Toobuilder
Well Known Member
Seems like we read an awful lot about alternator issues on this forum. I’m wondering why we seem to suffer disproportionately with these issues compared to the rest of the motorsports/industrial world? After all, when was the last time you had the alternator in your car go rogue and fry the computer? There has been a lot of attention given to OV crowbar protection, fuses, alternator shutdown schemes, etc, but shouldn’t we also focus on what can we do to minimize the frequency of failure in the first place?
I’m no expert, but it seems we are turning these units much higher than needed. Reading suggests that the output curve flattens out at around 6000 RPM, so if we take the typical 3.25 overdrive ratio of the large Lycoming pulley (9.75/3.0), we see we are spinning the alternator at almost 8800 RPM at takeoff (and maybe cruise for you FP guys).
To that end, I’ve added a 4.5 inch pulley to my alternator to slow it down. The resulting 2.16 ratio gives me a nice 5850 at takeoff and a comfortable 4750 at cruise.
Will it work? Only testing will tell. But I refuse to accept the dismal record of electrical systems installed in aircraft as “normal”. Alternator technology was figured out decades ago and there have been billions of hours racked up with what I suspect is a very acceptable MTBF rate. There MUST be something we are doing wrong. Off the top of my head, I suspect we could;
Slow the RPM (big pulley)
Cool the regulator (blast tube)
Proper teminations (General Practices)
Anybody have any other hints, tips, speculation?
BTW, the reason I picked the 4.5 inch pulley was simply a matter of availability. It was $29 bucks at Summit Racing so worth the experiment. Given a choice I would have gone a touch smaller.
I’m no expert, but it seems we are turning these units much higher than needed. Reading suggests that the output curve flattens out at around 6000 RPM, so if we take the typical 3.25 overdrive ratio of the large Lycoming pulley (9.75/3.0), we see we are spinning the alternator at almost 8800 RPM at takeoff (and maybe cruise for you FP guys).
To that end, I’ve added a 4.5 inch pulley to my alternator to slow it down. The resulting 2.16 ratio gives me a nice 5850 at takeoff and a comfortable 4750 at cruise.
Will it work? Only testing will tell. But I refuse to accept the dismal record of electrical systems installed in aircraft as “normal”. Alternator technology was figured out decades ago and there have been billions of hours racked up with what I suspect is a very acceptable MTBF rate. There MUST be something we are doing wrong. Off the top of my head, I suspect we could;
Slow the RPM (big pulley)
Cool the regulator (blast tube)
Proper teminations (General Practices)
Anybody have any other hints, tips, speculation?
BTW, the reason I picked the 4.5 inch pulley was simply a matter of availability. It was $29 bucks at Summit Racing so worth the experiment. Given a choice I would have gone a touch smaller.
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