, but
as soon as the alternator is working, they take over the job. (They
are integral to the idiot line scheme in cars, however, so may not
exist in aircraft units.)



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

David Lesher wrote:


There are also 3 small {~5A} diodes to provide field excitation. At
startup, the battery will supply the needed field excitation, but
as soon as the alternator is working, they take over the job. (They
are integral to the idiot line scheme in cars, however, so may not
exist in aircraft units.)

Negative. Neither the Prestolite (Ford style) alternators used on
Cessnas, nor the Chrysler alternators used in Pipers have the additonal
"diode trio". Only the aftermarket STCed INTERAV conversion uses
a Motorola alternator with the diode trio.

MikeM
Skylane '1MM