Checking alternator by turning it off?

Matthew K

Matthew K

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Hello all!

I was wondering what your thoughts were on turning off the alternator to check that the gauge responds/waiting to turn the alternator on until after engine is running.

I've heard that waiting to turn the alternator on can hurt the engine from the sudden opposite force from the alternator, but I find that pretty hard to believe that it'd actually do anything unless the gear is weaker than it should be. I've also seen Mraviation101 turn his alternator off on run-up to check for a response. On the plane I fly, a PA-28 180, it says to turn on a full load to check for alternator gauge response.

If it doesn't hurt cars to have the alternator suddenly turn on why the rumor that it'd hurt a plane?
 
Most cars on the road today were designed with electrical systems slightly less than 60 year old technology we have in airplanes.
 
I too have read the educated treatises on how it will over time cause harm, or not. Maybe we will get some of that again in this thread :) . I am of the belief you would never notice anything over the life of an electrical system by turning it on and off more than once each flight.
 
What seems to be the problem with checking it the old fashioned way by putting a momentary load on it?
 
Turning an alternator on after startup won't hurt the engine, and if it failed the drive gear things were pretty badly worn out already. A 60-amp alternator, at 14 volts, under full load, generates 840 watts of power. One horsepower is 746 watts, so you'd be absorbing 840/746 = 1.13 HP, plus a hair more for efficiency losses. Not much load at all. And that's at full load, which is usually more than the whole airplane can use.

An automobile's alternator is turned on by its regulator as soon as the regulator senses a small current from the stator winding, generated by residual magnetism in the rotor. A different setup from the airplane's.

Some airplanes have placards next to the alternator's switch warning you not to turn it off in flight unless necessary. I think voltage spikes generated when shut off under load might be the concern there. But the rotor is most strongly charged when the alternator is still on after the engine is shut down and before the master is turned off, so spikes generated then would be the largest and most damaging to electronics. The radios should be off if the master or alternator are turned off, anyway.

Testing the alternator by adding loads is perfectly adequate. The ammeter should stay at or above zero, or a loadmeter should show an increasing output as items are turned on.
 
Considering that to check operation of both alternators in light twins, you turn each off and back on in succession, and make sure each picks up whatever electrical load is present, during the pre-flight...

I'm going to hazard a guess that turning one off and on while NOT under heavy load (other than post-start battery charge -- and I'd wait at least long enough for that to not be some insane amperage draw before doing the test...) won't cause any problem at all.

Worst case, you did an extremely crappy job checking the belt during pre-flight, didn't even tug on it, and the tiny stress it would see during the flip back on, would pop it.

Might as well get that over with and fixed on the ground, anyway.

So much for physical damage.

Electrical damage to stuff downstream of it, is certainly a concern, but in most installations it shouldn't be much of one with the battery acting as a large but poorly rated capacitor. Not wanting to tempt fate, it's probably smarter to do the test before the avionics master switch is engaged. Stuff is too expensive.

It's also the only way to test the so-called "high voltage" light in most single engine Cessnas. Flip the split master's alternator side off, while leaving the battery on and the light will come on.

(And if you know anything about electronics, you'll quickly realize that it's not just a "high voltage" light, but it actually illuminates under at least three different conditions, driven by the voltage regulator, only one of which is related high voltage. ;)

It should be labeled "Light that the voltage regulator turns on for various reasons, see voltage regulator manual." but I suspect that placard is too wordy. ;)
 
turning the alternator on and off is exactly what the voltage regulator is doing to it to regulate the output voltage, so ....
 
The alternator in a car doesn't suddenly turn off.

The regulator controls the power to the field to control the output and does not cut off the power from the stator like the alternator switch.

The "risk" in using the alternator output on/off switch is from the potential for flyback voltages that have some small potential for blowing a diode or two if there is no other protection. The alternator stator is a big inductor, and the voltage drop across an inductor is equal to the inductance times the rate of change of the current. When you open the alternator output, you get a sudden drop in the current which results in a large voltage spike. Could be hundreds of volts if there is no protection. Just how bad it will be will depend on the current flowing at the time the switch is opened; opening the switch when the alternator is charging heavily results in a much larger inductive kick than if it is just idling. Many applications (such as automobiles) put in a fly-back diode across the alternator output that is normally reverse biased, but when you open the circuit it provides a path for the power stored in the inductor (in the form of a magnetic field) to be dissipated harmlessly. Dunno if your airplane has one. https://en.wikipedia.org/wiki/Flyback_diode

But, in practice, it appears that the risk of damaging the alternator diodes or other avionics from switching the alternator off is somewhere between small and minimal because it is not that uncommon for people to flip the switch without damage.
 
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Th alternator's stator is generating alternating current, which changes direction many times per second, passing through zero each time it does that. Shutting off the alternator doesn't generate a stator spike; it generates a rotor spike. Shutting off an AC coil doesn't make much spike at all unless the frequency is fairly low. The rotor (which is the field) is receiving a DC signal, and an abrupt shutoff of the field can do that.

A voltage regulator comes in a couple of forms. The old electromechanical regulator has a relay that buzzes at a fairly high frequency to rapidly shut off and turn on the field. The inductance of the field prevents the formation of square waves (rapid zero-to-max voltage), and any capacitor attached to the field will modify it further. Most systems have that capacitor as part of radio noise suppression systems. Electronic regulators modulate the field current more smoothly.

The "overvoltage" or "low voltage" light are one and the same. They are the same idiot lights we have in our cars are are fed by the same terminal on the regulator. The connection inside the regulator just looks for a differential between field and output feeds, and triggers the light if a significant differential shows up. Turning off the regulator (which is all the ALT switch does) will fire the light. A failed alternator will fire the light. A dead regulator will fire the light. The ALT switch's feed to the regulator runs through the overvoltage sensor, which is reading the voltage between the bus (the ALT switch's supply) and ground, and shuts off the regulator is the voltage rises to 15 volts or so. The light then triggers, but the overvolt sensor didn't feed the light.

One edit: A failed alternator will often not fire the light. If the failure is due to a regulator failure, it might. If the failure is due to worn brushes in the alternator, which is one of the most common failure modes (along with broken field wiring), the overvolt light won't fire. That's why some get caught with dead radios and lights at night; the only indication was an ammeter showing a discharge for some time before the lights went out.

The car's wiring is a bit different and the idiot light will show a failed alternator. The car is a bit smarter, but you can't shut the alternator off without turning the engine off.
 
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My preference is to turn on a high load device....like an older landing light and watch the load meter move.
 
Checkout_my_Six said:
My preference is to turn on a high load device....like an older landing light and watch the load meter move.
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Or pitot heat. I tend to turn the alternator on after engine start, before turning on the avionics. A CFI told me that it had something to do with the diodes..
 
Matthew K said:
...I've also seen Mraviation101 turn his alternator off on run-up to check for a response...
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<in my head> Oh no, he's claimed another one.

Whatever someone pretentious enough to call themselves "Mr. Aviation101" says, needs to be taken with a box of salt.
 
Timbeck2 said:
<in my head> Oh no, he's claimed another one.

Whatever someone pretentious enough to call themselves "Mr. Aviation101" says, needs to be taken with a box of salt.
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Lol he's a fairly popular YouTuber. I figured I'd mention him since I'd assume most on PoA had at least heard of him
 
GRG55 said:
What seems to be the problem with checking it the old fashioned way by putting a momentary load on it?
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Nothing. I just wanted to see if theirs actually anything backing up the idea to leave alternator on no matter what.
 
Matthew K said:
... a fairly popular YouTuber.
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Careful, people, do not make the common mistake of confusing popularity with quality.
This is a general comment, not aimed at anybody on this forum.
Do I need to throw around names like Adele and Gaga to make my point? :D

Back on topic: do what your checklist tells you. Usually, it calls for activation of the alt field switch once the engine is running. At that point, I check my ampmeter for proper indication. When I turn on other accessories, I check again. You can't go wrong with checking that you're charging, IMO.
 
Nothing wrong with waiting until the engine is running before turning the alternator on. As long as you don't forget. The alternator presents a load on the engine, which the starter has to overcome in addition to starting the engine. Not usually a problem, but on a cold day with a stiff engine and a battery that is far less productive at low temperatures, it can help to wait until after start to turn it on.

It is enlightening to try to turn an alternator by hand when it is turned on (engine off, of course, and belt disconnected). Turn it off and feel it spin freely.

Now, there are some strange systems that need the alternator switched on during start. An example is the Aerostar. Its regulators are fed directly off the alternator output, a system that should have died with the old generators. An alternator has a hard time bootstrapping itself due to the voltage drop across the stator diodes no matter how fast it is spinning. It needs a boost from the bus. An SB on the Aerostar called for a couple of diodes to feed the regulator from the starter contactor so it would come alive up when cranking and put the alternator to work so it would afterward power the regulator (and therefore the field). The alternator had to be switched on for it to stay alive.
 
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Dan Thomas said:
Nothing wrong with waiting until the engine is running before turning the alternator on. As long as you don't forget. The alternator presents a load on the engine, which the starter has to overcome in addition to starting the engine. Not usually a problem, but on a cold day with a stiff engine and a battery that is far less productive at low temperatures, it can help to wait until after start to turn it on.
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Or just an incredibly weak starter matched with an engine that is fussy to start and a feathered prop... :)

Seminole checklist calls for shutting off the Alternator on the side you're attempting an air-restart on, as well as shedding as much overall electrical load as possible so the other alternator can help out Ye Olde Battery as much as possible while you crank... and crank... and crank... and cough/sputter... and dive at 120 knots for some help from blade twisting to unfeather... and crank... and crank... and sputter/cough... and almost catch... and crank... :)
 
BigBadLou said:
Careful, people, do not make the common mistake of confusing popularity with quality.
This is a general comment, not aimed at anybody on this forum.
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Yea, just to be clear, I was just mentioning him to show an example of someone who does it. Not trying to say his method is good or bad.
 
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