I have one question about alternator check in Cessna 152. During engine run-up we should check whether alternator is working or not, so we switch it off and then the warning light should illuminate. But we do it on pretty high RPM (1700). Would this warning light illuminate if I switch off the alternator also on lower RPM i.e. idle?
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The problem is the light might come on on its own at idle, which is normal.
The warning light comes on when the system voltage is below 12.5 volts on a 14 volt system, and 25 volts on a 28 volt system. Automotive alternators (which most Lycomimg powered light aircraft have) need to spin at at least 2000 rpm to make adequate charging current to maintain system voltage under load.
The belt drive on the engine that overdrives the alternator relative to engine rpm may or may not maintain this rpm with the engine idling, so the bus voltage of the system may be just over 12.5/25 vdc or just below it, depending on the loads on the system (like radios and lights, etc.) and the alternator's condition.
The 152's POH specifically warns you that the light may come on after extended idle periods with electrical loads on the system, so this is to be expected, and it means that a test done with the switch can't be trusted to give a valid result (the light could be on because of "normal" idle low voltage, not because you switched it off).
Doing it at runup rpm just makes sure the alternator is spinning well above this marginal operating condition so it's producing normal charging current when you switch it off.
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$\begingroup$ Ok, I understand. One more question: so there is higher chance that the alternator light will illluminate "by itself" if the airplane is at idle for a long time; and there is almost no possibility, that the light won't illuminate if the alternator is switched off at idle engine RPM (assuming no warning system failure)? Because some time ago someone told me that the light won't illuminate if I switch the alternator off at engine RPM close to idle, because there is no load on it, what I assume is false. $\endgroup$– KonradCommented Jan 6, 2021 at 18:48
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2$\begingroup$ I believe so. All the light knows is there is less than 12.5 v at the bus. Whether that's because you turned the alternator off by switching off its field supply with the alt side of the master, or because it's current output isn't enough to keep the voltage over 12.5, shouldn't matter to the light itself, so it should come on if it's off at idle, then you switch off the alt. Maybe they mean if you turn the ALT and BAT sides off together you won't get a light. Anyway, try it yourself and see what happens. $\endgroup$– John KCommented Jan 6, 2021 at 20:47
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$\begingroup$ One more question, voltage in C152 electrical system is 24 V, so system allows voltage drop to 12,5 V? That's a lot imo. $\endgroup$– KonradCommented Jan 7, 2021 at 8:02
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1$\begingroup$ Oops you caught me out. I was talking about 14v systems and forgot that the Cessna changed their airplanes to 28v systems in the 80s I believe. I see that both 12 and 24 v batteries are available for 152s so it looks like the change was made sometime in mid production (with the 152 II?) but it's hard to find any info on what year the change was made. 150s were 14 v though. In any case, if the airplane has a 28 v system, the light would go on at 25v (12.5 x 2). Otherwise, same same. Note the battery voltage is 12/24, but the normal system voltage maintained by the alt is 14/28. $\endgroup$– John KCommented Jan 7, 2021 at 16:22
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2$\begingroup$ BTW the principle advantage of upgrading to 28 v is a tiny weight reduction because lighter thinner wires can be used to carry the same wattage, and a bit better electrical reliability (in theory) because the higher voltage makes circuits a bit more tolerant of degraded electrical connections. Operators were not too happy with the switch because if you had a fleet of 14 v airplanes you now had to also stock parts for 28 v systems if you acquired some 28 v ones. $\endgroup$– John KCommented Jan 7, 2021 at 16:29