mikem
September 18th 03, 05:38 PM
> List message posted by: Scott Bilinski >
>
> Is there any way to tell if your alternator is on its last leg?
> What are the signs?
Stator: Failure in Stator rectifiers. I have had diodes in the
stator rectifier fail open-circuit, meaning that the alternator
delivers only one-of-three or two-of-three phases. This causes a
reduction of max potential output current, so that the
alternator may not be able to carry the normal load.
The charging system still sort-of-appears-to-work, making
diagnosis difficult. If you monitor bus voltage with an
accurate dc voltmeter measured with engine running > 1500rpm,
most of the loads switched off (light load), then the bus
voltage would be normal (14.2-14.5V). The Voltage Regulator (VR)
is still doing its job by chopping the field current down to a
duty cycle of 5-50%. A +- ampmeter would show no net charge or
discharge in the steady state (proper indication).
If you turn on all of the loads (especially landing/taxi/nav
lights), then the bus voltage will sag due to the inablility of
the alternator to supply these loads. Under these conditions,
the bus voltage may sag to 12.6-13.5V, and the ammeter would
show a net discharge from the battery in the steady state. The
VR is likely doing its job by applying full (100%, unchopped)
field to the rotor, but with one third to two thirds of
the stator gone, the alternator cant keep up with the load...
Eventually, the battery will be chronically undercharged,
leading to cranking problems, especially in cold weather.
There is also a marked increase in bus ripple. If you normally
hear a (hopefully faint if you did your audio grounds right)
alternator whine in your headphone audio, then after the
stator/diode failure, the whine gets a lot louder, and lower
pitched.
Most stator failures are caused when the soldered electrical
connection between the stator wire and the diode stack comes
apart due to heat/vibration. Sometimes this is the only
problem, and can quite easily be repaired, but this requires
total disassembly of the alternator. You have to use a "hard
solder" to repair these connections (not Pb/Sn electronic
solder).
Rotor: The brushes can wear down to where they no longer "ride"
on the rotor slip rings. As they approach the wear limits, the
contact pressure between the brush and the slip ring is
insufficient to keep the slip ring clean, meaning that the brush
contact to the slip ring gets intermittent and electrically
"noisy". This reduces the available field current.
This manifests as "reduced max output current", with similar
behavior of the dc voltmeter and ammeter when all of the loads
are switched on. The noise in the headsets may sound different,
however. As before, this is not the VRs fault. It will probably
still try to do its job of keeping the bus voltage at 14.25V by
applying 100% field duty cycle, but the high resistance of the
brushes against the slip rings prevents the alternator from
developing much field current, thereby reducing output. This can
also manifest as the "pulsating ammeter syndrome", along with
unnaturlly high resistance in the external field wiring...
The alternator whine in headset audio will sound less musical,
more like hash (bacon sizzling). If you have an ADF or AM
radio, tune to the low end of the band (200Khz or 540Khz,
respectively) and you might hear the alternator hash. If you do
remember to this before you have alternator problems, the
increase in direct audio and/or ADF/AM radio RF noise as the
brushes wear down can give you a clue as to what is going on.
I have seen Lorans quit as the RF hash from the alternator
increases over time and finally overwhelms the Loran signals at
100KHz.
The fix is disassembly of the alternator, installing new
brushes, cleaning or possibly turning the rotor slip rings on a
lathe. This is part of a normal alternator overhaul done by
overhaul shops.
Mechanical: As bearings wear, the sideways pull of the belt can
cause the rotor to hit the stator pole piece causing
catastrophic failure and total destruction. Bearings are
normally replaced during overhaul. At annual, remove the belt,
and see how much play is in the bearings.
MikeM
Skylane '1MM
Pacer '00Z
>
> Is there any way to tell if your alternator is on its last leg?
> What are the signs?
Stator: Failure in Stator rectifiers. I have had diodes in the
stator rectifier fail open-circuit, meaning that the alternator
delivers only one-of-three or two-of-three phases. This causes a
reduction of max potential output current, so that the
alternator may not be able to carry the normal load.
The charging system still sort-of-appears-to-work, making
diagnosis difficult. If you monitor bus voltage with an
accurate dc voltmeter measured with engine running > 1500rpm,
most of the loads switched off (light load), then the bus
voltage would be normal (14.2-14.5V). The Voltage Regulator (VR)
is still doing its job by chopping the field current down to a
duty cycle of 5-50%. A +- ampmeter would show no net charge or
discharge in the steady state (proper indication).
If you turn on all of the loads (especially landing/taxi/nav
lights), then the bus voltage will sag due to the inablility of
the alternator to supply these loads. Under these conditions,
the bus voltage may sag to 12.6-13.5V, and the ammeter would
show a net discharge from the battery in the steady state. The
VR is likely doing its job by applying full (100%, unchopped)
field to the rotor, but with one third to two thirds of
the stator gone, the alternator cant keep up with the load...
Eventually, the battery will be chronically undercharged,
leading to cranking problems, especially in cold weather.
There is also a marked increase in bus ripple. If you normally
hear a (hopefully faint if you did your audio grounds right)
alternator whine in your headphone audio, then after the
stator/diode failure, the whine gets a lot louder, and lower
pitched.
Most stator failures are caused when the soldered electrical
connection between the stator wire and the diode stack comes
apart due to heat/vibration. Sometimes this is the only
problem, and can quite easily be repaired, but this requires
total disassembly of the alternator. You have to use a "hard
solder" to repair these connections (not Pb/Sn electronic
solder).
Rotor: The brushes can wear down to where they no longer "ride"
on the rotor slip rings. As they approach the wear limits, the
contact pressure between the brush and the slip ring is
insufficient to keep the slip ring clean, meaning that the brush
contact to the slip ring gets intermittent and electrically
"noisy". This reduces the available field current.
This manifests as "reduced max output current", with similar
behavior of the dc voltmeter and ammeter when all of the loads
are switched on. The noise in the headsets may sound different,
however. As before, this is not the VRs fault. It will probably
still try to do its job of keeping the bus voltage at 14.25V by
applying 100% field duty cycle, but the high resistance of the
brushes against the slip rings prevents the alternator from
developing much field current, thereby reducing output. This can
also manifest as the "pulsating ammeter syndrome", along with
unnaturlly high resistance in the external field wiring...
The alternator whine in headset audio will sound less musical,
more like hash (bacon sizzling). If you have an ADF or AM
radio, tune to the low end of the band (200Khz or 540Khz,
respectively) and you might hear the alternator hash. If you do
remember to this before you have alternator problems, the
increase in direct audio and/or ADF/AM radio RF noise as the
brushes wear down can give you a clue as to what is going on.
I have seen Lorans quit as the RF hash from the alternator
increases over time and finally overwhelms the Loran signals at
100KHz.
The fix is disassembly of the alternator, installing new
brushes, cleaning or possibly turning the rotor slip rings on a
lathe. This is part of a normal alternator overhaul done by
overhaul shops.
Mechanical: As bearings wear, the sideways pull of the belt can
cause the rotor to hit the stator pole piece causing
catastrophic failure and total destruction. Bearings are
normally replaced during overhaul. At annual, remove the belt,
and see how much play is in the bearings.
MikeM
Skylane '1MM
Pacer '00Z