REFLECTOR:Alternator Failure Analysis
Ronnie Brown
reflector@tvbf.org
Fri, 19 Sep 2003 08:26:44 -0400
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I found this excellent note on Aeroelectric.
Time: 09:22:42 AM PST US=20
From: MikeM <mladejov@ced.utah.edu>=20
Subject: Re: Signs of alternator starting to fail=20
=20
--> AeroElectric-List message posted by: MikeM =
<mladejov@ced.utah.edu>
=20
> List message posted by: Scott Bilinski <bilinski@??????.com>
>
> Is there any way to tell if your alternator is on its last leg?
> What are the signs?
=20
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.
=20
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 +- ammeter would show no net charge or
discharge in the steady state (proper indication).
=20
If you turn on all of the loads (especially landing/taxi/nav
lights), then the bus voltage will sag due to the inability 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.
=20
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.
=20
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).
=20
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.
=20
Also - the Field Terminal to Alternator Frame resistance should be =
around 4-6 ohms. This will vary as the alternator stator is rotated. I =
found this high (>100 ohms) on a recent alternator failure and =
discovered that the slip ring to field wire had broken. (Ronnie)
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
unnaturally high resistance in the external field wiring...
=20
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.
=20
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.
=20
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.
=20
One Polack to another...
=20
Mike Mladejovsky, Phd
Dr Alternator
Skylane '1MM
Pacer '00Z
=20
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<DIV><FONT size=3D2>
<DIV><FONT size=3D2>I found this excellent note on =
Aeroelectric.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" border=3D0>
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<TR>
<TD align=3Dleft width=3D"15%"><B><FONT face=3D"times new roman, =
times"=20
color=3D#000000 size=3D3>Time: </FONT></B></TD>
<TD align=3Dleft width=3D"85%"></FONT><FONT face=3D"times new roman, =
times"=20
color=3D#8f0000 size=3D3><B><I>09:22:42 AM PST=20
US</I></B></FONT></TD></TR></TBODY></TABLE>
<TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" border=3D0>
<TBODY>
<TR>
<TD align=3Dleft width=3D"15%"><B><FONT face=3D"times new roman, =
times"=20
color=3D#000000 size=3D3>From: </FONT></B></TD>
<TD align=3Dleft width=3D"85%"></FONT><FONT face=3D"times new roman, =
times"=20
color=3D#007000 size=3D3><B><I>MikeM <<A=20
=
href=3D"mailto:mladejov@ced.utah.edu">mladejov@ced.utah.edu</A>></I></=
B></FONT></TD></TR></TBODY></TABLE>
<TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" border=3D0>
<TBODY>
<TR>
<TD align=3Dleft width=3D"15%"><FONT face=3D"times new roman, times" =
color=3D#000000 size=3D3><B>Subject: </B></FONT></TD>
<TD align=3Dleft width=3D"85%"><FONT face=3D"times new roman, times" =
color=3D#707000 size=3D3><I><B><U>Re: Signs of alternator starting =
to=20
fail</U></B></I></FONT></TD></TR></TBODY></TABLE>
<DIV> <BR> =
-->=20
AeroElectric-List message posted by: MikeM <<A=20
href=3D"mailto:mladejov@ced.utah.edu">mladejov@ced.utah.edu</A>><BR>&n=
bsp; =20
<BR> > List message posted by: Scott =
Bilinski=20
<<A=20
href=3D"mailto:bilinski@??????.com">bilinski@??????.com</A>><BR> =
=20
><BR> > Is there any way to tell if =
your=20
alternator is on its last leg?<BR> > =
What are=20
the signs?<BR> =
<BR> =20
Stator: Failure in Stator rectifiers. I have had diodes in=20
the<BR> stator rectifier fail =
open-circuit,=20
meaning that the alternator<BR> delivers =
only=20
one-of-three or two-of-three phases. This causes=20
a<BR> reduction of max potential output =
current,=20
so that the<BR> alternator may not be able =
to=20
carry the normal load.<BR> =20
<BR> The charging system still=20
sort-of-appears-to-work, making<BR> =
diagnosis=20
difficult. If you monitor bus voltage with=20
an<BR> accurate dc voltmeter measured with =
engine=20
running > 1500rpm,<BR> most of the =
loads=20
switched off (light load), then the =
bus<BR> =20
voltage would be normal (14.2-14.5V). The Voltage Regulator=20
(VR)<BR> is still doing its job by =
chopping the=20
field current down to a<BR> duty cycle of =
5-50%. A=20
+- ammeter would show no net charge or<BR> =
discharge in the steady state (proper=20
indication).<BR> =20
<BR> If you turn on all of the loads =
(especially=20
landing/taxi/nav<BR> lights), then the bus =
voltage=20
will sag due to the inability of<BR> the=20
alternator to supply these loads. Under these=20
conditions,<BR> the bus voltage may sag to =
12.6-13.5V, and the ammeter would<BR> show =
a net=20
discharge from the battery in the steady state.=20
The<BR> VR is likely doing its job by =
applying=20
full (100%, unchopped)<BR> field to the =
rotor, but=20
with one third to two thirds of<BR> the =
stator=20
gone, the alternator cant keep up with the=20
load...<BR> Eventually, the battery will =
be=20
chronically undercharged,<BR> leading to =
cranking=20
problems, especially in cold weather.<BR> =20
<BR> There is also a marked increase in =
bus=20
ripple. If you normally<BR> hear a =
(hopefully=20
faint if you did your audio grounds =
right)<BR> =20
alternator whine in your headphone audio, then after=20
the<BR> stator/diode failure, the whine =
gets a lot=20
louder, and lower<BR> =20
pitched.<BR> =
<BR> =20
Most stator failures are caused when the soldered=20
electrical<BR> connection between the =
stator wire=20
and the diode stack comes<BR> apart due to =
heat/vibration. Sometimes this is the=20
only<BR> problem, and can quite easily be=20
repaired, but this requires<BR> total =
disassembly=20
of the alternator. You have to use a =
"hard<BR> =20
solder" to repair these connections (not Pb/Sn=20
electronic<BR> =20
solder).<BR> =
<BR> =20
Rotor: The brushes can wear down to where they no longer=20
"ride"<BR> on the rotor slip rings. As =
they=20
approach the wear limits, the<BR> contact =
pressure=20
between the brush and the slip ring is<BR> =
insufficient to keep the slip ring clean, meaning that the=20
brush<BR> contact to the slip ring gets=20
intermittent and electrically<BR> "noisy". =
This=20
reduces the available field current.<BR> =
</DIV>
<DIV>Also - the Field Terminal to Alternator Frame resistance should be =
around=20
4-6 ohms. This will vary as the alternator stator is =
rotated. I=20
found this high (>100 ohms) on a recent =
alternator=20
failure and discovered that the slip ring to field wire had broken.=20
(Ronnie)</DIV>
<DIV><FONT size=3D2></FONT><BR> This =
manifests as=20
"reduced max output current", with =
similar<BR> =20
behavior of the dc voltmeter and ammeter when all of the=20
loads<BR> are switched on. The noise in =
the=20
headsets may sound different,<BR> however. =
As=20
before, this is not the VRs fault. It will=20
probably<BR> still try to do its job of =
keeping=20
the bus voltage at 14.25V by<BR> applying =
100%=20
field duty cycle, but the high resistance of=20
the<BR> brushes against the slip rings =
prevents=20
the alternator from<BR> developing much =
field=20
current, thereby reducing output. This =
can<BR> =20
also manifest as the "pulsating ammeter syndrome", along=20
with<BR> unnaturally high resistance in =
the=20
external field wiring...<BR> =20
<BR> The alternator whine in headset audio =
will=20
sound less musical,<BR> more like hash =
(bacon=20
sizzling). If you have an ADF or AM<BR> =
radio,=20
tune to the low end of the band (200Khz or=20
540Khz,<BR> respectively) and you might =
hear the=20
alternator hash. If you do<BR> remember to =
this=20
before you have alternator problems, =
the<BR> =20
increase in direct audio and/or ADF/AM radio RF noise as=20
the<BR> brushes wear down can give you a =
clue as=20
to what is going on.<BR> I have seen =
Lorans quit=20
as the RF hash from the alternator<BR> =
increases=20
over time and finally overwhelms the Loran signals=20
at<BR> =
100KHz.<BR> =20
<BR> The fix is disassembly of the =
alternator,=20
installing new<BR> brushes, cleaning or =
possibly=20
turning the rotor slip rings on a<BR> =
lathe. This=20
is part of a normal alternator overhaul done=20
by<BR> overhaul=20
shops.<BR> =
<BR> =20
Mechanical: As bearings wear, the sideways pull of the belt=20
can<BR> cause the rotor to hit the stator =
pole=20
piece causing<BR> catastrophic failure and =
total=20
destruction. Bearings are<BR> normally =
replaced=20
during overhaul. At annual, remove the =
belt,<BR> =20
and see how much play is in the =
bearings.<BR> =20
<BR> One Polack to=20
another...<BR> =
<BR> =20
Mike Mladejovsky, Phd<BR> Dr=20
Alternator<BR> Skylane=20
'1MM<BR> Pacer=20
'00Z<BR> </DIV></FONT></DIV></BODY></HTML>
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