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Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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 Posted: Sun Feb 19, 2012 12:59 pm Post subject: Starter motor as generator |
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A friend of mine asked me a question I didn't know the answer to.
If the starter contactor sticks on, I assume it would act like a
generator, and there would be a current path back to the bus.
Would the starter motor spin fast enough to generate a high enough
voltage to hurt anything ?
The overvoltage circuit would kick in and disconnect the alternator,
but that would do nothing to help.
Is an indication the starter contactor is engaged useful ? It still
wouldn't indicate if the bendix gear stuck, just the starter
contactor, so still not always an indicator.
Is there enough noise from the bendix gear and starter being overspun
by the engine to alert the pilot ?
Jeff Page
Dream Aircraft Tundra #10
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 19, 2012 5:30 pm Post subject: Starter motor as generator |
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At 02:58 PM 2/19/2012, you wrote:
| Quote: |
A friend of mine asked me a question I didn't know the answer to.
If the starter contactor sticks on, I assume it would act like a
generator, and there would be a current path back to the bus.
|
No, all starters must be fitted with an over-running
disconnect clutch of some kind. Should a pinion gear
left solidly hooked to the starter's gearing it would
probably strip the armature pinion or spin windings
out of the armature slots
If it's a series wound starter, it will indeed spin
fast but because it is taking current out of the battery
an not because the engine is back-driving it. If you
browse through old patents on 'starter drives' you'll
an interesting array of de-clutching mechanisms designed
to prevent back driving. But as early as 1944 I found
the use of a 'sprag clutch' to prevent the starter from
being back-driven by the engine.
Figure 2 of http://www.freepatentsonline.com/2333765.pdf
http://en.wikipedia.org/wiki/Sprag_clutch
| Quote: | Would the starter motor spin fast enough to generate a high enough
voltage to hurt anything ?
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A stuck starter is an unloaded motor, not a driven
generator.
| Quote: | The overvoltage circuit would kick in and disconnect the alternator,
but that would do nothing to help.
|
Moot point. Such a condition does not generate
an OV.
| Quote: | Is an indication the starter contactor is engaged useful ? It still
wouldn't indicate if the bendix gear stuck, just the starter
contactor, so still not always an indicator.
|
Starter engagement disconnects are very simple mechanisms
but they're right up on the front of the engine to get
rained on, dusty, etc. If the pinion uses acceleration
inertia to engage it then it's a good candidate for
yearly tear down, cleaning and re-lubrication.
| Quote: | Is there enough noise from the bendix gear and starter being overspun
by the engine to alert the pilot ?
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Maybe not. If your starter is a modern device
with a solenoid driven pinion gear engagement,
a stuck starter contactor is about the only
thing that can cause the pinion to hang in the
engaged position. I've hear of airshow pilots
flying hopped up airplanes suffering a hung
pinion and flew their routine with the pinion
spinning at a gazaillion rpm. Didn't heare
it before takeoff but had a starter and ring
gear replacement job to do on the ground.
Modern, low area, high pressure, intermittent
starter contactors energized with a robust
battery rarely hang. Further, these contactors
often have an "I" terminal which can be used to
driven a "starter energized" light.
http://www.aeroelectric.com/Pictures/Contactors/s702wire.jpg
Bob . . .
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nuckolls.bob(at)aeroelect
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| Posted: Sun Feb 19, 2012 5:42 pm Post subject: Starter motor as generator |
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At 02:58 PM 2/19/2012, you wrote:
P.S. After sending the earlier reply I recalled the
origins of the OV legend. In airplanes with ammeters
(either minus-zero-plus battery ammeters =OR= an
alternator loadmeter), immediately after engine start
the ammeter would be hard-over feigning a strong effort
to 'recharge' the battery. In fact, the voltage regulator
was dumping current from alternator in response to
what it believed as a badly discharged battery. In
point of fact, the battery was fine but being heavily
loaded . . . perhaps to some value greater than alternator
output, by the free running starter motor.
This very high alternator output was mis-interpreted as
an OV event-in-progress as opposed to a misguided regulator
thinking it was being asked to recharge a dead battery.
Bob . . .
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Tundra10
Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario
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| Posted: Sun Feb 19, 2012 8:58 pm Post subject: Starter motor as generator |
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Bob,
You are full of wisdom as always.
Again, I appreciate you teaching me.
Thanks !
Jeff Page
Dream Aircraft Tundra #10
Do not archive
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