Fact Check: Alternator Test

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Mtns2Skies

Final Approach
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Mtns2Skies
In the FBO recently I overheard a CFI telling their student NOT to cycle the alternator in the runup pad because it's bad for it.

I've always performed this test as a part of runup, turn the alternator off, confirm the ammeter drops and the low voltage light comes on, turn it back on, confirm the ammeter shows a charge.

I wouldn't think de-energizing and re-energizing the alternator field would cause issues... but am I wrong? Is the CFI wrong?

Asking for myself.
 
I just check it right after startup. It should be showing a pretty good charge after a pretty good draw, no? No need to cycle it at all. Otherwise whats the concern?
 
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Your ammeter should be showing a discharge before starting, and a charge thereafter. We never cycled it.
 
I don't cycle it (them) either. I just check the ammeter moves when putting a load like the landing light or pitot heat on momentarily.
 
If you are in a discharged state, turning on the alternator could put a sudden load (resistance) on the system, possibly causing the belt to slip or stressing the mounting, but other than that...smart voltage regulators will slowly energize, AFAIK we don’t use those in aviation.
Generally it’s not a good idea with mechanical devices to do anything abruptly. You should ease the throttle in, not slam it forward for example.
 
I turn mine one after the engine is started, and off right before I go ICO on my mixture, you should be able to see if you have a problem after you switch it on.
 
To the original question, no, there's nothing wrong with turning the alternator off and back on.

However I've never done that check. Just look for voltage or positive amperage coming out of it with the engine running. On the 414 it had annunciator lights to tell you if an alternator failed, so usually in cruise I'd flip the alternators off one at a time to confirm those worked correctly.

Is yours a belt-driven or gear-driven alternator? If belt-driven, I was a fan of the PlanePower 150A alternators, put those on the 310 and really liked the high amperage delivered at low RPM. If gear driven, Hartzell ALV-9610 is my pick. Also provides a good amperage at low RPM and significantly lighter weight than the 9510s that those engines came with.
 
Cessna had a placard on some of their airplanes that warned you "Do not turn off alternator in flight except in emergency." I believe it was related to the advent of electronic voltage regulators. The rotor in an alternator is the field, and is a husky coil of wire on an iron core. Like any such coil, it is also an inductor and when the field current is abruptly cut off it generates a voltage spike. The old electromechanical regulators didn't much care about that spike, though that spike is also fed back to the alternator's output terminal, since that's where the field current is taken from, and if the diodes in the alternator are a bit marginal you might burn one out. Electronic regulators use a power transistor to regulate the field current, and semiconductors are generally much fussier about being spiked. In the flight school we were getting electronic regulators to replace the old, failing units that the airplanes were built with in the 1970s; electromechanical regulators aren't built anymore. Those electronic regulators were failing at an alarming rate, and I suspect that some instructor was demonstrating or teaching to check the alternator by switching it off and on in the runup. Used to be able to get away with that, but not now.

That was ten years ago. Perhaps the makers of regulators have installed Zener diodes or MOVs to shunt that spike and it doesn't matter anymore. I don't know. Even if they do, you still have the avionics to watch out for.

Some pilots argue that we can't switch automobile alternators off and on, thinking that they are fired by the ignition switch, and so we can't hurt anything in the airplane if we turn it off and on while the engine is running. Those guys don't understand that the car and airplane are wired differently. The car's regulator is turned on by the alternator stator, which generates a small current when the alternator starts turning; that's caused by residual magnetism in the rotor. That is fed to the S terminal on the regulator to close the internal relay (or semiconductor shutdown switch) and allows current from the A terminal, which is connected to the alternator's output, to flow through the regulating circuitry to the field. In such an installation, the alternator is not on at all until the engine is running. When we shut the ignition switch off, the alternator spools down and the regulator opens and a spike can be generated, but with the ignition off it can't reach electronic stuff.

In the airplane, now, that S terminal on the regulator is connected to the ALT switch so that we can shut the system off in case of malfunction in flight such as a stuck or shorted regulator causing massive overcharge, or if the alternator has failed (broken belt, maybe) and we want to stop losing field current through it to save the battery for more important stuff.
 
Never have switched off then back to check it. Put a load on it, like a landing light, to check it, yes.
 
I suspect that if an alternator is charging with a substantial current at the time of breaker or master switch induced shutdown, a massive negative spike is generated which can wipe out a lot of electronics, not just the voltage regulator. The battery might be able to prevent this from propagating thru the system providing it is still on line, but if a master relay or starter relay stutters, all bets are off for any silicon electronics. The fastest fuse is a backward biased silicon junction.
 
If you start on battery only, check gauges, then flip the alternator on and see the change on your gauges, no need for another cycle during run-up.

While we are on the subject, If you run a engine gear driven alternator, like those found on IO-520/550 models, every 500 hours get an IRAN completed. Not an overhaul or rebuild - an IRAN. These alternators are two pieces with a gasket in between. During assembly many times that gasket gets pinched and does not seat correctly. This allows engine oil to go places it should not. Once oil gets past the gasket bad things start happening.
 
If you want to know if the alternator is functioning properly, turn on higher load circuits like the pitot heat. If a student fails to
Mtns2Skies said:
In the FBO recently I overheard a CFI telling their student NOT to cycle the alternator in the runup pad because it's bad for it.

I've always performed this test as a part of runup, turn the alternator off, confirm the ammeter drops and the low voltage light comes on, turn it back on, confirm the ammeter shows a charge.

I wouldn't think de-energizing and re-energizing the alternator field would cause issues... but am I wrong? Is the CFI wrongAsking for myself.
Click to expand...

The test you are performing is easily confirmed by noting the indications pre and post engine start and turning on the pilot heat at runup and noting the indications.
 
Start the engine with the ALT field turned off (saves a few amps for starting too). Turn field on, note amps/volts.
 
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GRG55 said:
I don't cycle it (them) either. I just check the ammeter moves when putting a load like the landing light or pitot heat on momentarily.
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That's what I was taught. I used to use the landing light, but new LED light doesn't draw much current, so now use pitot heat.
 
The venture has low voltage lights so no need to check the alternator. In the Grumman I did as most others and flipped the landing light on and noted a change in amps.
 
Grum.Man said:
The venture has low voltage lights so no need to check the alternator. In the Grumman I did as most others and flipped the landing light on and noted a change in amps.
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That works for a loadmeter, which measures alternator output. Popular with Piper. In a Cessna and many others, an ammeter monitors current in and out of the battery, and if the alternator is working well, the needle will be close to the zero mark once the battery is recharged after start, and turning on heavy loads should make the needle stay the same. It might flicker as the load is applied and the regulator catches up, but it shouldn't drop below zero unless you're at idle RPM and you add a lot of load. And that's all the test you need: turn stuff on--turn everything on--in the runup, and see that the needle stays put. If it drops below zero, there's a problem somewhere.

That applies to alternators, which produce enormous power at relatively low RPM. They are sized to carry at least the entire continuous electrical load even at lower RPM. The old generators were much less capable, with most being around 35 amps and some only 20 or 25, and with old radios you'd be going dark on a long taxi at night. The alternators that replaced those were 60 amps.
 
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